3456
J . Phys. Chem. 1989, 93, 3456-3459
Pyrrole-Argon: Microwave Spectrum, Structure, Dipole Moment, and Coupling Constants
14N Quadrupole
Robert K. Bohn,* Department of Chemistry, University of Connecticut, Storrs, Connecticut 06268
Kurt W. Hillig, 11, and Robert L. Kuczkowski Department of Chemistry, University of Michigan, Ann Arbor, Michigan 481 09 (Received: July 12, 1988: In Final Form: October 13, 1988)
The rotational spectrum of the weak complex between argon and pyrrole has been observed by use of a Fourier transform microwave spectrometer with a pulsed supersonic nozzle molecular beam source. The complex is a nearly symmetric prolate top with the Ar atom located above the plane of pyrrole, 3.553 (10) A from the pyrrole center of mass. The line between the Ar atom and the pyrrole center of mass makes an angle of 5 . 5 (20)’ with the normal to the pyrrole plane and is displaced toward the N atom. The dipole moment of the complex, 1.707 (3) D, is smaller than that of free pyrrole, 1.767 (2) D, and the negative end of the complex’s dipole moment is rotated from the pyrrole plane toward the Ar atom. The I4Nquadrupole coupling constants have been determined. Based on the centrifugal distortion constant 0,and a pseudcdiatomic model, the binding energy of the complex is estimated to be 300 cm-I. The dipole moment and I4N quadrupole coupling constants of pyrrole were redetermined.
Introduction
rupole splittings were resolvable for the I4N species. Center frequencies were usually reproducible to f 1 kHz, and accuracies The microwave spectrum and molecular structure of a complex are estimated to be f 2 kHz. between argon and the aromatic molecule furan were reported Steel mesh parallel plates about 50 X 50 cm were placed about recently.] The Ar atom is located 3.54 A from the plane of the 30 cm apart straddling the microwave cavity. Dc voltages up to furan molecule. The line between the Ar atom and the furan 10000 V were applied to each plate, one negative and the other center of mass is tilted 1 1O from the normal to the furan molecule positive. For the Stark measurements, a trace of OCS was added toward the oxygen atom. Compared to furan, the isoelectronic to the gas mixture. At each voltage, the Stark-shifted microwave pyrrole molecule has a significantly larger dipole moment (1.74 0), [‘SN]pyrr~le(lo,l Oo,o), and transitions of OCS (1 vs 0.68 D)2*3and a more negative out-of-plane molecular quad(21,2 lo,l and 31,3 20,2) were sequentially [ISN]pyrrole-Ar rupole moment (-12.4 X vs -6.1 X e s u - ~ m ~ The ).~~~ measured. The dipole moments were determined from the relative direction of the dipole moment is also opposite in the two comshifts compared to those of OCS (0.71521 D).* p o u n d ~ . ~It, was ~ of interest to determine how these changes would Microwave Spectrum. The spectrum of pyrrole-Ar was preaffect the structure and binding energy of the pyrrole-Ar complex. dicted from a model based on the structure of furan-Ar.’ A This study reports the microwave spectrum of pyrrole-Ar. Q-branch (AKp = 2 1) series was predicted near 9.7 GHz. This Besides the structure and binding energy, the dipole moment and region was searched and transitions were quickly found. The I4N 14Nquadrupole coupling constants were determined. The dipole quadrupole splittings were comparable to the spacing between the moment of uncomplexed pyrrole has also been redetermined. Q-branch line centers, confusing their assignment. Therefore, R-branch transition were sought and found at 8665, 11 375, and Experimental Section 14 084 MHz. They were initially assigned as C-type transitions Materials. Pyrrole was obtained from the Aldrich Chemical which gave reasonable rotational constants except that the B Co. and used without purification. A sample of 99% enriched constant was smaller