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some, or even all, structures might be observed. In addition, the lowest energy conformer might not be the most abundant, or the most reactive with C2H2or C4H2,to form higher molecular mass precursors of soot. Indeed, as mentioned earlier, the most stable cyclic form is relatively unreactive with C2H2and C4H2,much less so then the propargylium f ~ r m . ~ ? ~ In attempts to locate various species present in flames, or in models of these flames simulated in ion cyclotron resonance exp e r i m e n t ~ ,we ~ have calculated IR frequencies and electronic transition energies. Especially of interest are those species with low-lying allowed electronic transitions below about 40 000 cm-I. Such states might be excited through modern laser techniques and observed through laser-induced fluorescence or through ex-
citation of those ions using afterglow techniques. Above 40 000 cm-' interference from other species, especially aromatics, might be expected. Acknowledgment. This work was sponsored in part through a research grant from the United States Air Force (Tyndall). This research was also supported in part by the Florida State University through time granted on its Cyber 205 supercomputer, and through a computer grant through the Office of Naval Research. We particularly thank John Eyler (Florida) and Lt. Floyd Wiseman (Tyndall) for useful and stimulating discussion. Registry No. I, 26810-74-2; 11, 21540-27-2; 111, 24858-94-4; IV, 57358-45-9;V, 117067-78-4.
'
Photophysical Consequences of External Perturbations on the Emissive (n,,3s) Rydberg State of Saturated Amines Arthur M. Halpern* and Ali Taaghol Department of Chemistry, Northeastern University, Boston, Massachusetts 021 15 (Received: April 25, 1988; In Final Form: July 6. 1988)
Absorptive and emissive transitions between the ground and lowest excited 3s Rydberg state of the saturated amine 1azabicyclo[2.2.2]octane (ABCO) were studied as a function of collisional perturbations by He, Ar, Xe, SF6,dimethyl ether (DME), and methylcyclohexane (MCH). The fluorescence lifetime of ABCO decreases sharply between 0 and ca. 50 Torr of perturber pressure and levels off at several hundred Torr. The fluorescence quantum efficiency also shows an abrupt decrease followed by a leveling off to a constant value well below 1 atm. Absorption spectra of ABCO are observed to be generally unaffected by the presence of these perturbers up to 1 atm. No evidence is found for an external heavy-atom effect in Xe quenching. SF6 quenches ABCO fluorescence at the gas kinetic rate. These effects are interpreted with a model in which excited ABCO and the perturber reversibly form a loosely bound, emissive excited complex. This model includes a step describing quenching collisions between excited ABCO and a perturber. Equilibrium constants for the excited complexes are found to be 25.4, 27.1, 34.9, 27.2, and 45.6 atm-' for the respective perturbers. Dissociation rate constants are estimated from gas kinetic formation rate constants. The data show that the most significant change in the ABCO*-perturber systems occurs for the nonradiative decay channel, suggested to be an electron-transfer-inducedinternal conversion. There are slight increases in the radiative rate constants of the ABCO*-noble gas complexes, while little change is noted for the DME and MCH complexes. The nonradiative rate constant for the ABCO*-MCH complex is about 0.44 that of ABCO* in cyclohexane solution; the 1/ n 2 normalized radiative rate constants are virtually identical. The lifetime of trimethylamine was found to be nearly independent of perturber pressure (He or 2-methylbutane) up to 1 atm. This insensitivity is interpreted as steric interference in forming the complex between the planar excited state and the perturber. The results suggest that the (nN,3s) Rydberg state of amines may have appreciable conjugate valence character.
