RL

FAD

EDGEWOOD ARSENALTECHNICAL REPORT

EATR 4568

PROBES OF NONREACTIVE ENVIRONMENTI

SIlL. THEORETICAL ASPECTS OF PYRIDINIUM

IODIDE CHARGE-TRANSFER ABSORPTION

by

Raymond A. Mockay

Edward J. Pozlomek

October 1971

D02

RoP!oduc bd y -

NATIONAL TECHNICAL DEPARTMENT OF THE ARMYINFORMATION SERVICEsp,j f..•o., va 22151 E D'IG EW O O D[ A R S EN A L• .

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1.

UN'CLAýSIRLEDSEcurity CIA a, c a"

DOCUME•NT CONTROL DATA - R & DS.. ,ýIocu¢i4, dfahti.c , .ottfll. bc,,j of f *,de oi olnow iio tl .n~ ftUOI nw i *lfor41 gjhon-1 fh I1 w llro pt I Io c lAaotI --

I 051 INAtIIQ ACTIVITY fC`0,)10?f ditihot) IV, FAEPORV 6*CYriITY CLA141FICATION|C0, Edgewood Ar'senal [ UNCL.ASSlIFIED)SAT'r t SMiUEA R PRE at? .. .. .Edeod Arsenal, Maryland 21010 N A •

PROBES OF NONREACTIVE ENVIRONMENT. Ill * THEORETICAL ASPECTS OF PYRIDINUM4IODID)E ClIARGL-TIUNSFER ABISORP'TION

4 09S!IPT-vt NO-T3 (T'p$ ropoll cid Inofo.It daI0.)

This work was performed In 1970.

Edward J. Poziomek , and Raymond A, Mackay

ncon flF O5 ATIE 70. 1 OTA6. NO~. uI0 PAOJS l.NO ~ M'

October 1971 20 15C, CONTRACT OR CRANT 0. - oo, COMIGINATO•RI AtPOR'T HUMBIfER13

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11 AGAYRACT

Pyridinium iodide salts possess iodide to ring charge-transfer (c-t) absorption; two c.tbands are generally observed in nonpolar solvents. The existence of two bands is importent for the researchtr in detection because the absolute and relative energies of thestbands could be a useful indication of changes to ring substituents and environment sur-rounding the ring, A question exists, however, as to whether the higher energy c-tband arises from an excited state of the donor or from a higher energy state of theacceptor, Self-consistent extended 1Itckel calculations have been performed for the uD-substituted and the 2-, 3-, and 4-cyano (and acetyl) pyridinium ions. The calculated....i ..... e bctwc'a. ',he euergy of the first (lowest) and second (next highest) vacantmolecular orbital of the pyridinium ion is in good agreement with the observed separa-tion of the two c-t bands in methylene chloride, This supports other experimentalresults, which indicate that: the second c-t band does not arise from the excited state(2p½) of the iodine atom. These and other results of the calculation are discussed interms of various experimeAtal parameters.

14. KEYWORDS

Defense systems Molecular complexesSpectroscopy Electron affinityPyridinium iodides Obbstituent constantsCharge-transfer SolventMolecular orbitals Environmant

F .... . .I 4v f 03 "a "- 00 "OI- m 141", - JA 4,4 UNWCLASIi

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I ~UNCLA'. m LINH A I..NK L0N C

___________ wOL Z.0 R-MT RL9 WT

SpactroscopyPyridinium iodidosCharge- trans ferMolecular orbit4luMolecular complexesElectron affinitySubstituent constants

E -nvironment

20 SoiyC~I~ta

'I.

