J. Phys. Chem. 1995,99, 501-504
501
Microwave Detection of a Key Intermediate in the Formation of Atmospheric Sulfuric Acid: The Structure of H20-SO3 J. A. Phillips, M. Canagaratna, H. Goodfriend, and K. R. Leopold* Department of Chemistry, University of Minnesota, 207 Pleasant St., S.E., Minneapolis, Minnesota 55455 Received: October 18, 1994@
The microwave spectra of five isotopically substituted derivatives of HzO-SO3 have been observed by pulsed nozzle Fourier transform microwave spectroscopy. The complex, which has long been regarded as an important precursor to HzSO4 in the atmosphere, has a structure in which the oxygen of the water approaches the sulfur of the SO3 above its plane, reminiscent of a donor-acceptor complex. The intermolecular S - 0 bond length is long (2.432 f 0.003 A), and the out-of-plane distortion of the SO3 is small (2-3”). The C2 axis of the water forms an angle of 103 f 2’ with the intermolecular bond. For an eclipsed configuration, this structure places the protons 2.67 8, from the SOs oxygens, indicating that a rather long distance must be traversed in order to transfer a proton to form sulfuric acid. The success of these experiments depended critically on the use of a molecular source in which liquid water was evaporated directly into the supersonic expansion. Such a source should be general for liquids of moderate vapor pressure, and its design is described.
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most recent calculations, however, predict a significantly smaller binding of 7.9 kcallmol, a higher barrier of 27.0 kcdmol, and a much longer bond length of 2.453 kl* Clearly, a definitive structural study of this complex would be quite useful. In this Letter, we report the observation of the SO3-HzO complex and its 34S and deuterated derivatives in the gas phase by pulsed nozzle Fourier transform microwave spectroscopy. Although intramolecular proton transfer is quite facile, the calculated barriers above suggest that the complex should be stable at the low temperatures of a supersonic jet, and indeed strong signals were observed for this species. Not surprisingly, we find a rigid rotor spectrum intermingled with additional transitions which do not fit a rigid rotor pattern and which presumably result from internal motions in the complex. Analysis of the rigid rotor spectra, however, it quite straightforward and provides concrete structural information which forms the subject of this report.
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Experimental Section
Introduction Sulfuric acid is one of the primary constituents of acid rain.’ It is also intimately related to the formation of atmospheric aerosols, whose influences on climate, the global energy budget, and stratospheric ozone depletion are all well e~tablished.,,~ Consequently, the mechanisms for formation of sulfuric acid in the atmosphere have been a topic of considerable interest. One homogeneous process which has long been thought to play an important role consists of the oxidation of SO2 to so,, followed by a two-step hydration involving the formation of an H2O-SO3 intermediate, viz.
+ OH -HSO, HSO, + 0, - HO, + SO, SO, + H,O -.SO,-H,O SO,
SO,-H,O
-
H,S04
(1)
(2)
The kinetics of reactions 1 and 2 have been studied in detail,19495 and the overall conversion of SO3 to H2S04 (reactions 3 and 4) has also been investigated.6 Direct observation of the HzOSO3 intermediate, however, has been an elusive goal, presumably because reaction 4 is fast, and indeed observation of this species has thus far only been possible via matrix isolation technique^.^-^ Although the structure was not determined, the data suggest that it is a donor-acceptor type, not a hydrogenbonded complex. In the most recent work, the symmetric stretch of SO3 was observed, indicating significant distortion from its normal planar configurationand suggesting a “stronger than van der Waals” intera~tion.~ From a theoretical standpoint, early calculations predicted 15.2 kcallmol for the binding energy of the adduct, 3.3 kcall mol for the barrier to formation of the acid,1° and an intermolecular S - 0 distance of 1.74 A. Later work gave a much more stable adduct and a higher barrier to the proton transfer,” with a binding energy of 21.4 kcallmol, a barrier height of 23.2 kcall mol, and an intermolecular S - 0 bond length of 2.027 A. The
