The electronic states of buta-1,3-diene studied by ab initio configuration interaction and DFT methods, and electron energy loss spectroscopy

The electronic vertical excitation energies for singlet and triplet valence, and Rydberg states of trans-buta-1,3-diene have been computed using ab initio multi-reference multi-root CI procedures with a [4s3p3d3f] set of Rydberg functions. Close numerical agreement between theory and experiment was found for a number of low-lying electronic states. The present CI and CASSCF [8MO,8e] calculations suggest that both the vertical and adiabatic order of the valence (pp) states is: A1Ag<B1Bu. An electron energy-loss spectrum, reported here, in which the incident electrons have near-threshold energies, supports this order. Adiabatic excitation energies and structures were obtained for several singlet and triplet states using CASSCF and B3LYP procedures; the results from these methods are generally in good agreement with each other. The C₁C₂ to C₂C₃ bond length ratio in the excited states varies widely, and is discussed. © 2010 Elsevier B.V. All rights reserved.