1148
RICHARD WAACK
AND l / I A H Y
A. DORAN
Electronic Spectra of Organolithium Compounds. 111. Effect of Methyl Substitution
by Richard Waack and Mary A. Doran The Dow Chemical Company, Easte~nResearch Lahoratory, Framingham. .Massachusetts (Received Dewmber 7 , 1963)
Substitution of a methyl group on an odd-alternant anion [e.g., benzyllithium, allyllithium, or styryllithium] causes, depending on position, either a red shift or a blue shift of the longest wave length absorption. This behavior is identical with that of norialternants and is consistent with the arrangement of the n-molecular orbitals of these molecules. Explanations are given in terms of resonance theory and rrdecular orbital theory.
The effect of a methyl substituent on the spectrum of an even-alternant hydrocarbon or a nonalternant hydrocarbon (e.g., azulene) is well-known. There are no reports in the literature, however, describing the behavior of the spectra of odd-alternant molecules (which must be free radicals or ions) upon methyl substitution. This paper is concerned with the effect of a methyl substituent on the spectra of the oddalternant anions’ benzyllithium, allyllithium, styryllithium, and 1,l-diphenyl-n-hexyllithium. The observed spectral changes are analogous to those of the nonalternant hydrocarbons in that, depending on the position of the methyl substituent, either a red or a blue shift in absorption maximum from that of the unsubstituted species results. A qualitative explanation of this finding is given in terms of resonance theory. LCAO-;\IO calculations of relative transition energies are in qualitative agreement with the spectral shifts. The use of molecular orbital coefficients to predict the shift direction, which was successful for evenalternants and norialternants, does riot give the correct shift for these odd-alternant anions.
Experimental The spectra were measured in tetrahydrofuran (THF) solution using the absorption cell and the procedures described previously.2 Renzyllithium was prcparcd from tribenzyltin chloride and phenyllithium in diethyl ether or from dibenzylmercury and metallic lithium in THF.3 Allyllithium was prepared in hexane from n-butyllithium The Journal of Physical Chemistry
and t e t r a a l l ~ l t i n . ~Solid allyllithium was washed with hexane and dissolved in THI’. a-hiethylbenzyllithium, p-methylbenzyllithium, a-methylallyllithium, ymethylallyllithium,~ and 8-methylallyllithium were prepared from the corresponding triphenyltin compounds6 in diethyl ether via transmetalation with phenyllithium. Insoluble tetraphenyltin was filtered off the chilled reaction mixture. Styryllithium, amethylstyryllithium, p-methylstyryllithium, o-methylstyryllithium, and m-methylstyryllithium were formed from n-butyllithium and the respective monomer in T H F solution. The monomers were dried over calcium hydride and vacuum distilled. The methylsubstituted 1,ldiphenyl-n-hexyllithium compounds ere formed from the corresponding 1,lditolylethylenes’ and n-butyllithium. I’henyllithium and ptolyllithium were prepared from metallic lithium and the corresponding bismercury compound in T H F solution. Phenyllithium was also prepared from chlorobenzene in T H F solution. (1) For convenience we refer t o organolithium compounds as anions although in solution they most probably exist as the ion pairs. (2) It. Waack and SI.A. Doran. J . A m . Chem. Soc., 8 5 , 1651 (1963). (3) R. Waack and Sf. A. Doran, J . Phgs. Chem., 6 7 , 148 (1963). (4) D. Seyferth and SI. A. Weiner, .I. Am. Chem. Soc., 8 3 , 3583 (1961). (5) Either a-methylallyltriphenyltin or y-rriethylallyltrighenyltin results in the same arganolithium compound, which is expected to be predominantly crotyllithium. (6) Kindly supplied to us by Dr. 1;. C . Leavitt and Xfzss Priscilla A . Carney of this laboratory. (7) Kindly supplied by Dr. Vernon Sandel of this laboratory.
