THEORETICAL AND EXPERIMENTAL STRUCTURES OF VINYL-CHLORIDE AND VINYL BROMIDE

The structures of vinyl chloride and vinyl bromide have been determined by ab initio molecular orbital calculations, and corrections applied to obtain theoretical r0 values. The theoretical r0 structures agree very well with experimental r0 structures derived from recent microwave spectral data. This contrasts with the situation for vinyl alcohol and vinyl fluoride for which significant discrepancies have been found between theoretical and experimental estimates of the CCHu angles involving the hydrogen atom geminal to the substituted center. The present results reinforce our earlier conclusion that the theoretical values for this parameter for vinyl alcohol and vinyl fluoride are more reliable than the experimental values. Difficulties in the experimental structural determinations for vinyl fluoride and vinyl alcohol are attributed to the unusual insensitivity of the experimental moments of inertia to the values of the CCHu angles, resulting in turn from the close proximity to a principal inertial axis of two of the atoms that define the CCHu angles in these molecules. In contrast, all three atoms defining the CCHu angles in vinyl chloride and vinyl bromide are far removed from the inertial axes. As a consequence, the experimental moments of inertia are sensitively dependent on the value of the CCHu angle, and theoretical and experimental structures agree well. We conclude that suggestions that substituted ethenes present unusual difficulties for ab initio structural determinations are unfounded. Sensitivity curves are suggested as valuable indicators of potential problems in experimental structural determinations.