A-'jila.colorado.edu/~wcl/wclgroup/Pub downloads/C2H and C2H...where MK is the molar mass of the...

J. Phys. Chem. 1991, 95, 1167-1177 1167

clusters, it has been necessary to estimate them by comparing with prior measurements on clusters of SF6 of similar size made with the same nozzle. It is straightforward to compute the velocity of a gas jet at its terminal Mach number. Clusters in a jet are swept along at the speed of the jet only until the "quitting surface", where the flow becomes too rarified to continue an effective transfer of momentum and energy between the gas molecules and the much more massive clusters. This surface is encountered well before the surrounding gas stops accelerating. Schwartz and Andres4I have shown how to calculate the ratio up,,,/uKm of the terminal velocities of small particles suspended in a gas and the gas, itself, in free jet expansions. For a gas of a given heat capacity ratio y, the velocity ratio is determined by a dimensionless slip number given by4'

(AI) where MK is the molar mass of the gas, Po is the initial pressure of the gas, D, is the diameter of the orifice through which the gas passes, and p p and Dp are the density and diameter of the particles. Schwartz and Andres present plots of the velocity ratio as a function of SLPN at two different y values. In flow through a Lava1 nozzle, collisions persist over a longer distance for a given nozzle diameter and necessitate a revised formulation. We have taken the simple, rather crude expedient of assuming that, for a given nozzle, we can calculate an effective nozzle diameter from a measured velocity ratio, adjusting D, to yield a value of SLPN corresponding to the observed uPm/ugm for the relevant y. This effective diameter, together with the appropriate remaining parameters in eq AI, yields a value for the effective SLPN and, hence, velocity ratio.

The velocity ratio for 104-A clusters of sF6 in neon at 3.9 bar was measured to be 0.87.57 From this information we estimate

SLPN = 3 MKPoD,/ ( 2ppDpR T)

that the velocity ratio for 11 5-A clusters of CCl, in neon at 1.4 bar should be approximately 0.80. To convert this ratio into a terminal cluster velocity, we modeled the flow of CCI, seeded into neon as described elsewhere.20 The inertial contribution of CCI, was retained in the gas-dynamic computations until the quitting surface and then removed for the remainder of the flow. By trial and error it was possible to locate the position in the flow at which the cluster speed uCI = uK was just 0.8uK,. This procedure gave a cluster velocity of 670 m/s. Therefore, the 3.0-mm intervals of sampling along the cluster beam correspond to time intervals of 4.5 ps. It is unlikely that the time scale calculated in this way is more than 10% in error. The effect of such an error would be trivial in the derivation of the nucleation parameter us,.

Determination of Fraction of Clusters Frozen. Because the cluster beam and gas jet both diverge as they flow into the diffraction chamber, and the degree of divergence is different, the diffraction patterns recorded at different positions are not strictly comparable. To adjust them to a common basis suitable for a least-squares analysis of the fraction of drop volume that is frozen, we first "level" them'* and subtract the smooth atomic background of intensity under the peaks. We then rescale the remainder to make the area of the first strong peak in the pattern the same for all patterns. Reference patterns corresponding to the liquid and to the completely frozen patterns are selected, and the observed pattern is fitted by a linear combination of the two reference patterns. The peak for the crystalline form, being much narrower than its liquid counterpoint, is therefore much taller. Least-squares analyses were based largely on the diffraction range 1 < s < 1.75 A-' .

(57) French, R. J.; Bartell, L. S. Unpublished research

Photoelectron Spectra of C,- and CPH-

Kent M. Ervint and W. C. Lineberger* Joint Institute for Laboratory Astrophysics, University of Colorado and National Institute of Standards and Technology, and Department of Chemistry and Biocheniistry, University of Colorado, Boulder, Colorado 80309-0440 (Received: July IO, 1990)

The photoelectron spectra of C2- and C2H- have been measured at a photon energy of 3.53 I eV. The electron affinities of dicarbon, EA(C2) = 3.269 f 0.006 eV, and ethynyl radical, EA(C2H) = 2.969 f 0.006 eV, are determined. The dissociation energy of dicarbon anion, &(Cy) = 187.2 f 2.5 kcal mol-I, is derived from the experimental electron affinities of C2 and C and the literature value for the dissociation energy of neutral dicarbon, D0(C2) = 14_1 .O f 2.5 kcal mol-'. The CC stretch fundamenial and CCH bending vibrational levels up to u = 3 are observed in C2H(X2Zt). The fundamental frequencies for C2H-(X'Zt) are 1800 f 20 cm-' (CC stretch) and 505 f 20 cm-' (CCH bend). The photoelectron spectrum of ethynyl exhibits strong non-Franck-Condon transitions, induced by vibronic coupling, to odd vibrational levels in the CCH bend. These transitions also exhibit different photoelectron angular distributions than the Franck-Condon allowed vibrational transitions. The C2H(A211) excited state was not observed within the experimental energy range, implying a lower limit of 3400 cm-l for its term energy.

I. Introduction Dicarbon, C2, and ethynyl radical, C2H, are abundant in in-

terstellar space's2 and are important in combustion processe~.~ The electronic states of C2 have been studied in detail by absorption and emission ~pectroscopy.~ The C2- anion is unusual in that it has two bound electronically excited state^.^-^ Both C2- and C2H- are candidates for interstellar observation of molecular anion^.^ Ethynyl is of interest spectr~sco_pically'@-'~ because of strong vibronic coupling between the C2H(X2Z+) ground electronic state and the low-lying C2H(A211) excited state. Reaction and quenching processes involving C2H have also been in~estigated. '~

'Present address: Department of Chemistry, University of Nevada, Reno, NV 89557-0020.

0022-3654191 12095-1 167$02.50/0

This work reports the photoelectron spectra of C2- and C2H- and their carbon- 13- and deuterium-substituted analogues. Be-

( I ) Bogey, M.; Demuynck, C.; Destombes, J. L. Astron. Astrophys. 1985, 144. L15-LI6. Combes, F.; Boulanger, F.; Encrenaz, P. J.; Gerin, M.; Bogey, M.; Demuynck, C.; Destomes, J . L. Ibid. 1985, 147, L24-L26. Vrtilek, J. M.; Gottlieb, C. A.; Langer, W. D.; Thaddeus, P.; Wilson, R. W. Astrophys. J .

(2) Lambert, D. L.; Gustafsson, B.; Eriksson, K.; Hinkle, K. H. Astrophys.

(3) Warnartz, J . A.; Bockhan, H.; M o w . A.; Wenz, H. W. Symp. (Int.) Combust. [Proc.] 1983, 19, 197-209. Bastin, E.; Delfau, J.-L.; Reuillon, M.; Vovella, C.; Warnatz, J. Ibid. 1988, 22. 313-322.

(4) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure. IV. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979.

1985, 296, L35-L38.

J. SUPPI. 1986, 62, 373-425.

0 1991 American Chemical Society

1168 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 Ervin and Lineberger

TABLE I: Vibrational Transitions of and ‘TI energy correction

transition eKE,” eV AE,,,C meV A E , , ~ cm-I EA(C2), eV FCFb

a3n,(u’=~) - ~2z;(~”=o) 0.190 f 0.010 - 1 . 1 612.3 3.268 f 0.010 0.748 x’s;(u’=o) - x2z;(u”=o) 0.261 f 0.010 +1.2 0.0 3.272 f 0.010 0.899 a~n,(o’=O) - x2z;(u”=l) 0.406 f 0.005 - 1 . 1 - I 145.7 3.270 f 0.005 0.227 x~z;(c~’=o) - x*z;(o”=l) 0.480 f 0.005 +1.2 -1 757.8 3.269 f 0.005 0.092 a311,(c’=O) - A21TU(u”=O) 0.676 f 0.005 -1.7 -3316.5 3.269 f 0.005 0.997

lJC2 t 13C2- ~ ~ I I , ( u ’ = o ) +- X ~ ~ ; ( U ” = O ) 0.187 f 0.010 - 1 . 1 616.3 3.270 f 0.010 0.738 x’z;(u’=o) - x2z;(u”=o) 0.261 f 0.010 +1.2 0.0 3.271 f 0.010 0.896 a3n,(u’=~) - x2z+(u”= I ) 0.398 f 0.005 - 1 . 1 -1073.6 3.268 f 0.005 0.234 x1z;(u’=o) - X2$(u”=I) 0.471 f 0.005 +1.2 -1689.9 3.270 f 0.005 0.095 a’~,(u’=O) t A211,(u”=0) 0.675 f 0.005 - I .7 -33 14.6 3.270 f 0.005 0.997

‘2C2 - ‘2C,-(C)

3.269 f 0.006c

3.270 f 0.006f

Electron kinetic energy of peak center. Franck-Condon factors calculated in the Morse oscillator approximation. Energy shift due to unresolved rotational contour and spin-orbit splittings. dVibrational and electronic term energy, calculated from molecular constants given in Table 11. rEA(12C2). fEA(”C2).

cause of their high electron affinities, the photoelectron spectra of C2H- and Cy have not been accessible with visible lasers. With recent modificationsI6 of the laser system in our negative ion photoelectron spectrometer for operation with near-ultraviolet light, we can now measure the photoelectron spectra of these species. We determine the electron affinities of C2 and C2H and discuss the electronic and vibrational structure of ethynyl.

The experimental methods are outlined in the next section. The experimental results for CF are presented and discussed in section 111, and the photoelectron spectra of C2H- are discussed in section IV. Thermochemical results are derived in section V, followed by a brief summary in section VI . 11. Experimental Methods

Negative ion photoelectron spectro~copy’~ is performed by crossing a mass-selected beam of anions with a fixed-frequency laser and measuring the kinetic energies of photodetached electrons. The experimental apparatus and procedures have been described in detail previ~usly.’~*~* Briefly, anions are produced in a microwave discharge flowing afterglow ion source, extracted, focused into a beam, and passed through a Wien filter for mass selection. The ion beam then intersects a 351.1-nm ( h v = 3.531 eV) laser beam in an optical buildup cavity with a total circulating power of 30-40 W. The kinetic energies of photodetached

( 5 ) Jones, P. L.; Mead, R. D.; Kohler, B. E.; Rosner, S. D.; Lineberger,

(6) Hefter, U.; Mead, R. D.; Schulz, P. A,; Lineberger, W. C. Phys. Reu.

