J. Phys. Chem. 1985,89, 3955-3960 Although branching ratio data do not provide direct evidence for the well depth for intermediate A in Figure 1, the failure to observe a metastable complex corresponding to A allows us to place an upper limit on the well depth. Rate constants for typical vibrational frequencies indicate that, for a well depth greater than 2.0 eV, the lifetime of the metastable complex is comparable to the flight time through our detector, about 60 ps. Thus, 2.0 eV represents an upper bound on the dissociation energy of Li((CH3)3COH)+. For purposes of comparison, the dissociation energy of Li(CH30H)+ is 1.65 eV, reported by Staley and Beauchamp.I5 (15) R. H. Staley and J. L. Beauchamp, J . Am. Chem. Soc., 97, 5920 (1975).
3955
The experimental measurements reported here with their accompanying calculations provide an example of how branching ratios for nonreactively scattered reagents relative to reaction products yield relatively precise information on the energetics of the isomerization barrier early on the reaction coordinate. We are presently working on other reactive systems of this type. More detailed results on this and other systems, including analysis of center-of-mass energy distributions, will be forthcoming. Acknowledgment. We thank the United States Department of Energy for support of this research. W.R.C. thanks the University of Rochester for a Sherman Clarke Fellowship and an Elon Huntington Hooker Foundation Fellowship. Registry No. Li+, 17341-24-1; tert-butyl alcohol, 75-65-0.
Ion Pairs from Electron Donor-Acceptor Systems: A Comparison between Picosecond and CIDNP Studies P. M. Rentzepis,* D. W. Steyert, H. D. Roth,* and C. J. Abelt AT& T Bell Laboratories, Murray Hill, New Jersey 07974 (Received: May 20, 19851
Electron donor-acceptor (EDA) interactions play an important role in many biological, chemical, and physical processes. To gain insight into the principal reaction intermediates and the mechanisms of product formation, a series of model reactions were studied. For example, the photoreactions of an electron donor-acceptor complex between 1,l-dimethylindene (DMI) and tetracyanoethylene (TCNE) and a donor acceptor pair, DMI/chloranil (CA), have been studied by picosecond and CIDNP spectroscopy. It was found that immediately after excitation of the EDA complex the corresponding ion radicals, DMI+. and TCNE-., are generated and decay within 50 ps. The decay of these transients is followed by a bleaching of the EDA complex which persists for -500 ps. The nature of this process is discussed. In contrast, the irradiation of the system DMI/CA at 532 nm gives rise to excited singlet CA, which undergoes rapid intersystem crossing to the triplet state, before it is quenched by electron transfer. The resulting ion radical pair, DMI+./CA--, reacts by reverse electron transfer and by coupling to form two adducts. Adduct B is formed first, but is later depleted in favor of adduct A.
STYPE
Organic photoreactions occur quite frequently by the transfer of an electron from an electron-rich donor (D) to an electrondeficient acceptor (A). These electron-transfer processes often proceed via electron donor-acceptor (EDA) complexes which promote coupling of the interacting molecules and thereby influence the stereochemistry of the reaction products. Most EDA complexes exhibit absorption in the visible or near-infrared due to charge transfer (CT) from the highest occupied molecular orbital of the donor to the lowest unoccupied molecular orbital of the acceptor.' Many EDA complexes are present in high equilibrium concentrations while others are only transient intermediabs2 In view of the importance of EDA complexes and electron-transfer processes in many reactions, we have investigated several EDA systems using various experimental approaches in order to assess the involvement of radical ion pairs, to identify the mechanism of their formation and decay, and to determine their importance in EDA complex photochemistry. Time-resolved picosecond spectroscopy has established that photoexcitation of EDA complexes in the C T band result in the formation of short-lived radical ion For example, ex(1) Mulliken, R. S. J . Am. Chem. SOC.1952, 74, 811-824. (2) Rehm, D.; Weller, A. Ber. Bumenges. Phys. Chem. 1%9,73,834-839.
