2811
J . Phys. Chem. 1991, 95,2811-2814
Infrared Spectrum of Sulfinic Acid HS02H in Solid Argon Matthew A. Fender$ Yasmin M. Sayed,t and Frank T. Prochaska* Department of Chemistry and Physics, Western Carolina University, Cullowhee. North Carolina 28723 (Received: October 1, 1990)
Mercury-xenon arc lamp photolysis of argon matrix samples at 12 K containing hydrogen sulfide and sulfur dioxide produced eight new infrared absorptions at 2591, 1209, 1093,762,476,450,340, and 270 cm-'. Extensive deuterium and oxygen-I8 isotopic substitution studies, and normal-wordinate calculations on all isotopomers, allow the assignment of these absorptions to eight vibrational modes of sulfinic acid. The HS02H molecule is formed by in situ hydrogen atom addition to matrix-isolated sulfur dioxide molecules. This is the first infrared spectroscopic observation of this reactive molecule.
Introduction Sulfinic acid, HS02H, is a reactive molecule that has been postulated as a reaction intermediate in many organic reactions. While sulfinic acid has been the subject of theoretical studies,l its infrared spectrum has never been reported. Considering the predicted stability of the sulfinic acid molecule relative to other isomers,' in situ hydrogen atom addition to sulfur dioxide isolated in an argon matrix seemed a reasonable method for generating this interesting molecule for infrared spectroscopic examination. Experimental Section Matrices were deposited on a CsI window cooled to 12 K by a Cryogenic Technology Inc. Model 21 closed-cycle helium refrigerator. Two diffusion-pumped stainless steel vacuum lines, each equipped with a 3-L stainless steel sample can, allowed deposition of one or two gas samples. In most experiments an Ar/H$/S02 sample of reagent mole ratio 500/4/ 1 was deposited at approximately 3 mmol/h for 15-1 8 h through one deposition line, and argon only was deposited through the second. In other experiments, an Ar/H2S sample of mole ratio 125/1 and an Ar/S02 sample of mole ratio 500/1 were deposited concurrently from the two separate vacuum lines. After deposition a scan was recorded from 3000 to 200 cm-I, and the matrix was then photolyzed for 30-90 min with the 200-1000-nm light from a Schoeffel high-pressure mercury-xenon arc lamp. Spectral regions of interest were rescanned after each photolysis period. Experiments were also performed in which hydrogen iodide was substituted for hydrogen sulfide. In addition, extensive isotopic substitution studies were carried out employing samples enriched with deuterium and/or oxygen-18 isotopes, including mixed-isotope experiments where hydrogen/deuterium and oxygen- 16/ 18 samples were mixed together. The reagents used were SO2 (anhydrous, Matheson), 69% 018-enriched SO2(Prochem), 99% 0-18-enriched SO2(Stohler), H2S, D2S (98% D enriched, MSD Isotopes), HI (Matheson), and argon (99.995% Matheson). Spectra were recorded on a Beckman 4260 infrared spectrometer at 1-cm-1 resolution. The spectrometer was interfaced to an Apple 11+ microcomputer, and Applesoft programs written in this laboratory allowed collection, storage, and manipulation of digitized spectra. Spectra were plotted on an Apple plotter. Results Upon deposition of samples of sulfur dioxide and hydrogen sulfide in solid argon, the only infrared absorptions observed are those of the precursor molecules.2 Photolysis of the matrix for 30 min with 220-1000-nm light from the mercury-xenon lamp generated eight new major product absorptions in the spectrum.
'Present address: Department of Chemical Technology, Asheville-Buncombc Technical Communitv Collene. Asheville. NC 28803. *Present address: DepaAment Gf 'Chemistry, University of Wyoming, Laramie. WY 82071. To whom correspondence should be addressed.
