Conformational stability of epifluorohydrin from temperature dependent FT-IR spectra of xenon solutions and ab initio calculations

The mid-infrared spectra (3200-400 cm(-1)) of epifluorohydrin (fluoromethyloxirane), c-OCH2CH(CH2F), dissolved in liquid krypton and xenon as a function of temperature (-150 to -55 degrees C) have been recorded. Utilizing several conformer pairs, the enthalpy differences have been determined between the most stable gauche-l conformer and the other two less stable conformers. These experimental data show that the second most stable conformer is the cis rotamer and the least stable is the gauche-2 form. Utilizing conformer bands at 772 (gauche-l), 807 (gauche-2), and 953 (cis) cm(-1) and appropriate van't Hoff plots the following enthalpy differences were obtained: Delta H (gauche-1 to cis) = 45 +/- 6 cm(-1) and Delta H (gauche-l to gauche-2) = 135 +/- 5 cm(-1). Using the cis band at 755 cm(-1) gave similar results. However, the predicted intensities from the ab initio calculations combined with the peak intensities utilizing all the temperatures gave a Delta H (gauche-l to cis) of 132 +/- 24 cm(-1) and a Delta H (gauche-l to gauche-2) of 153 +/- 16 cm(-1), where the former value appears to be too large. Structural parameters obtained from the MP2/6-311 + + G** calculations are in excellent agreement with the previously proposed parameters with the exception of the carbon-hydrogen stretches. The wavenumbers of the fundamentals for all three conformers are predicted to be better than 10 cm(-1) on average from the MP2/6-31G* calculations with a single scaling factor of 0.9. Complete vibrational assignments are given for all three conformers. These results are compared with the corresponding quantities for some similar molecules. (C) 1998 Elsevier Science B.V.