Introduction The influence of the medium on the potential energy hypersurfaces of ground and electronically excited molecules and, hence, the effects of environment on absorption and emission spectra have been of fundamental concern in molecular spectroscopy for many years. The solvent medium often plays a determining role, for example, in the relative ordering of the h a * ) and (a,**)states, both singlet and triplet, in N-heteroaromatic molecules.' In another context, currently receiving wide attention, molecules capable of achieving appreciable excited-state charge-transfer character consequent to conformational changes manifest profound solvent effects on emissive properties2 In this instance, the solvolytic properties of the excited molecule must be considered in the time domain because electronic state energetics depend on the dynamical coordination of both molecular conformations and solvent reorganization. In these examples of medium effects on
molecular spectroscopy and photophysics, electrostatic, hence energetic, consequences of particular molecular electronic states and their geometries can be considered in terms of intravalence excitations, Le., electronic configurations involving electrons within the valence shell (e.g., n = 2 for first-row elements). In considering extravalence, or Rydberg, states whose description requires the inclusion of atomic orbitals of higher principal quantum numbers, another sort of consideration must be made regarding the energetics of environmental interaction^.^ This issue stems from the much higher polarizability of excited Rydberg states as compared with intravalence states. For example, the polarizibility of the (n,3s) state of acetone has been estimated to be 450 A3; this contrasts with a ground-state value of 6.4 A3.4 In other cases where the polarizibilities of Rydberg states have been determined, values of several hundred cubic angstroms are found (for states terminating in 3s orbitals) and reflect the large
(1) (a) Liptay, W. Angew. Chem., Int. Ed. Engl. 1%9,8, 177. (b) Amos, A. T.;Burrows, B. L. Adu. Quantum Chem. 1973, 7, 289. (2) (a) Grabowski, Z. R.; Rotkiewicz, K.;Siemiarczuk, A.; Cowley, D. J.; Gaumann, W. N o w . J. Chim. 1979, 3,443. (b) Rettig, W. Angew. Chem., I n t . Ed. Engl. 1986, 25, 971.
(3) (a) Robin, M. B. Higher Excited States of Polyatomic Molecules; Academic: New York, 1974; Vol. 1, pp 208-229. (b) Runau, R.; Peyerimhoff, S. D.; Buenker, R. J. J. Mol. Spectrosc. 1977,68, 253; Avouris, P.; Rossi, A. J. Phys. Chem. 1981, 85, 2340. (4) Causley, G.C.; Russell, B. R. J. Am. Chem. SOC.1979, 101, 5573.
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I(nN,3s) Rydberg State of Saturated Amines spatial character of Rydberg statesas Accordingly, one would anticipate that the nature and magnitude of the interaction of such states with the environment would reflect the diffuseness of the Rydberg orbital. In fact, as Robin and -workers and others have shown, the susceptibility of an electronic absorption spectrum to the application of high pressures of a perturbing gas, Le., broadening and possibility shifting, is a hallmark of a Rydberg transition.6 Although intravalence transitions also manifest external perturbation effects, they are usually weaker, thus requiring considerably higher pressures for their observation.' In this paper we focus our attention on saturated tertiary amines whose lowest electronically excited states have been assigned as (n&) Rydberg.8 Generally, the absorption spectra of these compounds reflect very similar consequences of solvation as does the parent molecule, ammonia. Thus the vibrational structure that characterizes the 3s nN transition in these compounds is virtually obliterated in the condensed phase.g In the case of ammonia, the structureless absorption band seen in the condensed phase has a somewhat different Franck-Condon pattern relative to the envelope defined by the maxima of the vibronic features seen in the vapor.I0 Our strategy in choosing tertiary saturated amines is that these compounds are highly fluorescent, unlike secondary and primary amines (and ammonia). Thus we can study the effects of external perturbations not only on absorption spectra but also on fluorescence properties, e.g., radiative and nonradiative transition probabilities. We thus have the opportunity to correlate absorptive and emissive effects of external perturbations. In the latter case, we can directly probe interactions between the relaxed excited state and perturbing agents. Studies of this nature are relevant to the very nature of solvation because they allow one to examine certain properties of a solute molecule at the first critical step of solvation, namely, the interaction of just one, then several, solvent molecules with the target species.
