3218
J. Phys. Chem. B 2001, 105, 3218-3222
Photochemical Properties of Benzophenone Adsorbed on Ti-Al Binary Oxides: The Effects of the Surface Acidity H. Nishiguchi, J-L. Zhang,‡ M. Anpo,* and H. Masuhara† Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture UniVersity, 1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan, and Department of Applied Physics, Osaka UniVersity, Suita, Osaka 565-0871, Japan ReceiVed: September 23, 2000; In Final Form: February 5, 2001
Ti-Al binary oxides prepared by a coprecipitation method exhibit different chemical behaviors from those of physically mixed TiO2-Al2O3 oxides, and the dispersion of the TiO2 species in the Ti-Al binary oxides increases when the composition of Al of the starting materials is increased. When benzophenone is adsorbed on the Ti-Al binary oxides, the photochemical and photophysical properties of benzophenone are found to be greatly different from those of benzophenone adsorbed on porous silica glass such as Vycor glass. Namely, the phosphorescence properties of benzophenone adsorbed on Ti-Al binary oxides show the presence of the protonated form of benzophenone in addition to the benzophenone hydrogen-bonded to the surface OH groups. The characteristics of the surface properties, structures, and activities of these binary oxides are examined by monitoring the characteristics of the isomerization of cis-2-butene as a probe reaction. The phosphorescence properties of the protonated and hydrogen-bonded form of benzophenone changed with variations in the Ti/Al ratio, exhibiting a good correspondence with the results of the cis-2-butene to 1-butene and cis to trans isomerization reactions of cis-2-butene on the Ti-Al binary oxides, respectively. These good relationships suggest that the double bond shift isomerization reaction of cis-2-butene to 1-butene and the geometrical cis to trans isomerization occur at different sites, that is, the active sites for the cis-2-butene to 1-butene isomerization may be the surface Bro¨nsted acid sites and the active sites for cis to trans isomerization may be the surface OH groups located on Al2O3. Direct detection of the transient absorption spectra of benzophenone adsorbed on the oxides indicates that the benzophenone ketyl radicals are formed on the surfaces of Al2O3 through the hydrogen abstraction from the acidic surface OH groups by the excited triplet state of benzophenone.
Introduction There is considerable interest in the use of binary oxides to control the outcome of organic reactions and as tools to examine specific spectroscopic properties. Adsorption of the organic species on solids has become a powerful methodology to gain control of the selectivity of photochemical reactions. The features of the photophysical and photochemical behavior of adsorbed molecules are expected to be quite different from those of molecules in the gas phase and in solution; however, the relative lack of such studies on the adsorbed layers illustrate that it is one of the most unexplored fields in photochemistry, as compared with the number of studies of homogeneous systems. Therefore, it is of great interest to investigate the photophysics of adsorbed molecules in order to discover the effects the surface properties on the behavior of such adsorbed molecules and to examine the nature of the surface itself.1-6 It is also important to understand the general characteristics of the photolysis of adsorbed molecules, that is, how the reactivities of the excited states or the radical species themselves vary when they are formed on solid surfaces.7-15 Upon excitation of benzophenone in the (n, π*) band in both the gas phase and in various matrices, phosphorescence with a * To whom correspondence should be addressed. E-mail: anpo@ ok.chem.osakafu-u.ac.jp. † Department of Applied Physics, Osaka University. ‡ Permanent address: Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, P. R. China..
