Theoretical and experimental structures of vinyl fluoride and vinyl alcohol

Abstract

A new theoretical r0 structure for vinyl fluoride has been derived using ab initio molecular orbital procedures. In addition, new experimental r0 and rs structures have been obtained through a reanalysis of existing experimental microwave data. Significant discrepancies are found between the new theoretical (125.4°) and experimental (127.6°) r0 estimates of the CCHu angle involving the hydrogen atom geminal to the substituted center. This is attributed to the relative insensitivity of the experimental structural fits to the precise value of this bond angle, an insensitivity that is associated with the proximity of the relevant atoms to inertial axes. Because of these difficulties, the best estimate of the experimental r0 structure for vinyl fluoride is obtained by taking the theoretical r0 value for the CCHu angle and determining the remaining parameters by a weighted-least-squares fit to the experimental data. Weighted-least-squares fits are found generally to provide more consistency in the derived structural parameters than unweighted procedures. It is found that fitting of isotopic differences in rotational constants (as opposed to the conventional fitting of rotational constants themselves) provides the structure for vinyl fluoride most similar to the theoretical r0 structure. Similar observations are found to hold for vinyl alcohol, for which new theoretical and experimental results are presented.