Bonding in Singlet and Triplet Butalene: Insights from Spin-Coupled Theory

Spin-coupled (SC, equivalent to full generalized valence bond) calculations for the ¹A_<i>g</i> ground state of butalene at its optimal <i>D</i>_2<i>h</i> planar geometry show that cross-ring Dewar-like modes of spin coupling are of comparable importance to the more usually considered Kekulé-like modes. There are marked similarities to the SC description of one of the isomers of benzo­[1,2:4,5]­dicyclobutadiene. A complication for both of these systems is the existence of SC solutions in which some of the orbitals resemble in- and out-of-phase combinations of semilocalized atom-centered orbitals. The lowest triplet state, for which a nonplanar <i>C</i>_2<i>v</i> geometry is preferred, is somewhat more straightforward to analyze: the SC description of the ³B₂ state is dominated by a very simple pattern of two π bonds and two well-localized triplet-coupled orbitals