(7 ) Mead, R. D.; Hefter, U.; Schulz, P. A,; Lineberger, W. C. J . Chem.

(8) Oka, T. Philos. Trans. R. SOC. London, A 1988,324,81-95. Rehfuss, B. D.; Liu, D.-J.; Dinella, B. M.; Jagod, M.-F.; Ho, W. C.; Crofton, M. W.; Oka, T. J . Chem. Phys. 1988, 89, 129-137.

(9) Wallerstein, G . Asrron. Astrophys. 1982, 105, 219-220. Vardya, M. S.; Krishna Swamy, K. S. Chem. Phys. Lerr. 1980, 73,616-617. Herbst, E. Nature 1981, 289.656457. Sarre, P. J . J . Chim. Phys. 1980, 77,769-771.

( I O ) Carrick, P. G . ; Pfeiffer, J.; Curl, R. F., Jr.; Koester, E.; Tittel, F. K.: Kasper, J. V. V. J . Chem. Phys. 1982, 76, 3336-3337.

( I I ) Carrick, P. G . ; Merer, A. J.; Curl, R. F., J r . J . Chem. Phys. 1983,

(12) Curl, R. F., Jr.; Carrick, P. G . ; Merer, A. J. J . Chem. Phys. 1985,

(13) Yan, W. B.; Hall, J. L.; Stephens, J. W.; Richnow, M. L.; Curl, R . F., Jr . J . Chem. Phys. 1987.86. 1657-1661.

( 1 4) Jacox, M. E.; Olson, W. B. J . Chem. Phys. 1987,86, 3 134-42. ( I S ) For example: Stephens, J. W.; Hall, J. L.; Solka, H.; Yan, W.-B.;

Curl, R. F.; Glass, G . P. J . Phys. Chem. 1987, 91, 5740-5743. Lander, D. R.; Unfried. K. G.; Stephens, J. W.; Glass, G . P.; Curl, R. F. J . Phys. Chem. 1989, 93. 4109-41 16. Shokoohi, F.; Watson, T. A,; Reisler, H.; King, F.; Renlund. A. M.; Wittig, C. J . Phys. Chem. 1986, 90, 5695-5700.

(16) Ervin, K . M.; Ho, J.; Lineberger, W. C. J . Chem. Phys. 1989, 91,

(17 ) Mead, R. D.; Stevens, A. E.; Lineberger, W. C. In Bowers, M. T., Ed. Cas Phase Ion Chemistry; Academic: New York, 1984; Vol. 3, pp 213-248. Drzaic, P. S.; Marks, S.; Brauman, J. I . Ibid., pp 167-21 I .

( I 8) Leopold, D. G.; Murray, K. K.; Stevens Miller, A. E.; Lineberger, W. C. J . Chem. Phys. 1985,83, 4849-4865.

W. C. J . Chem. Phys. 1980, 73. 4419-4432.

A 1983, 28, 1429-1439.

Phys. 1985.82, 1723-1731.

6, 3652-3658.

82, 3479-3486.

5974-5992.

electrons are measured by a hemispherical electrostatic energy analyzer, which has a resolution of 8 meV fwhm for electron kinetic energies above 0.3 eV. Recording the photoelectron intensity as a function of the electron kinetic energy yields the photoelectron spectrum.

Dicarbon anions ( 1 0-50 PA) and acetylide anions ( 1 00-300 PA) are produced by the reaction19 and 0- with acetylene (Matheson, 99.6%) in the flowing afterglow ion source. Oxygen is added to the helium buffer gas ( O S Torr) upstream of a microwave discharge to produce 0-, and the acetylene is leaked in downstream of the discharge with flows optimized for the ion of interest. Isotopically substituted acetylene, I3C2D2 (Cambridge Isotopes; 99% 13C, 98% D), is employed to produce I3CF and I3C2D-. In some experiments, the reaction of 0- with ethylene or deuterated ethylene is used to make I2C2H- or I2C2D-; the acetylide anions in this case are produced by a secondary reaction.I6

Electron binding energies are given by the difference between the photon energy and the measured electron kinetic energy (eKE). The photoelectron spectrum of S-, produced from SO2 in the flowing afterglow ion source, is used to calibrate the electron energy scale. The absolute energy scale is calibrated against the electron affinity of sulfur,20 EA@) = 2.077 120 f 0.000001 eV. A linear correction factor for the relative energies, typically OS%, is based on the 9239.0-cm-’ spacing2’ of the S(lD2) +- S-(2P3/2) and S(3P2) - S-(’P3/2) transitions, which fall at eKE = 0.308 eV and eKE = I .454 eV, respectively, for 35 I . 1 -nm laser light. The experimental uncertainty of the absolute electron kinetic energy of well-resolved peaks in the photoelectron spectra is iO.005 eV. Relative energies, the spacing between peaks in the photoelectron spectrum, have uncertainties of i0.002 eV.

The sensitivity of the electrostatic energy analyzer is independent of the electron kinetic energy (within 20% over a I-eV interval) above eKE = 0.3 eV, based on diagnostic measurements of the Franck-Condon profiles in the photoelectron spectrum of 02-. At lower kinetic energies, the transmission function of the analyzer falls off precipitously because of the influence of stray fields on low-energy electrons; no photoelectrons are observed with kinetic energies below 0.1 eV. The origin transition for C; lies in this falloff region, eKE ;= 0.26 eV. To enhance the sensitivity below 0.3 eV as much as possible for CT, the electron optics were adjusted for some experiments to use a higher transmission energy through the electron energy analyzer. The relative sensitivity improves by a factor of 2 at eKE = 0.14 eV (measured for the 0 ( ! D 2 ) - 0-(2P3/2) transition) under these conditions, with a degradation of the resolution to 10-15 meV as a tradeoff.

(19) Viggiano, A. A,; Paulson, J. F. J . Chem. Phys. 1983, 79, 2241-2245. (20) Hotop, H . ; Lineberger, W. C. J . Phys. Chem. Re/. Data 1985, 14,

(21) Moore, C. E. Aromic Energy Leuels; US. National Bureau of 7 3 1-750.

Standards: Washington, DC, 1949; Vol. I. Circ. 467.

Photoelectron Spectra of C2- and C2H- The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1169

TABLE 11: Molecular Constants of C, and C,-

1854.5881 (83) 13.2730 (38)

I .8 19927 (24) 1.175231 (56) -2.022 ( I 3) 0.0 1.2425 22

-0.18242 (51) 1 1.66474 (24)

1.6325323 (36) 1.65452 (46) 0.212 (12)

1.3119 23

-15.2723 (28)

1781.189 (18) 11.6717 (48) -0.009981 (28) 1.74666 (32) 1.651 (46)

0.0 1.2683 8

“wez, = -0.01 53 (17) cm-I, Yso = -0.00041 (7) cm-’. Y31 = -0.000054 (5) cm-l.

Transitions observed at these lowest measurable kinetic energies tend to be broadened and slightly asymmetric, resulting in an increased uncertainty of *I0 meV for the positions of peaks below eKE = 0.3 eV.

111. Dicarbon A. Photoelectron Spectrum. The photoelectron spectrum of

C<, Figure I , shows a series of irregularly spaced peaks at binding energies from 2.85 to 3.34 eV. No additional transitions were observed in survey spectra covering lower binding energies. Table I lists the energies of the five observed transitions. Using the literature vibrational constants and term collected in Table I 1 for C2-(X2Z:), Cy(A211,), C2(XIZt), and C2(a311,), we find a unique vibronic assignment of the transitions as indicated in Figure 1 and Table I. We observe the vibrational origins of the C2(X1Z:)(r4) + C2-(X2Z:)(a4u1), C2(a311,)(a3ul) - C2-(X2Z:)(r4ul), and C2(a311u)(r3u1) - C~(A211,)(r3u2) electronic transitions, as well as the C,(X)(u’=O) - C2-(X)(u”=l) and C2(a)(u’=0) - C;(X)(u”=l) hot bands. The C2(XlZ:)(r4) 7 Cy(A211u)(r3u2) photodetachment transition is not observed: it would involve a two-electron process and is therefore very weak. The energy spacings of the observed transitions match the values calculated from the known molecular constants within the experimental uncertainty.

The vibronic assignments are further confirmed by the spectrum of I3Cy. The 0 + 0 origin bands show no isotope shifts (zero-point energy differences, EA(I2C2) - EA(I3C2) = 0.18 meV, are neg- ligible a t the present resolution), while the 0 + 1 hot band frequencies (Table I) decrease by 4%, as expected24 from the change in reduced mass.

The vibrational origin peaks of the C2(XlZ:) - Cc(X2Z:) (eKE = 0.261 f 0.010 eV) and C2(a311u) - C2-(X2Z:) (eKE = 0.190 f 0.01 0 eV) transitions lie in the low-energy region where the sensitivity of the electrostatic energy analyzer is reduced. Therefore, the intensities of these peaks are suppressed relative to the hot bands, and higher vibrational levels of the C2(XlZ:) and C2(a311,) states are beyond our energy range. The presence of the vibrational and electronic hot bands indicates that the C y anions are formed at elevated temperatures in the flowing afterglow ion source. Since the X - X and a + X vibrational origin transitions are in the region where the detection sensitivity is poor, however, the relative intensities cannot reliably be used to determine thc anion temperature or Franck-Condon intensities.

B. Electron Affinity. Because the energy spacings of the observed transitions are precisely known from literature molecular parameters (Table I I ) , we can use any one of the observed peaks to determine the adiabatic electron affinity, which corresponds to the electron binding energy of the C,(X’Z:)(u’=O) +

C2-(X2Z+)(u”=O) transition. Individual transition energies, listed in Table 1, are obtained from Gaussian tits to the peaks averaged from three independent scans for I2Cy and two for I3C2-. We

~ ~~~

(22) Davis, S. P.; Abrams, M. C.; Phillips, J. G.; Rao, M. L. P. J . Opt.

(23) Amiot, C.; Chauville, J.; Maillard, J.-P. J . Mol. Spectrosc. 1979, 75,

(24) Herzberg, G. Molecular Spectroscopy and Molecular Structure. I Specfra of Diatomic Molecules, 2nd ed.: Van Nostrand Reinhold: New York, 1950.

SOC. Am B 1988. 5, 2280-2285.