0022-3654/91/2095-2811$02.50/0
TABLE I: Product Absorptions (cm-') and Intensities (absorbance units) Produced by Mercury-Xenon Arc Lamp Photolysis of an Ar/H$/SO, = 500/4/1 Matrix Samrtle after 30-min after 90-min absorption photolysis photolysis assignt 0.010 HSOZH 2591 0.008 0.178 HSOZH" 1214 0.139 0.285 HSOZH 1209 0.225 0.285 HSOZH' 1204 0.215 0.045 HSOZH" 1101 0.042 1097 0.040 0.052 HSOZH" 1093 0.080 0.127 HSOZH 0.085 ? 79 1 0.060 0.660 HSOZH 762 0.518 0.140 HSOZH" 760 0.130 0.177 HSOZH' 755 0.140 486 0.060 0.078 HSOZH" 476 0.207 0.265 HSOZH 0.065 HS02H 450 0.047 0.077 HSOZH 340 0.067 0.025 HSOZH 270 0.023
Matrix site effect. The product bands continued to increase in intensity for photolysis times up to 120 min; photolysis for longer than 2 h caused the bands to begin to decrease in size. Results from the four major isotopic combinations are presented below. S1Q2 H2S: After photolysis eight major product absorptions appeared at 2591, 1209, 1093,762,476,450,340, and 270 cm-I. As seen in Figures 1-3, some of these bands were site-split into resolved or unresolved doublets or triplets. P O 2 D2S: The major product absorptions shifted upon deuterium substitution to 1196,844,758,471,425,319, and 260 cm-I. Figures 1-3 show that many of the deuterium product bands were site-split into doublets. S1802 H2S: The oxygen-18 and hydrogen major product absorptions were observed at 2591, 1168, 1083, 735, 474, 434, 336, and 268 cm-I (Figures 1-3). Site splitting of some of the major product bands was observed. SI8O2 D2S: When both deuterium and oxygen-18 isotopic substitutions are employed, the major product bands appear a t 1162, 840, 729, 465, 411, 311, and 252 cm-I. As seen in the figures, several of the deuterium product bands were again sitesplit into doublets. Table I lists the frequencies and absorption intensities of the product bands observed after photolysis in the normal-isotope experiment. The product absorptions in the deuterium and oxygen- 18 experiments had similar relative intensities. When hydrogen iodide is substituted for hydrogen sulfide in these experiments, the same eight product absorptions are observed
+
+
+
+
~~~
(1) Boyd, R. J.; et al. Can. J . Chem. 1980. 58, 331. ( 2 ) Hallam, H. E. In Vibrational Spectroscopy of Trapped Species; Hallam, H.E., Ed.; Wiley: New York, 1973; Chapter 3 and references
therein.
0 1991 American Chemical Society
2812 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991
Fender et al.
550
I
I
450
400
I
3 0
WAVENUMEW
W A V E "
Figure 1. Infrared spectra after 90 min of 220-1000-nm mercury-xenon arc lamp photolysis of an Ar/H,S/S02 = 500/4/1 matrix sample deposited at 12 K for 16 h. Different traces correspond to the isotopic combinations shown. Since the samples were condensed with a comparable amount of argon gas from a second deposition line, the reagent concentration in the matrix is half the sample value. The letter P indicates product absorptions, and S denotes sulfur dioxide precursor bands.
F i i 3. Infrared spectra after 90 min of 220-1000-nm mercury-xenon arc lamp photolysis of an Ar/H2S/S02 = 500/4/1 matrix sample deposited at 12 K for 16 h. Different traces correspond to the isotopic combinations shown. Since the samples were condensed with a comparable amount of argon gas from a second deposition line, the reagent concentration in the matrix is half the sample value. The letter P indicates product absorptions, and S denotes sulfur dioxide precursor bands.
Discussion Hydrogen sulfide and hydrogen iodide have been studied in mat rice^,^^^ and it has been shown5-' that hydrogen sulfide is an effective photolytic source of hydrogen atoms in matrices. It is also known&I0that gas-phase photodidation of hydrogen sulfide and hydrogen iodide occurs in the following way:
v
H2S
-
H
+ HS
HI-H+I
Figure 2. Infrared spectra after 90 min of 22O-1000-nm mercury-xenon arc lamp photolysis of an Ar/H2S/S02 = 500/4/1 matrix sample deposited at 12 K for 16 h. Different traces correspond to the isotopic combinationsshown. Since the samples were condensed with a comparable amount of argon gas from a second deposition line, the reagent concentration in the matrix is half the sample value. The letter P indicates product absorptions, and S denote sulfur dioxide precursor bands.