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B
citation of the tetracyanoethylene (TCNE)/9-~yanoanthracene (9-CNA) complex gives rise to the TCNE--9-CNA+ ion pair, which has a lifetime of 75 ps and decays by back electron transfer, regenerating the original EDA c o m p l e ~ .In ~ contrast, the radical ion pair formed by excitation of the TCNE/indene (IN) complex in the CT band decays within -60 ps, whereas the original EDA complex is regenerated only after 500 ps'. These results suggest the involvement of a transient intermediate which does not absorb in the 400-770-nm range. Biradical or zwitterionic coupling products of the radical ions and an EDA complex with a nonequilibrium geometry have been discussed as possible structures for this intermediate. In addition, the involvement of the indenyl radical must be considered. This species could be generated by proton transfer from the radical cation to an adventitious base. In an attempt to elucidate further the mechanism underlying this reaction and the nature of the unidentified intermediate, we have investigated the photochemistry of the TCNE complex with
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(3) Hilinski, E. F.; Masnovi, J. M.; Kffihi, J. K.; Rentzepis, P. M. J . Am. Chem. Soc. 1984. 106. 8071-8077. (4) Hilinski, E. F.; Masnovi, J. M.; Amatore, C.; Kochi, J. K.; Rentzepis, P. M. J . Am. Chem. SOC.1983, 105, 6167-6168. (5) Hilinski, E. F.; Huffman, J. C.; Kochi, J. K.; Rentzepis, P. M. Chem. P h p . Lett. 1984, 106, 20-25.
0 1985 American Chemical Society
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The Journal of Physical Chemistry, Vol. 89, No. 19, 1985
Letters tetracyanoethylene (Aldrich) was purified by repeated sublimation. The optical experiments were carried out in acetonitrile (Omnisol), which was used without further purification.
0.8
w
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06
a m U
a
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02
I
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I
400
500
600
700
Figure 1. Absorption spectra of indene/TCNE (---) indene/TCNE (-).
and dimethyl-
0.01
I
Picosecond Studies The charge-transfer absorption spectra of the TCNE complexes with indene and dimethylindene are shown in Figure 1. Both species show two bands, separated by -500 cm-' (-0.65 eV), which have been assigned to different ground-state CT conformers. The slight shift in the absorption maxima (-8 nm) of the dimethylindene relative to the indene complex is compatible with a minor change in oxidation potential (-50 mV). Although most factors influencing the intensities of the CT bands are difficult to assign accurately, it appears safe to conclude that the minor changes in relative intensities suggest similar populations of the conformers for the two donors. C T complexes of structures A, B, and C have been considered for the pair IN/TCNE. Because of the steric hindrance due to the geminal methyl groups structure C should be disfavored in the system DMI/TCNE.
WAVELENGTH (nm)
1,I-dimethylindene (DMI) and the reactions of tetrachloro-pbenzoquinone (chloranil, CA), irradiated at 532 nm, with indene and dimethylindene. Chloranil has a short-lived excited singlet state and undergoes intersystem crossing before it can be quenched by electron Accordingly, the radical ion pairs are formed in the triplet state and require intersystem crossing before they can undergo back electron transfer. In addition, the tetrachlorosemiquinone radical anion, the one-electron reduction product of chloranil, should be a stronger base than the T C N E radical anion. Therefore, it should deprotonate the indene radical cation more readily. On the other hand, DMI lacks acidic protons and, thus, cannot give rise to an indenyl species. Accordingly, these experiments can be expected to shed light on the involvement of the indenyl radical in the reactions of I N with TCNE and CA. Finally, we carried out nuclear spin polarization (CIDNP) experiments in these systems. The CIDNP method is based on the interpretation of greatly enhanced N M R signals in either absorption or emission, which may be observed during reactions proceeding via pairs of radicals or radical ions. The signal intensities and directions of the observed effects reflect the spin density distributions of these intermediates.8
Experimental Section The picosecond laser system used to record the transient absorption data was the same as that used in the previous TCNE/indene ~ t u d i e s . Excitation ~ of the EDA complex was achieved by a 532-nm pulse of 20-ps duration, generated by doubling the 1065-nm Nd3+:YAG pulse in a KDP crystal. The components used for the optical detection and decay measurement, including picosecond continuum, camera, vidicon, computer, and the analysis of the data were the same as described previously. The cw electronic absorption spectra were recorded on a Cary 15M spectrophotometer. Typical samples used for the CIDNP experiments were acetoned6 (Aldrich, Gold Label, 99%) solutions containing 0.02 M each of an electron-acceptor (chloranil, Eastman Organics) and an electron-donor hydrocarbon. These samples were purged with argon for 2 min and irradiated with the collimated beam of an Osram 200-W high-pressure mercury lamp in the probe of a Bruker WH90 Fourier transform N M R spectrometer. A pulse angle of 90° was employed to minimize the acquisition time for the spectra (typically 100 s for 25 pulses). 1,l-Dimethylindene (Wiley Organics) was purified by gas chromatography, whereas (6) Hilinski, E. F.; Milton, S. V.; Rentzepis, P. M. J . Am. Chem. Sor. 1983, 105, 5193-5196. (7) Delcourt, M. 0.;Rossi, M. J., personal communication to P.M.R. (8) Roth, H. D. In 'Chemicaily Induced Magnetic Polarization", Muus, L. T., et al., Ed.; Reidel: Dordrecht, The Netherlands, 1977; pp 39-76.