19-40,

1666.4 (io) 1969.542 (84) 10.80 (26) 15.100 (57)

-0 .135 (16)” 1.64305 (334) 1.87718 (27) 1.601 (44) 1.887 (28)

1 .15 (67)b -25.009 (1 5) 0.0 1.3077 I .2234 8 7

ELECTRON KINETIC ENERBV few

1 1 0 .o 0.2 0.4 0.6 0 .(I

B

I O.s 0 .o

Figure 1. (top) Photoelectron spectrum of I2C2- obtained with 351.1-nm laser light and the magic polarization angle, 0 = 54.7’. The electron binding energy (lower scale) is the photon energy (3.531 eV) minus the measured electron kinetic energy (upper scale). Experimental intensities below electron kinetic energies of 0.3 eV are suppressed by the falloff of the sensitivity of the electron energy analyzer. Electronic and vibrational assignments are labeled. (bottom) Franck-Condon simulation of the photoelectron spectrum of C; (see text). The vertical sticks represent calculated transition intensities; satellite transitions are vibrational sequence bands. The solid line gives the convolution of these transitions over the instrumental resolution function. The vibrational temperature for the simulation is arbitrarily set a t 1000 K.

must correct these energies for shifts in the peak positions due to unresolved rotational structure and spin-orbit splittings. For this purpose, the rotational line spectrum for each electronic transition is calculated by using molecular constants from Table I 1 and the usual selection rules and Hod-London factors24 for each electronic transition, neglecting nuclear spin statistics. Anion rotational level populations are specified by a Boltzmann distribution with the estimated rotational temperature of 300-350 K. Spin-orbit states are given statistical populations for the C,(A211,) ( J = 3/2) initial states and equal weightings for the C2(a311,) ( J = 0, 1,2) final states. The spin-orbit splittings are small enough that this approximate treatment does not introduce significant errors. The calculated line spectra are then convoluted with the instrumental resolution function, a Gaussian with 8 meV fwhm. The offset of the centroid of the resulting contour from the ro-

1170 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 Ervin and Lineberger

TABLE 111: Electron Affinitv of C, (eV) EA(C7) method vear ref . .,

3.268 f 0.007 3.3 0.1 3.374 5 EA(C2) 5

3.408 3.54 0.0s 3.3 0.2

3.1 or 4.0 3.1 12 3.30

22.9

photoelectron spectroscopy photoelectron spectroscopy autodetachment spectroscopy

photodetachment threshold electron impact electron impact graphite sublimation theory (MCSCF-MCEP) theory (Cl SDQ-I Ref)

- 1990 1988 1980

1970 1970 1963 1954 1987 1980

this work 26 5

25 27 28 29 31 30

tationless origin is used to correct the corresponding transition energies. These shifts are listed in Table I. Finally, the known term energy difference for each transition is subtracted or added to obtain the electron affinity. Averaging the electron affinities determined from the five observed transitions each of I2C2 and 13C2, we obtain EA(C2) = 3.269 f 0 . 0 0 6 eV.

Table I l l reviews previous experimental determinations of the electron affinity of dicarbon. The previously accepted value came from high-resolution autodetachment experiments on CF by Jones et al.,5 who reported limits on the electron affinity of dicarbon, 3.408 eV I EA(C2) I 3 .374 eV. The present result agrees with the upper bound from that work but disagrees sharply with the lower bound. The upper boundS is a strict experimental limit based on the observation of autodetachment from the u = 5 levels of the B2ZT states of I3CF, 12C13C-, and 13C<. For autodetachment to occur, neutral dicarbon must lie below the transition energy for the lowest ener y autodetaching state,s v = 2 7 4 9 0 cm-I (3.408

The lower bound from the autodetachment experiment^,^ EA(C2) 2 3 .374 eV, was chosen on the basis of the observed autodetachment rates. We discuss the autodetachment rates and alternative interpretations in section 1II.D.

Early low-resolution photodetachment threshold measurements by F e l d n ~ a n ~ ~ gave EA(C2) = 3.54 f 0.05 eV. Jones et aLs suggested that the observed threshold25 was not due to the ground-state transition, C2(X1Z:) - C2-(X2Z8+), which is expected to have a slow onset because it involves p-wave electron detachment, but rather corresponded to the production of the C2(a31’I,) excited state, which involves s-wave detachment and is therefore expected to have a sharper threshold. This reassignment would give EA(C2) = 3.46%;; eV. Even this lower value, however, is incompatible with the present result. It is probable that these early xenon lamp experiments simply lacked sufficient sensitivity to see the true threshold for either neutral state. The same photodetachment threshold e ~ p e r i m e n t ~ ~ overestimated the C2H electron affinity by 0.76 eV.

A recent measurement26 of the dicarbon electron affinity by time-of-flight photoelectron spectroscopy of carbon cluster anions produced by laser vaporization yielded EA(C2) = 3.3 f 0.1 eV. This determination is in good agreement with the present result, although the error limits also include the previous autodetachments value. Electron affinity values from electron impact experi- m e n t ~ ~ ~ * ~ ~ agree with the present determination within their uncertainties. An early graphite sublimation m e a s ~ r e m e n t ~ ~ gave EA(C2) = 3.1 eV or 4 . 0 eV, depending on the method of determination.

Accurate theoretical determination of the electron affinity of C2 is most challenging because extensive treatment of correlation energy is required. Two recent value^^^,^^ of 3.1 1 and 3.30 eV are in good agreement with experiment; indeed, the agreement is improved compared to the previous experimental value. C. Potential Energy Curves of C2 and CF. Potential energy

curves and vibrational levels for low-lying states of C2 and C2-

eV) for I3C2(B2ZU)(u=5). 8

(25) Feldman, D. Z. Naturforscfi. A 1970, 25, 621-626. (26) Yang, S.; Taylor, K. J.; Craycraft, M. J.; Conceicao, J.; Pettiette, C.

L.; Cheshnovsky. 0.; Smalley, R. E. Cfiem. Pfiys. Lett. 1988,44,431-436. (27) Locht. R.; Momigny. J. Cfiem. Pfiys. Lef t . 1970, 6 , 273-276. (28) von Trepka, L.; Neuert, H. Z. Narurforsch. A 1963, 18, 1295-1303. (29) Honig, R. E. J . Cfiem. Pfiys. 1954, 22, 126-131. (30) Dupuis, M.; Liu, B. J . Cfiem. Pfiys. 1980, 73, 337-342. (31) Nichols, J . A.; Simons, J . J . Chem. Pfiys. 1987, 86, 6972-6981.

3 5 i 3 0 .

> (3 u 15 W z W

I O

5

i

0 1 1 I O 1 2 1 4 1 6 1 8

r ( 8 ) Figure 2. Rydberg-Klein-Rees potential energy curves for IzC2 and ‘ T T calculated from molecular constants from the literature given in Table I1 and the present value for EA(Cz).

are shown in Figure 2. The energy curves are generated by the Rydberg-Klein-Rees, method32 from literature vibrational and rotational constants (Table 11). The spacing between the netural and anion curves is fixed by the electron affinity measured in this work. With EA(C2) = 3.269 f 0.006 eV, the u = 0 levels of both the C2(X) and C2(a) states lie below the u = 5 level but above the v = 4 level of the C2-(B) state.

Although only the origin and hot band transitions of the photoelectron spectrum of C y lie within our energy range, the vibronic assignments are unambiguous because the molecular parameters of the low-lying states of C2 and C c are precisely known. Figure 1 compares the experimental spectrum to a Franck-Condon simulation. The simulation uses the known term energies, vibrational constants, and bond lengths (Table 11) in a Morse oscillator approximation, which is satisfactory since only u = 0 and L; = 1 levels are observed. The Franck-Condon factors, listed in Table I, are found by numerical integration of Morse oscillator wave functions, which are calculated analytically by Laguerre series recursion The most important unknown parameter in the simulation is the electron affinity of C2, which is fixed by the observed peak positions as discussed above. The transition intensities are determined by the calculated Franck- Condon factors, by the anion vibrational and electronic temperatures, and by the relative detachment cross sections for the various electronic transitions. The line shapes are modeled as described above in section II1.A. Because the instrumental sensitivity falls off in the low-eKE region where the origin lies, the temperatures and electronic transition strengths cannot be extracted from the experiments. Instead, we use reasonable temperature estimates to show approximate transition intensities. As expected, the simulated spectrum, Figure 1, matches the experimental spectrum well except for the intensities at low eKE. The positions of the fundamental transitions ( 1 - 0), which are beyond our experimental energy range, are also shown in the simulation. The match of the experimental and simulated spectra further confirms our vibronic transition assignments.

D. Autodetachment Rates. The erroneous lower bound derived from autodetachment experiments, EA(C,) L 3.374 eV, compared to the present value of EA(C2) = 3.269 f 0.006 eV, was based on an interpretation of the observed autodetachment rates from

(32) Zare, R. N . J . G e m . Pfiys. 1964, 40, 1934-1944. (33) Halmann, M.; Laulicht, 1. J . Cfiem. Pfiys. 1965,43,438-448. Engler,

C. Z . Phys. Chem. (Leipzig) 1984, 6, 1193-1200.

Photoelectron Spectra of C2- and C2H-

various vibrational levels of the C2-(B2Z:) state. It was observed that the autodetachment from B(u=5) is more than IO times slower than from higher vibrational levels, B(u16), in IT2, W 3 C , and I3C2. Because autodetachment to C2(X'Z; from Cc(B2Z:) requires a two-electron transition, it is expected to be much slower than autodetachment to the C2(a311,) state, which is a oneelectron process.j4 To account for the large change in autodetachment rate between u = 5 and u = 6, it was postulated that the u = 5 level of Cc(B) lay between the u = 0 levels of the C,(X) and C2(a) states, thus energetically allowing the more favorable channel for u = 6 but not for u = 5. Since the C2(X) and C2(a) states are separated23 by only 61 2 cm-I, this conclusion placed tight limits on the electron affinity of C2. These argumentsS were compelling based on the data available a t the time, but the present results show that both the C2(X) and C2(a) ground vibrational levels lie below u = 5 of C2- (Figure 2). Therefore, the postulated mechanism cannot be responsible for the observed autodetachment rates.