In both of the above cases the excess energy appears primarily as translational recoil energy of the H atoms. Since the average energies of H-S and H-I bonds are approximately 377 and 297 W/mol, respectively, photolysis with 220-1000-nm light provides more than enough energy for photodissociation. €nthe present work, hydrogen atoms generated from these photolytic precursors thus have sufficient energy to translate through the argon lattice to sulfur dioxide sites, and addition can then take place to the SOz molecules in situ within the argon cage. The HS and I fragments are too large for translation through solid argon, and no evidence of reaction of these fragments was observed. The infrared spectrum after mercury-xenon arc photolysis revealed loss of HIS (or HI), and eight new absorptions attributable to reaction products were observed (Figures 1-3). Because these eight major product bands were observed to have the same relative absorption intensities a t all times in all experiments, it is clear that only one major product was being formed in these experiments, and the product was the result of hydrogen atom addition to sulfur dioxide molecules. No product absorptions were observed when only SO2or HIS was deposited in solid argon and
with the same relative intensity as in the H2S experiments. After photolysis the H2S and HI infrared absorptions were observed to have decreased in intensity. The relative concentrations of precursors sulfur dioxide and hydrogen sulfide were varied in several experiments over a wide range. In addition, the length of photolysis time was varied significantly in different experiments. In all experiments the major product bands always had the same relative absorption intensity compared to each other.
(3) Barnes, A. J.; Howells, J. D. R. J . Chem. Soc., Faraday Trans. 2 1972, 68. 729. (4) Engdalh, A.; Nelander, B. J. Phys. Chem. 1986, 90,6118. ( 5 ) Woodbridge, E. I.; Lee, E. K. C. J . Phys. Chem. 1986, 90, 6059. (6) Smardzewski,R. R.; Lin, M . C. J. Chem. Phys. 1977, 66, 7. (7) Tso, T.;Lee, E. K. C. J. Phys. Chem. 1984,88,2776. (8) Oldershaw, D. A,; Porter, D. A.; Smith, A. J. Chem. Soc., Faraday Trans. 1 1972,68, 2218. (9) Crompton. L. E.; Martin, R. M. J . Phys. Chem. 1969, 73, 3474. (IO) Cadman, P.; Polanyi, J. C. J. Phys. Chem. 1968. 72, 3715.
820
780 WAMNUWBERS
700
IR Spectrum of Sulfinic Acid in Solid Ar photolyzed with the mercury-xenon lamp. Observed Frequencies. The eight observed infrared absorption frequencies of this one major reaction product are discussed below. 1214,1209, and 1204 cm-l (Figure I ) . These three sites upon oxygen-18 substitution shifted to two sites at 1173 and 1168 cm-', indicating a shift of approximately 40 wavenumbers, with no evidence of an intermediate oxygen species. This 0-18 shift is characteristic of a vibrational mode of a single oxygen atom bonded to a sulfur atom and is thus assigned to a free S - 0 stretch. Deuterium isotopic frequencies were observed at 1204, 1202, and 1196 cm-I while deuterium and oxygen-18 counterparts appeared at 1 162, 1154, and 1 149 cm-I. A likely explanation for this small deuterium shift would be a secondary isotope effect of a D atom on an S - 0 stretching vibration. 1093 cm-' (Figure 1 ) . A small 0-18 shift of 10 wavenumbers to 1083 cm-' and a large deuterium shift of approximately 245 wavenumbers to a doublet at 849 and 844 cm-' were observed. With both deuterium and oxygen-1 8 substitution this doublet appeared at 847 and 840 cm-I. These isotopic shifts are indicative of an H-S-0 or S-0-H bending mode. 762 cm-l (Figure 2). This band exhibited an oxygen-18 shift of 27 wavenumbers to 735 cm-' with no indication of an intermediate absorption. Upon deuterium substitution a doublet at 761 and 758 cm-' was observed. Absorptions at 735 and 729 cm-' appeared with D and 0-18 substitution. The large oxygen-18 shift and small deuterium shift are characteristic of an S-OH stretching modes6 Each of these major isotopic absorptions showed one or two minor site-split absorptions that were shifted 5-10 wavenumbers to lower frequency (see Figure 2 and Table I). 