A
C
B
Selective excitation of the C T band is readily achieved because the spectra of the reactants and of the complex are well separated; at the excitation wavelength (532 nm) neither T C N E nor the indenes have appreciable absorption. Figure 2 shows the absorption spectra between 400 and 650 nm in units of AA (change in absorbance) at different time intervals before and after excitation. The regions of 430-450 and 520-540 nm are obscured by the notch filter regions used to eliminate the scattered 532-nm excitation light. Upon excitation, these spectra (Figure 2) show an immediate rise in absorption between 405 and 500 nm. This band is replaced within the 25-ps excitation pulse by two distinct broad bands which reach maximum intensities 15 ps after excitation without any apparent bleaching. The lower wavelength band at 400-450 nm is typically formed after excitation of TCNE charge transfer complexes and is assigned to the acceptor radical anion, TCNE--. The higher wavelength band at -600 nm is assigned to the 2D *Dotransition of the donor radical cation, DMI'., in analogy to a similar band observed in the I N / T C N E ~ y s t e m .This ~ assignment is further supported by the similarity with the radical cation spectrum observed for s t y r e i ~ e . ~ Within 50 ps of the excitation pulse the two absorption maxima decay to the original base line. However, as in the case of the IN/TCNE system the decay of the transient absorptions is accompanied by bleaching in the 530-nm region, which persists much longer than the transients assigned to the radical ions. It is obvious, therefore, that the reaction regenerating the ground state C T complex is not a simple back electron transfer but involves at least one additional intermediate as in the IN/TCNE case. These results are compatible with the following simple reaction scheme: DMI + T C N E + [DMI-TCNE]
-
[DMIsTCNE]
hv
[DMI+.TCNE-.] intermediate
[DMI+-TCNE-*]
-
4
intermediate
[DMI-TCNE]
The nature of the intermediates is not clear. We note that the bleaching observed for the DMI/TCNE complex is less pronounced than that observed for the IN/TCNE complex. This observation is in qualitative agreement with the involvement of biradical or zwitterionic adducts. The geminal methyl groups of (9) Shida, T.;Hammil, W. H. J . Chem. Phys. 1966, 44, 4372-4377.
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985 3957
Letters
it
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WAVELENGTH ( n m 1 Figure 2. Difference absorption spectra recorded between -8 and 50 ps after excitation of TCNE/DMI solution with a 532-nm 20-ps pulse.
the DMI radical cation are expected to interfere with coupling in the @-positionwhich is favored by electronic factors since it gives rise to more delocalized bifunctional intermediates. Nevertheless, it appears that the reactions of the I N / T C N E and DMI/TCNE complexes proceed via principally similar pathways. This conclusion tends to rule out the involvement of the idenyl radical in the reactions of I N with T C N E and C A and may be considered indirect evidence for biradical or zwitterionic intermediates.