ELECTRONIC SPECTRA
1149
OF ORGANOLITHIUM COMPOUNDS
Results The wave lengths of the absorption maxima in
THF solution of the parent lithium compounds, benzyllithium, allyllithium, styryllithium, 1,l-diphenyln-hexyllithium, arid phcnyllithium, and their methylsubstituted derivatives are listed in Table I. Table I : Electronic Absorption Spectra of Odd-Alternant Organolithiurn Compounds and Their Methyl Derivatives hnx, nip Benzyllithium a-Methylbenxyllithium p-Methylbenayllithium
330 333 315
Allyllithiurn a-Methylallyllithiuni yMethylallyllithiun1 p-Methylallyllithium Styryllithiurn a-Methyltityryllithium p-Methylstyryilithiurn o-Methylstyrylli thium rn-Methylstyryllithium
315 291 29 1 330 335 340 327 325 338
Phenyllithiuin p-Tolyllithium
292,268,261 292,273,268
1,l-Diphenyl-n-hexyllithium 1,l-Di-o-tolyl-n-hexyllithium 1,l-Di-p-tolyl-n-hexyllithium 1,l-IX-rn-tolyl-n-hexyllithium
495,315 501,328 490,318 498,310 (sh)
The following important features are shown by the data in Tablc I. A methyl group in a starred (or active) position* of the odd-alternant lithium compounds causes either a red shift or a bluc shift of the absorption maximum, depending on the position of substitution. R methyl group in an inactive position, such that it is in cross conjugation with the n-system, causes only a red shift in absorption maximum. Larger shifts result from methyl substitution in a smaller molecule. When the methyl substituent is removed from the chromophoric site, and there is no formal delocalization, e.g. , in phenyllithium, it causes little change in the spectrum. The deviation of o-tolyl-nhexyllithium from the general pattern is likely due to steric effects.
Discussion Methyl or alkyl substitution on an even-alternant hydrocarbon always causes a bathochromic shift of the conjugation On the other hand, methyl substitution of a nonalternant hydrocarbon, e.g., azuleneJi2causes either, depending on its location, a red shift or a blue shift of the longest wave length
absorption band. The spcctral changes caused by methyl substitution of the odd-altcmiant organolithium compounds are like those of the nonalternants, in that either a red shift or a blue shift in absorption maximum may occur. The perturbation of the parent molecule by a methyl group can be separated into a resonance effect (hyperconjugation) and an inductive effect. Considering the longest wave leiigth absorption, hypcrconjugation always produces a red shift independent of molecular structure, whereas the inductive effect can produce either a red shift or a blue shiftl3,l4depending on the molecule. In an even-alternant hydrocarbon, because the antibonding molecular orbitals are a mirror image of the bonding molecular orbitals, both bonding arid antibonding orbitals are shifted by equal a r n o u n t ~ ’ ~ * ’ ~ ; thus, the first-order inductive effect of a methyl group on the long wave length transition cancels 0 ~ t . l Red ~ shifts caused by methyl substitution of even-alternant hydrocarbons result from hyperconjugation.’G In nonalterriant hydrocarbons (e.g., aeulene) the shifts produced by methyl substitution are much larger and are mainly due to the inductive effect of the methyl substituent. 12,17 In odd-alternant organolithium compounds because the highest occupied orbital (to a first approximation) is the central noribonding orbital located between the lowest antibonding and highest bonding orbitals, both the hypcrconjugative and inductive action of a methyl substituent influence the longest wave length transition. Thus, methyl substitution on an odd alternant would be expected, depending on the position of the methyl group, to produce both red and blue shifts (8) An odd-alternant hydrocarbon is composed of two classes of carbon atoms, starred (the larger number) and unstarred, such that every other carbon atom belongs to a different class [A. Streitwieser, “Molecular Orbital Theory for Organic Chemists,” John Wiley and Sons, Inc., New York, N. Y., 1961, pp. 45--46. (9) G . W. Wheland, “ltesonance in Organic Chemistry,” John Wiley and Sons, Inc., New York, N. Y., 1955, p . 275. (10) F. A. Matsen, “Technique of Organic Chemistry,” Vol. 9, U’.West, Ed., Interscierice hblishers, Inc., New York, N. Y.,1956, p . 671. (11) Unless steric effects result in changes i n the geometry of the molecule. (12) E. Heilbronrier. “Nori-Benzenoid Aromatic Compounds,” D. Ginsberg, Ed., Iriterscienae Publishers, Inc., New York. N. Y., 1959, p. 224. (13) C. A. Coulson, Proc. Phgs. SOC. (London), A65, Gl (1952). (14) H. C. Loriguet-Iliggins and It. G. Sowden, J . Chem. Soc., 1404 (1952). (15) Experimental evidence for the effectiveness of purely inductive effects i n an even alternant is that Lhe spectrum of ariilinium ion, having a positive charge, absorbs a t almost the same wave length as does benzene.lO (16) D. Peters, .I. Chem. Soc., CA8, 4182 (1957). (17) B. Pullman, M . Mayot, and G. Rerthier, .I. Chem. l’hys., 1 8 , 257 (1950).