position. In actual experiments, 10-20 min elapsed before measurements were taken. Absorption spectra were obtained with a Varian 2300 spectrophotometer with a band pass of 0.1 nm; under these conditions the sequence structure was only partially resolved. In measurement of fluorescence intensities, a dc fluorimeter with right-angle excitation-emission geometry was used. It was equipped with a 60-W D2 source coupled to a 0.25-m Ebert (Jarrell-Ash) monochromator (band pass 3.2 nm), and a 0.3-m Czerny-Turner (Heath) monochromator (band pass 4 nm) that dispersed the fluorescence. ABCO was excited at 255.6 nm, and fluorescence was monitored at 285 nm. For presentation purposes, the fluorescence spectrum was obtained with a 150-W Xe source; the excitation and emission band passes were 3.2 and 0.6 nm, respectively. Fluorescence decay was studied with a time-correlated single-photon apparatus. Excitation was achieved with a D2 flash lamp (0.5 atm) operating at 8 kV and gated (30 kHz) by an EG&G 7782/HY-6 thyratron. Because of the long time scale required in these experiments (range 1640 ns), a 20-pF, IO-kV ceramic capacitor was placed between the charging electrode and ground. This arrangement produced not only more intense light pulses (hence more signal), but also a somewhat broader lamp pulse (ca. 4-11s fwhm). The latter feature has the desirable effect of reducing the importance of the troublesome afterpulse (at ca. 630 ns) in the observed decay curve. Because of the very low absorbance of ABCO at the excitation wavelength used (255.6 nm), count rates were low, and acquisition time were typically >60 min. Fluorescence was viewed at right angles through an interference filter centered at 270 nm (band pass 8 nm, Corion). The detector was an Amperex 56DUVP/03, the photocathode of which was configured at ground potential. The anode was capacitively coupled to the power supply (+2300 V). Another consequence of the low sample absorbance is the presence of scattered light in the decay curves. This effect was satisfactorily compensated for by using a very short lived component in the reconvolution analysis.12 The fitting procedure required threeexponential components because of the presence of long-lived emission (see below). Fitting the data to various mathematical models and extracting the respective optimized parameters were performed with an RS/ 1, which was installed on a laboratory PC. As a check, a global analysis of two measureables (Le., decay constant and fluorescence intensity) vs the common variable (pressure) was carried out by using EUREKA. Collision diameters for dimethyl ether and methylcyclohexane (5.68 and 6.29 A, respectively), used in calculating gas kinetic rate constants, were obtained from gas viscosities measured by using the evacuation technique.I3
+
Experimental Section
l-Azabicyclo[2.2.2]otane(ABCO or quinuclidine) (Aldrich) was dissolved in 2-methylbutane; lithium aluminum hydride was added to remove water. After gas evolution ceased, the solvent was evaporated, and the amine was sublimed from the solid residue. The fluorescence cell used in lifetime studies was cylindrical, 10 cm X 3.00 cm diameter, and constructed entirely of Suprasil. This cell allowed somewhat higher signals to be recorded relative to a 1-cm square cell; moreover, when properly masked, it kept scattered light to an acceptable level. The cell was filled with ABCO from a submanifold that contained the solid amine at 263 K; under these conditions, the (vapor) pressure of ABCO is 0.13 Torr." At this pressure, no evidence of excimer emission was obtained (even when ABCO was collisionally relaxed with a buffer gas), and hence it was deemed to provide collision-free conditions for the excited-state amine. In the fluorescence intensity measurements, a 1-cm square cell containing a magnetic stirrer was used; stirring action was used to hasten mixing of the amine and added perturber gas. In other measurements without the stirrer (e.g., in the cylindrical cell), the course of mixing was followed by monitoring the ABCO fluorescence signal as a function of time. It was found that about 2 min was required to achieve constant signal level; the magnitude of this intensity was acceptably close to that observed in the mixed sample of an identical com( 5 ) (a) Altenloh, D. D.; Russell, B. R. J . Phys. Chem. 1982,86, 1960. (b) Altenloh, D. D.; Ashworth, L. R.; Russell, B. R. J . Phys. Chem. 1983, 87, 4348. (6) (a) Robin, M. B.; Kuebler, N. A. J . Mol. Spectrosc. 1970,33, 247. (b)
Miladi, M.; LeFalher, J.-P.; Roncin, J.-Y.; Damany, H. J . Mol. Spectrosc. 1975, 55, 8 1. (7) Babb, Jr., S.E.; Robinson, J. M.; Robertson, W. W. J . Chem. Phys. 1959, 30, 421. (8) Parker, D. H.; Avouris, Ph. J . Chem. Phys. 1979, 71, 1241. (9) (a) Muto, Y.; Nakato, Y.; Tsubomura, H. Chem. Phys. Left. 1972, 16, 72. (b) Halpern, A. M. J . Phys. Chem. 1981, 85, 1682. (10) (a) Ley, H.; Arends, B. Z . Phys. Chem. 1932, B17, 177. (b) Dressler, K. J . Chem. Phys. 1961, 35, 165; (c) Ziegler, L. D.; Roebber, J. L. Chem. Phys. Lett. 1988, 144, 305. (11) Brown, H. C.; Sujishi, S.,J . Am. Chem. SOC.1948, 70, 2878.
