J. Phys. Chem. 1983, 87,1708-1712
1708
Experimental and Theoretical Studies on the Charge-Transfer Transition in SO,-P-Quinol and Related Compoundst John S. Tse‘ and John A. Ripmeester Dlvlsion of Chemistry, National Research Council of Canada, Ottawa, K1A OR9 Canada (Received: May 18, 1982; In Final Form: December 2, 1982)
Visible and UV spectra of solutions of SOz in substituted phenols and of the S02-(3-quinolclathrate were recorded. The yellow color of the solutions is best explained by formation of electron donor-acceptor complexes. The calculated energy of the complex does not vary appreciably with the orientation of SOz with respect to the phenol plane. We suggest that similar interactions in SOz-P-quinol clathrate are responsible for its distinctive yellow color which is unique to this clathrate because SO:, is the only known enclathratablespecies with low-lying vacant electronic orbitals.
Introduction Hydroquinone clathrates are molecular inclusion compounds of the general formula 3CGH4(OH),.xM,’where M is the guest species which are encaged in roughly spherical cavities formed by two interlocking three-dimensional hydrogen-bonded networks of hydmquinone.lJ Clathrates with a variety of encaged small molecules have been prepared and studied e x t e n ~ i v e l y . ~Of~ all the known pquinol clathrates only the sulfur dioxide-hydroquinone (S02-p-quinol) complex has a distinct yellow color.’ This unique property of the compound has aroused recent interest in the study of its physicochemical properties.’V8 Related SOz clathrates of phenol and substituted phenols are also colored, and recently it was shown that a pale yellow form of a-quinol containing small quantities of encaged SO2 also exists.8 It has been shown for a long time from UV spectroscopic studies that, when sulfur dioxide is dissolved in benzene and certain of its d e r i v a t i v e ~ , the ~ J ~absorption band of free SO2 at 290 nm (ca. 0.39 pm-l) is shifted to longer wavelength and its intensity enhanced. These observations have been taken as evidence of the formation of an electron donor-acceptor (EDA) complex between SOz and the benzene. A recent report7 further revealed that the electron absorption spectrum of the SOz-P-quinol in the clathrate and that of the solution state are almost indistinguishable. This observation strongly suggests that a similar charge-transfer (CT) interaction also is present in the SO,-P-quinol clathrate. It is not unreasonable to expect that the hydroxyl group in hydroquinone may push up the energy of the highest occupied molecular orbital (HOMO) of benzene and enhance the interaction with the lowest empty unoccupied molecular orbital (LUMO) of SOz. Thus, the interaction will lower the energy of the charge-transfer transition, causing the yellow color of the complex. In order to establish our anticipation, in this paper we report the electronic absorption spectra of the SO2-quinol clathrate and a series of phenols substituted with electron-donating and -withdrawing groups in carbon tetrachloride solutions saturated with SOz. Molecular orbital calculations were performed on model systems to assist spectral assignments and to identify the nature of the electronic transitions. Our results support the existence of a weak EDA complex between SO2 and phenol in solution. We also show that CT transitions are possible regardless of the orientation of SOz with respect to the ‘NRCC No. 21142.
phenol ring provided that this separation is within a reasonable range (-3 A). This observation complies with crystallographic’ and relaxations information on the mobility of the encaged SOz and strongly suggests that similar CT transitions will be observed in SO,-P-quinol and other phenol clathrates.
Experimental Section Electronic Absorption Spectra. Reagent-grade benzene, phenol, p-methylphenol, p-chlorophenol, and hydroquinone were used without further purification. The chemicals were dissolved in carbon tetrachloride or diethyl ether. Sulfur dioxide (Matheson) was then slowly passed through the solutions for a period of 15 min. The absorption spectra of the solutions were measured in the visible and ultraviolet region with a HP spectrometer with the appropriate solvent as reference. All the solutions except those of benzene showed a distinct yellow color. The electronic spectra of solid quinol and S02-P-quinol clathrate were measured with a photocoustic spectrometer. The optical portion of the spectrometer consists of a 1-kW xenon arc and a 0.25-m Ebert monochrometer. The amplified signal from the microphone in the sample cell is ratioed against that from a pyroelectric detector in order to compensate for the power variation in the light source. Spectra were obtained at a chopping frequency of 96 Hz and with a spectral resolution of about 3 nm. Theoretical Calculations. Owing to the large size of the molecules, it is not practical for us to carry out ab initio calculations. The semiempirical IND0/211 method using Zerner’s spectroscopic parametrizati~n’~J~ was used pri(1)D. E. Palin and H. M. Powell, J . Chem. SOC.,208 (1947). (2) T. C. W. Mak, J. S. Tse, C. Tse, K. S. Lee, and Y. Chong, J . Chem. Soc., Perkin Trans. 2, 1169 (1976). (3) M. C. Child, Jr., Q. Rev., Chem. SOC.,18, 321 (1964).