476 cm-l (Figure 3). Upon oxygen-18 substitution a broad band at 474 cm-' was observed, while a doublet at 47 1 and 459 cm-I appeared when deuterium was substituted for hydrogen. In the mixed hydrogen and deuterium experiment an intermediate absorption was observed, indicating the motion of two hydrogen atoms in this vibration. The small 0-18 and larger D shifts with the involvement of two hydrogen atoms leads to the assignment of an H-S-OH bending mode. 450 cm-' (Figure 3). This was a broad band in the unlabeled experiments; two partially resolved bands (or sites) appeared at 434 and 429 cm-' in oxygen- 18 experiments. In the mixed 0-16 and 0-18 experiment an intermediate absorption a t 441 cm-l appeared as a broad band, demonstrating the presence of two oxygen atoms in this vibrational mode. A doublet at 425 and 412 cm-' appeared on deuterium substitution, which shifted to 41 1 and 402 cm-l in the D and 0-18 experiment. Due to the pronounced sensitivity to both oxygen and deuterium substitution, this band is assigned to an 0-S-OH bending mode. 340 cm-l (Figure 3). This band showed an oxygen-18 shift of four wavenumbers to 336 cm-' and a deuterium shift to 319 cm-I. Upon D and 0-18 substitution, bands at 31 1 and 302 cm-' appeared, and no intermediate absorptions were observed. This isotopic behavior might be appropriate for a torsional mode or perhaps for an S - 0 wagging vibration in a small hydrogen-containing molecule. 270 cm-I. This absorption shifted to 268 cm-l upon oxygen-18 substitution and showed a deuterium shift to 260 cm-I. It appeared as a doublet at 264 and 252 cm-' in the doubleisotope experiment. As was the case for the 340-cm-' product absorption above, these isotopic shifts are reasonable for a torsional mode or an S - 0 wagging vibration. 2591 cm-l. Hydrogensulfur stretching modes absorb weakly in the 2600-cm-' region, and a careful search was made for an H-S product absorption. As seen in Table I, after 90 min of photolysis in the normal-isotope experiment, a weak ( A = 0.010) product band appeared at 2591 cm-I. An additional 3 h of photolysis reduced this product band by one-half to A = 0.005, and the other seven major product bands were also reduced by one-half. The major product formed in these experiments thus contains an H-S bond. Experiments were performed with D2S to search for the deuterium counterpart expected to absorb near 1900 cm-I, but this band was not observed. It may have been hidden under the D2S absorptions in this region.
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 2813 TABLE II: Observed Vibrational Frequencies of Suifinic Acid (HSOB) . - . in Solid Argon" HS1602H HSI8O2H DSI6O2D DS1*02D assigntC 259 1 2591 (1860)b 118601* H-SS 1209 1093 762 476 450 340 270
1168 1083 735 474 434 336 268
'1202 849 758 47 1 425 319 260
' 1 162' 847 729 465 411 311 252
S-0 s SQH b S-OH s H-S-OH b 0-S-OH b S-0 w t
"The 0-H stretching frequency region was not examined due to low matrix transmission. bCalculated using harmonic oscillator model. e~ = stretch, b = bend, w = wag, and t = torsion.
0 - H Stretching Frequency. Poor transmittance of solid matrices in the 3000-4000-cm-' region prohibited the 0 - H vibrational mode of sulfinic acid from being observed. Hashimoto" has reported the 0-H stretch of the HOS02 radical to be 3540 cm-' in solid argon, and the analogous mode for sulfinic acid would be expected to have a similar frequency. In the normal-coordinate calculations discussed below, the value of 3500 cm-' was used as the approximate 0-H stretching frequency of sulfinic acid. Normal-Coordinate Calculations. The eight major product absorptions observed in these experiments clearly demonstrate appropriate frequencies and isotopic shifts for assignment to the sulfinic acid molecule, HS02H, where one hydrogen atom is bonded to the sulfur and the second H atom is bonded to an oxygen. Normal-coordinate calculations were performed for all isotopomers in an attempt to further characterize the molecule and to remove uncertainties in the vibrational assignments of the 1093-, 340-, and 270-cm-' bands. A normal-coordinate program developed by the National Research Council of Canada was employed.'