than C. By reassigning the quantum numbers [lSN]pyrrole was purchased from MSD Isotopes. A GC-mass to the similar b-type transitions, the complete assignment including spectral scan confirmed its isotopic enrichment and chemical the Q-branch series fell into place. purity . The I4N quadrupole splitting of the transitions was analyzed Spectrometer. The rotational spectrum was observed between (see below), and the hypothetical unsplit frequencies were de7500 and 18 500 MHz in a Fourier transform microwave spectermined. Five a-type and 26 b-type transitions were fit to a trometer using a heated Bosch fuel injector pulsed nozzle gas centrifugal distortion model using Watson’s S reduction in the s o u r ~ e . ~Pyrrole ~’ was heated to 35 OC in a reservoir just upstream P representation. Rotational constants and four centrifugal from the nozzle. Argon at 1-1.5 atm swept this vapor through distortion constants were refined. The fifth centrifugal distortion the pulsed nozzle. Pyrrole comprised about 1% of the flowing constant, d2,could not be. determined from the data. Table I lists gas. Timing of the gas and microwave pulses was coordinated the transitions and the fit. The derived spectroscopic constants to minimize Doppler splitting of the transitions. The transitions are listed in Table 11. have full widths at half-maximum of about 25 kHz, and quadTwenty-three b-type lines of [lSN]pyrrole-Ar were measured and fit to the same centrifugal distortion model as the normal (1) Kukolich, S . G . J . Am. Chem. SOC.1983, 105, 2207. species. The assignments of the ISN-labeled species and its (2) (a) Mata, F.; Martin, M. C.; Sorenson, G. 0. J . Mol. Struct. 1978, spectroscopic constants are also listed in Tables I and 11, re48, 159. (b) Bak, B.; Christensen, D.; Dixon, W. B.; Hansen-Nygaard, L.; spectively. Andersen, J. R.; Schottlander, M. J . Mol. Spectrosc. 1962, 9, 124. (3) Nygaard, L.; Nielsen, J. T.; Kirchheiner, J.; Maltesen, G.; Rastrup14N Quadrupole Splitting. 14N quadrupole splittings from Andersen, J.; Sorenson, G. 0. J . Mol. Struct. 1969, 3, 491. pyrrole-Ar were predicted by using the quadrupole coupling (4) Sutter, D. H.; Flygare, W. H. J . Am. Chem. SOC.1969, 91, 4063. constants of p y r r ~ l e . ~The predicted pattern was very similar (5) Bak, B.; Hainer, H.; Sutter, D. H.; Dreizler, H. Z.Nuturforsch. 1972,
-
+-
-
-
-
27a, 705.
(6) Hillig, 11, K. W.; Matos, J.; Scioly, A,; Kuczkowski, R. L. Chem. Phys. Lett. 1987, 133, 359. (7) Balk, T. J.; Flygare, W. H. Reo. Sci. Instrum. 1981, 52, 33.
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(8) Muenter, J. S. J . Chem. Phys. 1968, 48, 4544. (9) Bolton, K.; Brown, R. D. Aust. J . Phys. 1974, 27, 147.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3451
Microwave Spectrum of Pyrrole-Argon TABLE I: Assigned Rotational Transitions for Pyrrole-Argon
transition J'
KPI
K,'
J"
3 2 11 10 9 8 7 6 5 4 3 2 2 3 4
0
3 2 9 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 9 10 4 2 4 3 3 4 5 2 1 1 0
2 1 11 10 9 8 7 6
5 6 7 8 9 10 11 4 4 4 3
5 4
5 3 3 2 2
1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 1 2 0 1 2 1 1 2 2 2 2
5 4 3 2 2 3 4 5 6 7 8 9 10 11 3 3 3 2 4 3 4 2 2 1 1
K/ 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 0 0 2 0 0 1 1 1 1
w 2 1 10 9 8 7 6
5 4 3 2 1 2 3 4 5 6 7 8 9 10 11 3 1 3 2 2 3 4 1 2 0 1
15n
14N
u(obsd)"
Aub
8 131.802 8666.356 9708.183 9713.185 9717.749 9721.866 9725.543 9728.767 9731.541 9733.854 9735.713 9737.099 9738.992 9739.489 9740.147 9740.969 9741.952 9743.079 9744.364 9745.789 9747.352 9749.056 10840.412 10841.073 10841.868 11375.672 13550.477 14084,284 16792.080 17868.308 17870.198
0.001 -0.002 -0.010 0.000 0.006 0.004 0.00s 0.003 0.003 -0.001 0.001 -0.008 -0.008 -0.005 -0.004 0.001 0.008 0.005 0.009 0.005 -0.003 -0.009 0.001 0.004 0.000 -0.001 -0.004 0.000 0.002 0.000 -0.003
