Evidence for inhomogeneous broadening in vibrational overtone transitions: Formation of 1, 3-cyclohexadiene from c i s-1, 3, 5-hexatriene

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Abstract
By pumping the 4–0 and 5–0 C–H overtones of cis‐1, 3, 5‐hexatriene, the rates of 1, 3‐cyclohexadiene formation are measured using a Stern–Volmer analysis. While the rate for 5–0 C–H excitation is faster than 4–0 C–H excitation, the latter is about one order of magnitude larger than that calculated from an RRKM treatment. Moreover, nonmonotonic variation in rates are found as a function of excitation wavelength within each overtone region. These observations cannot be explained by assuming pure homogeneous broadening of each overtone transition, but can be rationalized by including inhomogeneous contributions, which may originate from hot bonds in the initial state and/or from a rotational dependence of this electrocyclic reaction rate near threshold.