I'[)GIWOOI) ARSENAL ITCHNICAL REPORT

EATR 45o8

PROMI'S OF NONRI-ACTIVE _NVIRONMENTIII. IiHIOREIIiCAL ASIPc(TS OF PYRIDINIUMI())I)!: C!IAVGU-TRANSFER ABSORPTION

b~y1

Ralymond A. kfackay

Drexel University

Edward J. l'oziomek

I)el\knsive Research Depurtwe it

October 197 1

Approved for public release; distribution tio l'nited,

Task 113662710AD2901

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:dgcwco( Arsen~al, Maryland 21010

"DIGEST

Pyridinlum iodide salts possess iodide to ring chitugge-transfer (c-0) absorption; two c-t builds(ire generally observed in nonpolar solvents. Th1 existence of two bands is important for the Ic-0searcher in (detection because the absolute and relative energies of these binds could be a usefulindication of chtinges to ring substituents and ervironment surrounding the ring. A question exists,however, as to whether the higher energy c-t band arises from an excited state of the donor or froma higher energy state of the acceptor, Self-consistent extended Mickel calculations have bekn per-formed for the unsubsti tuted and the 2-, 3-, and 4-cynno (and acetyl) pyridinium ions. The caictL-lated difference between the energy ofr thie first (lowest) and seconld (neXt highest) vOCIcNt m'llIhculahrlorbital oh the pyridinium ion is in good agreement with thie .ob-rved separation of the two c-t bandsin methylene chloride. This supports other experimental results, which indicate that the second c-tband does not arise from the excited state (2P-• ) of the iodine alom. These and other results of thecalculation amn discussed iin terms of various experimental parameters,

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FOREWORD

Flie work described] ini th is re port was au thorized under Task I111662 71 OAD290 1, Cheloi-- l Ihcc lion anlld Identification Technology, Detection and identification Concepts. The work wits

j)V1rtitulnltI in aI070. -'

IVprOtliuliOn of this docLuimlent in whole or in part is prohibited except with permission ofilk' Comnuanding Officer, tLdgewood Arsenal, A1TN: SMUEA-TS-TIt, Edgewood Arsenal, Mary-lanid 2 1010: however, IMX' and the National Technical Informiotion Service lure authorized tortpmdtlkicc the dOCunw nlt Ir United States Government purposes.

Acknowledgments

The authors w ish to express thei appreciation to Dr. George Clarkc and Dr. Barry KalmanIlor use of their program and for assistance in tile perfornmance of the cailculations, alnd to Mr. Phillip

KInorr for iechinickl assistaunce. Espeeial thanks ore dute Dr. Kaliain for many helpful discussions,

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CONTIENTS

-- P~igc

1. INTR•.G IJ(C ION ... 7

I!. CAL('ULATIONS AND I-XPERIMENTATION .............. . 7

Ill. RISULTS AML) DISCUSSION ...................... 7

!• _A . Band t'nergies . . . . . . . . . . . . . . . . . . . . . .. . . 7

B. E-led ron A fiifity . . . . . . . . . . . . . . . .. . . .. . 10

C. Substituent Constunts ...... ....................... 10

IV. CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . I I

LITERATURE CITED . . . . . . . . . . . . . . . . . . . . . . . . . 13

DISTRIBUTION LIST ....... . 15

PRECEDING PAGE BLANK

r,4•4•I;•.

PROIES OF NONREACTIVE ENVIRONM LNT ]S~Iil, THIEOi.LT!CAL ASPEFCTS OF PY El IDNIUNiUv

-IODIDE l A iARGl-TtRANSFER ABSORPTION

I, INTRODUCTION.

Tile first (long wxavelength) electronic absorption bund of pyridixium iodide salts has beenstudied 1 tensivcty. It arises Imm ch rgQetrainsfer front iodide ion to the pyridinium ion., .2 Inixonlolar solvenlts a secon: C-t band, t tshorter wavelength than the first, is observed and huas beenascribed to tile flrst excited state of the iodinoe atom (2Pi)..' R m ,xperimental results indtcatetltx 1 the second bolnd may In fact result from ch Wrge.trwisxFer to tile becond vaeuint molecular orbitalotf the pyridiniwin ion) .' We have therefore performed self'consistent extended IHickel (SCEII)S~~calcullations oil variotis pyriditliun ions and cOmlpurced the results with expe, irimontal p,,ramoters fil-cluding C-t band energies. -

- , CALCULATIONS AND EXPERIMiNTATION.