u(obsd)"
Aub
8585.342 9423.214 9452.282 9478.981 9503.232 9524.966 9544.1 18 9560.638 9574.476 9585.593 9593.958 9609.854 9617.360 9627.364 9639.867 9654.868 9672.361 9692.347
0.001 -0.002 -0.000 0.002 0.002 0.003 0.000 0.000 -0.001 -0.003 -0.004 -0.002 0.001 0.002 0.003 0.004 0.001 -0.00s
11273.346
-0.001
17682.216 17698.153 1499 1.913 14997.223
-0.001 0.002 -0.002 0.001
"Unsplit center frequencies in MHz. buobsd - ucalCdin MHz. TABLE 11: Rotational and Centrifugal Distortion Constants for Pyrrole- Argon 14N I5N A/MHz 4601.423 (4) 4548.883 (3) 1355.701 (1) 1350.877 (1) B/MHz 1345.577 (1) 1355.070 (1) C/MHz 0.00464 (5) 0.00475 (2) Dj/MHz 0.02495 (1) 0.02503 (1) DJKIMH~ DK/MHz -0.026 12 (8) -0.025 9 (6) -0.000 32 ( 5 ) -0.00021 (T) dl/MHz K" -0.999 61 -0.996 69 0.002 8 u of fit/MHz 0.005 5 ' K
is the asymmetry parameter: ( 2 8 - A - C ) / ( A - C).
to that observed, and the quadrupole analysis was straightforward. Eighty-two well-resolved components of the 31 observed transitions were fit by least-squares methods (u = 1.5 kHz) to the two independent quadrupole constants. The transitions and fit are listed in Table SI as supplementary material (see paragraph at end of paper regarding supplementary material). The derived coupling constants are listed in Table 111. I4N quadrupole splittings from free pyrrole were measured in order to refine its quadrupole coupling constant^.^ Thirty wellresolved components from eight transitions were fit by least-squares methods (. = 2.4 kHz) to the two independent quadrupole coupling constants. The transitions and fit are listed in Table SI1 as supplementary material. The derived coupling constants are also listed in Table 111. The refined values agree well with the previous values.9 Dipole Moment. Stark-shifted trcnsitions of OCS, [15N]pyrrole and [15N]pyrrole-Ar were observed over a range of static electric fields up to 400 V/cm. The dipole moment of pyrrole was calculated directly from the ratio of the Stark shifts of pyrrole vs OCS. The dipole moment components of [I5N]pyrrole-Ar were similarly scaled to the dipole moment of OCS by multiplying the ratio of the frequency shift of the complex to the frequency shift
TABLE 111: "N Quadrupole Coupling Constants for Pyrrole and Pyrrole-Argon
pyrrole-argon exptlC calcdd x../MHz Xbb/MHZ x,/MHz
-2.581 (3) 1.315 (2) 1.266 (4)
-2.648 1.356 1.292
pyrrole'
pyrroleb
-2.704 (4) 1.412 (3) 1.292 (4)
-2.700 (1 1) 1.400 (8) 1.300 (8)
"This work. bReference 9. CAnalysisof 13 transitions with 82 components; u of fit = 0.0015 MHz. dCalculated from pyrrole by rotating 6.7O about the c axis of pyrrole-argon. TABLE I V Stark Coefficients" and Dipole Moments of Pyrrole and Pvrrole-Areon (lSN Swcies) obsd species transition 1 4 Au/r2 calcdb dipole: D pyrrole-argon 2,2-1,,, 0 1.532 0.002 pa = 0.092 (3) 1 6.064 0.002 /.ib = 1.705 (2) 313-202 0 -2.051 -0.003 /.LT= 1.707 (3) 1 0.155 -0.003 2 6.772 -0.003 pyrrole lol-000 0 1.123 pa = 1.767 (2)d
'Stark coefficients ( A u / c 2 ) in units of lo5 MHz/(V/cm)Z. Calculated Stark coefficients with dipole components in next column. CTheuncertainties are approximately 3u. dPreviousvalue, 1.74 (2) D; ref 3. of OCS by the Stark coefficient of OCS and least-squares fitting the resultant equations for p: and pb2. The results are summarized in Table IV. The Stark shifts were converted to the conventional units, MHz/(V/cm)2, by using the OCS shifts to calibrate the electric field. Molecular Structure. Since only two isotopic species have been assigned, few structural parameters can be determined. The rotational constants and dipole components are consistent with a model that has the Ar atom located above the plane of the
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The Journal of Physical Chemistry, Vol. 93, No. 9, 1989
cpyrl
Bohn et al.
