Conformational stability of 3-fluoropropene in rare gas solutions from temperature-dependent FT-IR spectra and ab initio calculations

The infrared spectra (3500-400 cm(-1)) of 3-fluoropropene (allyl fluoride), CH2=C(H)CH2F, dissolved in liquid argon, krypton, and xenon have been recorded at various temperatures ranging from -180 to -65 degrees C. From these studies, the enthalpy difference between the more stable cis conformer and the high-energy gauche rotamer has been determined to range from 60 +/- 8 cm(-1) (718 +/- 96 J/mol)in liquid xenon to 81 +/- 1 cm(-1) (969 +/- 12 J/mol) in liquid argon. These values have been extrapolated utilizing a linear relationship between the Kirkwood function of the solvent and the enthalpy differences in the solvents to give a value of 130 +/- 25 cm(-1) (1.56 +/- 0.30 kJ/mol) for the vapor. From the experimental enthalpy value, the gauche dihedral angle, torsional transitions for both rotamers, and better structural parameters, the potential function governing the conformational interchange has been recalculated. Ab initio calculations utilizing the 6-31G(d,p) and 6-311G(d,p) basis sets with electron correlation at the MP2 level predict the cis conformer to be the more stable rotamer, but from the MP2/6-311++G(d,p) calculation the gauche conformer is predicted to be more stable by 117 cm(-1) (1.40 kJ/mol). By combination of the ab initio predictions of the structural parameters with the previously reported microwave rotational constants for 11 different isotopic species of both conformers, complete r(o) parameters have been obtained for both rotamers. The results of these structural parameter determinations are compared to those previously reported.