Volume 68, Number 6
M a y , 1964
1150
from the absorption maximum of the parent species, as is observed. I t has been shown for even-alternant hydrocarbons that the shifts produced by a methyl substituent are proportional to the reciprocal of the number of carbon atoms in a molecule.12 This behavior is evident here, in that the shifts produced by methyl substitution on allyllithium are larger than those produced by substitution of benzyllithium. In p-tolyllithium the methyl substituent is considerably removed from, and not in conjugation with, the anionic chromophore. It has no effect on the longest wave length transition of phenyllithium, which our evidence indicates may be due to an n + **-type transition of an electron of the carbon-lithium bond. Thc methyl group can, however, interact with the Aelectrons of the phenyl ring. The shorter wave length transitions, which probably involve the Aelectrons, are moved to longer wave length by methyl substitution. The first electronic transition of an odd (or even) alternant carbonium ion is predicted to be of similar energy to that of the corresponding carbanion.211n The behavior to be expected from the molecular orbital arrangement in odd-alternant ions is that the inductive effect of a methyl group on the spectra of the carbonium ion should be opposite that of the carbanion. This will be illustrated in a following section. Resonance Theory Interpretation of Spectral Shifts. In this section an explanation is given for the effect of a methyl substituent on the spectra of odd-alternant anions in terms of resonance theory, which permits by inspection the prediction of the direction of the spectral shift. The methyl group may be either on an “active” or “inactive” position.Y A methyl substituent in an “inactive” position causes a red shift in absorption maximum of all three organolithium compounds, and may be considered the general behavior. When the methyl substituent is on an “active” position, shifts in absorption maximum either to the red or to the blue occur. To generalize tJhis behavior, consider the electron-donating effect of the methyl group on the relative stabilities of the resonance structures of the odd alternarits benzyllithium and allyllithium. The electron distribution probabilities (to a first approximation) for the Kekul6 structures of benzyllithium, allyllithium, and their methyl-substituted speci2s arc shown in Table II.19 In the benzyl anion the probability of t’he primary Kekul6 structure, Le., charge being in position 1, is four times greater than for structures 2, 3, or 4. Intuitively, owing to the electron-supplying behavior of a methyl group, the resonance structures of ~~-methylbenzyllithiumin which The Jozrrnal of Physkal Chemistry
RICHARD WAACKAND MARYA. DORAN
the charge is on the phenyl ring, Le., structures 2, 3, and 4, should be morc nearly equivalent in energy to the primary resonance structure 1 than they arc in benzyllithium; this is illustrated by their relative probabilities in Table 11. As the consequence of the a-
Table I1 : Probabilities of Kekulb Resonance Structures of Benzyl and Allyl Carbanions
J2/.
Calculated I,C.40-M0 charge densitiesn’6 ,-.--- ----_position 1 2 3 4
-
3
Benzyl a-Methylbenzyl p-Methylbenzyl o-Methylbenzyl
0.57 0.41 0.60 0.59
0.14 0.16 0.14 0.00
0 60 0 57
0 50 0 34
0.14 0.18 0.01 0.14
0.14 0.16 0.14 0.16
Zqf
0.99 0.91 0.89 0.89
A
1
2
.411yl y-Methylallyl
1.00 0.91
a Standard LCAO-MO methods arid t h e conjugation-induction model of Weitwieser and SairiV were used. Secular deterrninarits were diagonalized on a Rurroughs 220 m i n g a program written by Dr. James It. Scherer. T h e charge densities, which a r e analogous to probability distribution, are for the anions. Although no allowance was made in theae calculations for t h e effect of the negative charge o n the effective electronegativity o f t h e carbon atom skeleton, this should be similar in t h e niethylsubstituted and iionsubstituted compounds and not alter t h e comparison between the two. Charge distributed a t unmarked positions results in t h e sum of the charges at positions 1 + 4 n o t being equal to unity.
methyl substituent, therefore, the four Kekul6 resonance forms of a-methylbenzyllithium are more equivalent in energy than they are in benzyllithium. On the other hand, a methyl substituent in either the ortho or para position of benzyllithium would tend to push the charge out of the ring and thus make the primary Kekul6 structure of lower energy (Le., more stable) than those forms having the charge on the ring. Although resonance lowers the ground state energy of a compound, it usually, because the contribution of resonance structures is greater in the excited state than in the ground state, has an even greater stabilizing effect on the excited ~ t . a t e . ~Thus, - ~ ~ factors tending t o (18) D. I’. Craig, Chemical Society Annual Report, 1958. p . 173. (19) A. Streitwieser and 1’. M .Nair, Tetrahedron. 5 , 149 (1959). (20) (a) E. A. Brande, “Chemistry of Carbon,” Vol. I, E. H. Rodd, Ed., Elsevier Publishing Co., 1951, p. 83: (h) ref. 9, p . 676; (c) M. J. S. Dewar, Special Publicstion No. 4, The Chemiral Society, London, 1958. p. 64; (d) W. Kauarnanri, “Quantum Chemistry.” Academic Press, Inc., New York, N. Y., 1957, p. 678.
