Excited electronic states of HNC, hydrogen isocyanide

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Abstract
Walsh’s rules suggest that the excited states of HNC should have bond angles similar to the analogous states of HCN. To test this hypothesis, ab initio calculations have been carried out and compared to earlier calculations on HCN. The most surprising result is the prediction that several of the excited electronic states of HNC lie below the corresponding states of HCN. Also unanticipated are the unusually long CN bond lengths found for several of the lower excited states of HNC. The excited singlet states have bond angles qualitatively similar, although somewhat smaller, than those of HCN. For the triplet states, Walsh’s concept appears less successful, primarily owing to strong mixing between several different electronic configurations. Mulliken populations are used in the discussion of these results.