These studies were extended to electron transfer in the system DMI/chloranil (CA). These reactants do not form a strongly absorbing EDA complex, so that the acceptor is the principal absorbing species. Previous experimental evidence by means of picosecond6 and nanosecond' absorption spectroscopy of chloranil/arene in polar solutions has shown that excitation at the 355-nm ?r* n transition of chloranil results in the population of the triplet state via intersystem crossing. Picosecond experiments have allowed us to monitor the kinetics of this process and to completely determine its dynamics. Addition of naphthalene or indene led to the depletion of the triplet state via diffusioncontrolled electron transfer6 which generates the corresponding radical ion pairs [Np+-/CA--] or [IN+./CA-.]. In the case of naphthalene, the formation of a dimer ion radical (Np2+.) was
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Letters
3958 The Journal of Physical Chemistry, Vol. 89, No. 19, 1985
F r
also observed,6 but in the case of indene, no dimer ion radical spectra were recorded under similar experimental conditions. The system DMI/CA in acetonitrile exhibited the same behavior after excitation with a 355-nm picosecond pulse as the IN/CA system. Based on the picosecond data, the electron transfer reaction from DMI to CA proceeds via the following simple scheme: CA
1
1
-25pS
-.k'CA*
'CA* 3CA
I
+ DMI
isc
3CA
CA-.DMI+*
The absorption maxima of the transient species observed for the reaction of DMI with CA are practically identical with those observed for the reaction of IN with CA, namely 3CA* T, CA-. DMI'.
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D,-Do D,
ICA* S,-S1
TI -500 nm -430nm
Do -590 nm -460 nm
This existence of unrelaxed excited-state complexes is not supported by our results. However, neither DMI/CA nor IN/CA appear to be appropriate systems to detect these spectra, since we cannot exclude that unrelaxed excited state or discrete encounter complexes are masked by the triplet-triplet absorption of CA. In addition, chloranil is sufficiently insoluble in less polar solvents to preclude the detection of the longer-lived exciplex.2
W
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w Nuclear Spin Polarization Effects The second approach employed in the study of these EDA systems involves chemically induced dynamic nuclear polarization (CIDNP). This method is based on transient enhanced N M R signals which may be observed during radical pair or radical ion pair reactions if their lifetimes fall into the nanosecond range. Accordingly, this method frequently provides information about radical ion pairs generated by electron transfer to an excited triplet On the other hand, it often fails to give results for EDA photoreactions involving excited singlet states, because the resulting radical ions are too short-lived. Based on these considerations, it does not come as a surprise that no CIDNP effects are observed during the irradiation of either the I N / T C N E or the DMI/TCNE systems. Indeed, these experiments failed to provde any evidence for longer-lived radical ion pairs, even as a minor reaction pathway. The lack of results is compatible with the mechanism proposed above, but may not necessarily be considered additional support for it. In this context, we note that the failure to observe any CIDNP effects for the photoreactions of the IN/TCNE and DMI/TCNE pairs does not rule out the biradical adducts as intermediates. Although nuclear spin polarization has been observed in some biradical reaction^,^^,'^ there is a critical chain length requirement, and 1,4-biradicals cannot be expected to give rise to CIDNP effects. In contrast, the photoreactions of chloranil with both I N and DMI result in strong CIDNP effects which contribute significantly to the understanding of the reaction mechanism. During the irradiation, the aromatic and the olefinic signals of I N (Figure (10) Roth, H. D.; Schilling, M. L. M. Can. J . Chem. 1983,61, 1027-1035. ( 1 1 ) Roth, H. D.; Schilling, M. L. M. J . Am. Chem. SOC.1985, 107, 7 16-7 18. (12) Abelt, C. J.; Roth, H. D. J . Am. Chem. SOC.1985,107, 3840-3843. (13) Abelt, C. J.; Roth, H. D.; Schilling, M. L. M. J . Am. Chem. SOC. 1985, 107, 4148-4152. (14) Miyashi, T.; Takahashi, Y . ;Mukai, T.; Roth, H. D.; Schilling, M. L. M. J. Am. Chem. SOC.1985, 107, 1079-1080. (15) Kaptein, R.; Freeman, R.; Hill, H. D. W. Chem. Phys. Lett. 1984, 26, 104-107. (16) Closs, G. L . In 'Chemically Induced Magnetic Polarization",Muus, L. T., et al., Ed.; Reidel: Dordrecht, The Netherlands, 1977; pp 225-257.
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.ttt-+-+tct+-++++-+ +-+-++-
4
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/ . A , 500 WAVELENGTH
j
j
LA.