Tile cat.Writions w,;re performed by the mnethod suggested by I,,Hrris using ,x modifiedproced••e of Kalmani and Clarleu. All valt ene orbitals and electrons were included. Tile electronaffinities of the pyridinium ions were taken as the negative of the ionizaWic n potential of thecorresponding pyridinyl radlcals. The band energies trod the rate constants were determined inmethy~ene chlorldene

1!. RESULTS ANID DISCUSSION.

A. Bland Energies.

The lowest unoccupied molecular orbital (LUMO) in benzene is a doubly degenerate itorbital (eN, in D•,t ); on going to pyridine or pyridinuni ion, it splits into b, and a2 (in C1 ,. Theseoxbitals are shown below, The (lotted line Indicates the modus.

+~ 1

IX R

Kosower. E. M., and Skorcz, J. A. Pyridiniuto Complexes. 111. Churge-Trxansfvt Bands of iolyalkylpyftdixiiurnlodides. J. Amer. Chamit. Soc., 2, 2195 (1960).

2Nlackay, R. A., Lundolph, J. R., and Poriomck, l . J. ~perlxmentail Evidence Concernhin the Nature of the TwoChiarga-Ti'rtsrfr aixils in Pyridiixium Iodides. J. Amer. Chem. Soc. 93 (1971), and references therein.

31(osower, 1. M., Skorcz, Schwarz, W, M., Jr., and Patton, J. W. Pyridiniuma Complcxes. I. Tle Signlllcance of theSecoed Chluge-Trunsfet Ban1d of Pyridinium Iodides. J. Amer. Cihem. Soc. 82, 2188 (1960).

'lV'erhoeven, J. W., Dirkx, I, P., and de iBuer, Tt, J. Studios of Intet- and lntra4-holecular Donor-Acceptor lInter.actions. 11. hitermolectihir Chaoge Transfer Involving Subsflulted Pvlxxdimumt louv;. Te"lnlhedron 2S. 3395 (0969),

5 ]tllin.F F. Mel1-Cansistont helliods in 11ijckel Theory, 3. Clhem. Phiys. 484027 (1968).61<ahnan, B., und Clarke, G. J. Chomn. Phys. (1971" in press,

PRIVIDING P.AGE B.ANK

The lowest energy orbital has been assigned as b by Kosower and !Poziomek7 oil tilebasis of infrared evidence and by Verhoeven 8 on the basis of some simple HWickel linear combinationof atomic orbit-Is-inolecular orbitals (LCAO-MO) calculations. Our SCEH calculations confirmihese assignments. Both orbitals are essentially -t in character; the highest filled MO is mixed but is

largely a (in [lie plane). The energy separation between the bi and a2 orbitals depends on the natureand position of the substituent oil the ring. The first c-t band arises from donation of ant electronfrOMt iodide into JhC bl orbital of the pyridiniunt ring. If the second band arises from donationinto a, , then the energy difference between the two observed c-t bands (A,/'-) should be related tothe energy difference between the a2 and b 1 MO's.

Although these are orbital rather than state energies (i.e., conf'iguration interaction has notbeen included), it is to be expected that the additional terms will be largely self-canceling in A~E.tbecause of the nature of tile transitions and the similarity of tile two acceptor MO's,

The geometry used for the I-methyl group was such that one hydrogen atom was in theplane perpendicular to tile ring passing through the nitrogen and the 4-position. Except for the acetylsubstituents, the remaining geometry was fixed. The acetyl was positioned with the carbonyl oxygenin the plane of the ring.

As may be seen from tile results in table 1, there is good qualitative agreement between thecalculated and experimental values ofAEct. For all but the 3-substituted ions, the quantitativeagreement is excellent. Tme calculated values for the meta-substittuted species are somewhat low, butstill good considering that the energy gap is now fairly small. It may be that the parameters used 6

are n)ot quite optimal for this system.