high quantum yield due to the transition from the excited triplet to the ground state was observed. In addition, the phosphorescence spectra of benzophenone are sensitively changed by changes in its molecular environment. These basic properties of benzophenone molecules lead to an appreciable dependence of the phosphorescence spectra on the acidic properties of the support.16 Therefore, benzophenone molecules may be considered one of the most useful candidates to act as a “molecular probe” in studying the acidic properties of solid surfaces. It is known that various binary oxide catalysts often exhibit higher catalytic activity and selectivity than what can be expected from the properties of their components.17-20 As an additional advantage, with binary oxide catalysts, it is fairly easy to obtain detailed information on the relationship between the structure and the catalytic activity (including photocatalytic activity) by changing the composition continuously from 0 to 100%. However, few studies have been carried out on binary catalysts from this viewpoint. The characteristics of the phosphorescence of benzophenone adsorbed on Ti-Al binary oxides is investigated by measuring the phosphorescence, and in the present study, the photochemical properties of benzophenone on Ti-Al binary oxides with various Ti:Al ratios are investigated. The general relationship between the photochemical properties of benzophenone adsorbed on Ti-Al binary oxides and the surface structures of these binary oxides with variations in the Ti:Al ratio are examined. Moreover, the characteristics of the surface properties, structures,
10.1021/jp003443y CCC: $20.00 © 2001 American Chemical Society Published on Web 03/29/2001
Benzophenone Adsorbed on Ti-Al Binary Oxides and activities of these oxides are investigated by monitoring the characteristics of the isomerization of cis-2-butene as a test reaction. Experimental Section Ti-Al binary oxides with different atomic ratios of Ti to Al were prepared by the coprecipitation of desired amounts of a mixed aqueous solution of TiCl4 and AlCl3 (0.5 M/L concentration), by addition of an aqueous solution of ammonia as the precipitation reagents.16 For precipitation with ammonia, aqueous ammonia was added to the solution, which was kept cool in an ice bath. The final pH of the solution was 9.5. The precipitates were then filtered, washed, dried, and calcined at 773 K in air to convert to binary oxides before further treatment.21 Binary oxides were degassed at 290 K for 1 h, heated at 773 K under 20 Torr of O2 for 1 h, degassed at 773 K, and finally cooled to 290 K. Adsorption of Benzophenone (BP) was carried out both from the CCl4 solution of BP and the BP vapor. Prior to the adsorption of BP, the samples were evacuated to about 10-6 Torr at 473 K. After the adsorption of BP, the remained BP was measured. The amount of BP adsorbed was 1 × 10-6 mol/g. The phosphorescence spectra of BP were recorded at 77 and 290 K with a Shimadzu RF-501 spectrofluorophotometer, and their lifetimes were obtained by a streak scope using Hamamatsu C-4334. The excitation source was a N2 laser (337 nm). The photolysis of BP adsorbed on the oxides was carried out at 275 K using a high-pressure mercury lamp (λ > 300 nm). The photoreaction products of BP were analyzed by a GC-MS (Finnigan MAT TSQ-70). For the isomerization reaction, cis2-butene (10 Torr) was introduced onto the oxide catalysts at 273 K. The reaction products were analyzed by gas chromatography. Results and Discussion 1. Phosphorescence Properties of BP Adsorbed on TiAl Binary Oxides. Photoexcitation of BP in the (n, π*) band, a phosphorescence spectrum due to a radiative transition from the excited triplet state to the ground state is observed at around 400-500 nm.5,6 It is known that when BP are adsorbed onto the surface of the oxides without any strong acidic sites such as SiO2 and Vycor glass (PVG, major composition: SiO2 97% and B2O3 3%), they adsorb on the surface OH groups only through hydrogen-bonding. In such systems, the phosphorescence peak of BP is observed at around 430 nm with its excitation peak at around 330 nm. The phosphorescence spectra of BP adsorbed on PVG and in the CCl4 solution exhibit a wellresolved vibrational fine structure due to the stretching of the CdO bond of BP. The phosphorescence spectra of BP adsorbed on Ti-Al binary oxides was found to be quite different from those on PVG and in polar or nonpolar solvents. It should be emphasized that the phosphorescence wavelength peak position of BP adsorbed on the Ti-Al binary oxides was found to change dramatically by varying the excitation wavelengths.22 The phosphorescence spectrum exhibits no vibrational fine structures due to the CdO group and a red shift (max 445 to 475 nm) can be seen only when the sample is excited by the longer excitation wavelengths (280-360 nm). The maximum peak of the excitation spectrum of BP adsorbed on Al2O3 was observed at around 340 nm. Thus, the phosphorescence spectrum did not exhibit vibrational fine structures and only a red shift. At high Ti composition, the extent of the shifts in the phosphorescence of BP adsorbed became smaller (453-463 nm at Ti-Al (25:75)). Moreover, the position of the excitation
