JOURNAL OF MASS SPECTROMETRY, VOL. 31, 209-212 (1996)

[ CzOz] + * Ions of Unusual Structure

Lucian0 Pandolfo and Gastone Paiaro Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Universita di Padova, Via Marzolo 1,I-35131 Padova, Italy

Silvia Catinella and Pietro Traldit CNR Area di Ricerca, Corso Stati Uniti 4,I-35020 Padova, Italy

The structure of the C,O:' ion, obtained by electron impact ionization of C,O,, is reported. A double-focusing instrument was used to carry out metastable ion and collisional spectroscopic studies. Comparison of the data showed that a C,O:' species structurally different from those originating from other precursors is obtained by ionization of C,O,. A four-membered ring is proposed as the possible structure of the C,Of ' ion.

KEYWORDS: carbon suboxide; collisional experiments; ionization energies; squaric acid; oxalyl dichloride

INTRODUCTION

The study of linear and quasi-linear cumulenes is of great interest owing to their important role in the primi- tive atmosphere and as intermediates in the formation of interstellar species.' Schwarz and co-workers have been very active in this field and, by neutralization- reionization mass spectrometry (NRMS), were able to characterize oxycumulenes, thiocumulenes and mixed S-0 cumulenes.'

Chen and Holmes3 recently reported the generation of [C,O,]'- and neutral CzO,, the former showing a heat of formation of 940 & 10 kJ mol-'. To generate [C,O,] +. radical cations, electron impact (EI) ioniza- tion of oxalic acid and oxalyl chloride was used. Siilzle et aL4 employed butane-2,3-dione and the gas-phase ion-molecule reaction of CO with CO" to generate [C,O,] +' ions. The collisional spectra obtained for the C,O;' iow generated from the above precursors are very similar, showing the presence of ions at m/z 44 (CO:'), 40 (C,O+'), 28 (CO"), 24 (Cl'), 16 (0") and 12 (C"). These experimental data are in agreement with the trans planar OCCO+' structure proposed on the basis of ab initio molecular orbital s t ~ d i e s . ~ -8

Pursuing our interest in the gas-phase reactivity of C,O:' ions and the neutral carbon s u b ~ x i d e , ~ we observed a weak but clearly detectable C,Ol ' ion among the EI-induced decomposition products of C30:'. This paper reports results pertaining to its structural characterization and compares them with results on C,O:' obtained from other precursors as described above.

EXPERIMENTAL

Carbon suboxide, prepared by the literature method'' and stored at -2O"C, was injected into the instrument

t Author to whom correspondence should be addressed.

through a stainless-steel line (0.2 m x 1/16 in o.d.), kept at room temperature. Mass spectrometric measure- ments were performed using a VG ZAB2F instrument (VG Analytical, Manchester UK)."

Metastable ion studies were performed by mass- analysed ion kinetic energy (MIKE) spectroscopy" and collisionally activated dissociation (CID) spectra were obtained using air as the collision gas.',

Accurate mass measurements were performed using the peak-matching technique at 5000 resolving power (10% valley definition).

Squaric acid and oxalyl dichloride were commercially available products (Aldrich, Milan, Italy).

RESULTS AND DISCUSSION

The CAD-MIKE spectrum of C,O:' is shown in Fig. 1. The most abundant species is due to the ion at m/z 40, corresponding to a CO loss, as in the EI spectrum of C 3 0 2 . The complementary ion, CO", is also present at

52 28

W ) Figure 1. CAD-MIKE spectrum of C,O:'. E (V) represents the electrostatic sector voltage.

CCC l076-5174/96/020209-04 0 1996 by John Wiley & Sons, Ltd.

Received 5 September 1995 Accepted 24 October 1995

210 L. PANDOLFO ET AL.

C?+.

mi2 24

t

m/z 68

m/z 28 / \ d z 56

C d +.

c? + ' m/z 36

C,Ol+.

m/z 52

Scheme 1

m/z 28. Further fragments are due to the loss of 0, 0, and CO, leading to ions of m/z 52, 36 and 24, respec- tively (see Scheme 1). A further fragmentation channel leads to two complementary ions at m/z 12 (C") and 56

The simultaneous presence of the complementary ions is usually explained in terms of close ionization energies of the two species forming in the fragmentation process, in accordance with the Stevenson-Audier rule.14 This is confirmed by the data obtained from the fragmentation of C 3 0 2 , compared with the available energy data related to the observed fragment^'^ (see Table 1). Thus, detection of the C," ion and the absence of the complementary ion CO:' may be explained in terms of ionization energies ( I E ) of 10.9 and 13.77 eV, respectively. Analogously, the presence of the C3+' ion and the lack of 0;' is due to their differ- ence in I E s (11.1 vs. 12.12 eV). The application of the above-mentioned rule to the mass spectrum of C301 ' (Fig. 1) leads to the following conclusions: (i) the ioniza- tion potential of C,O (not available in the literature) must be lower than that of 0 (14.2 eV); (ii) the ioniza- tion potential of C,O must be comparable to that of

(c20;').