An alternate interpretation of the autodetachment rates discussed by Jones et aLS would require two vibrational levels of either C2(XIZ:) or C2(a311,) to lie between the u = 5 and u = 6 vibrational levels of C2-(B2Z:). This situation would decrease the minimum change in vibrational quantum number upon autodetachment for u = 6 compared to u = 5, which might be expected to increase the autodetachment rate. However, the present electron affinity is also incompatible with this mechanism, since it places only one vibrational level in each of the C2(X) and C2(a) states between the u = 5 and u = 6 levels of the Cc(B) state (Figure 2). Nichols and Simons31 have also discounted this possible mechanism on theoretical grounds, namely, that the propensity rulesS favoring smaller changes in vibrational quantum number are not applicable in this case. Furthermore, while the change in autodetachment rates was observed in I2C2, I2CI3C, and I3CZ, detailed examination of the vibrational spacings indicates that no reasonable value of the electron affinity could place two of the vibrational levels in C2(X) or C2(a) between u = 5 and u = 6 of C2-( B) simultaneously for all three isotopes.

Ab initio calculations by Nichols and Simon3' suggested another mechanism, that a crossing might occur between the electronic potential curves of C2-(B2Z:) and C2(a311,) between the u = 5 and u = 6 levels of the C2-(B) state. This would allow purely electronic autodetachment, which is much faster than vibrationally induced autodetachment, to occur for the u = 6 and higher levels but not for u = 5. The potential curves calculated by Nichols and Simons indicated such a crossing was possible, although these authors stopped short of declaring that it in fact exists. The RKR potential curves in Figure 2, however, indicate that the crossing does not occur until higher vibrational levels, near u = 9-10 in the C2-(B) state. Since the vibrational levels involved, u = 0-4 of C2(X) and C2(a) and u = 5-8 of C2-(B), have been directly observed by high-resolution rotational s p e c t r o s c ~ p y , ~ - ~ ~ ~ ~ ~ ~ ~ the RKR potential curves are believed to be accurate. Thus, the available experimental data do not support the suggestion that crossing of the C2(a) and C2-(B) electronic curves is responsible for the observed autodetachment rates.

The dramatic change in autodetachment ratesS between u = 5 and u = 6 of C2-(B) is not understood at present. One possibility would be a radiative or nonradiative transition from u = 5 in C2-( B) to another electronic state of C<. In order to explain the results, such a process would have to be fast compared to autodetachment of the C2-(B) state, and the resulting anion state would have to have a much slower autodetachment rate. Furthermore, these conditions would have to exist for the u = 5 level, but not for u L 6. We note that there is a near-resonance between C2-(B)(u=5) and C2-(A)(u=16). The high-lying levels of the C2-(A) state are known from perturbations in the C2-(B) state ~ p e c t r u m . ~ Using the vibrational constants in Table 11, we find the energy difference between these two levels is 187 cm-' for J = 0 (see also Figure 2). No such near-resonance exists for C2-(B)(u=6-9) and nearby states of C;(A). It is possible that a nonradiative transition involving the C,-(A) state could be

The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1171

ELECTRON KINETIC ENERGY (rV1 0.2 0.4 0 . 6 0 .e

ELECTRON EINOING ENERGY (eVJ

Figure 3. Photoelectron spectrum of l2C2H- obtained with 351 .I-nm laser light at the magic polarization angle, 8 = 54.7". The electron binding energy (lower scale) is the photon energy (3.531 eV) minus the measured electron kinetic energy (upper scale). The most-intense transition corresponds to the vibrational origin of the C2H(X2Z+) - C2H-(R'2+) electronic transition. Vibrational assignments of other prominent transitions are labeled.

responsible for the observed autodetachment rates. Circumstantial evidence for such a process consists of the near-resonance, the favorable Franck-Condon overlap of the two states near the inner classical turning point (see Figure 2), and the known perturbations7 between the A and B states of C2-. Although the relative rates for autodetachment versus a nonradiative transition from Cy( B) to CT(A) are unknown, the calculated radiative lifetimes of C;(A) states are relatively short,34 suggesting that radiative relaxation may be competitive with autodetachment from C,(A). Additional information is required to settle this question; detailed theoretical examination of possible mechanisms would be helpful.

IV. Ethynyl A . Photoelectron Spectrum. The photoelectron spectrum of

C2H- is presented in Figure 3. Survey spectra at electron binding energies lower than shown in Figure 3 indicate no additional photoelectron transitions. The spectra of 12C2H- and I3C2D- are compared in Figure 4 on a scale which shows the weaker transitions. Vibrational transition energies are listed in Table IV; these values represent averages from 20 independent scans for 12C2H-, three for 12C2D-, and six for I3C2D-.

B. C2H Vibrational Assignments. In the photoelectron spectrum of C2H- (Figure 3), there is a single predominant peak which is characteristic of a vertical transition with little change in geometry between the anion and the neutral species. In this case, the vibrational origin transition ( o f = 0 - u" = 0) has the strongest Franck-Condon intensity. Hence, the main peak at eKE = 0.56 eV can be identified as the vibrational origin. The shoulder on the right side of the origin peak is the 21 bending sequence transition.

Vibrational transitions can be assigned according to their characteristic frequencies and isotope shifts. The frequency of the second most intense transition, separated from the origin by 1850 f 20 cm-' in I2C2H, is characteristic of the CC stretch normal mode ( u 3 ) . This transition exhibits isotope shifts 5.1% to 1755 f 20 for I2C2D and 7.6% to 1710 f 20 cm-' for I3C2D. These isotope shifts are a few percent larger than what would be expected for a pure local CC stretch, indicating that the CC stretch normal coordinate includes some C H stretch motion. The CC stretch in I2C2H has been observed in high-resolution infrared spectroscopy experiment^^^ on the ethynyl radical; the reported value of 1840.5712 f 0.0004 cm-' agrees well with the present observation.

(34) Rosmus, P.; Werner, H.-J. J . Chem. Phys. 1984, 80, 5085-5088. (35) Kanamori, H.; Seki, K.; Hirota, E. J . Chem. Phys. 1987, 87, 73-76.

Ervin and Lineberger

for the 2; bend in I2C2H, 365 f 20 cm-I, matches the precise infrared value36 of 371.6034 f 0.0003 c d . The agreement with the infrared experiments confirms our vibrational assignments and also confirms that the observed spectrum is due to the X:Z,+ electronic ground state of C2H (as opposed to the low-lying A 2 n excited state).

The appearance of the 2; and 2; bend transitions in the spectrum of C2H- is surprising because the bend is a non-totally symmetric mode (since it reduces the C,, linear symmetry to C, planar symmetry). According to the Franck-Condon principle, the selection rules for non-totally symmetric modes are Au = 0, f2, f 4 , ...; Le., only even transitions are allowed. The appearance of both even and odd transitions indicates a breakdown of the Franck-Condon approximation. Mechanisms for the non- Franck-Condon behavior are discussed in section 1V.H. C. C2H- Vibrational Assignments. Transitions originating from

vibrationally excited acetylide anions are also observed. Vibra- tionally cooler anions are produced when an excess of acetylene is used in the reaction with 0-, such that most of the oxygen anions have reacted away before the end of the flow tube of the ion source. Hotter anions are produced when there is a deficiency of acetylene. The vibrationally hot anions are also largely removed by adding a quencher gas such as propylene or isobutane to the helium buffer gas downstream of the acetylene inlet. These observations indicate that the 0- + HCCH reaction initially produces vibrationally excited C2H-. The reduction of the excited population upon addition of excess acetylene suggests that acetylene rapidly quenches excited C2H- (collisionally or by proton-transfer reactions) or that excess acetylene leads to completion of the reaction early in the flow tube, allowing enough collisions with the helium buffer gas to quench the vibrations, or some combination of these two mechanisms.

The peaks that change relative intensity when the ion source conditions are changed can be assigned to transitions arising from vibrationally excited anions. Vibrational assignments of the hot and sequence bands are indicated in Table IV. ,We find that the CC stretch fundamental frequency in '*C2H-(X1Z+) is 1800 f 20 cm-I and the CCH bend frequency is 505 f IO cm-I. The transitions involving the CC stretch in C2H- show the greatest change in intensit . We observe changes of a factor of 50 in the intensity of the 31 CC stretch hot bands and a factor of 10 for the 21, 2:, and 2: sequence bands of the bending mode (which appear as partially resolved shoulders on the right side of the origin peak) and the 27 and 2: hot bands. Comparison of the intensities of the 3; and 37 transitions indicates the vibrational temperature for this mode varies from 450 to 2000 K, depending on ion source conditions. The lower frequency bend levels are more readily quenched by collisions with the helium buffer gas in the flowing afterglow ion source and therefore have closer to room-temperature populations, 450-750 K.

Our value for the CC stretch frequency in C2H-, 1800 f IO cm-l, conflicts with a frequency of 1758.621 f 0.003 cm-I determined by Gruebele, Polak, and Saykally'' using velocity modulation infrared laser spectroscopy. The assignment of the latter work has been questioned by B o t s ~ h w i n a , ~ ~ whose ab initio calculations yield 1815.5 cm-' directly, with a scaled prediction of 1807 f 5 cm-' based on calculated frequencies for HCCH. Botschwina's theoretical rotational constant, 1.389 f 0.002 cm-I, also disagrees with the value of 1.38 145 & 0.00026 cm-I for the transition observed by Gruebele et aL3' Our experimental frequencies are in excellent agreement with Botschwina's theoretical values38 for the CC stretch and also for the CCH bend frequency: u2 = 505 f 20 cm-l observed versus u2 = 510.5 cm-I calculated. We conclude that the assignment of the transition observed by Gruebele et aL3' to C2H- is in error. While the carrier of the observed transition is uncertain, recent thermochemical data

I K

1172 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991

ELECTRON BINDING ENERGY faVI

4000 2000 Transi t ion FrEQIJenCY ~ca-'I

Figure 4. Photoelectron spectra of I2C2H- (top) and "C2D- (bottom) obtained with 351 .I-nm laser light at the magic polarization angle, 0 = 54.7'. Thc electron binding energy (lower scale) is the photon energy (3.531 eV) minus the measured electron kinetic energy (upper scale). The top traces are magnified by a factor of 20 and offset from zero. Assignments of prominent vibrational transitions are labeled.