* To whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, January 1, 1995. 0022-365419512099-0501$09.0010
The spectrometer used in these studies was a newly constructed Balle-Flygare type pulsed nozzle Fourier transform microwave ~pectrometer,’~ the details of which will be given in a future publication. Briefly, the system is designed to operate in the 2- 18 GHz range and is equipped with a 20 OOO W s diffusion pump to maximize throughput. The interfacing is similar to that of Chang et al.14 but exploits new advances in computer hardware which allow direct, software-drivencontrol of the pulse timing sequence without the need for home-built digital electronics. Up to 15 free induction decay signals can be digitized per gas pulse at nozzle repetition rates up to 15 Hz. Gas pulses can be injected into the microwave cavity either perpendicular to its axis or axially through a small hole in one of the mirrors. The former configuration is fitted with an air lock, which facilitates repair of the source without shutdown of the vacuum system, and was therefore used in this work. Since SO3 and H20 react violently, the design of the molecular source was critical to the success of the experiments. Initial attempts to observe the complex employed a relatively standard “coinjection” nozzle15 in which a continuous flow of argon saturated with water vapor at room temperature was injected into a pulsed SO3lAr mixture. After a series of 1995 American Chemical Society
502 J. Phys. Chem., Vol. 99, No. 2, 1995
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TABLE 1: Observed Transition Frequencies of HzO-SO3 isotopic form S' S F' F' frequency (MHz)" H20-32S03 0-1 7213.129 1-2 14426.106 H20-34S03 0-1 7 195.480 1-2 14390.810 0- 1 HDO-32S03 1-1 6954.715 1-2 6954.739 1-0 6954.767 1-2 13909.318b D z O - ~ ~ S O ~ 0-1 6726.446b 1-2 13452.747b D20-34S03 0-1 6707.893b 1-2 13415.641 All frequencies are accurate to f 5 kHz. Line center of a nmow hyperfine multiplet.
-
-
(1
I
I
TABLE 2: Spectroscopic Constants for HzO-SOs isotopic form ( ( B C)/2) (MHz) DJ (kHz) eqQ (D)( ~ H z ) ~ ~ 0 - 3606.5774(27) ~ ~ ~ 0 ~ 6.35(47) ~ ~ 0 - 3 4 ~ 0 3597.7524(27) ~ 6.25(47) HDO-32S03 3477.3853(27) 6.97(47) -72(40) 3363.2351(27) 6.03(47) u D T O - ~ ~ S O ~ 3353.9585(27) 6.03(47) u
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The hyperfine structure was not analyzed.
TABLE 3: Structural Parameters for HzO-SO$ set Ib set2 set 3d set4e average 2.432 2.435 2.430 2.432(3) R(S-0) (8,) 2.430 92.6(1) 92.54 92.67 92.67 #I(OS-O) (deg) 92.54 a(SO/C2)(deg) 105.51 103.45 101.19 103.45 103(2) a Values obtained using r(OH) = 0.965 8, and the HOH angle of water equal to 104.8" (ref 20). The S-0 bond length in SO3 was taken as 1.4198 8, (ref 19). H z O - ~ ~ S OH20-34S03, ~, and HOD-32S03. H20-32S03, Hzo-%03, and DzO-~~SO~. DZO-~~SO~, DzO-~~SO~, and HOD-32SOs. e D z O - ~ ~ S ODzO-~~SO~, ~, and H20-32S03. In addition to the reported spectra, a number of weaker lines were also observed within 35 MHz of many of the rigid rotor transitions. None form exact 2:l ratios as do the assigned frequencies, though some quite plausibly correspond to the Kp = 1, J = 2 1 transitions predicted from the structure determined below. Others likely result from internal rotation andor proton exchange tunneling, and full confidence in the assignments of Kp = 1 lines will only be obtained once the internal motions are understood and the other neighboring transitions can be definitively accounted for. Independent of an analysis of the internal motions, the K = 0 rigid rotor spectra can be used to extract significant structural information about the system once several very reasonable assumptions are made. Preliminary analysis indicates a rather long bond and hence a relatively weak interaction, thus permitting the monomer bond lengths and the HOH angle to be constrained to their free-molecule value^.'