* Tetrahedral bond angles and standard bond lengths of 1.43 ( S - 0 ) , 1.35 (S-H), and 0.96 A (0-H) were used to describe the structure of the sulfinic acid molecule. Calculations were performed for each of the three possible staggered conformations, and the conformer in which the two hydrogen atoms are trans to each other gave the best results for all isotopic species. The average difference between the observed and calculated frequencies for the nine vibrational modes of the normal isotopic molecule was 5 wavenumbers (approximately 1%). The average difference was only slightly larger for the hydrogen and oxygen-18 isotopomer and increased to approximately 7% in the calculations for both deuterium molecules. The mode mixing discussed below may be one reason for the higher than expected percent difference in the deuterium calculations. The eigenvector results showed mode mixing to be occurring between the S-0 stretch and the S-0-H bend and between the S-OH stretch and the S-0-H bend. As would be expected, the calculations showed the hydrogen isotope S-0-H bend (1093 cm-I) interacted more strongly with the S-0 stretching mode, and the S-0-D bend (849 cm-I) interacted more strongly with the S-OD stretch. This mode mixing explains the larger than expected oxygen-18 shift of 10 wavenumbers observed for the 1093-cm-' S-O-H bending mode, and the calculations predicted an 0-18 shift of this magnitude. This type of mode mixing has been ~ i t e d ' ~as. 'occurring ~ in nitrous acid, and analogous isotopic shift behavior was seen for the oxygen- 18 HONO molecule. The sulfinic acid calculations also showed that mode mixing is occurring between the three lowest frequency vibrations, and this would account for the observed isotopic shifts of these absorptions. Table I1 summarizes the observed vibrational frequencies for the four major isotopic molecules and lists the best vibrational assignment for each absorption as determined by the normalcoordinate calculations. (1 1) Hashimoto, S.;Inoue, G.;Akimoto, H. Chem. Phys. Lcrr. 1984,107, 198. (12) Fuhrer, H.; et al. 'Computer Progrums f o r Infrared Spectrophotometry", Canadian National Research Council, 1976. (13) Hall, R. T.; Pimentel, G. C . Chem. Phys. 1963, 38, 1889. (14) Guillory, W. A.; Hunter, C. E. Chem. Phys. 1971, 51, 598.
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J . Phys. Chem. 1991,95, 2814-2817
Comparison to Suyenic Acid. Smardzewski and L i d produced sulfenic acid, HSOH (hydrogen thioperoxide), by the mercury arc photolysis of mixtures of O3and H2Sin solid argon. The 0-H stretching mode was observed at 3425 cm-I, the S-O-H bend at 1177 cm-I, the S-OH stretch at 763 cm-I, and a torsional mode at 449 cm-I. Except for the 1.4wavenumber oxygen-18 shift of the S-0-H bending vibration, the deuterium and 0-18isotopic shifts of the sulfenic acid absorptions were comparable to the observed sulfinic acid isotopic shifts reported here. The correlation between the vibrational frequencies and isotopic shifts for these two related molecules adds further support to the identification of sulfinic acid. Conclusion The photolysis of argon matrices containing hydrogen sulfide and sulfur dioxide produced sulfinic acid by in situ hydrogen atom
addition to SO2. Eight infrared absorptions of this reaction product were observed. Deuterium and oxygen- 18 isotopic substitution studies, normal-coordinate calculations, and comparison to the previously observed sulfenic acid molecule all support the identification of sulfinic acid. This is the first time the infrared spectrum of this reactive molecule has been reported.
Acknowledgment. The authors thank the National Science Foundation and the Cottrell Research Corporation for financial support. We thank Mr. Daniel R. Lorey I1 for his assistance with many parts of this work, including modification of the microcomputer software and preparation of the figures. The authors also thank Mr. Norris Bingham for his work on the H I experiments. Registry NO. HSOZH, 131865-77-5;D2,7782-39-0;lsO, 14797-71-8; Ar, 7440-37-1; SO2, 7446-09-5;HIS, 7783-06-4.
Photolysls of Nitric Acid In Solid Argon: The Infrared Absorption of Peroxynitrous Acid (HOONO)~ Bing-Ming Cheng,* Jeng-Wen Lee, and Yuan-Pern Lee* Department of Chemistry, National Tsing Hua University, 101, Sec. 2, Kuang Fu Road, Hsinchu, Taiwan 30043, R.O.C. (Received: October 1. 1990)
Nitric acid (HON02) in solid argon at 12 K was irradiated with ultraviolet light from various sources. Recombination of the fragments OH and NO2 from photolysis within the argon lattice site has led to the formation of peroxynitrous acid (HOONO). IR absorption lines at 3545.5, 1703.6, 1364.4,952.0,and 772.8 cm-I have been assigned to this molecule on the basis of isotopic shifts. Under certain conditions the lines at 3563.3,1708.3,1372.7,957.4,and 782.9 cm-l were also observed, and they have been attributed to HOONO in a less stable matrix site. The observed vibrational frequencies are in agreement with recent theoretical calculations on HOONO. The implication of the formation of HOONO from HONOz to atmospheric chemistry is also discussed.
