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J. Phys. Chem. 1991, 95, 2151-2762 stretches for the gauche conformer. Nevertheless, the normal vibrations are, in general, relatively pure considering the low symmetry of the molecule. Similarly, the cis-to-gauche barrier of 1096 cm-' for 3fluoro-2-methylpropene is not significantly different than the similar barrier of 1164 f 67 cm-' for 3-fluoropropene, but the gauche-to-gauche barrier of 1078 cm-' for the methyl-substituted molecule is nearly twice the value of 573 f 16 cm-l for this barrier in 3-fluoropropene. With the replacement of the hydrogen atom on the vinyl group with the methyl group, one expects a significant increase in the steric interaction for the gauche conformer which is exactly what is found experimentally. The potential barrier for the asymmetric rotor is nearly 3-fold, and since the V , and V2terms are relatively small, the relative uncertainties in their values are rather large. Even with the dihedral angle helping to fix the V2term there is still nearly 100% correlation between the VI and V2terms in these potential parameters. These uncertainties then lead to a relatively large uncertainty in AH, whereas the experimental uncertainty is about one-third the value obtained from the potential function. It should be noted that the experimental AH value is about one-third the value obtained for 3-fluoropropene. It would be interesting to see what effect the substitution of a halogen at the 2-position would have on the relative stability and the potential barriers for the substituted 3-fluoropropene molecule. Because the C-F and C = C bond distances are rather poorly determined from the a b initio calculations, the calculated dipole moment components are also rather poorly determined. In fact, for the gauche conformer, where the dihedral angle is calculated to be 140.7' with the 3-21G basis set, the calculated dipole moment components are very poorly determined. However, the relative differences in the total dipole moments calculated for the two conformers are very similar to the experimentally determined values.
The calculated structural parameters for the 3-flUOr0-2methylpropene molecule are very similqr to the values obtained for the corresponding parameters for the 3-fluoropropene molecule. Therefore, the methyl substitution does not significantly affect the other structural parameters in the molecule. From the ab initio calculations the C-F distance should be about 0.008 A longer for the gauche conformer than the corresponding distance for the cis conformer. This difference was not found from the parameters obtained from the microwave rotational constants, but this difference is about the determined uncertainty in this parameter. A similar difference was calculated for this bond distance in 3-fl~oropropene,~ but for the gauche conformer the C-C distance was determined to be 1.523 f 0.018 A, which is clearly too large compared to the value of 1.503 f 0.002 8, for this distance in the cis conformer. On the basis of the ab initio calculations, we fixed the C-C bond distance at 1.504 A in the gauche conformer and recalculated the C-F distance. The determined value was 1.390 f 0.006 A, which now agrees very well with the predicted difference from the ab initio calculations. Our value of 1.382 f 0.007 A for the C-F distance in cis-3-fluoro-2-methylpropeneis the same value determined for this parameter for 3-fluoropropene. Since the ab initio calculated value is the same for these two molecules, the value determined from the microwave rotational constants should be within experimental error of the actual value. One could probably obtain slightly better structural parameters from an electron diffraction study if the relative abundance of the conformers is taken into account along with the expected differences in the C=C and C-F distances for the two rotamers. Registry No. CH2C(CH,)CH2F, 920-35-4.
Supplementary Material Available: Listings of force constants (mdyn/A) for cis-3-fluoro-2-methylpropeneand gauche-3fluoro-2-methylpropene (Tables S1 and S2) (2 pages). Ordering information is given on any current masthead page.
Reinvestigation of Infrared-Induced Conformational Isomerizations of 1,P-Ethanedioi in Low-Temperature Ar Matrices and Reverse Reaction in the Darkt Chun Geun Park and Mitsuo Tasumi* Department of Chemistry, Faculty of Science, The University of Tokyo, Bunkyo-ku, Tokyo I 13, Japan (Received: September 24, 1990)
Infrared-induced conformational isomerizations of 1,2-ethanediol in low-temperature Ar matrices have been reexamined by using better experimental facilities. Three types of reactions (RI, R2, and R3) occur under infrared irradiation with different rates, and a reverse process of R1 takes place in the dark. The reactants of R1, R2, and R3 have been identified as conformers tG+g-, g+G+g-, and g-G+g-, respectively, all of which have the gauche conformation (G) around the CC bond. The products of RI and R2 are two conformers among tTt, tTg-, and g+Tg- which have the trans conformation (T) around the CC bond. A comparison of the observed frequencies and the normal frequencies obtained from ab initio molecular orbital calculations has suggested that tTt is the RI product and g+Tg- the R2 product. Although the product of R3 has not been specified, it must be one of the above-mentioned five conformers. Structure parameters of the six conformers have been optimized by ab initio calculations on the 6-31G** level, and total energies have been reevaluated by the MP2 method. The barriers to R1 and R2 are discussed by combining observed data with the results of ab initio calculations.
Introduction
Infrared-induced conformational isomerization of molecules isolated in inert-gas matrices at low temperatures is now a well-known phenomenon.' Since the first observation of the infrared-induced cis-trans isomerization of nitrous acid reported by Baldeschwieler and Pimente12 in 1960, studies of similar phenomena have been published for a variety of molecules, particularly in the past decade. However, most studies have treated molecules with one or two torsional degrees of freedom. For 'Dedicated to the memory of George C. Pimentel.
0022-3654/91/2095-2757%02.50/0
molecules having three or more axes of internal rotation, analysis of observed results becomes much more difficult due to increased numbers of conformers involved and complexity of spectral changes. Accordingly, only the cases of 1,2-ethanedi01,~-~ 1,2~~
~
(1) Frei. H.: Pimentel. G.C . Annu. Reo. Phvs. Chem. 1985.36.491-524. (2j Baldeschwieler, J: D.; Pimentel, G. 6.J . Chem. Phys. '1960. 33, 1008-1015. (3) Ha, T.-K.; Frei, H.; Meyer, R.; Garithard, Hs.H. Theor. Chim. Acto 1974, 34, 211-292. (4) Frei, H.; Ha, T.-K.; Meyer, R.; Giinthard, Hs.H. Chem. Phys. 1977.
.( 5- ) Takeuchi, H.; Tasumi, M. Chem. Phys. 1983, 77, 21-34.
25. 271-298. - - -
0 1991 American Chemical Society
2158 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991
Park and Tasumi
propanediol,6 2-aminoethano1,’ and 1,2-ethanediamine* have been studied. In 1983, a paper dealing with the infrared-induced conformational isomerization of 1,2-ethanediol was published from this laboratory.s In that study, a dispersive infrared spectrophotometer was used to observe infrared-induced spectral changes. In view of the superior power of recent Fourier-transform spectrometry in accurately determining minute spectral changes with high resolution and high signal-to-noise ratios, we decided to reinvestigate the infrared-induced conformational isomerizations of 1,2-ethanediol using a Fourier-transform infrared spectrophotometer, since this molecule offers a typical example of the three-axis case. The results are reported in this paper.
Materials and Methods 1,2-Ethanediol was purchased from Aldrich Chemical Co. (Anhydrous, 99+%, Gold Label), and it was dried by freezeand-thaw under vacuum several times. Vaporized 1,2-ethanediol was diluted with Ar gas (99.9999%) obtained from Takachiho Kagaku Kogyo to a matrix/sample ratio of 2000. This premixed gas was slowly sprayed onto a CsI plate maintained under vacuum (