^-^^ As in our previous work on donor-acceptor complexes of BF3,22923 however, we retain the out-of-plane distortion angle of the SO3 as an adjustable parameter. The zero-point excursion angle of the C3 axis of SO3 away from the intermolecular S - 0 bond axis is unfortunately not determinable from the data. However, it is likely to be small based on experience with complexes of BF3, where the monomer moments of inertia and Lewis acidity are ~ i m i l a r . ~Thus, ~ . ~ this ~ angle was set to zero in the analysis. Also, we assume that the water is oriented such that the hydrogens are equidistant from the SO3 plane, consistent with the observation of only one isomer of the HOD complex. Finally, the value of (B C)/2 is insensitive to the intermolecular torsional angle, which therefore remains undetermined in this study. With the above assumptions, the measured rotational constants can be used to determine the intermolecular S-0 bond length, R(S-0), the angle between the C2 axis of water and the S-0 bond, a(S-O/C2), and the out-of-plane distortion of the SO3, /?(0S-0)-90°. Since five rotational constants provide an overdetermined structure, different groups of three can be used to check for consistency provided that isotopic substitution on each moiety is represented in each set. Table 3 presents the results for four groupings, where it is seen that the calculated structure is remarkably well determined. For all structures given, the ( ( B C)/2) values of the isotopomers not used in the calculation are predicted to within 4 MHz. The reported uncertainties in the structural parameters reflect the scatter among determinations but do not include systematic model
-
7212.40
7213.70
-
Frequency (MHz)
Figure 1. The J = 1 0 transition of H20-SO3. The data collection time for this spectrum was approximately 40 s. unsuccessful spectral searches in this configuration, diagnostic tests using known transitions of H20-S0216 and H2S0417 indicated that significantly more water was needed than could be obtained from the room temperature vapor pressure. Thus, a reservoir of liquid water was placed behind a 0.004 in. i.d. injection needle oriented perpendicular to and backed off slightly from the axis of the expansion. A drip rate of a few drops per minute in air corresponds to a mass throughput which, when continuously evaporated into the vacuum chamber, raises the pressure by about 1 x Torr. Use of this source increased the H20-SO2 and H2SO4 signals by a factor of L 100 over those obtained in the initial tests and eventually led to successful observation of the H20-S03 complex. Optimum H20-S03 signals were obtained with a stagnation pressure of 2 atm behind a 0.8 mm nozzle and a partial pressure of SO3 equal to that above the polymerized material at 0 "C.
Results and Analysis Assigned transition frequencies for the five isotopomers studied are given in Table I, and a sample spectrum is shown in Figure 1. The J = 2 1 transitions lie at almost exactly twice the frequency of the J = 1 0 lines and were assigned to the K = 0, a-type rigid rotor spectra of a nearly prolate asymmetric top. Spectroscopic constants were determined according to the usual energy expression, viz.,
- -
+
= 2 ( ~ I)((B
+ 012)
- 4(5+ 1 ) 3 ~ J
(5)
where DJ is an effective constant, incorporating the effects of both centrifugal distortion and asymmetry. Deuterium quadrupole hyperfine structure for the HDO complex was analyzed by standard methods.'* The spectroscopic constants are listed in Table 2.
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J. Phys. Chem., Vol. 99,No. 2, 1995 503
8
\s
392.6"
/I
11
1033,
.................................................
2.432
HH
0
A
00
Figure 2. Structure of H20-SO3 drawn in the staggered configuration.
The intermolecular torsional angle is not determined. errors.z4 The agreement, however, is quite satisfactory and justifies the analysis. The average structure is shown in Figure 2. Finally, the deuterium quadrupole coupling constant of the HDO complex, though poorly determined, can be analyzed using the known value of eqQ(D) along the OD bond in HDO (318.6 ~ H Z ) Assuming .~~ only projective contributions to eqQ(D) in the complex, we obtain 65 f 7" for the average angle that the OD bond makes with the a axis. This may be compared with a value of 81" from the structure determined from rotational constants. The agreement is fair, but quite acceptable, as these angles are only crude deconvolutions of the large-amplitude averaging of substantially different quantities. Note that although the measurement uncertainties do not unambiguously exclude the opposite choice of sign for eqQ(D), only the negative value gives physically reasonable angles and is consistent with the observed intensities.