600
(nm)
Figure 3. Difference absorption spectra of DMI/chloranil monitored between -25 and 250 ps after excitation with a 20-ps pulse.
4) and DMI (Figure 5 ) showed enhanced NMR absorption. These results are compatible with the intermediacy of radical ion pairs; the observed signal direction is typical for reverse electron transfer
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985 3959
Letters
A
,
1
1
1
1
1
1
1
I
I
I
1
'
I
~
~
1
~
~
7.0 6.O 5.0 Figure 4. 'H NMR spectra of a 0.02 M solution each of chloranil and indene in acetone-d6in the dark (bottom) and during UV irradiation
(top). in a geminate pair. More importantly, the results provide an insight into the spin density distribution of the indene and dimethylindene radical cations. In both cases, the olefinic protons in the 2-position (p) are considerably more strongly enhanced than the olefinic protons in the 3-position ( a )or the aromatic protons. Therefore, the principal spin density of the indene radical cations, IN+-and DMI+., must be located at the @-carbons. Similar results were observed previously for reactions involving the cis- and trans- 1-phenylpropene radical cations, and similar spin density distributions were derived for this pair of intermediates." (17) Roth, H. D.; Schilling, M. L. M. J . Am. Chem. SOC.1980, 102, 4303-4310. Roth, H. D.; Schilling, M. L. M.J . Am. Chem. SOC.1979, 101, 1898-1900.
~
'
'
~
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~
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"
'
'
~
~
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'
'
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~
70 6 .O 5.0 Figure~ 5. IHl NMR spectra of a 0.02~ M solution each of chloranil and ~ ~ ' l ' ~ dimethylindene in acetone-d6in the dark (bottom) and during UV irradiation (top).
In addition to the polarized reactants, the CIDNP spectra also indicate the formation of reaction products. The photoreactions of chloranil with both IN and DMI give rise to a pair of emission features which are compatible with the formation of cycloadducts between the two reactants. Based on their N M R chemical shifts and coupling patterns and on the analogy with the cycloadducts between butadiene and benzoquinone'* and between methylenecyclopropane derivatives and ~ h l o r a n i l , 'these ~ products are identified as [2+2] cycloadducts of structure B. In these adducts a carbonyl oxygen of chloranil is connected to the B-position of the indenes. The observed signal directions (Figures 4 and 5) indicate that these products are formed from "free" radical cations, IN+. and DMI'., respectively, which have lost their spin correlation with the semiquinone anions. Interestingly, the DMI adduct of (18) Barltrop, J. A.; Hesp, B. J . Chem. SOC.1965, 5182-5188.
'
~
~
"
'
J . Phys. Chem. 1985,89, 3960-3962
3960
structure B can be observed in low yield during the early stages of the photoreaction. However, it is consumed as the irradiation is continued and in its place a second [2+2] adduct is built up. In this adduct, which has been isolated and characterized, a carbonyl oxygen of CA is connected to the a-carbon of DMI (structure A). The formation of two different [2+2] adducts and their different rates of buildup and decay are governed by several competitive reaction parameters. The spin density distribution of the indene radical cation favors coupling in the P-position, which ultimately leads to adduct B, over reaction in the a-position, which will product adduct A. This trend is further supported by the stabilities of the resulting singly linked bifunctional intermediates. On the other hand, the oxidation of adduct B may lead to a ring-opened radical cation and, ultimately, to fragmentation, whereas oxidation of adduct A is unlikely to lead to either ring opening or fragmentation. The assignment of ring-opened structures to the adduct radical cations is supported by the analogous intermediates in the electron-transfer-induced ring opening of DMI dimers.19 The existence of structures, which may undergo ring closure or suffer fragmentation, sets a precedent for understanding the bleaching (19) Roth, H . D.; Schilling, M. L. M. J . Am. Chem. SOC.1981, 103, 721 0-721 7 .