Introduction
The reaction of OH and NO2 to form nitric acid OH
+ NO2 + M
+
HONO2
+M
(la)
is important in the atmosphere because in this reaction the reactive HO, and NO, species are converted to stable nitric acid, an important component of acid precipitation. The rate coefficient for the reaction has been studied e~tensively.~-’In flash photolysis experiments, Robertshaw and Smith found that the rate coefficient of reaction 1 a has not yet reached its high-pressure limit under 6.5 X IO3 Torr of CF4 at 300 K.’ The possibility of a second reaction channel OH
+ NO2 + M + H O O N O + M
(1b)
was proposed to explain the observed results. Later, Atkinson et al. reported the formation of alkyl nitrate (RONOJ from the A mechanism reaction of alkyl peroxyl radicals (RO,)and consisting the rearrangement of an excited peroxynitrite (ROONO*) intermediate has been proposed to account for the production of RON02. The peroxynitrous acid (HOONO) is expected to be photolyzed more rapidly in the UV region than its isomer nitric acid (HONO2), and the photodissociation products of HOONO are likely to be H 0 2 and NO; accordingly the formation of HOONO via reaction I b would affect the catalytic ‘Dedicated to Professor George Pimentel. Y.P.L. was a student of Pimente1 and received his Ph.D. in 1979. *Research associate at Synchrotron Radiation Research Center, Taiwan, R.O.C. To whom correspondence should be addressed. Also affiliated with the Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan, R.O.C.
cycles of ozone destruction and the H N 0 3 / N 0 2 ratio in the atmosphere. HOONO has been variously proposed as a transient intermediate in aqueous solution in the reaction of nitrite ion and H2O2,*I2 in the photolysis of nitrate and in the reaction of OH and NO2.Is No spectroscopic information has been reported for HOONO except that a UV absorption a t -345 nm was tentatively assigned to HOONO in a continuous aqueous flow mixture of H N 0 2 , H202,and HC104. Recently, Burkholder et al. have studied the products of reaction 1 by means of a high-resolution FTIR spectrometer coupled with a rapid-flow multipass absorption ce11.I6 They failed to observe ( I ) Robertshaw, J. S.; Smith, I. W. M. J . Phys. Chem. 1982, 86,785. (2)Anderson, J. G.; Margitan, J. J.; Kaufman, F. J . Chem. Phys. 1974, 60,3310. (3)Howard, C. J.; Evenson, K. M. J . Chem. Phys. 1974,61, 1943. (4)Anastasi, C.; Smith, 1. W. M. J. Chem. SOC.,Faraday Trans. 2 1976, 72, 1459. ( 5 ) Atkinson, R.; Perry, R. A,; Pitts, J. N., Jr. J . Chem. Phys. 1976, 65, 306. (6)Wine, P. H.; Kreutter, N. M.; Ravishankara, A. R. J . Phys. Chem. 1979. 83. 3139. (7) Burrows, J. P.; Wallington, T. J.; Wayne, R. P. J . Chem. Soc., Faraday Trans. 2 1983, 79, 11 1. ( 8 ) Atkinson, R.; Carter, W. P. L.; Winer, A. M. J. Phys. Chem. 1983,
87. 2012. . ,_.._
(9)Gleu, K.; Hubold, P. Z . Anorg. ANg. Chem. 1935, 223, 305. (IO) Halfpenny, E.;Robinson, P. L. J . Chem. Soc. 1952, 928. ( 1 1 ) Anbar, M.; Taube, H. J . Am. Chem. Soe. 1954, 76.6243. (12)Benton, D.J.; Moore. P. J . Chem. SOC.A 1970, 3179. (13)Bart, F.;Hickel, B.; Sutton, J. Chem. Commun. 1969, 125. (14)Shuali, U.;Ottolenghi, M.; Rabani, J.; Yelin, Z . J . Phys. Chem. 1969, 73, 3445. (15) Grltzel, M.; Henglein, A.; Taniguchi, S.Eer. Bunsen-Ges. Phys. Chem. 1970, 74, 292. (16)Burkholder, J. B.; Hammer, P. D.; Howard, C. J. J . Phys. Chem. 1987, 91, 2136.
0022-3654/91/2095-2814%02.50/00 1991 American Chemical Society