Discussion From the above data, it is clear that H2O-SO3 resembles a donor-acceptor complex, with the oxygen of the water approaching the sulfur of the SO,. The 2.43 8, intermolecular bond length, however, is quite long compared with the 1.42 8, S-0 covalent bond distance in sulfuric acid, and the geometry at the sulfur is much more planar than tetrahedral. Because the hydrogens point away from the SO3, the intermolecular H-0 distance is 2.67 8, in the configuration of closest approach. The intermolecular 0-H and S-0 distances are 1.40 and 0.54 8, longer, respectively, than those of the transition state calculated by Hofmann and Schleyer,l2 and indeed the complex may owe its lifetime, in part, to the long distances which must be traversed for the proton transfer to take place. Since the complex is a nearly prolate asymmetric rotor, the effective distortion constants, DJ,reported in Table 3 in principle contain contributions from asymmetry. It is curious, however, that based on the above structure, the calculated deviations of the J = 2 1 transitions from twice the J = 1 0 transitions are 122,322, and 447 kHz for the H20, HDO, and D2O species, respectively, while the observed deviations are only 152, 160, and 145 kHz. Thus, the relative constancy of DJ among isotopes is surprising and is more reminiscent of true centrifugal distortion than of asymmetry. Although the use of DJ to determine an intermolecular force constant would be perilous in light of these observations, naive application of the usual formulaz6gives 0.14 mdyn/A. Though slightly larger than that for many weakly bound complexes, this value is not entirely unreasonable as the predicted binding energy of the complex also exceeds that of a typical van der Waals system. Further work will be needed to understand the true origin of DJ in this complex. Finally, Table 4 compares the results of this work with predictions from the previous two ab initio studies. Clearly, the most recent work by Hofmann and Schleyer is in excellent agreement with the experimental structure, and the agreement +
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TABLE 4: Comparison of Theory and Experiment for HzO-SO3 R(S-0)n U(S-O/C~)~ /?(OS-Oy binding energy source (A) (d%) (deg) (kcdmol) ref 11 2.027 154.8 98.2 21.4 ref 12 2.453 112 94.3 7.9 this work 2.432 103 92.6 Intermolecular S-0 bond length. Angle formed between the intermolecular SO bond and the CZaxis of the water. Angle formed between the SO bond in SO3 and the intermolecular S - 0 bond. thus lends support to their greatly reduced binding energy relative to that of previous reports. Further work is in progress to elucidate the potentially complex intemal dynamics which occur in this system and to further refine the experimental structure.
Conclusion The microwave spectrum of H20-SO3 has been observed by pulsed nozzle Fourier transform microwave spectroscopy using a source in which liquid water evaporates directly into a pulsed Sodargon expansion. The oxygen end of the water approaches the sulfur with a S--0 distance of 2.432 A and with bisector of the HOH angle at 103" with respect to the intermolecular bond. The SO3 is distorted only about 2-3" out of plane. These results are in good agreement with a recent ab initio study which places the binding energy of the complex at 7.9 kcaUmo1. Evidence of intemal motions is seen, and further work is in progress.
Acknowledgment. The support of the National Science Foundation (Grants CHE 9213635 and ATM 9322809) and the donors of the Petroleum Research Fund, administered by the American Chemical Society, is gratefully acknowledged. We are also indebted to Dr. Edward Campbell for the design and construction of the microwave spectrometer during his stay as a visiting scholar and to Dr. Tryggvi Emilsson and Professor Herb Gutowsky for generously sharing their technology and experience with us. We also thank Dr. Bruce Prezzavento and Mr. Bruce Moe of the Department of Chemistry at Minnesota for lending their electronics and computing expertise in the design of the computer interface. Note Added in Proof. Following submission of this manuscript, three new publications have appeared on the H20 SO3 r e a c t i ~ n , ~ ' and - ~ ~there is now evidence that the H20SO3 complex may react with H2O to form H2SO4, and/or that the latter is produced by the reaction of so3 with ( H z O ) ~ . ~ ~ . ~ ~ New ab initio calculations28of the structure of the H2O-SO3 complex are in good agreement with the results of this work.