Table 1. Ionization potentials of the species under study" (PE = photoelectron spectroscopy)

Ionization potention (eV)

10.6 (PE. c30,)

10.5 (El, C ) 10.9 (El. C2) 11.1 (El. C3) 14.01 (PE. CO) 13.77 (PE, CO,) Not available Not available Not available 14.2 (El, 0) 12.1 2 (PEP 0,)

CO (14.01 eV), as both the ionic species are present in the spectra; and (iii) the ionization energy of C 2 0 2 must be close to that of C (10.5 eV).

The fragmentation pattern of C 3 0 2 proves that extensive rearrangement processes or, alternatively, molecular ion(s) of structure(s) different from that of the neutral molecule must be present. Leaving aside the structural difference between neutral and odd-electron cations calculated for fundamental vibrational states, the observed fragmentation pathways are probably due to high internal energy species which activate decomposi- tion channels not available in the ground state.

Worthy of note is the presence, among the primary fragmentation products, of C 2 0 2 radical cations at m/z 56. These ions have never been described among the EI-induced decomposition products of C30 , . However, looking carefully at the EI mass spectrum one can detect them at a relative abundance of about 0.3%. Accurate mass measurements showed that these ions are a doublet composed of the species C,O:' (55.9898) and C,H,+' (56.0415) in an abundance ratio of 100:5. The latter species must necessarily originate from the instrumental chemical background. The high purity of the former species at m/z 56 led us to undertake further structural investigations on this ion, in order to estab- lish whether its structure is similar to those determined by Siilzle et aL4 and Chen and Holmes3 on the same species originating from different precursors.

First, in order to be confident of our CAD-MIKE data, we obtained the MIKE spectrum of the ion at m/z 56 in the background under the same sensitivity condi- tions as used for the experiment on C,O:'. The spec- trum is shown in Fig. 2 and the only related fragment ions are those at m/z 55 and 41; it is reasonable to assume that they originated from H' and CH,' losses from the C4HZ ' precursor.

The MIKE spectrum of C20:' ions originating from the EI of C,O, is shown in Fig. 3. Abundant fragments at m/z 44, 40, 32, 28 and 24 are present. Under collision conditions, CO loss is highly favoured (see Fig. 4), but all the species described in the MIKE spectrum of Fig. 3 are still present, together with the formation of C" ions (see Scheme 2). These data were so different from those of Siilzle et aL4 that we suspected that they might

E(v) Figure 2. MIKE spectrum of the ion at m/z 56 present in the instrument as background. € (V) represents the electrostatic sector voltage.

[CO,]" IONS OF UNUSUAL STRUCTURE 211

Figure 3. MIKE spectrum of C,O;' originating from C,O,. E (V) represents the electrostatic sector voltage.

have been due to different instrumental conditions. For this reason, we undertook a series of CAD-MIKE experiments on the C,O,f' ions originating from the EI of squaric acid [Fig. 5(a)] and oxalyl dichloride [Fig. 5(b)]. The two spectra turned out to be exactly the same as those obtained by Siilzle et al., and the differences found with respect to the C20$' ion formed by EI of C,02 necessarily indicate that these ions have different structures.

Returning to the MIKE and CAD-MIKE spectra of C,O:' ions generated from C,O$' (Figs 3 and 4), we emphasize that, in low internal energy regimes such as observed in MIKE spectra, the most favoured fragmen- tation channel is that related to 0, formation. Under collisional conditions, i.e. adding further internal energy to the C,O:' ions, the CO loss becomes the most favoured decomposition route, as observed in the case of C 2 0 z ' ions originating from different precursors. The differences observed between the MIKE and CAD- MIKE spectra may be explained by the fact that C,O:' ions generate C20: species, which are structurally dif- ferent from those originating from other different pre- cursors. The energy deposition occurring in collisional

32 40 1

E(v) Figure 4. CAD-MIKE spectrum of ion C,Ot' originating from C,O,. E (V) represents the electrostatic sector voltage.

b

Figure 5. CAD-MIKE spectra of ion C,O;' originating from (a) squaric acid and (b) oxalyl dichloride. E (V) represents the electro- static sector voltage.

experiments leads to partial isomerization of such a structure towards that occurring in all other cases, i.e. a trans planar structure.