observed in the IN/TCNE and DMI/TCNE reactions. Although excitation of the C T bands in these systems does not lead to [2+2] adducts, singly linked intermediates with a limited lifetime may well be formed from the reactant radical ion. Since these species contain relatively localized a-systems, their involvement may account for the bleaching observed in the 570-nm region. Conclusion The different techniques applied to the C T systems discussed here probe different aspects of their chemistry. The picosecond experiments follow the optical excitation and deexcitation of the principal absorbing species and provide clear-cut evidence for the intermediacy of radical ions. However, because of the relatively low quantum yields, picosecond experiments cannot probe the mechanism of product formation. The nculear spin polarization studies, on the other hand, fail to give any information concerning the fast physical processes, but they do provide insight into the structure of the indene radical cations IN+. and DMI+-, and reveal the nature of the primary CA adduct. Because of the substantial enhancements typical for this method, the product formation can be probed despite the low quantum yields. Finally, product studies reveal the early buildup and the ready depletion of the primary CA adduct (type B) and the accumulation of the second one (type A). Altogether, the combination of techniques employed here reveals different facets of a complex system.
Torsional Dynamics of 9-Carbonyl Substituted Anthracenes V. Swayambunatban+ and E. C. Lim* Department of Chemistry, Wayne State University, Detroit, Michigan 48202 (Received: June 18, 1985)
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It is shown that So Sl(m*)excitation of 9-carbonyl substituted anthracenes leads to a geometry change from nearly perpendicular to nearly coplanar form.
Introduction Conformation of carbonyl-substituted anthracenes is determined by the balance between resonance interaction (between the anthracene ring and the carbonyl group) which favors a coplanar structure and the steric effect which favors a nonplanar structure. For substitution at the 9-position, the steric hindrance by the peri-hydrogens is so large, relative to the resonance stabilization, that the carbonyl group cannot lie coplanar with the anthracene ring in the ground electronic state (So) of the molecule. In solution at room temperature, the angle between the plane of the carbonyl group and that of the anthracene ring has been reported to vary from close to 25" in So 9-anthraldehyde to approximately 80" in So 9-acetylanthracene.' Electronic excitation from So to the lowest excited singlet state (S,) of m*character is expected to increase the resonance interaction2 and hence favor the attainment of more coplanar structure. Although the lack of the mirror image relationship between absorption and fluorescence in 9-anthroic acid and its esters has been presented as evidence for the conformational no conclusive spectroscopic data exists concerning the geometry change between So and SI. In this Letter we report the results of our recent work on the fluorescence of 9-carbonylanthracene in solution, as well as in supersonic free jet, which conclusively establish the occurrence of a large conformational change along the torsional coordinates. Lubrizol Foundation Predoctoral Fellow, 1983-84, and Wayne State University Graduate, Fellow, 1984-present.
0022-3654/85/2089-3960$01.50/0
Experimental Section The laser-induced fluorescence spectra of jet-cooled molecules were obtained with a continuous free-jet apparatus described earlier.5 A quartz nozzle with a -0.2-mm-diameter orifice was used for the supersonic expansion. The sample, contained in the nozzle chamber, was heated (to 120-180 "C) and seeded in -200 torr of argon. The fluorescence was generated with the output (-0.6 cm-' fwhm) of a Nd:YAG-pumped dye laser, which crossed the jet 10 mm from the nozzle. The fluorescence excitation spectra were obtained by scanning the laser while monitoring the total fluorescence through a sharp cutoff filter. The dispersed fluorescence spectra were measured by using 0.22-m double monochromator with the spectral resolution of about 18 A. The fluorescence decays in solution were measured by timecorrelated photon counting, using a cavity-dumped dye laser which was synchronously pumped by a mode-locked argon ion laser. The dispersed fluorescence and the fluorescence excitation spectra in condensed phase were recorded with an Aminco SPF-500 spec-
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(1) R. J. W. LeFebre, L. Radom, and G. L. D. Ritchie, J . Chem. Soc. B, 775 (1968). and references herein. (2) See, for example, H. Suzuki, *Electronic Absorption Spectra and the Geometry of Organic Molecules", Academic Press, New York, 1967. (3) T. C. Werner and D. M. Hercules, J . Phys. Chem., 73,2005 (1969); T. C. Werner and R. M. Hoffman, J . Phys. Chem., 77, 161 1 (1973). (4) S. Suzuki, T. Fujii, N. Yshiike, S. Komatsu, and T. Iida, BUN. Chem. SOC.Jpn., 51, 2460 (1978). (5) H. Saigusa and E. C. Lim, J. Chem. Phys., 78, 91 (1983).
0 1985 American Chemical Society