ELECTRONIC SPECTRA OF ORGANOLITHIUM COMPOUNDS
favor resonance hy making the resonance structures more equivalent in energy21 should lower the excited state energy relative to the ground state energy and produce a red shift in absorption maximum. Conversely, factors tending to inhibit resonance by making one of t,he resonance forms of lower energy than the others should cause a blue shift. in absorption maximum. Identical reasoning applies to allyllithium and methylallyllithium. The two resonance forms of allyllithium arc equal in energy. I n butenyllithium the resonance forms are not equivalent in energy. The crotyllithium resonance form is expected to be considerably more stable than the a-methylallyllithium resonance form, since the secondary carbanion of this latter structure is more energetic than a primary carbanion. (See the charge density probabilities in Table 11.) Konequivalence of these resonance forms is substantiated by proton magnetic resonance studies of butenylmagnesiuni bromide by Roberts, et a1.,22 which establish that its predominant, if not exclusive structure is crotylmagnesium bromide. Thus, methyl substitution makes resonance less favorable in aand 7-methylallyllithium, and its absorption is blue shifted relative t,o allyllithium. This discussion leads to the general rule that when the position of a methyl substituent on a starred C atom of an odd-alternant organolithium compound is such that the principal resonance structures of the molecule are made more equivalent in energy, its absorption is shifted to longer wave length, but if methyl substitution is such as to inhibit resonance in that the principal resonance structures are made less equivalent in energy, the absorption is shifted to the blue from that of the parent compound. Molecular Orbital Interpretation of Spectral Changes and Calculation of Spectral Shifts. By perturbat,ion methods both Coulsonl* and Longuet-Higgins and Sowden14have shown that the change in the long wave length transition energy of even alternants or nonalternants due to the inductive effect of a substituent is given by23 A ( h V ) i n d u o t = [Czn+l,r
-
C2n,r]8ar
(1)
where refers to the coefficient of atom r (which is substituted) in wave function n, where n is the highest occupied orbital. Because 601, is positive for a methyl substituent, Le., the methyl-substituted carbon has a decreased effective electronegativity, red shifts owing to methyl inductive effects are expected when c2,,,. > cPn+1,? and blue shifts when c ~ ~ >+ c~~ , ,, ~ .~ This formula, and a similar one to account for the red shifts owing to hyperconjugation, correctly predicts with
1151
qualitative agreement the direction of spectral shifts in methyl-substituted azulenes.12-14 This is not so with the odd-alternant anions. The shifts predicted according to eq. 1 from the coefficients of the highest bonding and lowest antibonding *-orbitals [LCAO-MO] of henzyllithiurn or allyllithium are not in the correct direction. (See Scheme I.) Successive iterations to allow for the negative charge, using the w-technique, l 9 does not, in either molecule,24alter the direction of the predicted spectral shift. Scheme I : Direction of Spectral Shifts Predicted from Equation 1
&;;(;
YYR)
Red (B)
Red (B)
Benzyllithium a
Allyllithium
Shift found experimentally.
The fact that the orbital coefficients do not predict the correct spectral shift is interpreted as indicating that the orbitals obtained from LCAO-MO are not, in these systems, an adequate approximation to the true molecular orbitals, even with iterationlg to allow for the charge. Electron interaction, which is more important in ions,25s26presumably makes the simple descript,ion inadequate. The formulas derived via perturbation methods of I,CAO-MO theory are not, therefore, expected to be generally applicable to the odd-alternant ions. T’ullman, Mayot, and Berthier17 calculated qualitative relative transition energies for the methyl-substituted azulenes, using LCAO-RlO methods and the parameters of aiulliken, et, to describe the inductive and conjugative effect of methyl. Continuing (21) Ref. 9, pp. 20-24. (22) J. E. Nordlander. W. G. Young, and J . D. Roberts, .I. Am. Chem. Soc.. 83, 494 (1961). (23) Although odd-alternant molecules were not considered in these theoretical studies, there seems to be no reason that the same conclusions should not be valid,for them species. We are indebted to Prof. C:. A. Coulson for confirmation by personal communication. (24) Unfortunately. as found for the benzyl cation,18 the iterations of benzyl anion do not converge [ w = 1.41. The allyl anion does converge. (25) 11. C. Longuet-Higgins and J. A. Pople, Proc. P h y s . Soc. (London), A68, 591 (1955). (26) A. Brinstock and J. A. Pople, Trans. Faraday Soc., 50, 901 (1954). (27) R. S. Mulliken, C . A. Itieke, and W. G. Brown, J . A m . Chem. Soc., 63, 41 (1941).