TABLE I V C2H- Vibrational Transition Frequencies (an-')'' transition

- I800 -1 705 -1655 3P -1 100

-790 -1015 ( -1065 2a3:

-795 2: -505 [-3951 -385 2: -220 -205 2: -135 -120 -105 21

365 270 275 2; 710 555 2: 790 605 605 2;

1230 930 925 2;

1850 1755 1710 3A

0 0 0 0-0

1595 1595 1525 2t3; 1685 2:3;

2120 2015 1920 2A3; 2455 [2240] 2170 2:3; 2550 2255 2i3A 2935 2555 2i3;

2805 ?C

(34351 [3310] ?C

a Measured peak positions relative to the 0-0 origin; *20-cm-' un- CH stretch; v2, CCH

The bending normal mode ( v 2 ) transitions (2 ; , 2& and 2 3 have frequency spacings of a few hundred wavenumbers and shift dramatically (20-26%) upon deuteration. The observed frequency

certainty [&40 cm-' for values in brackets]. bend; v3, CC stretch. eUnassigned (see text). (36) Kanamori, H.; Hirota, E. J . Chem. Phys. 1988, 89, 3962-3969.

(37 ) Gruebele, M.; Polak, M.; Saykally, R . J. J . Chem. Phys. 1987, 87, 1448-1449. Owrutsky, J.; Rosenbaum, N.; Tack, L.; Gruebele, M.; Polak, M.; Saykally, R. J. Philos. Trans. R . Soc. London, A 1988, 324, 97-108.

(38 ) Botschwina, P. In Maier, J. P., Ed. Ion and Cluster Ion Spectroscopy and Structure: Elsevier: Amsterdam, 1989; pp 59-108.

Photoelectron Spectra of C2- and C2H- The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1173

ELECTRON KINETIC ENERGY (eV) 0.2 0.4 0.6 0 .e TABLE V: Electron Affinity of C,H (eV)

'%,H- 0-0

e = 0'

= e = "

5 lo' e B E 10' Y i

102

101

3.4 3.2 3 .o 2 .e ELECTRON BINDING ENERGY IoVJ

Figure 5. Laser polarization dependence of the photoelectron spectrum of W2H- . The solid line represents a laser polarization angle of 0 = Oo, and the connected circles represent 0 = 90°, where 0 is the angle between the electric field vector of the light and the direction in which the photodetached electron is ejected. The two spectra were taken with identical ion current, laser power, and collection time. Vibrational assignments are labeled.

provide an explanation for the lack of a large C2H- population under the discharge conditions utilized by Gruebele et a1.37939 Their discharge used a mixture of NF3, as a source of F, with acetylene. Recent work40 on the gas-phase acidity of acetylene indicates that the reaction F + HCCH s C2H- + H F is more endothermic, AG,,,,298 = +4.27 f 0.20 kcal mol-], than the 2.9 f 2.0 kcal mol-' given in the earlier l i t e r a t ~ r e . ~ ~ The equilibrium therefore favors F over C2H-, providing an explanation for the difficulty in finding the C2H- vibrational transition in the discharge.39

D. Photoelectron Angular Distributions. Most of the photoelectron spectra reported in this work are taken with the laser polarization at the "magic" angle of 6 = 54.7', where 6 is the angle between the electric field vector of the laser light and the direction in which photodetached electrons are detected. The magic angle spectra yield inensities proportional to the total cross section.4z Figure 5 presents photoelectron spectra of C2H- at different po- larizations, 6 = Oo and 6 = 90°. We observe a striking pattern: the origin and all of the other Franck-Condon allowed transitions have strong intensity maxima at 6 = 0'; all non-Franck-Condon transitions (those with odd changes in the CCH bend vibrational quantum number) exhibit a maximum at 6 = 90°. This is the first case in negative ion photoelectron spectroscopy where vibrational bands within a single electron transition show markedly different photoelectron angular distributions.

Detailed measurements of the angular distributions of the origin and 26 transitions yield asymmetry parameters42 of B = I .5 f 0.1 and 0 = -0.8 f 0.1, respectively, where 0 has a range from 2.0 (cos2 9 distribution) to - 1 .O (sin2 6 distribution). Spectra taken at Oo and 90° indicate that the allowed transitions all have asymmetry parameters in the range /3 = 1.4-1.6, while the nonallowed transitions have /3 = -0.5 to -0.8, for both I2C2H- and I3C2D-. The opposite angular distributions of the odd bending transitions could be used-along with isotope shifts and temperature dependence-to help assign the less intense vibrational transitions (Table IV).

E. Electron Affinity. The accuracy of electron affinity determinations by negative ion photoelectron spectroscopy depends

(39) Grucbele, M. H. W. Ph.D. Thesis, University of California, Berkeley, 1988.

(40) Ervin, K. M.; Gronert, S.; Barlow, S. E.; Gilles, M. K.; Harrison, A. G.; Bierbaum, V. M.: DePuy, C. H.: Lineberger, W. C.; Ellison, G . B. J . Am. Chem. SOC. 1990, 112, 5750-5759 and references therein.

(41) Lias, S. G . ; Bartmess, J . E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W . G . J . Phys. Chem. ReJ Data 1988, 17 (Suppl. No. I ) .

(42) Cooper, J.; Zare, R. N . J . Chem. Phys. 1968, 48, 942-943.

2.969 f 0.006

2.94 k 0.10 2.2 f 0.4 2.1 k 0.3 3.73 h 0.05

2.96 1 k 0.0 I5 3.15 3.54

2.71 3.18 f 0.25 2.14 2.5

22.8

EA(CJ+) method year ref photoelectron spectroscopy 1990 this

photodetachment threshold endothermic electron transfer electron impact photodetachment threshold electron impact theory (QCI-CBS) theory (CCD S-MBPT(4)) theory

theory (RCISD(Q)/6-31 l++G(2d,Zp)) theory (SCF) theory (SCF-CISD) theory (scaled SCF)

(MP3/6-3 l++G**//HF6-3 I+G*)

work 1979 45 1973 46 1970 27 1970 25

1990 52 1988 47 1987 51

1986 50 1979 45 1978 49 1977 48

1963 28

on the correct assignment of the vibrational and electronic origin transition. In the present case, the vibrational origin assignment is ambiguous because the transition is vertical, with the origin having the strongest Franck-Condon intensity. Furthermore, isotopic substitutions show frequency shifts that are consistent only with our origin assignment. The assignment of the obterved electron ba>d to the ground electronic state transition, CzH(X22+) - C2H-(XIZ+), is confirmed by agreement of the vibrational frequencies with experimental values for the neutra13s,36-43 and calculated values3s for the anion.

The assigned vibrational origins lie a t eKE = 0.564 f 0.005 eV for I2C2H-, eKE = 0.560 f 0.005 eV for I2C2D-, and eKE = 0.562 f 0.005 eV for I3C2D-. These values must be corrected for the rotational shifts to obtain the electron affinity. The rotational contour is modeled in a manner similar to that described above for C2, using the experimental rotational c o n ~ t a n t ~ ~ , ~ for ethynyl radical and Botschwina's calculated geometry38 for the anion. The calculated contours for each of the isotopes indicate that the rotational shifts are less than 1 meV, which is small compared to the instrumental resolution. Since the origins are resolved from the 2; sequence bands and the 3; transitions are small, additional shifts due to underlying sequence bands are also not significant. The final adiabatic electron afinities are EA- (I2C2H) = 2.969 f 0.006 eV, EA(I2C2D) = 2.973 f 0.006 eV, and EA(I3C2D) = 2.971 f 0.006 eV. Differences in the electron affinities due to zero-point energies cannot be predicted inde- pendently because the C H stretch frequencies of the anions and neutrals are not known experimentally. Taking into account only the observed CC stretch and CCH bend frequencies, the electron affinities of I2C2D and I3C2D would be estimated at 7.4 and 3.1 meV higher, respectively, than EA(I2C2H) due to zerepoint energy differences, in reasonable accord with experiment.

The electron affinity of ethynyl radical obtained in this work, EA(C2H) = 2.969 f 0.006 eV, is in agreement with the photodetachment threshold value of 2.94 f 0.10 eV of Janousek, Brauman, and S i m o n ~ . ~ ~ Other e ~ p e r i m e n t a l ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ and theo- r e t i ~ a l ~ ' - ~ ~ electron affinities are reviewed in Table V.

F. Vibrational Energy Levels. Figure-6 shows the vibrational energy levels of C2H(X22+) and C2H-(X'2+) derived from the transitions assigned in Table IV. As noted above, frequencies from high-resolution infrared experiment^^^,^^*^^ on the neutral species are in good accord with the present results for the (010) and (001)

(43) Kawaguchi, K.; Amano, T.; Hirota, E. J . Mol. Spectrosc. 1988, 131,

(44) Kanamori, H.; Hirota, E. J . Chem. Phys. 1988, 88, 6699-6701. (45) Janousek, B. K.; Brauman, J. 1.; Simons, J . J . Chem. Phys. 1979, 71,

(46) Hughes, B. M.; Lifshitz, C.; Tiernan, T. 0. J . Chem. Phys. 1973,59,

(47) Lima, E. G . ; Canuto, S. Int . J . Quantum Chem. Symp. 1988, 22,

(48) Pacansky, J . ; Orr, G . J . Chem. Phys. 1977,67, 5952-5955. (49) Vasudevan, K.; Grein, F. J . Chem. Phys. 1978, 68, 1418. (50) Baker. J.; Nobes, R. H.; Radom, L. J . Comput. Chem. 1986, 7,

(51) Li, W. K.; Nobes, R. H.; Radom, L. THEOCHEM 1987,34,67-79. (52) Montgomery, J. A,, Jr.; Petersson, G . A. Chem. Phys. Lett. 1990, 168,

58-65,

2057.

3 162-3 181.

199-205.

349-358.

75-78. Montgomery, J. A,, Jr. Private communication.


(031) 2935

9 2 1 ) 2550


(050) [2195] (2166.21) (011) 2120 (2090.82) l00l) 1850 (1840.57)

(040) [1700]

(030) 120

(020) 790

(010) 365 (371.60) (ooo) 0

C,H x 'P+ (v,v2v3)

(011) 2300

(001) Is00

(OU)) 1015

(010) 505

(m) 0

GH- x '2' (vIv2v3)

Figure 6. Vibrational energy level diagram for I2C2H(g2Z+) and 12C2H-(%12t). The energy levels (cm-I) are obtained from the observed transition frequencies given in Table IV. Vibrational energy levels within each state are drawn to scale, but the energy separation between I2C2H and I2C2H- is much larger than shown. The energies in brackets for (040) and (050) of I2C2H are predictions of a model potential for the CCH bending mode fit to the lower levels (see text and Figure 7). En- ergy levels from high-resolution infrared spectroscopy (refs 35, 36, 44, and 43) are given in parentheses. The energy level for (050) of C2H was originally assigned to (070) in the IR experiments (see text).

levels. In the worst case, the energy of the (01 1 ) level is 2120 f 20 cm-I from this work, compared to the infrared value43 of 2090.823 f 0.009 cm-'. Deviations between the two experiments may result from the fact that our determinations involve peaks that are unresolved rotationally and that transition energies of the weaker transitions may be shifted slightly by partially overlapping peaks. Nevertheless, the agreement with the high-resolution experiments is satisfactory.