(4) L. Mandelcorn, Ed., “on-Stoichiometric Compounds”, Academic Press, New York, 1964. (5) C. A. Fyfe, “Molecular Complexes”, R. Foster, Ed., Elek Science, London, 1973. (6) D. C. McKean, ‘Vibrational Spectroscopy of Trapped Species: Infrared and Raman Spectroscopy of Matrix-Isolated Molecules, Radical and Ions”, H. E. Hallman, Ed., Wiley, New York, 1973. (7) P. S. Santos and P. C. Isolani, Chem. Phys. Lett., 67,487 (1979). (8) E. Davis, D. Leaist, S. R. Gough, J. A. Ripmeester, and D. W. Davidson, to be submitted for publication. (9) L. J. Andrews and R. M. Keefer, J . Am. Chem. Soc., 73, 4169 (1951). (10) D. Booth, F. S. Dainton, and K. J. Ivin, Trans. Furuday Soc., 55, 1293 (1959). (11)J. A. Pople and D. L. Beveridge, “Approximate Molecular Orbital Theory”, McGraw-Hill, New York, 1970. (12) J. Ridley and M. C. Zerner, Theor. Chim. Acta, 32, 111 (1973).
0022-3654/83/2087-1708$01.50/00 1983 American Chemical Society
Charge-Transfer Transition in SO,-@-Quinol
marily because of the reliability of this method in reproducing reasonable electronic spectra of large aromatic compounds. The computer program for this calculation employs a modified Mataga-Nishimoto formula to estimate two-electron Coulomb integrals ym14
where RABis the distance between the two centers and fr is set to 1.2 as suggested by Weiss. To improve the calculation of the electronic spectrum, the two-center oneelectron integral is approximated as
The Journal of Physical Chemistty, Vol. 87, No. 10, 1983 1709
TABLE I: Theoretical Ionization Potential and Electronic Transition for SO, and Phenol
(1)so* ionization potential. eV Koopman’s
2ph-TDA
orbital
I
I1
I
11
exptll’
4a, la, 3b,
11.89 10.99 12.14
15.43 16.79 14.55
9.15 10.58 8.91
14.53 15.98 13.27
12.31 13.01 15.99
electronic spectra, n m oscillator strength
energy
where PA is a characteristic bonding parameter of atom A. S is a modified orbital overlap related to the ordinary orbital overlap S by
orbital 4a1 -+ 2b1* 1b,-+2bI* la,-+2b1* 3a1 2b1*
I
I1
I
I1
170 297
344 200 198 172
0.084 0.402
0.0423 0.0151 0.0178 0.0411
-+
where g, and g, are the geometric factors necessary for converting from the local diatomic coordinate system to the molecular system, and f, and f,, which are set to 0.585 and 1.266, respectively, are empirical factors adjusted to give the best agreement with experiment. In our present study, the electron spectra were calculated with a limited configuration interaction (CI) procedure with all possible singly excited configurations constructed from molecular orbitals having energies between -0.6 and 0.5 au (generally 150-200 singly excited configurations). Inclusion of higher energy excitations seems inappropriate as we have not included double excitations and polarization wave functions. The method of selecting configurations in electronic spectrum calculations has been reviewed extensively recently.16 The oscillator strength of an electronic transition was calculated by the dipole length operator method.16 For the calculation on SO2,it is controversial whether the S 3d wave function should be included in the basic functions. Previous ab initio calculations17J8 on SO2 showed that meaningful results could not be obtained unless the S 3d orbitals were included. In order to assess the necessity of S 3d orbitals in our semiempiricalmethod, we have computed valence ionization energies and electronic spectra for SO2 with (basis set 11)and without (basis I) S 3d orbitals and compared them with experiments. The results are summarized in Table I. The ionization potentials were calculated with the diagonal 2ph-TDA (two-particle-hole Tamm-Dancoff app r o x i m a t i ~ n ) to ’ ~ account ~ ~ ~ for electron correlation and relaxation accompanying ionization. It is interesting to note that the Koopman’s theorem21orbital orderings for SO2are quite different for the two basis sets. However, the same sequence is recovered after correction for the Koopman’s defect. The 2ph-TDA approximation used gives ionization energies within 1-2 eV from the experiment.22 It is obvious from Table I that the ionization (13)J. Ridley and M. C. Zerner, Theor. Chim.Acta, 42, 223 (1976). (14)N. Mataga and K. Nishimoto, 2. Phys. Chem., 13, 140 (1957). (15)B.Dick and G. Hohlneicher, Theor. Chim.Acto, 53,221(1979). (16)A. J. McHugh and M. Gouterman, Theor. Chim.Acto, 13, 249 (1969). (17)J. H. Hillier and V. R. Saunders, Mol. Phys., 22, 193 (1971). (18)L. S. Cederbaum, W. Domcke, W. von Niessen, and W. K. Kraemer, MOL Phys., 34, 381 (1977). (19)L. S. Cederbaum and W. Domcke, Adu. Chem. Phys., 36, 205 (1977). (20)D. Saddei, H.-J. Freund, and G. Hohlneicher, Surf. Sci., 96,527 (1980). (21)T.A. Koopman, Physica (Amsterdam), 1, 104 (1933).
