Methodological traps encountered in theroretical treatments of the photoisomerization of triplet butadiene

Abstract

The relative triplet energies of various conformations of butadiene were computed with reference to the ground state using a variety of conventional unrestricted Hartree—Fock (UHF) and restricted Hartree—Fock (RHF) methodologies correlationally corrected at the Møller-Plesset second- and third-order levels. It is shown that the geometries and relative energies of the various triplet species cannot be adequately estimated at either the UHF or the RHF level alone. In particular, at the RHF open-shell level, the geometry of the singly-twisted allyl—methylene diradical triplet shows the same properties as those encountered in the classic RHF instability of the allyl radical. However, the relative energies computed at the UHF level alone are poor, possibly reflecting the differing levels of spin contamination obtained for different geometries. Therefore, in contrast to the case of the ethylene triplet, where the relative energies of the triplet conformations are not correlation sensitive, the relative energies of the various conformations of the butadiene triplet require a correlation-energy correction.