8660
J. Phys. Chem. 1996, 100, 8660-8664
High-Resolution FTIR Spectrum of Jet-Cooled CH3CHF2 Don McNaughton* and Corey Evans Centre for High Resolution Spectroscopy and Optoelectronic Technology, Department of Chemistry, Monash UniVersity, Wellington Road, Clayton, Victoria, 3168, Australia ReceiVed: NoVember 7, 1995; In Final Form: February 29, 1996X
The mid-IR spectrum of HFC-152a, CH3CHF2, has been recorded using a supersonic jet-FTIR spectrometer system, at high and low resolution. Molecular constants have been obtained from a vibration-rotation analysis of the A-type band ν5 (1413 cm-1) and the B-type band ν16 (941 cm-1). The resultant constants have been used to simulate band profiles, from which the rotational temperature in the jet has been estimated to be 50 K. The vibrational assignment of CH3CHF2 has been revised using the low-resolution jet spectrum and Raman and far-infrared spectra recorded at room temperature.
Introduction Environmental concerns resulting from the ozone depleting properties of halocarbon emissions have placed major industries under considerable pressure to find suitable replacements for these compounds. The 1987 Montreal protocol is focused on phasing these compounds out, including the widely used CFCs, over the next decade. In fulfillment of this international agreement, suitable replacements with low ozone depletion potentials must be found. To function as refrigerants, blowing agents, propellants, and fire extinguishing gases these replacements must have similar physical and chemical properties to their less desirable counterparts. Hydrofluorocarbons (HFCs), long proposed as replacements, are now gaining acceptance as substitutes because the C-H bonds are susceptible to attack from OH• radicals in the atmosphere, thus reducing their ozonedepletion potential and tropospheric lifetime. In addition, the HFC’s lack chlorine or bromine atoms which effectively catalyze the breakdown of ozone in the stratosphere. HFC-152a (1,1difluoroethane) has been proposed as a replacement for Freon12 as a refrigerant gas. The vibrational structure of CH3CHF2 was originally analyzed at low resolution by Smith et al.1 and more recently reanalyzed using both IR and Raman spectroscopy by Guirgis and Crowder.2 In both of these studies parts of the vibrational assignment were considered by the authors to be unsatisfactory or not conclusive. Subsequently, Lucas et al.3 performed ab initio molecular orbital calculations at the MP2/6-31G** and MP2/ 6-311G** levels of theory and compared the predicted vibrational frequencies with past assignments. Absolute strengths of IR absorption bands have also been measured at various temperatures, to assist in modeling the infrared trapping capacity of HFC-152a.4 During the course of this work Villaman˜an et al.5 assigned the microwave spectrum of HFC-152a in the ground state and four lowest excited vibrational states. A full and unambiguous knowledge of the vibrational modes of CH3CHF2 is desirable in order to calculate thermodynamic quantities like heat capacity and to detect the gas in the atmosphere by infrared spectroscopy. In particular, the molecular constants obtained from high-resolution vibration-rotation analyses are useful in modeling the temperature-dependent spectral profiles of these molecules for quantitative analysis. However, large molecules, such as CH3CHF2, have an extremely * Address correspondence to this author. Email: D.McNaughton@ sci.monash.edu.au. X Abstract published in AdVance ACS Abstracts, April 15, 1996.