The difference in energy between the 2p, /2 and 21'3,2 states of the iodine atom is 21.7kcal mote- 1.2 If the second c-t band also arises from donation into the b, orbital of the ring, butwith the iodine atom being left in its first excited (2P1 /2) state, then in the absence of any stronginteractions AE,., should be about 22 kcal mole- I regardiess of the substituent. In other words,'fet should be approximately independent of the acceptor, as is observed in alkali iodide c-tspectra. In tropylium iodide, which has only one (doubly degenerate) acceptor orbital, two iodideto ring c-t bands are observed in CH 2 Cl2 with a AE,.t of 19.9 kcal mole- 1.9 The spectra oftropylium chloride and bromide substantiate that the two bands (1o arise from the 2P3 /2 and 2P1)/2states of iodine.t 0 This is in accord with the expectation that AE,.t should not vary more than afew k•al mole- I from the gas phase 2P- /2 " 2P3/ 2 separation. Thus, one should expect to observetlhree c-t bands in tile pyridinium iodide spectra, one about 21 kcal mole- I higher than the first(lowest energy) band, and another varying between 10 and 30 kcal mole" I higher than the first,depending on the substituent, and its position. However, only two bands have been definitelyobserved. 2 In fact, there should be four bands, the last corresponding to transfer of an electroninto the second vacant MO of the py!,i. tun l ,hi g, 1inq It- ;•,!;n.. . .. . . . . . .,, 12 state,. Howver.thisNM ban 1dwol be "buried" under tile intense local transitions of tile ring.

7 Kosower, E1,1., and Poziomek, E. J. Stable Free Radicals. I. Isolation and Distillation of I -Ethyl-4-carb-,nethoxy-

pyridinyl. j. Amer. Chem. Soc. 86, 5515 (1964).a Verhoeven, J. W. Intramolecular Electron Donor-Acceptor Interaction in N-Aralkyl-Pyridinium Ions. Thesis.

Laboratorium voor Organische Scheikande dier Universiteit van Amsterdam. 1969.9 Kosower, 1. M. The Solvent Sensitivity of lie Charge-Transfer Brand of Tropylium Iodide. J. Org. Chem. 29, 956

(1964).-oHarmon, K. M.. Cummings, F. F., Davis, D. A., and l)iestlcr, D, J. Carbonium ,, Salts. V. The Spectra a1d --

Decomposition o T"ropylium Hahides in Me hylene Chloride. j. Amer. Clen,. Soc. 84, 3349 (1962).

S!

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Table I. Charge-Transfer Transition Energies for Sevendl I.Methylpyridiliunm Iodides

SRing .!<1",i IaI~

substituet ~Found Caled

4-CN 58.2 85.3 27.1 29.04-COC11 3 63.8 92.5 28.7 30.82-CN 59.8 81.2 21,6 20.12-COCH 3 64.7 87,2 22.5 19.7I-CH3 76.7 98.2 21.5 19.5