\
Results and Discussion We have chosen to confine this investigation to the bicyclic cage amine l-azabicyclo[2.2.2]octane(ABCO) rather than to a, perhaps, more simple compound, such as trimethylamine (TMA). The reason for this choice is that the absorption spectrum of TMA is very diffuse, owing to the strong coupling of methyl rotations with the out-of-plane bending motion, which is strongly excited in electronic tran~iti0ns.l~Evidently, ABCOs rigid cage structure, which eliminates internal rotational degrees of freedom, restores considerable vibronic character to the absorption and fluorescence spectra. (Broadening at ambient temperature arises from sequence transitions built up from a low-frequency cage torsional mode of ca. 65 cm-l.)15 Hence these features provide sensitive and definitive probes of collisional perturbation effects. The high vapor pressure of ABCO at ambient temperature (ca. 2 Torr at 300 K)," as well as its very high intrinsic fluorescence quantum efficiency,I6 (12) Frye, S.L.; KO,J.-J.; Halpern, A. M. Photochem. Photobiol. 1984, 40, 555. (1 3 ) Halpern, A. M.; Reeves, J. H. Experimental Physical Chemistry; Scott, Foresman-Little, Brown: Glenview, IL, 1988; pp 247-252. (14) Halpern, A. M.; Ziegler, L. D.; Ondrechen, M. J. J . Am. Chem. SOC. 1986, 108, 3907. (15) Gonohe, N.; Ygtsuda, N.; Mikami, N.; Ito, M. Bull. Chem. Sm. Jpn. 1982, 55, 2796.
Halpern and Taaghol
146 The Journal of Physical Chemistry, Vol. 93, No. 1, 1989
31
35
33
37
41
39
43
1 o3 WAVENUMBERS
Figure 1. Absorption and fluorescence spectra of ABCO vapor at 293 K. The 0-0 band is located at 39080 cm-I; the feature at 43 100 cm-’ is a hot band associated with the S2 So transition.
-
2CG
1
I
1
I
I
,
1 1
-
1
1
1
I
1
l--_--L
TIME
Figure 3. Fluorescence decay curves of ABCO, 0.13 Torr, at 293 K in the presence of (A) 0 and (B) 37 Torr of methylcyclohexane; the lamp profile is also shown. Excitation and analyzing wavelengths are 255.6 and 280 nm, respectively. Each major division is 200 ns. The smooth curves (drawn through the decay curves) are the optimized reconvolutions.
-
I o3 WAVENUMBERS
+
Figure 2. 0-0 band of the SI SOtransition in ABCO vapor at 293 K: (A) in the absence of perturber gas; (B) in the presence of 1 atm of Ar. The band pass is 0.1 nm.
also recommended this molecule for such studies. There are, however, several experimental drawbacks encountered in studying ABCO: (1) it undergoes excimer formation and emission and (2) it has a moderately long fluorescent lifetime in the zero-pressure limit (300 ns).16 The result of these complications is that one must work at a low enough ABCO partial pressure to prevent intermolecular ABCO associations from masking the effects of perturber-ABCO interactions. In these studies, we have kept the ABCO pressure below ca. 0.13 Torr so that the ABCO*-ABCO collision frequency is always less than decay rate of the excited state (see Experimental Section). The choice of perturbers is dictated by the requirements that they must be optically transparent in the near UV (below ca. 220 nm) and that they have high saturation pressures at ambient temperature. We also sought to use perturbers that span a range of desirable properties, such as polarizibility and polarity. In the study reported here, we used the noble gases He, Ar, and Xe, as well as SFs, methylcyclohexane, and dimethyl ether. One of the key findings of our investigation is that effects on the photophysical properties of ABCO brought about by collisional perturbation occur at surprisingly low pressures, Le., > kc furnishes the expression
+
hv,
A+P
and presence of methylcyclohexane vapor. Perturbation Effects. We observed that the addition of perturbers causes abrupt changes in the fluorescence lifetime and quantum intensity of ABCO. At relatively low pressures, e.g., the dynamic equilibrium condition discussed above. It is especially noteworthy that the fluorescence quantum efficiencies of the ABCO* perturber complexes, qc, are about one-third that of the free amine ( q A = 1.0). Collisionally induced increases in the fluorescence decay parameters of vapor-phase cage amines have been noted before.16 Here, we see that the decay constants of the complexes are roughly 3 times larger than kA. Thus we see that the primary effect of collisional perturbation on the 3s Rydberg state of ABCO is nonradiative decay. The nature of this deactivation channel (or channels) is not known, but we suggest that internal conversion, perhaps mediated by electron transfer, is responsible. The lack of a significant increase in k,c for the heavy-atom perturber, Xe, relative to He and Ar fails to support an intersystem crossing mechanism. Interestingly, we see little evidence of changes in the radiative transition probabilities of the complexes vis-5-vis ABCO*. Thus kfc values are close to that for the free amine, kfA = kA = 3.3 X lo6 s-l. This is at least consistent with the failure to observe changes in the absorption strength of ABCO in the presence of the perturbers (up to ca. 1 atm). Nevertheless, we can discern a slight, but real, increase in krc values for the noble gases. Perhaps this reflects the change in effective “refractive index” of ABCO* when complexed with these atoms, in view of the fact that, in the condensed phase, the radiative rate constant is proportional to n2, whereas f n.23 As an example, it has been noted before that kfA/n2for ABCO is nearly constant, from the vapor phase to different solvent environment^.