S0022-3654(95)03275-8 CCC: $12.00
congested rovibrational spectrum at room temperature owing to the large number of heavily populated rotational states and the hot bands arising from low-frequency vibrational modes. To resolve and assign the individual vibration-rotation transitions, it is necessary to reduce the number of populated energy levels. Such molecules also have a large number of fundamental vibrational modes and at room temperature, where the lowenergy vibrational modes are heavily populated, the lowresolution infrared spectra are dense and difficult to assign. A reduction in the number of populated levels also greatly simplifies the low-resolution analysis. One convenient way of achieving a reduction in the number of populated energy levels is to take advantage of the properties of a supersonic jet expansion in which the degrees of freedom of a gas undergo cooling. We have constructed an apparatus capable of recording high-resolution FTIR spectra of species in a supersonic jet and used it to assign vibration-rotation bands of HFC-134a (CF3CH3F),6 Freon-12 (CCl2F2),7,8 and Halon-1211 (CBrClF2).8 In this work we report measurements of the high-resolution jet-cooled FTIR spectrum of CH3CHF2 and an analysis of the rovibrational structure of the ν5 and ν16 bands. Analyses of the low-resolution jet-cooled mid-infrared and room temperature far-infrared and Raman spectra of CH3CHF2 have enabled the vibrational modes to be reassigned. The vibrational assignments are compared with MP2/6-311G(d,p) predictions. Experimental Section The jet FTIR apparatus consists of a sample chamber evacuated by a Varian HV12 cryopump and externally coupled to a Bruker IFS 120HR interferometer. A narrow near-parallel beam of infrared radiation is directed into the chamber and passed 11 times through the supersonic expansion region. Commercially available CH3CHF2 (BOC, Australia), at a stagnation pressure of ca. 600 kPa, was allowed to flow directly through a Pyrex tube and expanded into the chamber through a 240 µm diameter hole. Under these flow conditions the cryopump was overloaded, so that at high resolution only five scans could be taken before the pressure in the lower region of the chamber began to rise above 0.4 Pa. The cryopump was isolated and allowed to recover between each set of five scans. In this way, a total of 19 × 5 scans of high-resolution jet-cooled spectra were recorded at 0.0035 and 0.008 cm-1 unapodized resolution using 850-1550 and 2400-3950 cm-1 filters, and a KBr beamsplitter. Jet-cooled spectra of CH3CHF2 between 650 and 4500 cm-1 were also recorded at 0.l cm-1 by co-adding © 1996 American Chemical Society
FTIR Spectrum of Jet-Cooled CH3CHF2
J. Phys. Chem., Vol. 100, No. 21, 1996 8661
Figure 1. (a) Room temperature spectrum of CH3CHF2 at 0.1 cm-1 resolution, 150 Pa and a path length of 8 m. (b) Jet-cooled spectrum of CH3CHF2 at 0.1 cm-1 resolution.
100 scans. A more detailed description of the experimental system is given elsewhere.7 Far-infrared spectra of CH3CHF2 at room temperature were recorded on the Bruker HR120 at 1.0 and 0.1 cm-1 resolution using an IR Laboratory Inc. liquid helium cooled silicon bolometer with a 660 cm-1 cold pass filter and a 3.5 µm Mylar beamsplitter. The CH3CHF2 was isolated at pressures from 100 to 200 Pa and at 6.5 kPa in an Infrared Analysis Inc. multiple pass cell with polyethene windows and a path length set to 8 m. Room temperature mid-infrared spectra were recorded in the same cell equipped with KBr optics at 0.1 cm-1 resolution and at a sample pressure of 150 Pa using a HgCdTe/InSb liquid nitrogen cooled detector. Raman spectra at 1.0 and 4.0 cm-1 resolution were recorded by Bruker Analytische Messtechnik GmbH, Karlsruhe, Germany on a RFS-100 FT-Raman spectrometer, with a Ge detector and excitation at 1064 nm using an 600 mW laser and a long path length gas cell. The details of this system have been described by Lehner et al.9 Molecular orbital calculations for the harmonic wavenumber values and intensities were carried out on a Dec Alpha system and a SIG IRIX CHALLENGE system using the GAUSSIAN 94 suite of programs10 at the MP2/6-311G(d,p) level of theory. For the Raman intensities a HF/6-311G(d,p) basis was used. Results and Discussion Low-Resolution Band Analysis. CH3CHF2 is an oblate asymmetric top (κ ) 0.75) which belongs to the Cs symmetry point group, so that its 18 vibrational modes belong to the symmetry species A′(A/C-type bands) or A′′ (B-type bands). As can be seen in Figure 1, where both room temperature and jet-cooled spectra are shown, the result of rotational cooling in the jet expansion is a decrease in spectral bandwidth leading to sharper Q-branches, allowing better determination of band centers and revealing hidden features. It can also be seen in Figure 1 that some spectral features diminish significantly relative to others, implying that there is significant vibrational
TABLE 1: Observed IR/Raman Bands of CH3CHF2 assignment
obsd band center/cm-1
spectrum type
band type
ν1 ν12 ν2 ν3 ν4 ν5 ν14 ν6 ν7 ν8 ν15 ν16 ν9 ν10 ν11 ν17 ν18 2ν9 + ν18 ν9 + ν15b 2ν15b ν5 + ν9 ν5 + ν8 2ν7 + ν11
3016.08 3001 2975.16 2958.52 1456.97 1413.22 1364.08 1359.77 1171.13 1145.08 1134.96 941.76 868.71 569.88 470.14 390.48 221 1958.27 1999.72 2273.07 2279.19 2560.23 2809.31
jet Raman jet jet jet jet jet jet jet jet jet jet jet 25 °C 25 °C 25 °C 25 °C jet jet jet jet jet jet
A/C B A/C A/C A/C A/C B A/C A/C A/C B B A/C A/C A/C B B
A/C A/C
strength m w s w mw s mw w m mw vs s w m mw w vw w w w w w w
calcd band center/cm-1
1413.277a
941.748a
1957.4 2003.6 2269.8 2281.9 2558.3 2810.5
a Fitted band origin from high rovibrational analysis. b Possible assignment.