3.CN 65.0 79.0 14.0 7.23.COCIt 3 70.6 80.8 10.2 7.1

lit, and Et, are the firs( and second c.-t bald1 inergie' respectively in kcal

jnole- tpiecision i0.2 kcal mole- ;accuracy A0.4 kcal mole-1) a deter-

7-inlned in C1t 2C12-

b,•C.~t (Fouid) = Et2 - ti. 4Et (Caled) 1-' (02) - E (bI i, i kcal Iokm.e-

-• The energy difference for 2,4,6-trimethylpyryliuni iodide c-t bands (in CHC13 ) was foundto be 14.5 kcal mole- 1.1 1 Here, however, local transitions are close in energy to the c-t bands and Ican interact with them. In particular, the ni-r* transition can interact with the c-t band, whichleaves iodine in the 2p,12 state, possibly giving it intensity and shifting it to lower energy. In con-trast to the pyryfliun ion, however, the pyridinium ion does not possess a lone pair o' electrons, andthis interaction is not possible. Thus, the absence of the third band is puzzling. A possible explana-tion for the failure to observe this c-t band is that it has been reduced ill intensity because of whatmay be considered as a "ligand field" s,.litting of the iodine 5p orbitals by the pyridiniuni ion. Thespace part of the wave functions for the 2 'P3,2, and 71]/2 states of iodine contain contributionsfrom the p~, p, , and( 1), orbitals. Assuming that the electron transferred from the iodide ion to thering in the c-t process is donated essentially from an iodine 5p orbital, the relative contribution ofthe Px, p," and P, orbitals to the overall transition dipole moment may be calculated. For a modelin wilch the iodide ion is located on the z-axis above the center of o "benzene" ring (idealizedC6 , symmetry), the contribution ofpx and p), are equal and very much greater than that of p,Calculations show that this is still true even when the actual pyridinium ion is used and the iodidei i: ion is loca ted over tile position of minimunm electrostatic potential energy, which is somewhat dis-

placed from the center of the ring.

For zero ligand field splitting (A ), tho, two c-t bands, (LIle to the iodinIc being left in) thle,2p3 /2 and 2p ,/2 states, are thus of comparable intensity. In the limit of A>>r; (spin-orbit

couplilg con)stant), the 22p3/- • dP 2 states go over to two otler states, designated 21. and 2 Arespectively. The 2E state contains only P, and py, whereas "A contains only p.. In this case, thec-t transition leaving iodine in tile 2A state has neligible intensity ir the idealized case (C 6 , ), zerointensity] compared to the lower energy transition, which leaves iodine in the 2E state. There issome experimental support for this explanation from other c-t spectra. In the case of tropyliumliodide, the higher energy c-t band is less intense than the lower energy one; 9 for the bromide, bothbands (although not completely resolved) are clearly of comparable intensity. The higher energyband is considerably less intense in 2,6-dichlorobenzylquinoliniuln iodide,1 2 which might be

11Balaban, A. T., Mocanlu, M., and Siimoiin Z, Charge-transfei Specina of Pyryliunm lodides. Tetrahedr on 20, 119

(1964)."2Bricgieb. G ,, Jun,, W., and Herre, W. Anionen-und LdsUnlgsniiitel-AbliiingigkeP cites interionaten Elektronen-

naustausches in lonenepairen (am Beispiel von Salzen des N,2-6.l)ichlorbenzylchinoliniums). Z. Phys. Chem.

Nene Polge .38, 253 (1963).

9

expected ' ve a "iigmid field strength" closer to that of the pyridifliulfl ion. III the case of

gaseous iodide, both bands would, as observed, be expected to be of comparable intensityregardless of the value of a because of the spherical symmetry of the acceptor orbital on the alkalimetal ion. Thius, the I" .,ive intensities of the possible c-t transitions between iodide ion and anacceptor will depend on the nature of the acceptor orbital(s), the geometry of the complex, and theligand field A trength of the accepA0tor species.

13. Electron Affinity,

The electron affinity (EA) of the pyridinium ion has been taken as equal to the negativeof the calculated ionization potential (11) of the pyridinyl radical. Although the calculation iscapable of giving absoILute values of the IP within 10 to 1.5 ev, the relative values for different posi-lions o1 the same substituent are considerably better. A plot of Et versus EA (data given in table11) shows the correct general trend. The individual correlations for the 2-, 3-, and 4-CN or COCI1 3substituted ions are quite good.

C. Substituent Conslants.

Correlation of u substituent constants with E, I (in CHCl3 ) has been reported by Kosowerand Skorcz1 3 for 4-CN, 4-COOCH 3 , 3-COOCH 3 , H-, 3-CH3 , and 4-CH 3 substituted pyridiniumiodides; ou values were used for the 4-substituents that are electron withdrawing. Similarly, there isa good correlation of a substituent constants with E, 1 (obtained in CH 2 CI2 ) for the compoutndslisted in table 111.