exptlZ3qQ 333 257
( 2 ) Phenol electronic spectra. nm exptl
calcd
oscillator strength
252 228 196
270 231 198
0.019 0.084 1.619
Experimental oscillator strength ca. 1OM*.
energies calculated with S 3d wave functions are in better agreement with experiment than those calculated without the S 3d energy for the 3bz orbital by ca. 2 eV. The agreement with experiment is very encouraging. A similar effect can also be observed in the calculation of the electronic spectra. Calculations which neglect the S 3d orbitals fail to reproduce the spectrum completely, whereas those which include the S 3d functions improve both the transition energies and oscillator strengths drastically. Our assignment of the SO2 electronic spectra agrees with experiment. We conclude that the S 3d orbital is important in the present INDOI2 calculations. For the purpose of saving computation expenses, we chose to use phenol to mimic hydroquinone in the supermolecule calculations. This choice is primarily based on the similarity in the chemical and physical properties of these two compounds. The electronic spectrum of phenol in c y ~ l o h e x a n eshowed ~~ three absorption bands centered at 3.55,4.36, and 5.07 pm-l. The calculated values of 3.68, 4.30, and 5.02 pm-* agree well with the observed values.
Results and Discussion The UV-visible spectra of benzene, phenol, substituted phenols, and hydroquinone in saturated SO2solution are shown in Figure 1. The general features of all the spectra are very similar. Each spectrum consists of a broad absorption band centered at ca. 300 nm and a weak but sharper shoulder at 400-430 nm. The broad band at 300 nm has been observed in previous solution spectra of SO, in benzene and o l e f i n ~ . ~This J ~ absorption can be identified with the lA1 ‘B1transition in free SOz. To the best of our knowledge, the peak at ca. 400 nm has not been reported before. We assign this peak to the phenol-S02 CT transition (see below). It is evident from the spectra that increasing the donor properties of the substituents
-
(22)H. Bock, B. Solouki, P. Rosmus, R. Steudel, and W. Schultheis, Angew. Chem., 85,987 (1973). (23)L. Lang, Ed., “Absorption Spectra”, Vol. 11, No. 62647,Academic Press, New York, 1961.
1710
Tse and Ripmeester
The Journal of Physical Chemistry, Vol. 87, No. 10, 1983
&
(o.u.) -0.3220
-0.3436
a
-0.4504
-0.5185
b'
Flgure 3. Schematic diagram for (a) HOMO of phenol, (b) LUMO of SO,, and (c) SO,-phenol interaction sites.
It is well-known that the zero differential overlap (ZDO)" approximation often predicts aritfact structures
~
250
I 300 350 400 WAVELENGTH (nm)
450
Flgure 1. UV electronic spectra of SO2 in (a) CCI,, (b) benzene, (c) quinol/CCI,, (d) phenol/CCI,, (e) methylphenol/CCl,, and (f) chlorophenollCCI,.
a
W V
z d
m (L
0
v)
9 W
s
5a W
r
1
WAVELENGTH (nm)
Flgure 2. Photoacoustic spectra of solid (a) hydroquinone and (b) SO,-P-qulnol clathrate.
on the phenol shifts the band at ca. 400 nm to longer wavelength (lower transition energy) and increases the intensity. The solid-state photoacoustic spectra of quinol and the SO2 clathrate are compared in Figure 2. The saturation of absorption signal due to the intense K K* transitions prevented us from making detailed assignment. However, the spectra clearly showed the cutoff of the absorption band shifted from 320 nm in hydroquinone to 475 nm in S02-P-quinol clathrate. This observation suggests that new absorption occurred at the 400-nm region in the clathrate.