r-Rcm
a
@
P I I I
-
b
7pyrrole
Figure 1. Structure of the pyrrole-Ar complex. The (Ipyr, bPYr,and cPy axes are the principal axes of free pyrrole. Rm, the line between the Ar atom and pyrrole's center of mass, is indicated by the dashed line and 8 is the angle between R,, and the normal to the pyrrole plane.
TABLE V: Determination of R , , and 0 from Fitting Pairs of Inertial Moments or Combinations of Rotational Constants and the Moments Calculated with the Derived Parameters
eji'kg
9.1424.; 9.14600 5.48089 I,,/amu A* 109.8309 109.8309 110.9236 Ibb/amu A2 372.7808 373.8729 372.7808 I,,/amu AZ 371.8619 372.9542 372.9542
5.48077 110,8736 372.2361 372.4018
5.5 (20) 1O9.831Ob 372.7806b 372.9541b
,K = (2B - A - C ) / ( A - C) is the asymmetry parameter where A , etc. are the rotational constants. bExperimental moment of inertia.
pyrrole ring. The assumption is made that the pyrrole molecule is unchanged upon complexation and that the complex retains C, symmetry. Two structural parameters, R , and 8, defined in Figure 1, can be determined by fitting two of the moments of inertia. The inertial tensor of the complex can be written as' Ixx
= Ibb(pyrrole)
IYy = I,,(pyrrole) I,, = I,,(pyrrole)
+ &m2
+ pRcm2cos2 8 + pRcm2sin2 8
Iyr= -pRCm2sin 8 cos 8 where x , y , and z are respectively the bpyr,spy, and cpylaxes shown in Figure 1 and p is the reduced mass of the argon and pyrrole. By adjusting R , and 0 and diagonalizing the inertial tensor, any of three pairs of moments can be fit. An alternative procedure was to fit the rotational constant combinations, B + C and K = (2B - A - C ) / ( A - C), which are sensitive to R,, and 8, respectively. The parameters obtained for the four cases are given in Table V. It is seen that no fit is entirely satisfactory; Le., poor agreement is obtained upon calculating at least one of the moments of inertia with the derived R , and 8. This disparity arises from the large amplitude vibrations which have been neglected in interpreting the moments of inertia. These vibrational effects are also apparent upon noting that P,,(complex) - Pbb(pyrrole)Io should ideally be zero while the observed value is 0.5463 amu A2. There is little reason to choose between the four calculations based on the numerical agreement. However, the observed pb selection rules are only consistent with structures having 8 less than about 6.5'. Thus, the parameters derived from I b b and I,, or E + Cand K seem preferable. This leads to the values listed in Table V as the best parameters (with large uncertainties). The sign of the tilt angle, Le., whether the Ar atom is tilted toward or away from
Figure 2. Projection of the structure of the pyrrole-Ar complex in the a-6 plane. The dipole moments of pyrrole and the two possible orien-
tations for pyrrole-Ar are shown to scale. the N atom, cannot be determined from the data from only one isotope. However, the principal axis coordinates of the N atom can be obtained from Kraitchman's equations" and the rotational constants of the normal isotope and the IsN species. Agreement between the Kraitchman coordinates and those calculated from models with the Ar tilted toward and away from the N atom is much better for the model with the Ar atom tilted toward the N atom as shown in Figure 1. The Kraitchman a and b coordinates of the N atom are 1.16 and 1.19 A, respectively. The calculated values from a model with Ar tilted toward the N atom are 1.19 and 1.14 A, respectively, while those from a model with Ar tilted in the other direction are 1.42 and 1.09 A, respectively.
