The electronic states of benzene and the azines. I. The parent compound benzene. Correlation of vacuum UV and electron scattering data with ab initio CI studies

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A multi-reference ab initio CI study of benzene as parent molecule of the azines is presented. The results for both Rydberg and valence states are correlated with experimental VUV and electron scattering data, and with some exceptions ( 1B 1u and 1E 1u) good correlations are obtained. The difficulty in computing reasonable values for the second and third ππ* singlet states by CI arise from the paucity of virtual orbitals for the upper state MO (e 2u); this is largely overcome by quadruple and penta zeta p c basis sets, which have the effect of lowering the 1B 1u excitation energy below the Rydberg 3s state.