cooling. This vibrational cooling allows for easy identification of bands arising from excited vibrational states. For most bands it is possible to determine from their appearance whether they are A/C-type or B-type. Both A- and C-type bands have prominent Q-branches, whereas the Q-branch transitions in B-type bands do not overlap in the same way and generally merge into the P and R structure. The strongest bands in the mid-infrared region have been assigned to the fundamentals in a manner consistent with the results of the ab initio calculations and expected band types. Table 1 shows the band assignments, relative strengths, and band types of the observed vibrational bands. The room temperature spectrum in the region 1100-1200 cm-1 shows four bands that
8662 J. Phys. Chem., Vol. 100, No. 21, 1996
McNaughton and Evans
TABLE 2: Experimental Wavenumber Values and ab Initio Values (cm-1) for CH3CHF2 assignment
infrareda
infraredb
this work
MP2/6-311G**c
mode description
3018 2979 2963 1460 1414 1365 1143 1129 868 571 470
3016 2975 2959 1466d 1413 1362 1171 1142 868 569 469
3016.0 2975.2 2958.5 1456.9 1413.2 1359.7 1171.1 1145.1 868.7 569.9 470.1
3111 (14.3/111) 3054 (44.9/37) 3009 (2.6/119) 1461 (6.4/9.3) 1426 (84.8/2.2) 1367 (1.2/0.6) 1148 (54.1/2.0) 1141 (76.7/4.5) 865 (9.4/6.0) 559 (6.9/0.8) 461 (13.6/0.9)
CH3 asym str CH str CH3 sym str CH3 sym def C-C str CCH bend CF2 str CF2 str CH3 rock CF2 bend CF2 wag
3001 1460 1360 1171 930 383
3016 1457 1164 1149d 942 383 222e
3001d
3108 (8.9/56.4) 1464 (0.001/5.9) 1397 (32.1/7.9) 1153 (144/2.7) 948 (55.2/3.9) 377 (0.03/0.25) 249 (0.09/0.01)
CH3 asym str CH3 asym def CCH bend CF2 str CH3 rock CF2 twist CH3 torsion
A′ ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11 A′′ ν12 ν13 ν14 ν15 ν16 ν17 ν18
1364.1 1134.9 941.7 390.5 221
a Reference 1. b Reference 2. c The ab initio predictions have been multiplied by a scaling factor of 0.97 and the numbers in brackets are the IR (km mol-1) and Raman intensities (Å4 amu-1). d Raman values. e Reference 9.