The bending vibrational levels of ground-state C2H(g2Z+) exhibit strongly increasing spacings with increasing bend quantum number u2. The energy levels (with the spacing from the next lower level given in parentheses, all f20 cm-I) are 365 cm-' for u2 = 1 , 790 (425) cm-' for u2 = 2, and 1240 (450) cm-I for u2 = 3. The increasing vibrational spacings can be attributed to a bending vibrational potential which is flatter than quadratic near the linear geometry, Le., with significant qu_artic and higher terms. Kraemer et alas3 have calculated the C2H(X2Z+) potential energy surface and used the nonrigid bender model to calculate vibrational energy levels. For the series given above, they predict 298 cm-I for u2 = 1,649 (351) cm-' for u2 = 2, and 990 (341) cm-' for u2 = 3. The calculated absolute frequencies are too low, but the calculations correctly predict the general trend in vibrational spacings.

The bending vibrational energies of C2H(gZB+) can be modeled by a one-dimensional vibrational potential with quadratic and quartic terms. Figure 7 shows such a two-parameter potential energy curve obtained as the best fit of the observed energy levels v2 = 0-3. The potential for the neutral shows substantial deviation from a harmonic oscillator, as is required to reproduce the increasing frequency spacings for the higher bend vibrational energy levels. Kraemer et aLs3 used terms up to powers of 12 to fit the bending part of their ab initio potential energy surface. That potential cannot be compared directly to the present potential, because thc ab initio potential is part of a multidimensional surface while the empirical model is a simple one-dimensional fit. However, the low vibrational frequencies predicted by the theoretical calculations indicate that the ab initio curve is flatter near linear geometry than the model potential.

(53) Kracmer, W. P.; Roos, B. 0.; Bunker, P. R.; Jensen, P. J . Mol. Spectrosc. 1986, 120. 236-238.

c

\ I

1 ' 1 ' 1 ' - i .O 0 .o 1 .o

CCH bend

Figure 2. Bending vibrational potential for the ground states of IzC2H(X2Z+) and I2C2H-(k1Z+) as a function of the CCH bend normal coordinate. The anion bend potential is represented by a quadratic potential with the observed frequency of 505 cm-I. The neutral potential includes quadratic and quartic terms fit to the observed energies of the (OIO), (020), and (030) levels, V = 1575QJ + 1960Q: (for Q2 i n A amu1/2 and V in cm-I). The energy separation between the anion and the neutral is actually much larger than shown.

The unobserved u2 = 4 and u2 = 5 bending vibrational levels are predicted by our model potential to occur at about 1700 and 2195 cm-l, respectively. The latter frequency compares with a state a t 2 166.2 1280 f 0.00028 cm-I observed in a (Ou20)-(01 0) absorption transition by Hirota and c o - ~ o r k e r s . ~ ~ They assigned the upper state to u2 = 7, although u2 = 5 was also considered. The present results suppori a revised assignment to u2 = 5.

The (01 1 ) level of C2H(X2Z+) is observed in the present work at 2 120 f 20 cm-I, corresponding to the high-resolution value of 2090.823 f 0.009 cm-' obtained by Hirota and c o - ~ o r k e r s ~ ~ and 2104 cm-' in argon matrix isolation spectra of Shepherd and Graham.54 Higher bend quantum levels in combination with the u3 = 1 excitation of the CC stretch are observed at 2550 f 20 cm-' for (021) and 2935 f 20 cm-l for (031). As also shown in Figure 6, this series of levels has uneven frequency spacings. In particular, the (01 1)-(001) spacing (270 f 20 cm-' in this work, 250.25 cm-l from the IR experiment^^^) is anomalously smaller than the corresponding (010)-(000) spacing (365 f 20 cm-' here or 371.60 cm-' from IR work36). Hirota and c o - ~ o r k e r s ~ ~ have attributed this effect to vibronic interaction _between the C2H(X2Z+) ground state and the low-lying C2H(A2n) excited state. Their high-resolution found a large difference in the spin-orbit coupling constant between the (010) and (01 1) levels, which was also attributed to the vibronic interaction. The proximity of the (050) and (01 1) levels suggests that a Fermi interaction could also play a role in the anomalous spacing. G. Unassigned or Unobserved Transitions. A few transitions

at low electron kinetic energy, near the end the sensitivity range of the electron energy analyzer, could not be positively assigned. These transitions merit-some discussion because they are in the region where :he C2H(A211) state might be expected. The term energy of the A211 state is unknown. Experimental e ~ t i m a t e s ' ~ * ~ ~ * ' ~ and ab initio calculationsSS of the term energy range from 1600 to 5500 cm-'.

(54) Shepherd, R. A,; Graham, W. R. M. J . Chem. Phys. 1987, 86,

(55) Calculated values of T(C2H(A2n)) include 5480 cm-' (vertical) [Hillier, I . H.; Kendrick, J.; Guest, M. F. Mol. Phys. 1975, 30, 1133-1 1381; 4000 cm-' [Shih, S.-K.; Peyerimhoff, S. D.; Buenker, R. J. J . Mol. Specfrosc. 1979, 74. 124-1351; 2000 cm-' [Fogarasi, G.; Boggs, J. E.; Pulay, P. Mol. Phys. 1983, 50, 139-1511; 3650 cm-l [ref 531; 5835 cm-' [Reimers, J . R.; Wilson, K. R.; Heller, E. J.; Langhoff, S. R. J . Chem. Phys. 1985, 82, 5064-50771; and 5420 cm-' [Osamura, Y.; Mitsuhashi, F.; Iwata, S. Chem. Phys. Lett. 1989. 164, 2051.

2600-2605.

Photoelectron Spectra of C2- and C2H-

The most prominent unassigned transition is at 3435 f 40 cm-l (relative to the origin) in I2C2H- and 3310 f 40 cm-l in I3C2D-. (High-quality spectra for I2C2D- were not obtained in this energy region.) The peaks in the low-eKE region are broadened, leading to large relative uncertainties; however, the I2C2H level does not appear to be the same as levels at 3366.36 cm-I in IR experimentss6 or 3380.4 cm-l in argon matrix i~ola t ion . '~ The downward frequency shift for the heavier-isotope indicates that the transition is not the origin of the C2H(A211) state, nor a vibrational hot band of the C2H(A211) - C2H-(XlZ+) transition. The frequency shift is much too small for the transition to correspond to a bend or the C H stretch. The isotope shift is only 3.895, suggesting that it is also not a pure overtone of the CC stretch normal mode (Le., 3 3 , for which the 3; transition has a 7.6% isotope shift. This possibility also seems unlikely because twice the 3; frequency would be 3600 and 3420 cm-I for I2C2H and I3C2D, respectively, and because the Franck-Condon factors are unfavorable. On the other hand, vibronic mixing with the A211 state could alter the energy levels and intensities s ~ b s t a n t i a l l y . ~ ~ Other possibilities are a sequence band from a vibrationally excited anion state, transitions for the two isotopes that do not correspond to the same vibrational transition, or overlapping unresolved transitions.

An additional unassigned transitions7 lies a t 2805 f 20 cm-I relative to the origin in I3C2D and has no obvious counterpart in I2C2H. One possible assignment is to the CD stretch fundamental, 1; which Jacox and Olson14 assigned to a level at 2798.5 cm-l for I2C2D and at 361 1 cm-I for I2C2H in argon matrices. The assignment for I3C2H is not definitive,14 however, and has been d i s p ~ t e d . ~ ~ - ~ * Recent high-resolution results by Hirota and co- workerss9 indicate that the 2798.5-cm-l band in I2C2D is a 211-2Z transition and therefore cannot be the pure vI CD stretch but could be the stretch in combination with a bend. Unfortunately, the transitions in the 3600-8000-cm-1 region observed by are beyond our experimental energy range.

The origin of the C2H(A211) state is not observed. The characteristic feature of an electronic origin is the absence of an isotope shift:, the only observ_ed transition meeting this criterion is the C2H(X2Z+) - C2H-(X'Z+) origin. We would expecj the electronic transition probability of the origin of the C2H(A211) state to be comparable to that of the C2H(X2Z+) origin (at least within a factor of IO), since both involve one-elecpon transitions. According to calculated geometries for the A211 state, the Franck-Condon factor for the origin transition is also favorable. Therefore, the lack of observation implies that the origin ?f the excited state lies beyond our energy range, giving To(C2H(AZII)) > 3400 an-'.

H . Non- Franck-Condon Behavior. The observation of transitions with odd changes in the vibrational quantum number of the CCH bend indicates a clear breakdown of Franck-Condon behavior. Only Au = 0, f 2 , f 4 , ... transitions are allowed for non-totally symmetric modes in the Franck-Condon approximation. The CCH bend is non-totally symmetric because the motion in the CCH bend normal coordinate reduces the symmetry from C,, to C,,. Another clue that these transitions are unusual is the observed striking difference in the photoelectron angular distributions compared to the allowed transitions.

We shall consider three possible mechanisms for the existence of the odd transitions in the CCH bending mode. First, the odd transitions would be Franck-Condon allowed if the anion and neutral were bent rather tkan linear. However, the high-resolution s p e c t r o ~ c o p y ~ ~ * ~ ~ of C2H(X2Z+) clearly identifies it as linear, and calculation^^^*^^ leave no doubt that C2H- is also linear. Second, a rotational-vibrational interaction, i.e., Coriolis coupling, could

(56) Stephens, J . W.; Yan, W.-B.; Richnow, M. L.; Solka, H.; Curl, R . F. J . Mol. Sfrucf. 1988, 190, 41-60.

(57) The asymmetry parameters for the unassigned peaks are approximately fl -05, which suggests that the final states have r vibronic symmetry. However, these peaks lie near threshold, where the angular distribution may be a strong function of the electron kinetic energy.

(58) Yan, W. B.; Dane, C. 8.; Zeitz, D.; Hall, J . L.; Curl, R. F., Jr. J . Mol. Spectrosc. 1987, 123, 486-49s.

