Variational calculations on the A g vibrational states, the automerization, and the predicted Raman spectrum of cyclobutadiene

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Abstract
Variational calculations of vibrational levels of three Ag modes are reported for cyclobutadiene, its vicinally 13C-dilabeled derivative, and the tetralabeled derivatives 13C4H4 and C4D4. These calculations use the ab initio two-configuration GVB/4-31G potential surface for the antisymmetric CC stretch (automerization), symmetric CC stretch, and symmetric CCH bend. The three vibrational modes are strongly coupled so that one-dimensional and adiabatic multidimensional approaches are not justifiable in this case. The splitting of the ground vibrational state of di-13C -cyclobutadiene is calculated to be 4 cm−1, which implies the rate constant for automerization is k=2.4×1011 s−1 at temperatures near absolute zero. The Raman spectrum of cyclobutadiene is predicted to have two strong bands in the region of 3100–3200 cm−1. The other transitions are predicted to be of considerably lower intensity, though three of them should be strong enough to be observed. Two of the three are predicted to be accompanied by satellite lines which are due to the automerization splitting of the Ag levels. Uncertainties involved in the theoretical approach are discussed.