were previously assigned as fundamentals; however the jetcooled spectrum over the same region shows only three bands. The loss of intensity of the 1164 cm-1 band indicates that it arises from a vibrational excited state and is not a fundamental as assigned in the previous work of Guirgis and Crowder.2 In addition, the ν4 and ν15 bands previously thought to be at 1466 and 1149 cm-1, respectively, have been reassigned to 1456.9 and 1134.9 cm-1. Analysis of the infrared spectra failed to conclusively establish the positions of the ν12 and ν13 bands. Previously, ν12 has been assigned to 3001 and 3016 cm-1, but there is no evidence of any structure in the infrared spectrum around 3000 cm-1 other than that assigned to ν1 and ν2. To help with the assignment, Raman spectra were recorded and analyzed. Several bands were observed around 3000 cm-1 but most of these appeared to have the wrong symmetry. Raman selection rules predict that A′′ modes should have type Bb or Ba Raman bands. In other spectral regions, these bands were found to be broad, flat and fairly weak. The only band in the C-H stretching region which appeared to match this profile was centered at 3001 cm-1 and we have tentatively assigned this as ν12. Previous assignments have placed ν13 at 1460 and 1457 cm-1, but it could not be confidently assigned in this work. The Raman spectra did show two weak bands in this region, at 1464 and 1448 cm-1; however, from the observed overlapping profiles of the bands we were not able to confidently assign them. Given the very low intensity expected for ν13 from the ab initio predictions as shown in Table 2 this is not surprising and we assume this fundamental is too weak to be observed. A number of weaker bands in the jet-cooled spectrum have been assigned to combinations and these are listed in Table 1. Analysis of most of the room temperature far-infrared spectrum at 1.0 cm-1 resolution was straightforward and consistent with previous assignments. The torsional mode assigned by Fateley and Miller9 at 222 cm-1 is observable in our spectrum only at very high pressures (6.5 kPa). This weakness is to be expected given the predicted infrared intensity of this mode from the ab initio calculations. In Table 2 the fundamental vibrational wavenumber values assigned in this work are compared with previous experimental values and our ab initio MP2/6-311G(d,p) predictions, which have been scaled by a factor of 0.97. This scaling factor is the average of the scaling factors derived from the ratio of the MP2/6-311G(d,p) calculated wavenumber values and experimental values for
HFC-134a.6 The resulting predictions are in agreement with the observed values to within 3.5% apart from the torsional mode. Considering the excellent agreement for the other lowfrequency modes and the corresponding modes of HFC-134a6 the experimental value of 221 cm-1 for the torsional mode is badly predicted at this level of theory. In addition to the changes in assignment in this work, the mode descriptions obtained from the MP2/6-311G(d,p) calculation differ somewhat from those of Guirgis and Crowder 2 suggesting that the force-field calculations from that work are unreliable and need to be reexamined. Vibration-Rotation Analysis. MACLOOMIS,12 an interactive program for the Macintosh computer that is based on a Loomis-Wood (Fortrat) format was used to examine peak lists generated from the high-resolution spectra of the strongest bands in the spectrum. The program, described in ref 12, aids in the process of assigning transitions by enabling the regularly spaced series of transitions possessing the same Kc values but differing J to be selected. For all but low values of J and K even highly asymmetric molecules like HFC-152a display these regular spacings that facilitate initial assignment. All assignments were checked by calculating ground state combination differences and comparing the results with differences calculated from the ground state parameters. Unique assignments were obtained only for ν5 (C-C str) and ν16 (CH3 rock). Transitions that appeared to be overlapped on the MACLOOMIS display, or asymmetry doublets calculated to be split by more than 0.0004 cm-1, were assigned an experimental uncertainty of 0.002 cm-1, otherwise an uncertainty of 0.0004 cm-1 (10% line width) was used in the weighted least-squares fits. The assigned transitions were then transferred to an asymmetric rotor fitting program, in which Watson’s A-reduced Hamiltonian 13 in the Ir representation was used. Typically the IIIr representation is used for oblate tops; however, it was found by Villaman˜an et al.5 that the Hamiltonian, in this representation, did not converge well, and a Ir representation was found to give the best fit. Results of the least-squares analysis are given in Table 3. A set of ground state constants derived from the ground state combination differences and the early microwave results agreed with those calculated from the mmwave data by Villaman˜an et al.5 Subsequently, fits to the available data were carried out with the ground state constants held to the values of Villaman˜an.