(59) Hirota, E. Private communication. (60) Lee. T. J.; Schaefer, H. F., 111 J . Chem. Phys. 1985.83, 1784-1794.

The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1175

lead to an intensity transfer between vibrational levels of different symmetry. Coriolis coupling can be important when two modes have frequencies close together, which is not the case here.

The probable explanation for the occurrence of the odd transitions is vibronic coupling. The total cross section for photode- tachment16,61 is given by

0 a v.l(~"(Q")lc((Q)l\k'(Q))l2 ( 1 )

where v is the asymptotic electron velocity and p(Q) is the electronic transition moment between the vibrational wave function of the initial state (anion) W' and of the final state (neutral) W, as functions of normal coordinates Q. In the Franck-Condon approximation, the electronic transition moment is assumed constant over the range of vibrational coordinates with significant overlap, Le., p(Q) = wo. Vibronic interaction between the odd bendicg vibrational levels (7 symmetry) and levels the low-lying C2H(AZII) excited state causes this approximation to break down in the case of the bend normal coordinate.

In analyzing photoelectron spectra, we have often profited from Franck-Condon simulations to extract physical information such as geometry changes and shapes of the vibrational energy surfaces from the observed intensities.16f'2 In the-present case, dejailed Franck-Condon simulations of the C2H(X2Z+) - C2H-(XlZ+) photoelectron spectra have not been attempted, for two reasons. First, the non-Franck-Condon transitions would have to be added as a perturbation, involving an adjustable parameter without a direct physical interpretation. Attempts to replace the Condon assumption of constant electronic transition moment with a linearly varying function of the bend normal coordinate, i.e., 4 Q 2 ) = plQ2 + po, indicated that no single value of pI could reproduce all of the transition intensities. This result is reasonable because the vibronic mixing with the excited state that induces the non- Franck-Condon transitions will depend strongly on the energies of the perturbing levels. The second difficulty in applying a Franck-Condon analysis is the irregular spacings of the 0u21 vibrational levels, which means that the combination bands cannot be described by a simple independent oscillator model. Rather, an adequate theoretical treatment would require detailed calculations of the vibronic energy levels and transition moments.

A qualitative, physical picture of the origin of the vibronic interaction which induces the non-Franck-Condon transitiocs may be acquired by considering the molecular orbitals of C2H-(XIZ+), C2H(X2Z+), and C2H(A211). The anion has a closed-shell configuration (isoelectronic with acetylene), with two electrons in the lone pair on the terminal carbon atom. The stability of this configuration accounts for the high electron affinity of C2H. Removing an electron from-the lone pair of the anion creates the ground-state neutral, C2H(X2Z+), which retains a nominal triple CC bond. Removing inste_ad an electron from a pn orbital of the anion produces the C2H(A211) excited state, in which the triple bond has been partially broken. Now consider what happens to the orbitals when the C2H(X2Z+) ground state is bent. Bending the molecule destroys the sp hybridization of the medial carbon atom and therefore breaks the triple-bond character, leading to an sp2 hybridized medial carbon and a double CC bond for the bent molecule, as shown below:

H-C=C* - 'C=C* /

H

Thus, the molecular orbital configuration changes radically when the molecule is bent. The dipole moment of the molecule per- pendicular to the CC axis changes from zero in the linear geometry to a value for the bent molecule (in-plane) which depends strongly on the bond angle. This change in electronic character leads to a strong dependence of the electronic transition moment on the bend coordinate and hence the breakdown of the Franck-Condon approximation.

(61) Massey, H. S. W. Negafiue fons; Cambridge University Press:

(62) Ervin, K . M.; Ho, J . ; Lineberger, W. C. J . Phys. Chem. 1988, 92, Cambridge, 1976; p 417 ff.

5405-541 2.


The molecular orbital scheme can also qualitatively explain the large difference in photoelectron angular distributions for the transitions with odd changes in the bend quantum number. Photoelectron angular distributions generally depend on the symmetry of the orbital from which the electron is detached. Vibronic transitions that retain the linear symmetry ( u vibronic states) involve removal of a carbon lone-pair electron, which has spo character. Detachment of s-like electrons leads to ejection along the electric field vector of the light, leading to a cos2 19 distribution, as is observed for the origin and other allowed transitions. The same electron is removed in the photodetachment transitions leading to the A vibronic levels, but the rehybridization of the remaining electrons leads to the effective net removal of a pa electron. Detachment of a 2p-like electron 0.5-2 eV above threshold leads to preferential ejection perpendicularly to the electric field vector:) resulting in a sin2 0 distribution, as we observe for the non-Franck-Condon transitions.

V. Thermochemistry A. Bond Dissociation Energies of C2 and Cy. The electron

affinities of C and C2 are related to the bond dissociation energies of C2 and C2- according to

(2) The electron affinity of dicarbon is determined in the present work, EA(C2) = 3.269 f 0.006 eV, and the electron affinity of atomic carbon is also precisely EA(C) = 1.2629 f 0.0003 eV. The dissociation energy of neutral C2 is less well established than might be expected, considering the extensive spectroscopic information available on this diatom. The bond dissociation energy for neutral dicarbon determined by Messerle and K r a u d 5 from an extrapolation of rotationally dissociating levels of the C2(c'I'I,) state, D0(C2) = 49300 f 300 cm-' or 141 .O f 0.9 kcal mol-', is adopted in the JANAF Thermochemical Tables.66 Huber and Herzberg4 eschew the spectroscopic extrapolation and cite instead D0(C2) = 6.21 eV or 143.2 kcal mol-', an average of measurements of the enthalpy of C2 sublimation from graphite. However, sublimation experiments give67 a range of values for D0(C2): 144 f 5 kcal mol-' from a 1962 review by Brewer et a1.68 of Swan emission and mass spectrometric (MS) measurements; 142.0 f 1.7 kcal mol-' by Drowart et al. (MS);69 143.0 f 1.6 kcal mol-' by Kordis and Cingerich (MS);70 139.3 f 0.6 kcal mol-' by Zavitsanos and Carlson (MS);71 and a lower limit of > 137 f 9 kcal mol-' by L'vov et al. (Swan emission and a b ~ o r p t i o n ) . ~ ~ In a 1979 papcr, Brewer and Hagan7) adopted the Messerle and Krauss spectroscopic value of D0(C2) = 141.0 f 0.9 kcal mol-', citing AH,,b,o(C,(a311,))/R = lOlOOO!~~ K, which they find to be consistent with measurements of the oscillator strengths of the C2 Swan bands. Lambert et aL2 also selected the spectroscopic value. Wc adopt the Messerle and Krauss spectroscopic value but choose the uncertainty limits conservatively to include the various sublimation measurements, D0(C2) = 141.0 f 2.5 kcal mol-'. Combining this bond energy with the electron affinities

(63) Hanstorp, D.; Bengtsson, C.; Larson, D. J . Phys. Reo. A 1989, 40,

(64) Hotop, H.; Lineberger, W. C. J . Phys. Chem. ReJ Data 1985, 14,

(65) Messerle, G.; Krauss, L. Z . Naturforsch. 1967, 22A, 1023-2026. (66) Chase, M. W., Jr.; Davies, C. A,; Downey, J. R . , Jr.; Frirup, D. J.;

McDonald, R . A.; Syverud, A. N. J . Phys. Chem. Ref. Data 1985, 14 (Suppl. No. I ) .

(67) We use AH,o(C(g)) = 170.0 f 0.1 kcal mol-' and heat capacities from JANAF [ref 661 to convert literature C2 heats of formation or subli-

DO(C2-) = Do(C2) + EA(C2) - EA(C)

670-675.

73 1-750.


given above using eq 2, we obtain Do(CT) = 187.2 f 2.5 kcal mol-'. The larger dissociation energy of the anion compared to the neutral reflects the addition of a bonding electron from

B. CH Bond Dissociation Energies of Acetylene and Ethynyl. The electron affinity of a hydrocarbon radical, EA(R), can be combined in a thermochemical cycle with the gas-phase acidity, the enthalpy for the process RH - R+ + H-, AHac,d(RH), and the ionization potential of the hydrogen atom to obtain the ho- molytic bond dissociation energy, D(R-H). This cycle is represented by the following relation:

Using the present measurement of EA(C2H), the gas-phase acidity of acetylene from proton-transfer kinetics experiment^,^^ AH,,id,o(HCCH) = 376.4 f 0.6 kcal mol-', and the ionization potential,66 IP(H) = 313.587 kcal mol-', we obtain the bond dissociation energy of acetylene, D,(HCC-H) = 131.3 f 0.7 kcal mol-'. Proton-transfer bracketing experiments'''' provide a lower bound for the gas-phase acidity of ethynyl, AHacd,o(HCC) > 347.0 f 2.0 kcal mol-', which from eq 3 leads to a lower limit for the ethynyl bond dissociation energy, Do(H-CC) > 108.7 f 2.1 kcal mol-'. An absolute value for the ethynyl bond dissociation energy can be derived40 from our experimental values for Do(HCC-H), combined with literature values for D0(C2) and the heat of formation of acetylene, yielding Do(H-CC) = 116.3 i 2.6 kcal mol-'.

Further thermochemical derivations are given in our recent report in collaboration with Ellison, DePuy, Bierbaum, and co- w o r k e r ~ ~ ~ on the bond dissociation energies of acetylene and ethylene. That paper provides details of the proton-transfer kinetics experiments and a full discussion of other experimental determinations of the CH bond dissociation energy of acetylene. High-level theoretical calculations of Do(HCC-H) which have appeared since preparation of our previous are in close agreement with our experimental value: 131.0 f 1.0 kcal mol-' from multireference configuration interaction calculations by Bauschlicher et al.,74 13 1.54 f 0.5 1 kcal mol-' from quadratic configuration interaction with complete basis set extrapolation by Montgomery and P e t e r s s ~ n , ~ ~ and 129.7 f 1 kcal mol-' from generalized valence bond theory and correlation-consistent configuration interaction by Wu and Carter.75 As this manuscript was being completed, we learned of two experiments that cor- roborate our value for Do( HCC-H): 1 3 1 . I f 0.7 kcal mol-' from the threshold for photoelectron-induced dissociative attachment in acetylene,76 HCCH + e- - C2H- + H (combined with the present value for EA(C2H)), and 13 1 f 1 kcal mol-' by kinetic energy analysis of H atoms produced in the 201-206-nm photodissociation of acetylene.77

Recent theoretical values for the ethynyl bond dissociation energy include Do(H-CC) = 102 kcal mol-' by Wu and Carter,75 Do(H-CC) = 1 1 1.6 f 0.5 kcal mol-' by Montgomery,s2 and 1 12.4 f 2.0 by Bauschlicher and L a n g h ~ f f . ~ ~ The latter value has overlapping error limits with the experimental value of Do( H-CC) = 116.3 f 2.5 kcal mol-', which as discussed above is derived by using the somewhat uncertain value for D0(C2). The prediction of Wu and Carter7s can be excluded based on our experimental lower limit, Do(H-CC) > 108.7 f 2.1 kcal mol-'.