Volume 68,Numbw 6 May. 1964
1152
RICHARD WAACKANI) MARYA. DOHAN
this approach, using 1he inductive and conjugative parameters of Streitwieser and Xairlg and iterating by the w-techniyuc to allow for the negative charge, the relative transition energies (in units of p) for benzyl and allyl anions in Table 111 arc obtaiiied. Two proccdurcs were used to calculatc relative transition energies: (1) combiiiiiig the methyl group corijugatioii and iiiductiori parameters with a zeroth order Huckel calculation and (2) introducing the conjugation and induction parameters into thc sixth iteration of the respective anion skeleton. The values given are averages of the first two iterations. Although for some molecules the individual iterations are somewhat extreme, the average of the first two iterations is cxpected to be a good approximation to the converged values. Streitwieser has shown that this is true for odd-alternant cations. 1 q , 2 x I t is also true for allyllithiurn.**
Table I11 : Calculated Relative Transition Energies Yobsd
X 10-2 Cnl.-'
Allyllithiurii Unsubstituted y-Methyl @-Methyl Renzyllithium Unsubstituted a-Methyl p- Me t hy 1 o-Methyl m-Met,hyl
--Procedure I---At;@) ucnled"*
318 344 303
1.207 1.229 1.114
303 301 318
0.750 0.760
0.7!)7 0.74:Y
0 . 703
a 327 294
b 304 318
---Procedure 2--
As(@)
Voalod
1.222 1.240 1.122
a 322 292
0.786 0.783 0.837 0.790 0.782
b 302 322
Transition energies are relative to aIiyIIithiurn. * Transit.ion energies are relative to benzyllithiurn. ' This is t h e only value not qualitatively in agreement with observation.
The allyl anion transition energies are in qualitative agreement (i.e., correct order) for both calculation procedures. Since iteration of allyl anion converges, procedure 2, in which self-consistency of the negative charge was approached and then the effect of the methyl group introduced, might be preferred. The benzyl anion calculations are less satisfactory, possibly because thc benzyl anion does not convergc using w = 1.4. h different value for w should result in convergance,2xand perhaps better agreement. Improvcment might also be obtained if allowance were made for changing p29 as well as a. These result8 indicate that T,CAO-MO calculations are potentially useful for providing qualitative predictions of the spectral behavior The Journal of P h y s h l Chemistry
of odd-alternant ions, although further work is necessary. Assuming a value for p and calculating the relative magnitudes of the transitions shows that quantitativci agreement of these simple calculations is poor, which is to be expected. These calculations of relative transition entqies provide better agreement with experiment than the perturbation formulas based on IEAO-lIO, indicating iteration by thc w-tcchnique partially compensates for electron correlation.2x Cornpanson of the &#'ect of a M e t h y l Substatuent on the Spectra of Cations and Anions. To a first approximation thc bonding and antibonding molecular orbitals of odd alternants are mirror images of each other centered around a central nonbonding orbital. The lowest transition in the anion can he considered as a one-electron excitation from the filled nonbonding orbital to the lowest empty antibonding orbital, whereas the lowest energy transition in the cation will be from thc filled highest bonding orbital to the empty nonbonding orbital. The coefficients of the lowest antibonding and highest bonding orbitals are equal. For the the anion the former is given a positive sign [in eq. 1] and the nonbonding coefficient has a negative sign. l'he situation is reversed for the cation. The inductive action of a methyl substituent, therefore, should have an opposite effect on the spectrum of the cation than it does on the anion. The limited data available on the electronic spectra of odd-alternant cations generally support this prediction. For example, the absorption of a-methylbenxyl cation, A,,,,, 435 mp,m is blue shifted relative to benzyl cation, A,,,,, 470 mp.30 The absorption of a,adimethylbenzyl cation is further blue shifted, A,,,,, 390 mp. The absorption of butenyl cation, A,, 290 mp,31 is red shifted relative to the allyl cation, A,,, 273 mp.31 The absorption of P-methylallyl carbonium ion, A,, 256 m/1,31 is blue shifted relative to allyl carbonium ion. These shifts are all opposite those of the corresponding carbanions. The shift of the latter two cations is also opposite that predicted from eq. 1. The absorption of o,o,p-trimethylbenzyl cation (I), A,,, 470 mp, is strongly blue shifted by dimethyl sub360 mp,32 again 8 stitution in the a position, A,, behavior opposite that expected for the carbanion.