^^ The role of collisional perturbers in affecting the reaction field associated with emissive transitions is unclear; thus it is not known how many perturber “solvent” molecules are required to cluster about an ABCO* solute to approach or achieve solvated conditions. It is interesting that the quantum efficiency of the ABCO*-MCH complex, 0.29, is very close to that reported for ABCO in cyclohexane (CH) solution at 293 K (0.27).24One cannot go too far in making a quantitative comparison between these efficiency values because they are determined for different phases and, hence, experimental errors possibly cloud the picture; nevertheless, the qA values are very close to each other. The significant finding here is that the nonradiative decay constant measured for ABCO in C H solution, 1.8 X IO’ s-’, is more than twice the value for the ABCO*-MCH complex (see Table I). Thus it appears that “solvation” by (presumably) one hydrocarbon molecule is sufficient in inducing about 44% of the radiationless decay ultimately achieved in the condensed phase. A comparison of the radiative rate constants of isolated ABCO*, ABCO*-MCH complex, and ABCO in C H solution is also remarkable. The two vapor-phase values are 3.3 X lo6 s-l and 3.4 (23) Reference 19, pp 51, 87-103. (24) Nosowitz, M.; Halpern, A. M. J . Phys. Chem. 1986, 90, 906.
J. Phys. Chem. 1989, 93, 149-154
x lo6 s-l, respectively (see Table I), and the solution value, when normalized by n2, is 3.3 X lo6 s-1.22 Thus we can conclude that collisional or environmental perturbation brings about little, if any, effect on the radiative transition probability for the 3s Rydberg state in ABCO. The success in applying Scheme I1 to the data relative to Scheme I shows how sensitive the quenching rate parameter, kQ (KQ), is in achieving consistency in K, values (see above). We list in Table I ratios of kQ to the gas kinetic rate constants for the respective perturbers. It can be seen that the fraction of such quenching collisions is very small, ranging from 0.0017 for He to 0.01 1 for DME and MCH. These data may possibly indicate that very strong (Le., energetic) collisions directly induce nonradiative decay, perhaps through some high-energy channel. To wit, the fluorescence excitation spectrum of ABCO under isolated conditions reveals a significant reduction in intensity at or near the onset of the S2 So transition.” Finally, we discuss results of a similar study of the fluorescence of trimethylamine vapor (TMA). As noted above, the lowest excited state of this amine has been interpreted as a 3s Rydberg.8 Surprisingly, we observe that the fluorescence lifetime of TMA under isolated conditions (0.45 Torr) is significantly less sensitive to perturbing collisions that ABCO. Thus between 0 and 1 atm of added He, the TMA lifetimes are found to be 37.8 and 35.9 ns, respectively. Similar results are obtained for 2-methylbutane. Although we note that the intrinsic lifetime of TMA is ca. 8 times shorter than that of ABCO and thus it is a kinetically less sensitive probe, we still fail to observe the behavior shown in Figures 4-6
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149
at correspondingly higher perturber pressures. One interesting interpretation of these results is that electronically excited, planar, TMA is sterically restricted from forming excited complexes because of the impeding H atoms, even in the case of He. ABCO, which is structurally prevented from achieving planarity, allows a perturber to approach the N atom with closer proximity. The ability of ABCO to undergo efficient excimer formation and the failure of many other amines to manifest such upper-state complexation (e.g., TMA) have been attributed to steric effects.25 If this structural property plays such an important role in affecting perturbing collisions, it suggests that the lowest excited state is more compact than one might have thought for an (nN,3s) Rydberg state. It is also possible that the degree of Rydbergconjugate valence m i ~ i n g ~is~appreciable, ,~’ causing the electron density to be more associated with the three a-C-N atoms than expected. Acknowledgment. C. J. Ruggles participated in many helpful discussions. We acknowledge the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Registry No. ABCO, 100-76-5; TMA, 75-50-3; MCH, 108-87-2; DME, 115-10-6; Hc, 7440-59-7; Ar, 7440-63-3; Xe, 7440-63-3; SFs, 255 1-62-4; 2-methylbutane, 78-78-4. (25) Ruggles, C. J.; Halpern, A. M. J. Am. Chem. SOC.,in press. (26) See ref 3a, pp 24-25. (27) Mulliken, R. S.Acc. Chem. Res. 1976, 9, 7.