VI. Summary We have observed the photoelectron spectra of C2- and C2H-.

Unambiguous assignments of the origin peaks yield the electron affinities of dicarbon, EA(C2) = 3.269 f 0.006, and ethynyl, EA(C2H) = 2.969 f 0.006 eV. The value for dicarbon differs significantly from the previously accepted electron affinity based

C2( X ' Z 3 f f 4 ) to C2-(X2Zg+)( *4o').

Do(R-H) = AH,,.,,j,o(RH) + EA(R) - IP(H) (3)

mation enthalpies to D,(C,).

I R2-l RR. (68) Brewer, L.; Hicks, W. T.; Krikorian, 0. H. J . Chem. Phys. 1962, 36,

. - - . - -. (69) Drowart, J . ; Burns, R . P.; DeMaria, G.; Inghram, M. G. J . Chem.

(70 ) Kordis, J . ; Gingerich, K. A. J . Chem. Phys. 1973, 58, 5058-5066. (71) Zavitsanos, P. D.; Carlson, G . A . J . Chem. Phys. 1973, 59,

Phys. 1959, 31. 1131-1132.

2966-2973. (72) L'vov, B. V.; Novotny, 1.; Pelieva, L. A. J . Appl. Spectrosc. (Engl.

Transl.) 1980, 32, 553-559. [Zh. Prikl. Spektrosk. 1980, 32. 965-9731, (73) Brewer, L.; Hagan, L. High Temp. Sei. 1979, 1 1 , 233-263. Brewer

and Hagan cite AH,o(C(g))/R = 85538-& K or AHf,,(C(g)) = 169.987;,' kcal mol-' (cf. ref 67).

(74) Bauschlicher, C. W., Jr.; Langhoff, S. R.; Taylor, P. R. Chem. Phys.

(75) Wu, C. J . ; Carter, E. A. J . Am. Chem. Sac. 1990, 112,5893-5895. (76) Ruscic, B.; Berkowitz, J. J . Chem. Phys., submitted for publication. (77) Baldwin, D. P.; Buntine, M. A.; Chandler, D. W. Abstract FB5, 45th

Ohio State University Symposium on Molecular Spectroscopy, Columbus, OH, June 1990.

(78) Bauschlicher, C. W., Jr.; Langhoff, S. R . Chem. Phys. Lett., submitted for publication.

Lett. 1990, 171, 42-48.

J. Phys. Chem. 1991, 95, 1 177-1 183 1177

on indirect observations in the autodetachment spectroscopy5 of C2-( B2Z:).

The spectrum of the CzH(g2Z+) - C2H-(g1Z+) transition highlights the rich vibronic structure of the ground state of ethynyl. We observe the CC stretch and CCH bend modes of both C2H and C2H-. The observed fundamental frequencies of C2H- are 505 f 20 cm-' for the v2 bending mode and 1800 f 20 cm-' for the v j CC stretch mode. I n the neutral ground state, overtones and combination bands are observed up to v = 3 in the bending mode and L' = I in the CC stretch. The determination of these vibrational levels may aid the identification of additional bands in the high-resolution infrared spectra of C2H radical. Strong Franck-Condon nonallowed transitions are observed for odd

quantum levels of the CCH bending vibration in C2H(g2Z+). These transitions are induced by vibroni_c interactions of these 7r vibrational levels with the low-lying C2H(AZII) excited state. From the absence of the AZII origin transition within the available energy range, y e infer a lower limit of 3400 cm-' for the term energy of the AZII state.

Acknowledgment. Joe Ho and Mary K. Gilles have provided valuable experimental assistance. We are grateful for stimulating discussions with Professor G. Barney Ellison, Dr. C. Tom Wickham-Jones, and Dr. Kermit K. Murray. This research is supported by the National Science Foundation, Grants PHY86- 04504 and CHE88-19444.

Wavelength Dependence of the Multiphoton Ionization and Fragmentation Dynamics of Cr( CO),/Methanol van der Waals Heteroclusters

William R. Peifer and James F. Garvey* Department of Chemistry, State University of New York at Buffalo. Buffalo, New York 14214 (Received: July 18, 1990)

Mixed van der Waals clusters of Cr(C0)6 and methanol, generated in a pulsed free-jet expansion of seeded helium, are irradiated by strongly focused UV laser pulses, corresponding to intensities on the order of 1012-1013 W/cm2. Photoions created via multiphoton dissociation (MPD) and ionization (MPI) of these neutral clusters are analyzed by time-of-flight mass spectrometry. We find that the multiphoton dissociation and ionization dynamics of Cr(CO)6 within van der Waals clusters are quite unusual in comparison to those of the naked molecule, since the observed photoion yields are highly wavelength dependent and since complete ligand stripping does not appear to dominate the photophysics in the neutral ladder. The observed photoions are accounted for in terms of a dynamical scheme wherein the solvated Cr(C0)6 first undergoes single-photon photodissociation, yielding a solvated coordinatively unsaturated photoproduct, S,Cr(CO),. The extent of unsaturation in the primary photoproduct is dependent on photon energy, analogous to gas-phase photodissociation of naked Cr(C0)6. This neutral species subsequently undergoes MPI, giving rise to a nascent parent cluster ion, S,Cr(CO),+. If this parent cluster ion is sufficiently excited, it can relax to a distribution of daughter ions by at least three different mechanisms: further ligand loss; intracluster bimolecular reaction with an adjacent solvent molecule, leading to a more thermodynamically stable product; or intracluster V-V energy transfer to the solvent bath. The first of these three alternatives appears to be the dominant energy disposal channel following MPI at 248 nm, while the second of these three alternatives appears to dominate following MPI at 350 nm. CD30D is apparently more efficient than CH30H in cooling the various photoions via intracluster V-V energy transfer. This may be due to a more favorable correlation of vibrational frequencies and symmetries. Our ability to prepare and spectroscopically interrogate specific chromium carbonyl species within van der Waals clusters holds promise as a general technique for probing the electronic structures of coordinatively unsaturated organometallic complexes.

Introduction Coordinatively unsaturated transition-metal carbonyls are of

fundamental significance in organometallic chemistry.' Many of these species are thought to be intermediates in industrially important catalytic cycle^.^,^ These highly reactive molecules can be synthesized conveniently in the gas phase by pulsed UV laser photolysis of stable, saturated transition-metal carbonyls. Monounsaturated species can be easily generated in condensed phases by UV or near-UV single-photon photodissociation of saturated precursor^.^ Photodissociation of saturated transition-metal coordination compounds in solution and subsequent coordination of a solvent molecule to the unsaturated photoproduct occur on the picosecond time scale.5 The dynamics of these solution-phase processes have been studied by a variety of transient visible,b* IR,"' and Raman12 spectroscopic techniques. One

( I ) Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1982, 21, 71 I . (2) Meyer, T. J.; Caspar, J . V. Chem. Reo. 1985, 85, 187. (3) Poliakoff, M.; Weitz, E. Ado. Organomet. Chem. 1986, 25, 277. (4) Geoffroy, G . L.; Wrighton, M. S. Organometallic Photochemistry;

( 5 ) Welch, J. A.; Peters, K. S.; Vaida, V. J. Phys. Chem. 1982, 86, 1941. (6) Simon, J. D.; Xie, X. J. Phys. Chem. 1986, 90, 6751. (7) Joly, A. G . ; Nelson, K. A. J. Phys. Chem. 1989, 93, 2876. (8) Lee, M.; Harris, C. B. J . Am. Chem. SOC. 1989, 1 1 1 , 8963.

Academic: New York, 1979.

would also like to be able to study coordinatively unsaturated molecules in an environment free from energy transfer to and/or reactions with solvent molecules, preferably under collisionless conditions. Although powerful mass spectrometric techniques have been applied to the study of the structure and reactivity of coordinatively unsaturated ionic species in the gas phase,l3-l5 studies of the excited electronic states of the corresponding neutral species are not as numerous.16 The photolability of transition-metal carbonyls makes their study by UV/visible absorption spectroscopy difficult. Excited-state lifetimes are short, and excited species tend to photodissociate rather efficiently; consequently, absorption bands are quite diffuses4 Although some unsaturated metal

(9) Moore, J . N.; Hansen, P. A.; Hochstrasser, R. M. J . Am. Chem. Soc. 1989. 1 1 1 . 4563. - --. - - - , - - -

( I O ) Wang, L.; Zhu, X.; Spears, K. G. J. Am. Chem. Soc. 1988,110,8695. ( I I ) Wang, L.; Zhu, X.; Spears, K. G. J. Phys. Chem. 1989, 93, 2. (12) Yu, S.-C.; Xu, X.; Lingle, R., Jr.; Hopkins, J . B. J. Am. Chem. Soc.

( I 3) Russell, D. H., Ed. Gas Phase Inorganic Chemistry; Plenum Press:

(14) El-Shall, M. S.; Schriver, K. E.; Whetten, R. L.; Meot-Ner, M. J .

( 1 5 ) Jayaweera, P.; Blades, A. T.; Ikonomou, M. G.; Kebarle, P. J. Am.

(16) Weitz, E. J. Phys. Chem. 1987, 91, 3945.

1990, 112, 3668.

New York, 1989.

Phys. Chem. 1989, 93, 7969.

Chem. SOC. 1990, 112. 2452.

0022-3654/91/2095-1177$02.50/0 0 1991 American Chemical Society

A-'jila.colorado.edu/~wcl/wclgroup/Pub downloads/C2H and C2H...where MK is the molar mass of the...

Documents

Transcript of A-'jila.colorado.edu/~wcl/wclgroup/Pub downloads/C2H and C2H...where MK is the molar mass of the...