-
in supermolecule calculation^.^^^^^ Therefore, we do not attempt to identify the most stable conformation of the SO2-phenol EDA complex by the INDO method. Instead, we took a more intuitive frontier orbitals26approach to deduce possible conformations for the EDA complex. It can be shown by perturbation theory that the stability of an EDA complex is proportional to the overlap between the occupied orbitals of the donor and the vacant orbitals of the acceptor.25 The decisive role is governed by the interaction between the frontier orbitals of the donor (HOMO) and the acceptor (LUMO). In our case, the donor is the HOMO of phenol and consists mainly of the K electrons of the carbon atoms. The acceptor is the LUMO of SO2 which is largely S 3p, in character (see Figure 3). Application to the SO2-phenol system leads to the four possible structures depicted in Figure 3. In structure 1, the lone pair from the hydroxyl oxygen atom donates an electron into the empty LUMO of SOz forming a dative bond. To achieve maximal overlap, the SO2 molecule should be perpendicular to the phenol. Similar structure have been suggested for the structure of the amine-SO2 EDA c~mplexes.~'For structures 2-4, the primary interaction involves the K electrons on the phenol. I t is difficult to deduce the most stable conformation by inspecting the schematic orbital interaction diagrams. Although one should not accept the total binding energy of molecules calculated by semiempirical methods too literally, our calculations for a SO2-phenol separation of 3.0 A showed that structure 1 does not lead to any extra stability, whereas structures 2-4 are stabilized with respect to isolated SO2and phenol molecules. The predicted order of stability is 4 > 3 > 2 > 1 with an energy difference between 2 and 1 of 24 kcal/mol, between 3 and 2 of 2.3 kcal/mol, and between 4 and 3 of 1.3 kcal/mol. This ordering seems to be quite reasonable as one may expect the interaction between the SO2LUMO with the x electrons to be much stronger than with the lone pair of ox~~
~
~~
~~~
~~~~
~~
(24) W. A. Sokalsi, P. C. Hariharan, H. E. Pokie, and J. J. Kaufman, Int. J. Quantum Chem., 18, 189 (1980). (25) P. Hobaz and R. Zahradnik, "Weak Intermolecular Interaction in
Chemistry and Biology", Elsevier, Amsterdam, (1980). (26) B. M. Gimarc, "Molecular Structure and Bonding", Academic Press, New York, 1979. (27) R. R. Lucchese, K. Haber, and H. F. Schaefer, 111, J . Am. C h m . SOC.,98, 7617 (1976).
The Journal of Physical Chemistty, Vol. 87,No. 10, 1983 1711
Charge-Transfer Transition in SO,-@-Quinol
TABLE 111: Theoretical Electronic Spectra of SO,-Phenol Complexes
TABLE 11: Molecular Orbitals Relevant to Bonding and Electronic Transition of SO,-Phenol EDA Complexesa orbital energy, au 21 22 23 24 25 26 _27. 28 29 30 31 32 33
_
_
21 22 23 24 25 26 ~
~
-27 28 29 30 31 32 33 21 22 23 24 25 26 27
______
28 29 30 31 32 33 21 22 23 24 25 26 _ _ _ 27 --_ 28 29 30 31 32 33
-0.5300 -0.5196 -0.4911 -0.4665 -0.4560 -0.3497 _ -0.3292 __ -0.1643 -0.0024 0.0129 0.0165 0.0708 0.1004
description
(1)Position 1 SO, (22%) t phenol (78%) phenolC n phenolC A phenolCn phenol: OH (29%) + C (71%) p h e n o l C IT __ pheno! _ _C-? _ -- -_- _- - - - - -- - -- --S 3p, (88%) SO, antibonding p h e n o l C n* phenol C n* SO, antibonding phenoln*
-0.5208 -0.5087 -0.5083 -0.4718 -0.4644 -0.3655
:P:3.?6.1. -0.1457 -0.0021 -0.0000 0.0235 0.0840 0.0917
--
no.
