Effect of superexchange interactions on conformations and barriers to rotation in molecules containing a double bond

Exchange perturbation theory of the Rayleigh–Schrödinger type is applied, in first and second order, to a calculation of the conformations of simple molecules containing a double bond and of barriers to internal rotation around a single or a double bond. Indirect exchange (’’superexchange’’) interactions between nonbonded atoms via the double bond are considered explicitly. In the model, the electrons of the nonbonded atoms are replaced by one effective electron per atom, those of the double bond by two spin-paired effective electrons. The charge distribution of the effective electrons are of Gaussian form. The method is applied to acetaldehyde, the acyl halides and related compounds, and to the ethylene molecule; calculated results are in good agreement with experiment. A decomposotion of the interaction energy is given, for different conformers, in terms of two- and many-atom interactions.