J. Phys. Chem. B, Vol. 105, No. 16, 2001 3219 TABLE 1: Isomerization of cis-2-Butene over Ti-Al Binary Oxides at 273 K enrichment 2 mol% of yield (nmol/m .h) total yield of Al ions at surface TiO2 cisftrans cisf1 (nmol/m2.h) area (m2/g) surfacea 0 3 12 25 50 75 90 97 100 a
320.8 91.6 86.0 50.7 34.5 55.2 17.3 12.0 4.7
16.7 21.4 22.6 24.2 21.2 18.2 13.7 2.9 1.3
337.5 107.4 114.2 74.9 55.7 73.4 31.0 14.9 6.0
250 256 272 286 316 309 305 240 173
1.00 0.93 0.96 1.00 1.04 1.05
Data derived from XPS spectra.21
spectra changed from around 340 to 370 nm when the Ti mole ratio became larger than 50%. These results clearly suggest that more than two emitting species of BP are involved in the phosphorescence spectra, that is, for Ti-Al binary oxides, two phosphorescence components having different peak positions and lifetimes were observed with the extent of their contribution to the total phosphorescence spectrum strongly depending on the composition or Ti/Si ratio of the binary oxides. These phosphorescence components are well assigned to the phosphorescence from the excited triplet state of the BP molecules physisorbed on the surface OH groups by hydrogen bonding and the phosphorescence from the excited triplet state of the protonated BP molecules formed through a proton transfer from the Bro¨nsted acid surface sites on the oxides to BP.6,23 Differing from these results observed with the Ti-Al binary oxides, when the oxides prepared by the physical mixing of TiO2 and Al2O3 were used, only the phosphorescnce of BP adsorbed on the Al2O3 moieties could be observed, and its yields changed with changes in the composition of the oxides although the peak position remained constant, that is, increasing the composition of Al2O3 of the physically mixed oxides led to an increase in the yield of the phosphoresence of the BP adsorbed on Al2O3 at the same wavelength region of around 460 nm. However, the phosphorescence of the BP adsorbed on TiO2 moieties was not observed. This may be explained by the concept that energy transfer (and/or electron transfer) occurs from the excited triplet state of BP to the conduction band of TiO2 moieties. 2. Relationship between the Characteristics of the Phosphorescence Spectra of Benzophenone Adsorbed on the TiAl Binary Oxides and the Characteristics of the Rates of Isomerization of Cis-2-Butene on them at 273 K. The cis2-butene isomerization reaction was chosen as a test reaction to examine the catalytic properties and the surface structure of Ti-Al binary oxide catalysts because it is very sensitive to surface acidic properties.24 The reactivities and selectivities change with the properties of the catalytic surfaces. The yield of cis to 1-butene isomerization (double bond migration reaction) first increases on the addition of a small amount of TiO2 into Al2O3, reaching a maximum of Ti-Al (25: 75) and then decreases with an increase in the TiO2 composition, as shown in Table 1 and Figure 1. The yield of cis to trans-2-butene isomerization (geometrical isomerization) is the highest on the pure Al2O3 catalyst. Upon increasing the mole fraction of TiO2, the yield of the geometrical isomerization decreases. Figure 2 shows the relationships between the rates of the cis-2-butene to 1-butene isomerization reaction on the Ti-Al binary oxides and the phosphorescence lifetimes of the protonated BP on the oxides as a function of the Ti-Al compositions. It is likely that the formation of the protonated BP species is linked to the
3220 J. Phys. Chem. B, Vol. 105, No. 16, 2001
Figure 1. Yields of cis-2-butene isomerization over Ti-Al binary oxides at 273 K.
Figure 2. Relationship between the yield of 2 f 1-butene isomerization of cis-2-butene over Ti-Al binary oxides and the phosphorescence properties of the adsorbed BP. ([A]; hydrogen bonded form, [B]; protonated form).
surface acidity of the oxides. In other words, surfaces with high acidic properties result in a high efficiency of the phosphorescence from the protonated BP species, that is, yield of the phosphorescence reflects the acidity of the surfaces. The amounts of acidic sites and their acidity with various acid strengths on the Ti-Al binary oxides were determined by Tanabe et al. using the n-butylamine titration method.25 The maximum acidity was observed on the binary oxide having a molar Ti-Al ratio about 10/90. It could be concluded from these results that there is a good relationship between the acidity of the oxides and the rates of the isomerization of 1-butene on the oxides, as shown in Figure 2. It can be also seen that there is a close relationship between the phosphorescence properties of BP adsorbed on the Ti-Al binary oxides such as their yields and lifetimes and the rates of cis-2-butene to 1-butene isomerization on these oxides, as shown in Figure 2. The phosphorescence peak positions of BP adsorbed on the binary oxides show a red shift upon the addition of a small amount of titania into the alumina component, exhibiting a maximum at Al (10-20%) and then a blue shift with an increase in the Al component.22 The lifetimes of the protonated BP species became longer upon the addition of a small amount of titania into the alumina, exhibiting the longest lifetime at
Nishiguchi et al.
