Conformational analysis, barriers to internal rotation, vibrational assignment, and ab initio calculations of 3-fluoro-1-butene

The far-infrared spectrum of gaseous 3-fluoro-1-butene, CH2=CHC(CH3)FH. has been recorded at a resolution of 0.10 cm(-1). The asymmetric torsional fundamentals of the most stable form HE (hydrogen atom eclipses the double bond) and the higher energy FE (fluorine atom eclipses the double bond) conformations have been observed at 88.7 and 105.9 cm(-1), respectively, each with several excited states falling to lower frequencies. From variable-temperature (-55 to --100 OC) measurements of the infrared spectra of xenon solutions, the enthalpy difference between the HE and FE conformers has been determined to be 87 +/- 6 cm(-1) (250 +/- 17 cal/mol). The same determination yields an enthalpy difference of 292 +/- 5 cm(-1) (835 +/- 15 cal/mol) between the HE and the least stable ME (methyl group eclipses the double bond) conformer. From these data, the asymmetric torsional potential function governing internal rotation about the C-C bond has been determined. The potential coefficients are V-i = -212 +/- 11, V-2 = 381 +/- 12, and V-3 = 576 +/- 6 cm(-1) for the cosine terms and V-1' = 322 +/- 17, V-2' = -214 +/- 10, and V-3' = -240 +/- 13 cm(-1) for the sine terms. A complete assignment of the vibrational fundamentals observed from the infrared spectra (3200-30 cm(-1)) of the gas and solid and the Raman spectra (3200-10 cm(-1)) of all three physical states is proposed. The vibrational data have been compared to the corresponding quantities obtained from MP2/6-31G(d) ab initio calculations. Additionally, complete equilibrium geometries have been determined for the three conformers using the 6-31G(d) and 6-311+G(d,p) basis sets at the RHF and/or MP2 levels. The results are discussed and compared with the corresponding quantities obtained for other similar molecules.