(28) A. Streitwieser. J . Am. Chem. Soc.. 8 2 , 4123 (1960). (29) 1'. C. Den Boer-Veenendaal, J. A. Vliegenthart, and D. H. W . Den Boer, Tetrahedron. 18, 1325 (1962). (30) J . A. Grace and M.C. R. Symons, .I. Chem. Soe., 958 (1959). (31) J. Kosenhaum and M.C . R. Gymons, ihid., 1 (1961). (32) N. C. Deno. J. J . Jaruzelski. and A. Schriesheim, J. O r g . Chem.. 19, 155 (1954).
TEMPERATURE DEPENDENCE OF PHOTOISOMERIZATION
R'Iethyl siibstitution of both mela positions of I causcs a red shift, A,,, 380 mp,32 which is the only carbonium ion shift in the same direction as might be expected for the carbanion. Interestingly, the spectral shifts found
1153
for benzyl carbonium ions on methyl substitution are larger than the corresponding shifts in the spectrum of benzyl carbanions, whereas the shifts in the allyl systems arc of tho same magnitude.
Temperature Dependence of Photoisomerization.
111.'
Direct and Sensitized Photoisomerization of Stilbenes
by Shmuel Malkin and Ernst Fischer Photochemical Laboratory, The Weizmunn Institute of Science, Rehoaoth, Israel
(Received December 9 , 1963)
A quantitative and comparative study was made of the direct and sensitized photoisomerization and of the fluorescence of stilbene and several derivatives, in a wide temperature range. Quantum yields for all three processes were evaluated and their interdependence was studied. The yields of the direct trans cis photoconversion and of the fluorescence were found to change sharply on cooling, probably owing to a potential barrier of close to 2 kcal./mole in the photoconversion. No such temperature dependence was observed in the photoisomerization sensitized by benzophenone, down to - 140'. This indicates that the observed barrier is situated somewhere between the first excited singlet level and the active intermediate responsible for the actual isomerization. A comparison between stilbene and its p-bromo derivative makes it plausible that this barrier is due to the necessity, In stilbene, to pass a second, somewhat higher excited singlet level, unattainable directly, and by-passed in bromostilbene. The nature of this level is discussed. -+
Introduction froin tlhis laboratory, it was I n farlier showii that the quantum yield of the trans cis photoisonierization of aromatic azo compounds drops sharply with decreasing temperatures. These results were taken to indicate the existence of potential barriers somewhere along the path from the excited singlet trans niolecule to the ground singlet cis molecule. Similar preliminary results were obtained with stilbene' and confirmed by Dyck and J I c C l ~ r ethus , ~ indicating the existence of potential barriers also in this case. The photoisomerization of stilbene has been invcstigated by several authors. all of whom conclude that the first excited singlet state, reached by light absorption, is not directly responsible for the isomerization, which -+
probably occurs in a lower excited state reached subsequently. Evidence that this intermediate state is a triplet was advanced by Schulte-Fr~hlinde,~ Stegemeyer,5 and Dyck and M c C l ~ r c ,the ~ latter authors paying particular attention to the role of excited states as iritcrniediates in the course of energy degradation. Corninentirig on the potential barriers observed by the present authors, Stegenieyer5 suggests that these (1) Part 11: S. Malkin and E. Fischer, Symposium on Reversible Photorhernical Processes, Durham, N. C., April. 1962; J . Phys.
Chem., 66, 2482 (1962). (2) E. Fischer, J . A m . Chern. Sac., 82, 3249 (1960). (3) H . H. Dyck and D . S. McClure, J . Chem. Phys.. 36, 2326 (1962). (4) D. Schulte-Frohlinde, H. Blurne, and H. Gusten, J . Phys. Chem., 66,2486 (1962). ( 5 ) H . Stegemeyer, ibid., 66,2555 (1962)
Volume 68, Number 6
May, 1964