FTIR Spectrum of Jet-Cooled CH3CHF2
J. Phys. Chem., Vol. 100, No. 21, 1996 8663
TABLE 3: Molecular Constants (in cm-1) of the ν5 and ν16 Bands of CH3CHF2 (Ir Representation)a parameter
ground state5
ν16
ν5
νj0 A B C ∆J ∆JK ∆K δJ × 108 δK × 107 ΦJ × 1013 ΦJK × 1013 ΦKJ × 1013 φJ × 1013 φK × 1012
0.316618424 0.298964583 0.172467785 1.58504 × 10-7 6.03364 × 10-8 9.71422 × 10-8 5.764504 1.550315 2.537 -3.231 5.200 1.204 1.8506
941.748628(53) 0.3165893(13) 0.2982457(11) 0.17205485(58) 1.603(11) × 10-7 1.413(80) × 10-7 -1.11(11) × 10-7 5.764504b 1.550315b 2.537 -3.231 5.200 1.204 1.8506
1413.277194(95) 0.315457(42) 0.298667(34) 0.1733078(10) 1.58504 × 10-7b -2.213(37) × 10-6 3.692(70) × 10-6 5.764504b 1.550315b 2.537 -3.231 5.200 1.204 1.8506
456 0.91 4.04 × 10-4
319 1.09 5.05 × 10-4
no. of trans std dev rms error
Correlation Matrices for the Fits ν16
νj0
A
B
C
∆J
∆JK
νj0 A B C ∆J ∆JK ∆K
100 11 -15 -67 -52 27 -25
100 -99 -44 -51 74 -82
100 44 50 -72 82
100 94 -72 52
100 -87 68
100 -91
ν5
νj0
A
νj0 A B C ∆JF ∆K
100 13 -15 -69 35 -25
100 -99 -59 82 -89
B
100 59 -81 88
C
∆JK
∆K
100 ∆K Figure 2. (a) Part of the jet-cooled spectrum of the ν5 band of CH3CHF2 at 0.0035 cm-1 unapodized resolution. (b) Simulated highresolution band profile at 50 K.
100 -86 74
100 -97
100
a Fitted parameters are italicized. The numbers in parentheses are 1 standard deviation (expressed in the last two digits) from the leastsquares fit. b Constrained to the ground state values.
The spectral transitions of the B-type band ν16 occur between the A′ and A′′ states and have the selection rules ∆Ka ) (1, ∆Kc ) (1, and ∆J ) 0,(1. Least-squares fits of the molecular constants were performed on the Kc ) 3-22, 24 subbands in the R-branch and Kc ) 4-22 in the P-branch. A few series were excluded because of strong perturbations in the Kc ) 1419 levels, resulting in a fit to 456 weighted transitions in the final fit. An interaction between ν16 and 2ν11 is the most likely cause of the observed resonance. In the least-squares analysis of the A-type band ν5, which has the selection rules ∆Ka ) 0, ∆Kc ) (1, and ∆J ) 0,(1, fits were performed on the Kc ) 3-6, 8-12 subbands in the R-branch and Kc ) 4-14 in the P-branch. Initial fits of ν5 revealed that the centrifugal distortion parameter DJ could not to be accurately determined and so it was subsequently fixed to the ground state value. The other distortion parameters, although determined with some accuracy, are much different from the ground state values and strongly correlated, possibly due to the small number of transitions fitted (319) or to a small perturbation in the energy levels. The standard deviation from the resulting fits of both bands is less than 1.1, indicating a good fit to the data. The derived rotational constants were used to simulate the observed vibration-rotation structure at a number of temperatures and a rotational temperature of 50 K was found best to reproduce the profile of the ν5 and ν16 bands. Under similar experimental conditions, the molecules CHClF2,14 CH3CCH,15 and CF3I16 were all found to have a similar rotational temperature to HFC-152a. The estimated rotational temperature is only
an approximation, since the molecules in the sampling area of the jet are not in equilibrium and so the population distribution is not truly Boltzmann-like. Lower rotational temperature could be achieved through seeding in argon but this would have the undesirable effect of decreasing absorption strength and a temperature of 50 K proved cool enough to resolve most of the rotational structure at 0.0035 cm-1. A portion of the R-branch structure of the ν5 band at 0.0035 cm-1 resolution is shown in Figure 2 along with a highresolution simulation. Even though centrifugal distortion has been neglected, the spectrum is fairly well reproduced. The relative intensities of the peaks in the two