Oxygen wagging and ring puckering in ? 1A2 cyclobutanone

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Abstract
Analysis of the near ultraviolet spectrum of cyclobutanone shows that in its excited state the oxygen atom is bent out of the C2C1C4 plane, producing an inversion doubling of the carbonyl out-of-plane mode in the excited state. The out of plane angle is 39° at the potential minimum, from a fit to the eigenvalues for a quadratic Lorentzian-type potential function. The barrier to inversion of the varbonyl group in the excited state is 1550 cm−1. The interval of 98 cm−1 which is observed throughout the spectrum is assigned as a v′=1, v″=1 sequence transition in the ring puckering mode Q20, from which we have ν′20=134 cm−1, compared to the ground state value of ν″20=36 cm−1. The quartic + quadratic potential for the upper state, which was evaluated from this data, was found to contain only a single minimum, which indicates that the C1C2C3C4 carbon atoms are coplanar in this state. CNDO‖2 potential energy curves for the C1C3 and C2C4 ring puckering coordinates in the two electronic states were found to predict correctly the trends in the spectroscopically derived potentials. The eigenvector coefficients from the CNDO‖2 calculation were subjected to a Mulliken population density analysis. The bond density differences obtained in this calculation attribute the large increase in ring strain energy which occurs on electronic excitation to transannular interaction in the C2C4 direction.

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