Table 1I, Calculated Electron Affinities for SeveralI -Methylpyridinium Iodides

PRing it b 1substituent /A

I.CH 3 7.83 3.323.CN 7.96 2.86

3-COCH 3 8.54 3.06

2.COdI 3 8.73 2.80

4-COCH 3 8.76 2.78

2-CN 8.23 2.58 :0

4-CN 8.32 2.52Tr+c 8.85 2.16d

"aElectron affinity.bl~irst c-t band as observed in CH 2CL2 .CTropylium ion,dcatculated from datum from Kosower, E.M. The Solvent

Sensit ivity of the Charge-Trrmsfor Band of Tropyliumt

Iodide. J, Org, Chem. 29, 956 (1964).

13Kosower, E. M., and Skorca, 1. A. In Advances in Molecular Spectroscopy. p 4 13 . Pergamnon Press, New York,

New York. 1962.

101

1VMS

Table Ill. Comparison of the Euoigy of the First c-. Band (Efj) of Varioms Py ridlinimnl Iodide.s(in CH2CI2 ) With u Substituent Constants and Calculated Charges (QN) oil

the Pyridinium Nitrugen

Ring Et 0 mb opb bcJQsubstitucnt kcal/moik

II 76.7 0 0 0 1,00

3.CN 65.6 +0.56 - - 1.54

2.CN 60.1 - - 1.774.CN 58.5 -. 1I0.66 +1.00 1.57

3.COC" 3 70.6 +0.38 - - 1.29

2.COCI1 3 04.7 - - - 1l.3 1

,i.COCH3 63.8 - +0.50 +0.87 1.37 A

4.C1 3 80.70 - -0.17 -

3.CH3 78,0c .0.069 -3.COOC2115 71.3 10.37 - - -

4-COOC 2I15 65.2 - 10.45 +0.68 --

2.COOC 2H15 67.8 ... I41,aroi Mackay, R. A., Landolph, J. Rl., ane, Poziosntk, E. J. Experimental Evidence Concerning tle Nature of the TwoCCharge-Transfor Bands in Pyridipnium todides. J, Amer. Chem. Soc. 93 (1971).

b,+0 11, HIline, J. Pilysical Organic Chemistry. p. 87, McGraw-hlill Book Companly, lc., New York, New York. 1962.0c,'o4m Jamf", H. H. A Reexamination of the Hammett Equation. Chem, Rev. S3, 191 (1953).dThe calculated charges on nitrogen are normalized to the unsubstiluted I -methyl pyridinimn ions.0 From CIICI3 data as "corrected" to Ctt2CI2 , )atsfrfom Kosower, E. NJ., and Skorcz, J. A. In Advances in

Molecular Spectroscopy. ) 413. Pergamon Press, New York, New York, 1962,

Steic hindrance by a substituent towards iodide ion approaching the pyridinium ringwould not be expected to vary appreciably with ring position. This allows an estimate of oitho a-values from c-t transitions observed with the 2-substituted compounds. For example, the ortho a-values for 2-COOC 2 1-15 , 2-COCH 3 , and 2-CN as estimated fromn a plot of the data in table Ill are A+0.48, +0,65, and +0.89 respectively, These fall in between the values of p and a for the Ucorresponding p-substituent.

A reasonable correlation also exists between o values (at and o( ) and the calculatedcharges on the nitrogen (QN ), However, fr'om a plot of thcse data (given in table I1l) and the calcu-lated values of QN for the 2-substituents, o, for the CN appears to be larger than at;<,N , whereasao O-OCH13 is less than at, 4OCllt 3 anld approximtately equal to G',l -COCII 3. Though the data are

interesting, their interpretationi m terms of the various possible eftects is difficult to achieve satis-factoriiy without additional results with more compounds.