spectra are not perfectly matched, but this is to be expected when the experimental spectrum has a considerable level of noise. The small remaining spectral features unaccounted for by the simulation are probably due to transitions arising from lowenergy excited vibrational levels such as the torsional mode. Unique assignment of the very strong B-type band ν15 proved to be impossible. Analysis of the high-resolution spectrum revealed several perturbed series of the R-branch but no series from the P-branch could be assigned. Analyses of the highresolution spectra of the ν1 and ν2 bands proved, as a result of low signal to noise, to be ineffectual. Conclusion The jet-cooled infrared spectrum of CH3CHF2, together with Raman and far-IR room temperature spectral data, has allowed a reassignment of the vibrational modes of this hydrofluorcarbon. The present assignments now compare favourably with scaled MP2/6-311G** ab initio predictions and provide the necessary data for a new force field analysis and for thermodynamic calculations. High-resolution analyses of the ν5 and ν16 bands have been carried and the molecular constants
8664 J. Phys. Chem., Vol. 100, No. 21, 1996 subsequently used in the simulation of band profiles, which show that a rotational temperature of approximately 50 K has been achieved in the jet. The constants for the ν5 band in particular, which is in the atmospheric infrared window, provide data that will be useful for modeling and monitoring HFC-152a in the atmosphere. This work shows that rovibrational analysis of large asymmetric molecules, such as CH3CHF2, is possible using broad-band infrared jet-spectroscopy techniques combined with the appropriate interactive assignment software. Acknowledgment. The financial assistance of the Australian Research Council is gratefully acknowledged. The authors also thank Bruker Analytische Messtechnick GmbH and Dr. J. Sawatzki for recording the FT/Raman spectra. Supporting Information Available: Tables of vibrationrotation assignments and the output of the least-squares fits to these assignments (18 pages). Ordering information is available on any current masthead page. References and Notes (1) Smith, D. C.; Saunders, R. A.; Nielsen, J. R.; Ferguson, E. E. J. Chem. Phys. 1952, 20, 847. (2) Guirgis, G. A.; Crowder, G. A. J. Fluorine Chem. 1984, 25, 405. (3) Lucas, K.; Delifs, V. B.; Speis, M. Int. J. Thermophys. 1993, 14, 993. (4) Cappelani, F.; Restalli, G. Spectrochim. Acta 1992, 48, 1127.
McNaughton and Evans (5) Villaman˜an, R. M.; Chen, W. D.; Wlodarczak, G.; Demaison, J.; Lesarri, A. G.; Lo´pez, J. C.; Alonso, J. L. J. Mol. Spectrosc. 1995, 171, 223. (6) McNaughton, D.; Evans, C.; Robertson, E. G. J. Chem. Soc., Faraday Trans. 1995, 91, 1723. (7) McNaughton, D.; McGilvery, D.; Robertson, E. G. J. Chem. Soc., Faraday Trans. 1994, 90, 1055. (8) McNaughton, D.; McGilvery, D.; Robertson, E. G. J. Mol. Struct. 1995, 348, 1. (9) Lehner, C; Kawai, N. T.; Sawatzki, J.; Van der Velen, B. Presented at the 14th International Conference on Raman Spectroscopy, Hong Kong University of Sciience and Technology, Aug. 1994. (10) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L., Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; HeadGordon, M.; Gonzalez, C.; Pople, J. A. GAUSSIAN 94, ReVision A; Gaussian Inc.; Pittsburgh, PA, 1995. (11) Fateley; W. G.; Miller, F. A. Spectrochim. Acta 1961, 17, 857. (12) McNaughton, D.; McGilvery, D.; Shanks, F. J. Mol. Spectrosc. 1991, 149, 458. (13) Watson, J. K. G. In Vibrational Spectra and Sructure; Durig, J. R., Ed.; Elsevier: Amsterdam, 1977; Vol. 6. (14) Amrein, A.; Luckhaus, D.; Merkt, F.; Quack, M. Chem. Phys. Lett. 1988, 152, 275. (15) Amrein, A.; Quack, M.; Schmitt, U. Z. Phys. Chem. 1987, 154, 59. (16) Bu¨rger, H.; Rahner, A.; Amrein, A.; Hollenstein, H.; Quack, M. Chem. Phys. Lett. 1989, 156, 557.
JP953275N
