J. Phys. Chem. 1989, 93, 511-517
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UV Absorption Spectra and Photochemical Reactions of Simple Aromatic Hydrocarbons in the Cryogenic Oxygen Matrix Satoshi Hashimoto and Hajime Akimoto* Division of Atmospheric Environment, The National Institute for Environmental Studies, P.O.Tsukuba-gakuen, Ibaraki 305, Japan (Received: April 26, 1988; In Final Form: June 29, 1988)
The UV absorption spectra and photochemical properties of benzene, toluene, p-xylene, mesitylene, durene, styrene, and naphthalene in solid oxygen at 11 K have been studied. The well-resolvedcontact charge transfer (CCT) absorption spectra have been observed. Participation of valence interaction to the CT state has been discussed on the basis of the evidence that the spectra of the methyl-substituted benzenes consist of two absorption bands, and a vibrational structure of the intrinsic bands disappears. The vertical transition energy of the main CCT band plotted against the ionization potential correlates linearly with the slope of 0.46 f 0.13, which is far from unity. Photochemical excitation within the CCT absorption band for all the compounds studied (except styrene) c a w oxidative cleavage of the aromatic ring to form the corresponding dicarhyl compounds and C 0 2 as main products. Styrene reacts as an alkene giving benzaldehyde and formaldehyde,which are specific for the oxidative cleavage of a side chain rather than an aromatic ring. It has been concluded that the oxidative cleavage of a double bond, either olefinic or aromatic, is the common reaction pathway characteristic of the ion-pair state or [ D'. .O,].
Introduction Well-resolved contact charge transfer (CCT) absorption bands of a series of alkene-oxygen pairs were first observed in our previous studies'2 on the photochemistry of simple alkenes isolated in the cryogenic oxygen matrices (the matrix/reactant ratio, M / R , is 250/ 1). Characteristic photochemical reactions were found'q2 to proceed by excitation to the C T states and were classified into the following pathways: (i) photooxygenation to give an oxygen adduct (for 2,3-dimethyl-2-butene), (ii) cis-trans isomerization, (iii) oxidative double bond scission to give two corresponding carbonyl compounds, and (iv) photofragmentation to form C 0 2 , CO, etc. The relative importance of these pathways varied with the excitation energy and the molecular structure of alkenes. These. results prompted us to extend our study to a series of aromatic hydrocarbons in solid oxygen. Benzene, toluene, p-xylene (1,4dimethylbenzene), mesitylene (1,3,54rimethylbenzene),durene (1,2,4,5-tetramethylbenzene),styrene, and naphthalene were selected as electron donor molecules. The UV absorption spectra of the aromatic molecules isolated in oxygen matrices ( M / R = 250/1 or 1000/1) and the CCT photochemistry of these compounds will be reported in the present study. The CCT absorption spectra of aromatic moleculeoxygen pairs have attracted much interest since Evans3a reported oxygen-enhanced absorption spectra of oxygen-saturated liquid benzene, toluene, styrene, and several other aromatic compounds 1953, although the reported absorption maxima were later ascribed to an instrumental e r r ~ r . ~In~ the . ~ 1960s several studies4-' were reported on the C C T absorption spectra of the aromatic molecule-oxygen system in the liquid phase, while a theoretical study was advanced by Tsubomura and Mulliken.8 In all these studies, however, only enhanced absorptions overlapping the long-wavelength tails of the first singlet symmetry-forbidden bands were generally observed, and no well-resolved absorption maximum was revealed. Solely for benzene, Lim and Kowalski4 presented the spectrum of the C C T absorption in the liquid phase as the difference between spectra obtained with and without pressurized oxygen, which showed the absorption maximum at around 220 nm. Birks and co-workers9later reported the absorption maximum (1) Hashimoto, S.; Akimoto, H. J . Phys. Chem. 1986, 90, 529. (2) Hashimoto, S.; Akimoto, H. J . Phys. Chem. 1987, 91, 1347. (3) (a) Evans, D. F. J. Chem. Soc. 1953,345. (b) Evans, D. F. Chem. Ind. (London) 1953, 1061. (4) Lim, E. C.; Kowalski, L. V. J . Chem. Phys. 1962, 36, 1729. ( 5 ) Jortner, J.; Sokolov, U. J . Phys. Chem. 1961, 65, 1633. (6) Chien, J. C. W. J . Phys. Chem. 1965, 69, 4317. (7) Slifkin, M. A.; Allison, A. C. Nature (London) 1967, 215, 949. (8) Tsubomura, H.; Mulliken, R. S. J . Am. Chem. SOC.1960, 82, 5966.
