EFFECTS OF METHYL SUBSTITUTION ON THE EXCITED-STATES OF BUTADIENE

The effects of methyl substitution on the excited-state properties of butadiene are studied. Hyperconjugation and inductive effects are considered in detail and it is shown that the semiempirical parametrization normally used to account for the latter may be justified on the basis of ab initio calculations. The spectroscopic red shift resulting from methyl substitution in butadiene has two origins. In 1-methylbutadiene it is due to hyperconjugation while in 2-methylbutadiene inductive mixing between the optically forbidden Ag- state and the allowed Bu+ state is found to be important. The description of these effects within the framework of MO theory requires that doubly excited configurations be included in the CI scheme; however, they are intuitively obvious from consideration of valence bond structures. Charge distributions are strongly affected by the inclusion of doubly excited configurations and the Ag- and Bu+ states are found to have reverse directions of polarization. The CNDO/S method is shown to be incapable of properly treating inductive effects and may be replaced, for the compounds under consideration, by a π electron scheme in which the orbitals of the methyl group have been appropriately parametrized. © 1979, American Chemical Society. All rights reserved.