--
(2) Position 2 SO, (41%) + phenol (59%) SO, bonding phenol C u phenolCu SO, (19%) t phenol C n (81%) phenolCn - SP,-(8%!?$?enqlC S 3p, (64%) phenol C n* phenol C n* SO, antibonding phenol C n* phenol C U *
..(??%I - - -
oscillatory energy, strength, nm nm
1
355
0.0305
2 3 4 5 6 7 8 9
269 239 216 192 191 190 190 189
0.0146 0.0024 0.1476 0.4119 0.5099 0.2706 0.0128 0.6366
1 2
47 5 359
0.2610 0.0535
3 4 5 6 7 8 9
215 208 194 194 193 188 187 183 171
0.1382 0.0140 0.4814 0.1478 0.0255 0.7398 0.2434 0.0314 0.0587
description
(1) Position 1 SO, ( u -+ u ) *
-+
10 11
phenol ( n
-+
n*)
-+
phenol ( n -+ n * ) mixed ( p h e n o l ) mixed ( p h e n o l ) mixed (phenol)
SO, ( u -+ u * ) phenol ( n -+ n * ) (2) Position 2 CT CT
phenol ( n CT mixed phenol ( n
-+
ygen. Furthermore, the energy differences among structures 2-4 are very small. This suggests that, even though at room temperature the SO2 molecules may possess a certain amount of kinetic energy, it is conceivable that there will be instantaneous interaction with the a electrons of the hydroquinone molecules forming the wall of the cages. To faciliate the discussion of the bonding and the electronic properties of the SO2-phenol EDA complexes, we have tabulated the energy and character of the relevant
16 -+ 28 26 -+ 30 27 28 21 28 25 -+ 28 27 -+ 30 28 27 -+
-+
n*)
-+
-+
n*)
n+u*
27 30 28 23 26 -+ 29 mixed 18'28 26 -+ 3 1 -+ -+
phenol ( n n * ) So, ( u -+ u * ) mixed phenol ( n -+ u * ) -+
( 3 ) Position 3 (3) Position 3 0.2958 'CT 1 483 SO, (65%) t phenol (35%) -0.5278 0.0201 CT 2 309 SO, (73%) + phenol (27%) -0.5062 0.0033 phenol ( n -+ n * ) 3 266 SO, (26%) t phenol (74%) -0.5057 phenol C u -0.4713 4 0.0234 CT 243 SO, (10%) + phenol C n (90%) -0.4654 0.0867 phenol ( n -+ n * ) 216 5 phenol C n -0.3580 0.1413 SO, phenol -+ n* 198 6 + phenol -0.3439 . _ _____-_SO, . .- -(7%) --_ _ _ _C_n -(93%) - - - - . - - _ _7_ _196 _ _ _ 0.6349 _ _ _ conjugate of 3 S 3p, (64%) -0.1415 0.0081 n u * (SO,) 194 8 phenol C II* 0.0000 0.8181 phenol ( n -+ n * ) 191 9 phenol C n* 0.0031 10 0.0218 SO, ( u -+ u * ) 182 SO (7 8%) antibonding 0.0244 11 0.0229 phenol -+ SO, 181 phenol C n* 0.0863 0.1250 phenol SO, 12 178 phenol (-n*) 0.0922 (4) Position 4 4 ) Position 4 0.3270 CT 1 407 SO, (41%) + phenol (59%) -0.5270 0.0127 CT 2 307 SO, (91%) + phenol (8%) -0.5069 SO, (9%) + phenol (91%) -0.5046 3 0.0472 phenol ( n -+ n * ) 27 0 phenol C o -0.4679 4 239 0.0350 phenol ( n )-+ SO, ( u * ) SO, (10%)+ phenol (90%) -0.4650 219 0.2016 phenol ( n -+ n * ) 5 SO, (8%) + phenol (92%) -0.3660 0.0652 C T 6 197 -0.3362 CT - - . _ n_ _ _ _ _ _ _ _ _ _ _ _ _ _7_ _194 _ _ _ 0.0072 .____ _ - - _ - _ _ - _phenol 0.1003 mixed 8 193 S 3p, (65%) -0.1415 0.6456 mixed 9 189 phenol C T I * -0.0014 10 0.6400 mixed 188 phenol C IT* 0.0057 phenol ( T I -+ T * ) 1 1 0.0046 187 SO, antibonding 0.0227 so, ( u -+ u * ) 0.0404 12 180 SO, (12%) t phenol n* (88%) 0.0859 mixed 13 0.605 175 SO, (80%) t phenol U * (20%) 0.0933
a MO listed below dashed line are unoccupied in t h e ground state.