Figure 3. Relationship between the total yields of isomerization of cis-2-butene over Ti-Al binary oxides and the total yields of the phosphorescence of BP, [A] + [B]. ([A]; hydrogen bonded form, [B]; protonated form).
Ti-Al (3:97) and then becoming shorter with an increase in the Al composition. These phosphorescence properties show that the protonated BP became more stable by the addition of small amounts of TiO2 (10-20%), namely, by increasing the number of surface Bro¨nsted acid sites, which play a significant role as proton donor sites on the binary oxides. The stability of the protonated BP is sensitive and easily affected by the surface acidic properties. These results clearly suggest that the active sites for the cis to 1-butene isomerization reaction may be the surface Bro¨nsted acid sites, which act as good proton donors to 2-butene to produce a cationic intermediate species to 1-butene. The results of the TPD measurements of NH3 adsorbed on the Ti-Al binary oxides clearly show that there are at least two different Bro¨nsted acid sites. Being in agreement, the existence of the Bro¨nsted acidic sites were also observed for the Ti-Al (10:90) catalyst by Tanabe et al.25 It can be reasonably assumed that the surface Bro¨nsted acidic sites do, in fact, exist and act as proton donor sites toward butene in its ground state and also as proton donors to BP in its excited state to form the protonated BP species. On the other hand, as can be seen in Figure 3, there is a parallel relationship between the yield of the phosphorescence of BP adsorbed on Ti-Al binary oxides (hydrogen bonding BP (B)) and the rates of cis to trans-2-butene isomerization on the oxides. Tanabe et al. have reported that the isomerization reaction of cis-2-butene over the TiO2 catalysts proceeds through carboanion intermediates, and the rate of cis to trans of 2-butene is very low, compared to that on Al2O3.26 Furthermore, it is reported that the values of trans-2-butene/1-butene for cis-2butene isomerization is much lower than 1 over solid bases, in which the π-allyl carboanion is considered to be intermediate species.27 In the case of the Ti-Al binary oxides with higher mole fractions of Al, the values of trans-2-butene/1-butene are much higher than 1 (about 19 at Al2O3 catalyst), as shown in Table 1. These results clearly suggest that the cis to trans isomerization reaction of 2-butene on the Ti-Al binary oxides does not proceed through the π-allyl intermediates. In other words, the observed parallel relationship between the rates of cis to trans isomerization of 2-butene, and the yield of the phosphorescence of BP adsorbed on the Ti-Al binary oxides suggests that the cis to trans isomerization proceeds on the OH groups on the surface of the Al2O3 moieties.
Benzophenone Adsorbed on Ti-Al Binary Oxides
J. Phys. Chem. B, Vol. 105, No. 16, 2001 3221 TABLE 2: Yields of the Photolysis of BP Adsorbed on Various Oxides at 273 K (%)a composition of oxides(%)
Al100 Ti0
Al97 Ti3
Al75 Ti25
Al50 Ti50
Al25 Ti75
PVC
products yields(%) benzhydrol benzpinacol
10.0 3.9
13.2 3.5
28.1 4.2
30.3 5.3
33.3 5.8
3.2
a Product yields(%) ) yields of products/amount of adsorbed BP on oxides.
Figure 4. Effect of the UV-irradiation time on the phosphorescence spectra of BP adsorbed on Al2O3: 1, before UV-irradiation; 2, after UV-irradiation for 10 min; 3, for 240 min irradiation; excitation wavelength, 340 nm at 77 K.
