Geometries of the excited electronic states of HCN

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Abstract
Ab initio quantum mechanical electronic structure calculations have been carried out for the ground state and 12 low‐lying (< 10 eV) excited states of HCN. A contracted Gaussian basis set of essentially double zeta quality was employed. A new theoretical approach, which should be widely applicable, was applied to the excited electronic states. First one selects a physically meaningful set of orbitals, which, hopefully, will be about equally suitable for all the electronic states of interest. After selecting a single configuration to describe each electronic state, configuration interaction is performed including all configurations differing by one orbital from any of the selected reference configurations. The method appears to be one of the simplest capable of treating several states of the same symmetry. The predicted geometries have been compared with the experimental results of Herzberg and Innes, as well as the appropriate Walsh diagram. The ab initio calculations and the Walsh diagram concur that Herzberg and Innes's assignment of the B̃ 1A″ state, with bond angle 114.5°, is incorrect. Although the theoretical predictions are in several cases at variance with the Walsh diagram, these differences can in most cases be justified in terms of a breakdown of the single configuration picture of electronic structure. One modification of Walsh's diagram is suggested, a change in the shape of the 5a′ orbital binding energy. Without this modification, the Walsh prediction is in serious disagreement with Herzberg and Innes's 141° bond angle for the C̃ 1A′ state. The present theoretical study predicts a bond angle of 141.2° for the third 1A′ electronic state of HCN.