19 -+ 28 22 -+ 28 23 -+ 28 25 28 27 + 30 25 28 27 -+ 3 1
'
+
-+
-+
27 -+ 28 21 -+ 29 26 -+ 30 27 -+ 29 25 -+ 28 27 -+ 30 27 -+ 3 1 23 -+ 28 26 -+ 29 mixed mixed mixed 26 28 19 -+ 28 21 28 27 -+ 29 25 -+ 28 27 -+ 30 27 -+ 3 1 23 -+ 28 -+
-+
27 -+ 34 26 -+ 3 1
molecular orbitals in Table 11. It is apparent from the table that the stability of the EDA complex is proportional to the amount of mixing of the SO2 and phenol molecular orbitals. The SO2-phenol complex with structure 1 can be regarded as two separate entities with very little or no interaction. Strong orbital mixing is observed for structures 2-4. As a result the EDA complexes appear to be more stable. The theoretical electronic spectra are summarized in Table 111. The assignment of the spectra is based on the dominant configuration(s) which appear in the coefficient(s) of the CI vector for each electronic state. Transitions at lower energies are usually dominated by a single
1712
The Journal of Physical Chemistty, Vol. 87, No. 10, 1983
configuration whereas those observed at higher energies are strongly mixed. The theoretical electronic spectrum of structure 1 is simply the overlapping spectra of noninteracting SOz and phenol. Both the transition energies and the corresponding oscillator strengths strongly resemble those of the isolated moieties (cf. Table I). This observation is consistent with the ground-state electronic structure of this complex which suggests that there is no binding between the SOz and phenol. The lowest energy electronic transition calculated a t 355 nm (2.80 Wm-') can be assigned to the 4a, 2bl* transition in free SO2. The small oscillator strength for this transition rules out the possibility of a CT excitation. The theoretical electronic spectra for structures 2-4 are very similar and quite different from those for structure 1. The most distinctive feature is the prediction of a low-energy transition a t 400-480 nm with oscillator strength about ca. 10 times stronger than the lowest transition in SOz. Analysis of the CI vector for this transition indicates it to be an excitation from an occupied phenol 7~ orbital to a vacant SOz orbital. This kind of electronic excitation can be classified as intermolecular CT transition. The transition energy is in good agreement with the low-energy absorption (ea. 400 nm) observed in the SOz-phenol solution. The absorption in the blue region of the visible spectrum is responsible for the yellow color of the solutions. Higher excitations of these SO2-phenol EDA complexes occur in the 180-220-nm region. The character of the transitions is complex. Since their assignments are not of primary interest in the present study, we have not pursued them here. However, these transitions can be attributed collectively to a mixture of intermolecular and intramolecular transitions. One very important consequence may be inferred from the theoretical analysis. We have shown that regardless
-
Tse and Ripmeester
of the relative orientation of the SOz molcule with respect to the plane of the phenol, long as the contact separation remains in a reasonable range, an interaction will exist between the two species and an intermolecular CT transition can be observed a t low energies (ca. 400 nm). We anticipate that in the SOz-quinol clathrate, even though SOz trapped inside the cage can reorient very rapidly,8 instantaneous interaction with the a electrons of the hydroquinone may still persist. Within the time scale of the Franck-Condon principle, a CT electronic transition is possible.
Conclusion We have shown from theoretical studies of the SO2substituted phenol complex that the formation of an EDA complex is the likely explanation for the observed yellow color of the solutions. The energy of the CT transition does not change appreciably with the orientation of SO2 with respect to the phenol plane. We suggest that similar interactions exist in SOz-/3-quinol clathrate and account for the distinct yellow color of this compound. That SOz-/3-quinol is the only colored hydroquinone clathrate known probably arises from the fact that SO2 is the only enclathratable molecule which possesses a low-lying vacant orbital. Acknowledgment. We thank Dr. Singleton is recording the photoacoustic spectra and to Dr. Davidson for helpful discussion. Registry No. SO2--quinol clathrate, 85235-38-7; S02-benzene clathrate, 4328-16-9; SO2-phenol clathrate, 85235-35-4; S 0 2 - p methylphenol clathrate, 85235-36-5; S02-p-chlorophenol clathrate, 85235-37-6; SO2-hydroquinone clathrate, 1786-27-2; P-quinol, 123-31-9.
