Conjugation and Polar Effects in Butadiene

Abstract

Various valence bond structure functions are projected onto approximate ground‐state wave functions of butadiene. The most important structure, C☒C☒C☒C, contributes approximately 70% of the total. The next most important structure corresponds to two attractive dipoles and contributes over 40% to the ground state, while structures with double bonds between the central carbons contribute relatively little. It appears that ionic contributions may be important to the central carbon‐carbon bond length, with the dipole‐dipole interaction behaving like an effective repulsion because of the variation of the electronic wave function. The problem of the nonorthogonal valence bond representation is examined: it is found that conjugation is strongly inherent even in a ``nonconjugated'' structure C☒C☒C☒C, and that conjugation and dipolar interaction are nearly separable.