IV. CONCLUSIONS.

SCEII calculations for I -methylpyridinitm ion and 2-, 3-, and 4-cyano (and acetyl)l-methylpyridinium ions give data on energy differences between the lowest and next highest vacantmolecular orbitals of the pyridinium ion, which are in good agreement with the observed separation

NA IIY

m

of (lie two pyridinium-iodide c-t bands in methylene chloride. This result supports previous indica-tions that tile higher energy c-t bNIa relts ft'ro11 clhargeC-transfer to the second vacant moleculal-ol'bilall of' tile pyl'idilliufn ionl,

calctulated electron a(lliitics of the pyi'idinlitinl ions show expected trends in plots withthe low energy pyridilnitIil-iodide c-t transition. A reasonable correlation was also found to existbIetVeen o valueS (01) and 0111 ) and the calculated charges on the pyridiniuiii nitrogen.

A possible explanation ['or the failure to observe three c-t bands lias boen proposed, which,it valid, allows prediction of relative intensities from simple. group theoretical considerations usingidealized •1ynnletry.

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LITERA'rURE CITED

1. Kosower, E, M., and Skorcz, J. A. PlyridilliunM Coplloxes. I1l. Charge-Transfer Bandsof Polyalkylpyridinium lodides. 3. Ame,. Chem. Soc., 82, 2195 (1960).

2. Mackay, R. A., Landolph, J. R., and Poziomek, E, J. Experimental Evidence Concern-ing the Nature of the Two Charge-Transfer Bands in Pyridiniun lodides. J. Amer. Chem. Soc. 93(1971).

3. Kosower, E. M., Skorcz, Schwarz, W. MW, Jr., and Patton, J. W. Pyridinium Complexes.1. The -Significance of' the Second Charge-Transfer Band of Pyridiniwnm lodides. J. Amer. Chem.Soc. 82, 2188 (1960).

4, Verhoeven, J. W., l)irkx, 1. P., and de Doer, Th. J. Studies of later- and Intra-Molecular Donor.Acceptor Interactions. 11. Intermolecular Charge-Transfer Involving SubstitutedPyridinium Ions. Tetrahedron, 25, 3395 (1969).

5. Harris, F. E. Self-Consistent Methods in H~iikel Theory. J. Chem. Phys., 48, 4027l(19(8)g)

6. Kalman, B., and Clarke, G. J. Clhe. Plhys. (1971) in press,

7. Kosower, E. M., and Pozioiek, E. J. Stable Free Radicals, I. Isolation and Distilla-tion of l-Ethyl-4-carbomethoxypyridinyil J. Amer. Chem. Soc. 86, 5515 (1964).

8. Verhoeven, J. W. Intramolecular Electron Donor-Acceptor Interaction in N-Aralkyl-

Pyridinium lens. Thesis. Laboratorium voor Organische Scheikunde der Universitelt vanAmsterdam. 1969.

9. Kosower, E. M. The Solvent Sensitivity of the Cluirge-'rransfer Band o1f TropyliumIodide. J. Org. Chem. 29, 956 (1964).

10. Harmon, K. M., Cummings, F. E., Davis, D. A., and Diestler, D. J. Carbonium IonSalts. V. Thc Spectra and Decomposition of Tropylium Halides in Methylene Chloride, J. Amer.Chem. Soc. 84, 3349 (1962).

11. Balaban .A. T ,, ,a , and Shu011 Z. .harge-Trunisfer Spectra of' Pyrylituml,,,dides. Tetrahedron, 20, 119 (1964),

12. Briegleb, G., Jung, W., and Hlerre, W. Anionen-und Lcsungsmittel-Abhingigkeit eniesinterionaren Elektronetaustausches in lonenepatiren (am Beispiel von Salzen des N-2,6-Diclhlorbenzylchinoliniums). Z. Phys. Clhew, Neue Folge, 38, 253 (1963).

13, Kosower, E. M., and Skorcz, J. A. In Advances in Molecular Spectroscopy. p 4 13.Perganion Press, New York, New York. 1962.

14. Hine, J. Pihysical Organic Chemistry. p 87, McGraw-Hill Book Company, Inc., NewYork, New York. 1962.

15, Jaffe, H. H. A Reexamination of the lamniett Equaition. Chew Rev. 53, 191 (19.53).

13