Discussion Molecular Structure. The structural parameters reported above place the Ar atom almost directly above the midpoint between the two ring C atoms adjacent to the N atom. The slightly larger distance (3.5S3vs 3.54 A) and the smaller tilt angle (5.5' vs 11') in pyrrole-Ar compared to furan-Ar appear to correlate better with the presumably slightly larger size of the N atom compared to the 0 atom than with the dipole and quadrupole moments of free pyrrole and furan noted in the Introduction. Binding Energy. The stretching force constant between Ar and pyrrole can be estimated from the centrifugal distortion constant, Dj, using the pseudodiatomic model with the relationship
k, = 1 6 r 2 B 3 p / D J This gives a force constant of 0.034 mdyn/A and a harmonic frequency of 48 cm-I. Assuming a Lennard-Jones 6/12 potential, the binding energy was estimated from the force constant as 300 cm-I. The analogous calculation for argon-furan gave k = 0.032 mdyn/A, w = 46 cm-I, and 6 = 270 cm-'.I2 The calculations suggest that pyrrole-Ar is a slightly stronger complex than furan-Ar, which would be consistent with the larger dipole and quadrupole moments of pyrrole. However, the inherent uncertainties in estimating binding energies in this manner are probably about *20%, precluding any firm conclusion on this point. Dipole Moment. Figure 2 displays a projection of the molecular structure on the a-b plane, the dipole moment of pyrrole, and the dipole moment and components of the complex. We assume that p b of the complex is directed like p of pyrrole. The sign of pg of the complex is not determined by experiment. The two possibilities are shown in Figure 2. Regardless of the choice, the negative end of the complex's dipole moment is shifted out of the pyrrole plane toward the Ar atom. (11) Kraitchman, J . Am. J . Phys. 1953, 21, 17. (1 2) These parameters differ somewhat from the values in ref 1 since they were recalculated by use of the simpler relationship employed here.
3459
J. Phys. Chem. 1989, 93, 3459-3465 14N Quadrupole Coupling. Table I11 shows the coupling constants of pyrrole-Ar (first numerical column), the coupling constants of pyrrole relabeled to most closely correspond to the principal axes of the complex (third and fourth columns), and the coupling constants predicted by rotating pyrrole's principal axes 6 . 7 O to coincide with the principal axes of the complex (second column). The agreement between the observed coupling constants and those predicted by this transformation is very good given the uncertainties in the structural parameters and vibrational averaging effects. The observed constants are 2-3% smaller than the predicted values, suggesting that the electric field gradient at the N
atom is the same in the complex as in pyrrole except that there is some essentially isotropic vibrational averaging in the complex.
Acknowledgment. This research was supported by the National Science Foundation, Washington, DC, Grant CHE-8614340. Registry No. Ar, 7440-37-1; pyrrole, 109-97-7. Supplementary Material Available: Tables SI and SI1 listing the quadrupole split transition frequencies of pyrrole-argon and pyrrole (4 pages). Ordering information is given on any current masthead page.
Vibrational Level Structure and Intermode Coupling of S, trans-Stilbene As Studied by Laser-Induced Fluorescence Spectroscopy in a Supersonic Free Jet Taeko Urano,+ Megumi Maegawa,$ Kaoru Yamanouchi, and Soji Tsuchiya* Department of Pure and Applied Sciences, College of Arts and Sciences, The University of Tokyo, Komaba, Meguro- ku, Tokyo 153, Japan (Received: July 19, 1988; In Final Form: October 31, 1988)
The laser-induced fluorescence (LIF) spectra of trans-stilbene in a supersonic free jet have been investigated to discuss the vibrational level structure in the SIstate. Since the rotational temperature is reduced to as low as 1.5 K, the congested vibronic bands can be resolved to uncover the level structure in the SI state. The rotational band contours have been measured to determine structural parameters of the SI state. It is concluded that in the SI state the C=C bond length, r c e , is larger by 0.05 A and the C=C-C angle, t9c,c--c, is smaller by 10' than those in the So state. Most of the transition lines to the SI vibrational levels above 800 cm-' exhibit splittings caused by Fermi coupling. The number of the observed split bands is larger in a transition to a higher Si vibrational level. Above 1200 cm-' from the band origin, the line broadening is found for respective lines, and this is attributed to a congestion caused by Fermi coupling with dense background levels. The observed change in the level structure caused by 13Cor D isotope substitution is ascribed to differences between the level coupling schemes in the normal and isotopic species.