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References and Notes (1) (a) Wayne, R. P. The Chemistry of the Atmosphere, 2nd ed.; Claredon Press: Oxford, 1991. (b) SO2, NO and NO2 Oxidation Mechanisms: Atmospheric Considerations; Calvert, J. G., Ed.; Acid Precipitation Series, Vol. 3 (Teasley, J. I., Series Editor); Butterworth: Boston, 1984. (c) Calvert, J. G.; L m s , A,; Kok, G. L.; Heikes, B. G.; Walega, J. G.; Lind, J.; Cantrell, C. A. Nature 1985,317,27. (d) Kellogg, W. W.; Cadle, R. D.; Allen, E. R.;L a z ~ s A. , L.; Martell, E. A. Science 1972, 175, 581. (e) Graedel, T. E. Rev. Geophys. Space Phys. 1977, 15, 421. (2) (a) Hofmann, D. J. Science 1990,248,996. (b) Langner, L.; Rodhe, H.; Crutzen, P. J.; Zimmerman, P. Nature 1992, 359, 712. (c) Charlson, R.J.; Vanderpol, A. H.; Covert, D. S.; Waggoner, A. P.; Ahlquist, N. C. A m o s . Environ. 1974, 8, 1257. (d) Parks, W. P.; Scolnick, E. M.; Kozikowski, E. H. Science 1974, 184, 156. (e) Turco, R. P.; Whitten, R. C.; Toon, 0. B. Rev. Geophys. Space Phys. 1982,20, 233. (3) (a) Solomon, S.;Garcia, R. R.; Rowland, F. S.; Wuebbles, D. J. Narure 1986, 321, 755. (b) Solomon, S. Rev. Geophys. 1988,26, 131. (c) Hoffman, D. J.; Solomon, S. J . Geophys. Res. 1988,94,5029. (d) Solomon, S. Nature 1990,347,347. (e) Toon, 0. B.; Hamill, P.; Turco, R.P.; Pinto,
Letters
504 J. Phys. Chem., Vol. 99, No. 2, 1995 J. Geophys. Res. Lett. 1986, 13, 1284. (f) Hamill, P.; Toon, 0. B.; Turco, R. P. Geophys. Res. Lett. 1986, 13, 1288. (g) McElroy, M. B.; Salawitch, R. J.; Wofsey, S. C. Geophys. Res. Lett. 1986, 13, 1296. (h) Molina, M. J.; Tso, T.-L.; Molina, L. T.; Wang, F. C.-Y. Science 1987, 238, 1253. (i) Tolbert, M. H.; Rossi, M. J.; Golden, D. M. Geophys. Res. Lett. 1988, 15, 847. (i) Wolff, E. W.; Mulvaney, R. Geophys. Res. Lett. 1991, 18, 1007. (4) (a) Margjtan, J. J. J. Phys. Chem. 1984, 88, 3314. (b) Wine, P. H.; Thompson, R. J.; Ravishankara, A. R.; Semmes, D. H.; Gump, C. A,; Torabi, A.; Nicovich, J. M. J . Phys. Chem. 1984, 88, 2093. (c) Stockwell, W. R.; Calvert, J. G.Atmos. Environ. 1983, 17, 2235. (d) Calvert, J. G.; Su, F.; Bottenheim, J. W.; Strausz, 0. P. Atmos. Environ. 1978, 12, 197. (5) (a) Gleason, J. F.; Sinha, A.; Howard, C. J. J . Phys. Chem. 1987, 91, 719 and references therein. (b) Gleason, J. F.; Howard, C. J. J. Phys. Chem. 1988, 92, 3414. (c) Martin, D.; Jourdain, J. L.; Le Bras, G. J. Phys. Chem. 1986, 90, 4143. (6) (a) Hofmann-Sievert, R.; Castleman, A. W., Jr. J . Phys. Chem. 1984, 88, 3329. (b) Wang, X.; Jin, Y. G.; Suto, M.; Lee, L. C.; O'Neal, H. E. J . Chem. Phys. 1988, 89, 4853. (7) Tso, T.-Y.; Lee, E. K. C. J. Phys. Chem. 1984, 88, 2776. (8) Bondybey, V. E.; English, J. H. J. Mol. Spectrosc. 1985,109, 221. (9) Schriver, L.; Carrere, D.; Schriver, A.; Jaeger, K. Chem. Phys. Lett. 1991, 181, 505. (10) Holland, P. M.; Castleman, A. W., Jr. Chem. Phys. Lett. 1978, 56, 511. (1 1) Chen, T. S.; Moore Plummer, P. L. J. Am. Chem. SOC.1985, 89, 3689. (12) Hofmann, M.; von R. Schleyer, P. J. Am. Chem. SOC. 1994, 116, 4947. (13) Balle, T. J.; Flygare, W. H. Rev. Sci. lnstrum. 