for the benzene-oxygen charge-transfer band in the vapor phase at 219 nm. More recently, Rest et a1.I0 studied the absorption bands of benzene, toluene, styrene, and indene in the cryogenic solid phase at low mixing ratios (lO/l-1/60) and reported the difference spectra between those obtained in the O2and Ar mixture as the CCT bands with oxygen. The reported absorption maximum of the benzene-oxygen (1/1) system was 235 nm. Photooxidation of 02-saturated neat benzene was studied by Wei et al.," and the characteristic oxidative rupture of the aromatic ring to form mucondialdehyde (trans-tram-2,4-hexadiendial) was reported. In the vapor phase, Irina and Kurien12 observed the formation of phenol in addition to mucondialdehyde when oxygen with benzene vapor was irradiated at 254 nm. Formation of phenol has also been reported in more recent studies in the gas phase. Thus, optoacoustic measurements at 248 nm by Hunter et al.,13 photolysis of the benzene-O2 cluster at 193 nm by Knee et al.,I4 and bulk photolysis at 193 nm by Nakashima and YoshiharaI5 all indicated the production of phenol together with other ill-defined products. Lin and TevaultI6 studied the photooxidation of benzene in 10 K Ar matrices doped with 02.Production of C O and C02was observed and attributed to secondary photodissociation of initial peroxy adducts or their dialdehyde isomers from the reaction of triplet benzene with 02.Photooxidation of 02-saturated neat toluene has also been studied, and an oxidative chain reaction to form benzaldehyde, benzyl alcohol, and benzyl hydroperoxide has been reported by Wei and Adelman" and by Chien.I6 More recently, Tokumaru and mworkersl8 studied CCT photochemistry of a series of organics-oxygen systems in solution. Although the reaction mechanism is not necessarily clear, since the secondary reactions involving reactive solvents often complicate the product distribution, a reactive intermediate for each system has been discussed. (9) Briks, J. B.; Pantos, E.; Hamilton, T. D. S. Chem. Phys. Lett. 1973, 20, 544. (10) Rest, A. J.; Salisbury, K.; Sodeau, J. R. J. Chem. Soc., Faraday Trans. 2 1977, 73, 265. (11) Wei, K.; Mani, J.-C.; Pitts, J. N., Jr. J . Am. Chem. SOC.1967, 89, 4225. (12) Irina, J.; Kurien, K.C . J . Chem. Soc., Chem. Commun. 1973, 7 3 8 . (13) Hunter, T. F.; Rumbles, D.; Stock, M. G. Chem. Phys. Lett. 1977, 45, 145. (14) Knee, J. L.; Otis, C. E.; Johnson, P. M. J. Phys. Chem. 1982, 86, 4467. (15) Nakashima, N.; Yoshihara, K. Bull. Chem. Soc. Jpn. 1982,55,2783. (16) Lin, M. C.; Tevault, D. E. Combust. Flame 1981, 42, 139. (17) Wei, K. S.; Adelman, A. H. Tetrahedron Lett. 1969, 3297. (18) (a) Onodera, K.; Furusawa, G.; Kojima, M.; Tsuchiya, M.; Aihara, S.; Akaba, R.; Sakuragi, H.; Tokumaru, K. Tetrahedron 1985,41,2215. (b) Kojima, M.; Sakuragi, H.; Tokumaru, K. Tetrahedron Lett. 1981,2889. (c) Onodera, K.; Sakuragi, H.; Tokumaru, K. Tetrahedron Lett. 1980, 2831.
0022-3654/89/2093-0571$01.50/00 1989 American Chemical Societv
572 The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 This subject is also interesting because of its relevance to atmospheric chemistry since the photocatalytic oxidation process on metal surfaces,I9 which may contribute to the atmospheric degradation of organic compounds under certain conditions, has D'. .OF]. been postulated19a to proceed via a similar ion pair [ Very efficient COz production from th e photooxidation of toluene on TiOz has been reported by Ibuski and TakeuchLzo Relevance. to ozone chemistry has been pointed out by Martinez21 since both state processes are thought to involve a similar biradical (>'-Oo') as an intermediate.
Hashimoto and Akimoto
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Experimental Section Cryostat. The cryostat, cooled by a two-stage closed-cycle helium refrigerator, has a transmitting optics that can be applied to IR and UV-vis spectroscopy by selecting the suitable sample plate and the windows. The details of the cryostat system have been described W Spectral Measurement. To minimize the optical scattering, a pulsed deposition method similar to those used by Perutz and TurnerZZand Rest et al.IOwas employed. Ten portions of an oxygen-aromatic gas mixture (250/ 1 for benzene and methylsubstituted benzenes, 1000/1 for styrene and naphthalene) in a volume of a few cubic centimeters loaded from a 4-L glass bulb at about 100 Torr were deposited successively on the sapphire sample plate at 10 K. The total amount of the mixture deposited was 0.25 f 0.01 mmol. Each deposition took 30 s with a 90-s interval. The spectrum between 200 and 900 nm with a resolution of 2 nm was measured with a UV-vis spectrophotometer (Hitachi, 220A) interfaced with a data processing system (NEC, PC-9800 system). A spectrum of pure solid oxygen prepared under the same conditions was also measured as a reference for subtraction. The apparent optical densities of the pure oxygen reference were 1.71, 1.06, 0.60, and 0.27 at 200, 220, 250, and 300 nm, respectively. Besides the Herzberg band absorption of 0 2 (