ESR Studies on Structures and Dynamics of Propane and n-Butane Radical Cations: Determination of the Activation Energies of Methyl Group Rotation and the Hyperfine Coupling Constants of the Out-of-Plane Methyl Protons Kaoru Matsuura, Keichi Nunome, Kazumi Toriyama,* and Machio Iwasakit Government Industrial Research Institute Nagoya, Hirate, Kita, Nagoya 462, Japan (Received: April 26, 1988)
The activation energies of the restricted reorientation of methyl groups around an electron-deficient C-C u-bond have been determined for the first time for propane and n-butane radical cations by a simulation using the modified Bloch equations. They are 2.5 kcal/mol for CH3CD2CH3+in SF6and 2.3 and 2.4 kcal/mol for n-C4Hlo+ in CFC1, and n-C4Flo,respectively, being close to those for the parent molecule. The unresolved hyperfine coupling constants of the out-of-plane methyl protons are also determined together with their signs. They are -3.0 G for CH3CD2CH3+in SF6 and +7.8 G for n-C4Hlo+in CFC13. The positive value obtained for n-C4Hlo+is unexpected, so the geometries of the radical cations are discussed with the aid of INDO and MIND013 calculations.
Introduction Since our first detection of C2H6+ in SF6,1we have studied the structures and reactions of the radical cations of linear, branched, and cyclic alkanes systematically by the low-temperature matrix isolation method.*” One of the important concepts derived from these studies is u-delocalization of the unpaired electron over the molecular frame, which was found in linear and cyclic alkanes. The ESR spectra of linear alkane radical cations with an extended form are composed mainly of three lines of 1:2:1 due to the two outermost in-plane protons regardless of the chain length. This indicates that the methyl groups of these a-radical cations are fied at 77 K in spite of their weakened a-frames. This is in marked contrast with the a-methyl groups of well known T-radicals where they are usually rotating freely even at 77 K. However, we have recently found that the barriers to internal rotation of the methyl Deceased in 1987.
0022-365418912093-Ol49$01 S O / O
groups in linear alkane radical cations are not so high that the restricted reorientation of the methyl groups must be taken into consideration to analyze the spectra above 77 K.6 In the preliminary report, we have shown that the ESR spectrum of nC4HI0+observed at 150 K in CFC1: is broadened by the restricted (1) Iwasaki, M.; Toriyama, K.; Nunome, K. J. Am. Chem. SOC.1981,103, 3591. (2) Toriyama, K.; Nunome, K.; Iwasaki, M. J . Phys. Chem. 1981, 85, 2149. (3) Toriyama, K.; Nunome, K.; Iwasaki, M. J . Chem. Phys. 1982, 77, 5891. (4) Nunome, K.; Toriyama, K.; Iwasaki, M. J . Chem. Phys. 1983, 79, 2499. (5) Iwasaki, M.; Toriyama, K.; Nunome, K. Faraday Discuss. Chem. SOC. 1984, 78, 19. (6) Iwasaki, M.; Toriyama, K. J . Am. Chem. SOC.1986, 108, 6441. (7) Wang, J. T.; Williams, F. Chem. Phys. Lett. 1981, 82, 177.
0 1989 American Chemical Society
