Bonding in singlet and triplet butalene: insights from spin-coupled theory

Spin-coupled (SC, equivalent to full generalized valence bond) calculations for the (1)A(g) ground state of butalene at its optimal D(2h) 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 C(2v) geometry is preferred, is somewhat more straightforward to analyze: the SC description of the (3)B2 state is dominated by a very simple pattern of two π bonds and two well-localized triplet-coupled orbitals.