I. Introduction The intramolecular vibrational energy redistribution (IVR) in the photoexcited molecule has been extensively studied in the context of the state-selected photochemical reactions.'-5 For detailed discussion of IVR, the vibronic structure in the electronically excited state affords an important basisS6 trans-Stilbene (tSB) is a typical molecule that undergoes the isomerization reaction to the cis form through photoexcitation to the S, In the past decade, a number of papers have been published on the laser-induced fluorescence (LIF) spectroscopy of the Si-So transition of tSB.5,'20 The supersonic beam technique combined with laser spectroscopy enables selective excitation to single vibronic levels in the SI state, which is free from spectral congestion caused by hot-band transitions. Consequently a great deal of progress has been made in studies on intramolecular processes and isomerization dynamics initiated by a single vibronic level excitation. Zewail et al. measured the decay curves of the dispersed fluorescence from selected several vibronic levels of the SI tSB.5*'2 It was found that the features of the decay curves, such as the number of the beat modulations, beat frequencies, and depth of the modulation, are dependent significantly on the initially prepared level. The IVR rate becomes approximately 100 times as large as the isomerization rate at 1500 cm-l. Therefore, it was concluded that the energy of the initially prepared vibrational level is randomized completely before isomerization takes place and that the isomerization rate is not dependent on the initially prepared vibrational mode and is dependent only on the vibrational Graduate student: Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan. f Undergraduate student: Chemistry Course of Domestic Science Department, Japan Women's University, Mejirodai, Bunkyo-ku, Tokyo l 12,
Japan.
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energy. In other words, the necessary condition for the RRKM theory is satisfied for the isomerization." Information on the intermode coupling obtained from the (1) Parmenter, C. S. Faraday Discuss. Chem. Soc. 1983, 75, 7. (2) Felker, P. M.; Lambert, W. R.; Zewail, A. H. J . Chem. Phys. 1985, 82, 2961. (3) Felker, P. M.; Lambert, W. R.; Zewail, A. H. J. Chem. Phys. 1985, 82, 2975. (4) Felker, P. M.; Lambert, W. R.; Zewail, A. H. J . Chem. Phys. 1985, 82, 2994. (5) Felker, P. M.; Lambert, 82, 3003.
W. R.; Zewail, A. H. J . Chem. Phys. 1985,
(6) van Herpen, W. M.; Meerts, W. L.; Drabe, K. E.; Kommandeur, J. J . Chem. Phys. 1987,86, 4396. (7) Saltiel, J.; d'Agostino, J.; Megarity, E. D.; Metts, L.; Neuberger, K. R.; Wrighton, M.; Zafiriou, 0. C. Org. Photochem. 1973, 3, 1. (8) Hochstrasser, R. M. Pure Appl. Chem. 1981, 52, 2683. (9) Syage, J. A.; Lambert, W. R.; Felker, P. M.; Zewail, A. H.; Hochstrasser. R. M. Chem. Phvs. Lett. 1982. 88. 266. ( I O ) Syage, J. A.; Lambeh, W. R.; Felker, P. M.; Zewail, A. H. J . Chem. Phys. 1984, 81, 4685. (1 1) Syage, J. A.; Lambert, W. R.; Felker, P. M.; Zewail, A. H. J . Chem. Phys. 1984,-81, 4706. (12) Felker, P. M.; Zewail, A. H. J . Phys. Chem. 1984, 88, 6106. (13) Amirav, A,; Jortner, J. Chem. Phys. Lett. 1983, 95, 295. (14) Malors, T. J.; Even, U.; Jortner, J. J . Chem. Phys. 1984, 82, 2330. (15) Zwier, T. S.; Carrasquillo, M. E.; Levy, D. H. J . Chem. Phys. 1983, 78,
5493.
(16) Taatjes, C. A,; Bosma, W. B.; Zwier, T. S. Chem. Phys. Lett. 1986, 128, 127. (17) Suzuki, T.; Mikami, N.; Ito, M. J . Phys. Chem. 1986, 90, 6431. (18) Spangler, L. H.; van Zee, R. D.; Zwier, T. S. J . Phys. Chem. 1987, 91, 2182. (19) Spangler, L. H.; van Zee, R. D.; Zwier, T. S. J . Phys. Chem. 1987, 91, 6077. (20) Urano, T.; Hamaguchi, H.; Tasumi, M.; Yamanouchi, K.; Tsuchiya, S. Chem. Phys. Lett. 1987, 137, 559.
0 1989 American Chemical Society
