Ring puckering in the 1B2(n, 3s) Rydberg electronic state of cyclobutanone

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Abstract
The first vacuum ultraviolet absorption spectrum of cyclobutanone has been photographed under conditions of moderately high resolution. This system, which is given the assignment ?1B2(n, 3s) ←?1A1, has been observed to have unusually discrete rotational and vibrational fine structure. To higher frequencies from a weak (0–0) band are a pair of weak vibronic bands at (+105) and (+196) cm−1 which are assigned to the ring puckering sequence transitions (1–1) and (2–2). From a fit of the energy levels derived for the upper state to a quartic–quadratic potential function it was determined that the C1C2C3C4 carbon atoms are coplanar in this state. The increase in puckering frequency from 35 to 140 cm−1 is interpreted in terms of a large increase in ring strain on excitation.

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