3. Photochemical Properties of Benzophenone Adsorbed on Ti-Al Binary Oxides. To examine the effect of the surface properties of the binary oxides on the photochemical reactions of the adsorbed BP, their photolysis has been investigated. After the adsorption of BP on the binary oxides, the systems were UV-irradiated, and benzhydrol and benzpinacol were detected as the major products in the photolysis. Their yields increased with the UV-irradiation time, and the intensity of the phosphorescence of BP simultaneously decreased. On the other hand, only the production of benzpinacol could be observed in the photolysis of BP adsorbed on PVG and SiO2. Their reaction scheme is as follows: SCHEME 1
These results clearly suggest that the existence of the surface acidic sites are closely associated with the formation of benzhydrol. As shown in Figure 4, the wavelengths of the peak position of the phosphorescence spectra of BP adsorbed on Al2O3 shift to shorter wavelength regions by UV-irradiation, its extent depending on the UV-irradiation time. It was found that by increasing the UV irradiation time, the phosphorescence spectrum of BP first decreases in intensity at longer wavelength regions, clearly indicating that the phosphorescence from the protonated BP first decreases in intensity and suggesting that these species are closely associated with the photolysis of BP but not the hydrogen bonded BP. It can thus be concluded that the BPH+ species play an important role in the photoreactions of BP adsorbed on the oxides. Table 2 shows the yields of the photolysis of BP adsorbed on various oxides. The photoproduct at 273 K of BP adsorbed on PVG (where only hydrogen bonded BP species are formed) is only benzpinacol. On the other hand, the major photoproducts
Figure 5. Time-resolved Triplet-Triplet transient absorption spectra of BP adsorbed on Al2O3 following excitation at 355 nm, (pulse duration ) 1.6 µs, pulse intensity ) 10 mJ). 1, 0 µs; 2, 3 µs; 3, 5 µs; 4, 100 µs; 5, 24 000µs after laser flash.
from BP adsorbed on the Ti-Al binary oxides (where the hydrogen bonded BP and the protonated BP species are formed) are benzhydrol and benzpinacol. It is seen that the yields of benzhydrol strongly depend on the Ti/Al ratio. Together with the results mentioned above, it is concluded that the more predominant the BP protonated species observed on the surfaces, the larger the yield of benzhydrol. These results suggest that the observed photoproducts are mainly produced from the BPH+ species, in agreement with the fact that the acidic strength of the Ti-Si oxides is much higher than those of Ti-Al oxides, the yields of benzhydrol on the Ti-Si oxides are much higher than those on the Ti-Al oxides.28 Figure 5 shows the time-resolved T-T transient absorption spectra of BP adsorbed on Al2O3 at 298 K. The transient absorption exhibits two absorption peaks at 540 and 570 nm. The 540 nm band is found to be very similar to these of the BP triplet (BP microcrystalline),16 therefore, the band of 540 nm is attributed to the T-T transition absorption of the BP triplet. In the case of the T-T absorption of BP in the solution system, the transient spectrum is found to exhibit its peaks at 520-530 nm and 540-550 nm. The 520-530 nm band and 540-550 nm band are attributed to the BP triplet and BP ketyl radical.29,30 Topp31 and Obi et al.32 have shown that the band of BP ketyl radical exhibits a red shift by about 10 nm in comparison with the band of BP triplet. Thus, the T-T absorption band at 570 nm band observed in this study can be attributed to BP ketyl radical. The transient absorption spectrum due to the ketyl radicals could be detected by laser flash photolysis in these
3222 J. Phys. Chem. B, Vol. 105, No. 16, 2001 systems so that it is likely that the observed photoproducts are generated through the formation of ketyl radicals from the BPH+ species. These BPH+ species play a significant role in the photolysis of BP adsorbed on these binary oxides to produce the ketyl radical intermediates. The T-T absorption spectrum of BP adsorbed on pure TiO2 was not observed even at the first stage of delay time, suggesting that the BP triplet or BP ketyl radicals were not formed on the pure TiO2. In other words, as mentioned above, the energy transfer from the excited triplet state of BP into the conduction band of the semiconductor TiO2 may occur faster than nsec time scale. Conclusions The dispersion of the TiO2 species in the Ti-Al binary oxides prepared by a coprecipitation method increased when the amounts of the composition of Al of the starting materials of the mixture of AlCl3 and TiCl4 was increased. The phosphorescence properties of the protonated and hydrogen-bonded forms of BP on Ti-Al binary oxides changed with changing the Ti/Al ratio, exhibiting good relationship with the results of the cis-2-butene to 1-butene and cis to trans isomerization reactions of cis-2-butene over the Ti-Al binary oxides, respectively. The major photoproducts from BP adsorbed on the Ti-Al binary oxides (where the hydrogen bonded BP and the protonated BP species are formed) were benzhydrol and benzpinacol, the yields of benzhydrol strongly depending on the Ti/Al ratio. It was concluded that the more predominant the BP protonated species on the surfaces, the more yield of benzhydrol was observed. References and Notes (1) Anpo, M.; Che, M. AdV. Catal. 1999, 44, 179. (2) Photochemistry on Solid Surfaces; Anpo, M., Matsuura, T., Ed.; Elsevier: Amsterdam, 1989. (3) Zhang, J.; Yamashita, H.; Anpo, M. Chem. Lett. 1997, 1027.
