Polarities of hydrocarbons and non-neighbour resonance integrals

Abstract

A study is made of the polarities predicted for alternant hydrocarbons by the VESCF procedure with non-neighbour resonance integrals included. Four categories of unsymmetrical hydrocarbons emerge: purely cyclic six-membered ring compounds (μ ≤ 0.1 D), systems containing conjugated chains (μ = 0.1 D), quinodimethanes (μ = 0.3 D), systems containing four-membered rings (μ = 0.6 D or more). The theoretical values are close to experiment in the few cases where the latter is available but the need for further precise experimental studies is emphasized. For comparison, analogous calculations on non-alternant hydrocarbons are presented. The results suggest that alternants containing four-membered rings are as polar as typical non-alternants and that on polarity grounds there is no clear-cut separation into two categories of hydrocarbon. The discrepancy between theory and experiment for azulene and possible discrepancies for one or two other non-alternants 1s discussed. It seems possible that the neglect of possible σ-bond polarities should be questioned and that recent theoretical techniques embracing all valence electrons might be needed for full understanding of hydrocarbon polarities.