1981, 52, 33. (14) Chuang, C.; Hawley, C. J.; Emilsson, T.; Gutowsky, H. S. Rev. Sci. lnstrum. 1990, 61, 1629. (15) (a) Rego, C. A.; Batten, R. C.; Legon, A. C. J . Chem. Phys. 1988, 89, 696. (b) Legon, A. C.; Wallwork, A. L.; Rego, C. A. J. Chem. Phys. 1990, 92,6397. (c) Lovas, F. J.; Suenram, R. D.; Fraser, G. T.; Gillies, C. W.; Zozom, J. J. Chem. Phys. 1988,88, 722. (d) Gutowsky, H. S.; Chen, J.; Hajduk, P. J.; Keen, J. D.; Emilsson, T. J . Am. Chem. SOC. 1989, 11 1, 1901. (e) Emilsson, T.; Klots, T. D.; Ruoff, R. S.; Gutowsky, H. S. J . Chem. Phys. 1990, 93, 6971. (16) Matsumura, K.; Lovas, F. J.; Suenram, R. D. J . Chem. Phys. 1989, 91, 5887.
(17) Kuczkowski, R. L.; Suenram, R. D.; Lovas, F. J. J . Am. Chem. SOC. 1981, 103, 2561.
(18) Gordy, W.; Cook, R. L. Microwave Molecular Spectra;Wiley: New
York, 1970. (19) Kaldor, A,; Maki, A. G. J . Mol. Struct. 1973, 15, 123. (20) Harmony, M. D.; Laurie, V. W.; Kuczkowski, R. L.; Schwendeman, R. H.; Ramsay, D. A,; Lovas, F. J.; Lafferty, W. J.; Maki, A. G. J . Phys. Chem. Ret Data 1979, 8, 619. (21) These choices are fully consistent with the ab initio results of ref 12. Also, we note that the S-0 single bond distance in H ~ S O Jis 1.422(10) A [ref 171, while the so3 bond length is 1.4198 8, [ref 191. Thus, whether the complex is truly weakly bound with no distortion of monomer structures or strongly bound with a fully formed SO bond as in sulfuric acid, the S=O distances on the so3 should be essentially the same. Similarly, the 0 - H distances in HzO and H2S04 are 0.965 A [ref 201 and 0.97 A [ref 171, respectively, and the choice is of little consequence. (22) Dvorak, M. A,; Ford, R. S.; Suenram, R. D.; Lovas, F. J.; Leopold, K. R. J . Am. Chem. SOC. 1992, 114, 108. (23) Reeve, S. W.; Bums, W. A.; Lovas, F. J.; Suenram, R. D.; Leopold, K. R. J . Phys. Chem. 1993, 97, 10630. (24) The sensitivity of the reported structure to assumptions made in the analysis was checked. If the H20 or so3 bond lengths are changed to their values in HzS04, or if the HOH angle in water is changed by even as much as 5", the structure of the complex remains the same to within the uncertainties given. Only the zero-point oscillation amplitude of the SO3 potentially introduces uncertainties beyond those quoted, with a 10" average angle between the C3 axis of the SO3 and the intermolecular S - 0 bond reducing R(S-0) by 0.03 A, a(S-O/C2) by 3S0, and /?(OS-0) by 0.6". (25) Thaddeus, P.; Krisher, L. C.; Loubser, H. N. J . Chem. Phys. 1964, 40, 257. (26) Aldrich, P. D.; Kukolich, S. G.; Campbell, E. J. J. Chem. Phys. 1983, 78, 3521. (27) Kolb, C. E.; Jayne, J. T.; Worsnop, D. R.; Molina, M. J.; Meads, R. F.; Viggiano, A. J. Am. Chem. SOC. 1994, 116, 10314. (28) Morokuma, K.; Muguruma, C. J.Am. Chem. SOC.1994,116, 10316. (29) Reiner, T.; h o l d , F. J . Chem. Phys. 1994, 101, 7399. Jp9427894