Nishiguchi et al. (4) Zhang, J.; Matsuoka, M.; Yamashita, H.; Anpo. M. Langmuir. 1999, 15, 77. (5) Leermakers, P. A.; Thomas, T. L.; Weis, D.; James, F. C. J. Am. Chem. Soc. 1966, 88, 5075. (6) Okamoto, S.; Nishiguchi, H.; Anpo, M. Chem. Lett. 1992, 1009. (7) Wilkinson, F.; Willsher, C. J. Tetrahedron. 1987, 43, 1197. (8) Kubokawa, Y.; Anpo, M. J. Phys. Chem. 1974, 78, 2442. (9) Anpo, M.; Kubokawa, Y. J. Phys. Chem. 1974, 78, 2446. (10) Kubokawa, Y.; Anpo, M. J. Phys. Chem. 1975, 79, 2225. (11) Anpo, M.; Wada, T.; Kubokawa, Y. Bull. Chem. Soc. Jpn. 1975, 48, 2663. (12) Anpo, M.; Kubokawa, Y. Bull. Chem. Soc. Jpn. 1976, 49, 2623. (13) Anpo, M.; Wada, T.; Kubokawa, Y. Bull. Chem. Soc. Jpn. 1977, 50, 31. (14) Anpo, M. Chem. Lett. 1987, 1221. (15) Anpo, M.; Yamamoto, Y.; Suzuki, S. Chem. Lett. 1989, 1339. (16) Anpo, M. et al., to be published. (17) Kodama, S.; Nakaya, H.; Anpo, M.; Kubokawa,Y. Bull. Chem. Soc. Jpn. 1985, 58, 3645. (18) Anpo, M.; Nakaya, H.; Kodama, S.; Kubokawa, Y.; Domen, K.; Onishi, T. J. Phys. Chem. 1986, 90, 1633. (19) Mau, A. W. H.; Huang, C. B.; Kakuta, N.; Bard, A. J.; Campion, A.; Fox, M. A.; White, J. M.; Webber, S. E. J. Am. Chem. Soc. 1984, 106, 6537. (20) Domen, K.; Naito, S.; Soma, M.; Onishi, T.; Tamaru, K. J. Phys. Chem. 1982, 86, 3657. (21) Anpo, M.; Kawamura, T.; Kodama, S.; Maruya, K.; Onishi, T. J. Phys. Chem. 1988, 92, 438. (22) Nishiguchi, H.; Zhang, J-L.; Anpo, M. Langmuir, submitted for publication. (23) Nishiguchi, H.; Yamashita, H.; Anpo, M. Bull. Chem. Soc. Jpn., to be submitted. (24) Ferin, V. A.; Shvets, V. A.; Kazanskii, V. B. Kinet. Katal. 1979, 19, 1041. (25) Shibata, K.; Kiyoura, T.; Kitagawa, J.; Sumiyoshi, T.; Tanabe, K. Bull. Chem. Soc. Jpn. 1973, 46, 2985. (26) Itoh, M.; Hattori, H.; Tanabe, K. J. Catal. 1974, 35, 225. (27) Hattori, H.; Itoh, M.; Tanabe, K. J. Catal. 1975, 38, 172. (28) Shibata, K.; Kiyoura, T.; Kitagawa, J.; Sumiyoshi, T.; Tanabe, K. Bull. Chem. Soc. Jpn. 1973, 46, 2985. (29) Shizyka, H.; Hagiwara, H.; Fukushima, M. J. Am. Chem. Soc. 1985, 107, 7816. (30) Shaefer, C. G.; Peters, K. S. J. Am. Chem. Soc. 1980, 102, 7566. (31) Topp, M. R. Chem. Phys. Lett. 1976, 39, 423. (32) Kajii, Y.; Fujita, M.; Hiratsuka, H.; Obi, K.; Mori, Y.; Tanaka, T. J. Phys. Chem. 1987, 92, 2791.
