ABINITIO STUDY OF THE TRANS-BUTADIENE PI-VALENCE STATES USING THE EFFECTIVE VALENCE SHELL HAMILTONIAN METHOD

Low-lying pi-electron vertical excitation energies of trans-butadiene are calculated using the effective valence shell Hamiltonian method. The results are compared with previous experimental and theoretical analyses of this molecule's congested electronic spectra. The computations employ a large basis set (126 functions) which includes both diffuse functions on the carbon atoms and polarization functions on all atoms. Good agreement is obtained with the experimentally well known vertical excitation energies to the 1 3B(u), 1 3A(g), and 1 1B(u) states where deviations from experiment are only 0.01, 0.01, and 0.22 eV, respectively. We confirm the experimental assignment of a valence like 1A(g) state around 7.4 eV (calculated at 7.49 eV). Likewise, a member of a symmetry allowed 3p Rydberg series (of A(u) or B(u) symmetry) in the electron impact spectrum with origin at 7.07 eV is assigned as the 2 1B(u) state (with calculated vertical excitation energy of 7.00 eV). Most experiments place the 2 1A(g) state above the 1 1B(u) state; however, a resonance Raman assignment places it below. Our calculated excitation to the 2 1A(g) state is 0.05 eV above the 2 1B(u) state, about 0.5 eV lower than previous ab initio determinations. The computed vertical excitation energies are in good agreement with the interpretation of experimental electronic spectra, are in much better agreement with experiment than previously published ab initio calculations, provide the first definitive assignment of the 2 1B(u) state at 7.08 eV, and conclusively assign the 3 1A(g) state at 7.4 eV. The accuracy of the large basis effective valence shell Hamiltonian is, in part, due to retention of both valence and Rydberg orbitals in the valence space, a feature which has a bearing on intruder state problems and on current semiempirical pi-electron theories.