Tracer Study of Photochemically Excited Cyclobutanone-2-t and Cyclobutanone. II. Detailed Mechanism, Energetics, Unimolecular Decomposition Rates, and Intermolecular Vibrational Energy Transfer

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Abstract
A detailed photochemical transformation mechanism for the electronically excited cyclobutanones is proposed. It is inferred that the quantum yield of the radiationless, internal conversion process (S ↝ S 0 ) is significantly large and varies from 0.7 at 3130‐Å to 0.5 at 2480‐Å excitation, while the remaining quantum yield is being attributed to the competing intersystem crossing process (S 1 ↝ T 1 ) . Energetic and mechanistic differences of two nondegenerate photochemical decomposition pathways are discussed and compared to the singular thermal decomposition pathway. The rates of internal conversion, intersystem crossing (S 1 ↝ T 1 ) , and the decomposition of CB* (T 1 ) could be greater than 1 × 1011 sec−1 since no observable perturbations of these rate processes by various gases appear at a gas kinetic collision frequency of 1 × 1010 sec−1. The highly vibrationally excited CB** (S 0 ) produced by an internal conversion process following the monochromatic photoactivation of the ground state CB(S0) decomposes unimolecularly. The observed specific rate constant k(E) of CB**(S0) for its decomposition varies from 0.7 × 1010 sec−1 at E = 93 kcal / mole to 5 × 1010 sec−1 at E = 114 kcal / mole , within the limitations of the strong collision assumption, and these experimental values of k(E) 's are in good agreement with the values of k(E) obtained with an approximate RRKM calculation. Efficiencies per collision of deactivating CB** (S 0 ) with E = 93 kcal / mole have been determined for monatomic gases (He, Ne, Ar, Kr, and Xe), diatomic gases (H2, D2, and O2), and organic molecules (C3H6 and C3H8), and they vary from 0.06 for He to 1.00 for C3H6.

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