For the low internal energy structure(s), looking at the formation of 0;. which was never observed in any other case, we propose a four-membered intermediate, which readily gives rise to 0;. formation. However, it

C Z + ' i

-co C J +, - C

d z 56 d z 28 d z 44

I -O

m/z 40

S c h e m e 2

212 L. PANDOLFO ET AL.

a b

It O=C

*c=Ol+ C

Scheme 3.

cyclic structure of C20:' leading to C, and 0, frag- ments is charge localization on C , species, and not, as in the present case, on the 0, molecule. Consequently, different mechanisms must be proposed.

One hypothesis is to consider 0:. and C:* ions with structures different from the molecular ion of C, and 0, molecules ; however, of course, this hypothesis is improbable. An alternative is that the 0:. formation originates through sequential losses of two C atoms, i.e. as the result of a fast process followed by a slow one (or vice versa). The possible intermediacy of the CO;. ion in 0;. formation agrees with the energy data (ZEco2 = 13.77 ev), although the same approach indicates that the COZ- structure must necessarily be different from that of neutral CO, (as suggested by the same energy data: ZE, = 10.5 eV). On the basis of these experimental data, we propose the four-membered cyclic structure a reported in Scheme 3 for the C2Q:' radical cation gen- erated by electron impact of C,O, . This structure, through sequential C losses, would lead to the 0:. ions through the intermediacy of ions b.

Under collisional conditions, the equilibrium between structures a and c is strongly shifted in the direction of c, i.e. the C,O:' ions already described as originating from other precursors.

must be emphasized that such a hypothesis s completely contrary to the Stevenson-Audier rule. In fact, as reported in Table 1, the I E of C, is 10.9 eV, whereas that of O2 is 12.12 eV. What would be expected from unimolecular fragmentation of the four-membered

REFERENCES

1 . B. E. Turner, Astrophys. Lett. 23, 21 7 (1 983) ; D. Smith and N. G. Adams, J. Chem. Soc., faraday Trans. 2 161 3 (1 989).

2. D. Siilzle and H. Schwarz, in Fundamental of Gas Phase /on Chemistry, edited by K. R. Jennings, pp. 237-248, Kluwer Academic Publisher, Amsterdam (1 991 ) .

3. H. Chen and J. L. Holmes, Int. J. Mass Spectrom. /on fro- cesses 133, 1 1 1 (1 994).

4. D. Sulzle, T. Weiske and H. Schwarz, lnt. d . Mass Spectrom. /on Processes 125, 7 5 (1 993).

5. R . C. Haddon, 0. Poppinger and L. Radom, J. Am. Chem. SOC. 97, 1645 (1 975).

6. L. B. Knight, Jr., J. Steadman, P. K. Miller, 0. E. Bowman, E. R. Davidson and D. Feller, J. Chem. Phys. 80, 4593 (1 984).

7. J. T. Blair, J. C. Weisshaar, J . E. Carpenter and F. Weinhold, J. Chem. Phys. 87, 392 (1 987).

8. W. E. Thompson and M. E. Jacox, J. Chem. Phys. 95, 735 (1991).

9. L. Pandolfo, G. Paiaro. A. Somogyi, S. Catinella and P. Traldi, Org. Mass Spectrom. 29, 540 (1 994).

10. A. Klemenc, R . Wechsberg and G. Wagner, Monatsh. Chem. 66, 337 (I 935)

11 . R. P. Morgan, J. H. Beynon, R. H. Bateman and B. N. Green, lnt. J. Mass Spectrom. Ion Phys. 28. 171 (1 979).

12. R. G. Cooks, J. H. Beynon, R . M. Caprioli and R. G. Lester, Metastable lons, p. 11 2, Elsevier. Amsterdam (1 968).

13. R. G. Cooks, Collision Spectroscopy. Plenum Press, New York (1 978).

14. H. E. Audier, Org. Mass Spectrom. 2,283 (1969). 15. R. 0 . Levin and S. G. Lias, lonization Potential and Appear-

ance Potential Measurements. National Bureau of Standards, Washington, DC (1 982).