A b i n i t i o study of the π-electron states of t r a n s-butadiene

Abstract

Extensive ab initio configuration interaction calculations were carried out on the π‐electron states of trans‐1,3‐butadiene. A double‐ζ contracted Gaussian basis set, augmented with two diffuse 2p π functions on each carbon atom, was used in the calculations, which were based on a frozen σ core obtained from a ground‐state SCF calculation. All excitations fro and to π orbitals were included in the CI treatment. Natural orbitals were obtained for many of the wavefunctions,and their spatial extenty was determined. Only five of the calculated excited states were found to have a valencelike charge distribution (computed vertical excitation energies in eV are given in parentheses): 1 ³B_u (3.45), 1³A_g(5.04), 2¹A_g(6.77), 3³B_u (8.08), and 1⁵A_g (9.61). These states all correlate with the valence N and T states of ethylene and can be readily described in terms of the ’’two‐vinyl model’’ as either N T (the first two) or TT (doubly excited, the last three). Except for the doubly excited 2¹A_g, all low‐lying singlet excited states were found to involve diffuse excited orbitals (as in the case of similar calculations on ethylene). Their identification as V states and/or Rydberg states is not always clear‐cut. The calculated vertical excitation energies for the NT states are in good agreement with electron impact data, but owing to limitations of the frozen σ core, computed excitation energies for the diffuse states (particularly those related to the ethylene V state) are too higy by as much as 1 eV. The absorptions in the butadiene spectrum at 5.9 and 7.1 eV are believed to be due to the 1¹B_u and 2¹B_u states, computed at 7.05 and 8.06 eV, respectively.