THE NATURE OF MULTIPLE BONDS .1. SIGMA,PI-BONDS VS BENT BONDS, A COMPUTATIONAL SURVEY

We examine the nature of multiple bonds within an independent-particle picture, climbing a hierarchy of approximations to the full generalized valence bond (GVB) description, the most general wave functions for which an independent particle interpretation of the bonding is possible. We begin with a systematic computational survey of molecules with multiple bonds, comparing sigma, pi and bent bond descriptions, using the strongly orthogonal perfect-pairing (SOPP) approximation to the GVB method. Although in several cases the bent-bond model is energetically favored using SOPP-GVB wave functions, in a number of other cases the usually accepted sigma, pi bond description is energetically favored. In order to obtain a definitive answer to the long-standing question of the better orbital description of the bonding, we conclude that the full GVB theory must be employed without restrictions. We outline a procedure by which generalized valence bond wave functions can be rigorously obtained without spin coupling or orbital orthogonality constraints and apply it to a sequence of molecules directed toward finding the best qualitative description of the bonding in multiple bonds: sigma, pi bonds or bent bonds? Our calculations lend support to Slater's and Pauling's sixty-year-old conjectures, finding the bent bond description energetically superior to the symmetry-restricted sigma, pi bond representation in the theoretical limit of wave functions able to meaningfully address this question.