Vapor phase photochemistry of cycloheptanone

Abstract

By means of excitation spectroscopy and product analysis, the photochemical transformation of cycloheptanone in the vapor phase (1 torr) has been investigated as a function of the excitation energy, temperature, and foreign gas pressure. Following S 1 (nπ*) ← S O excitation, it is shown that the two photochemical channels leading to the aldehyde, and to CO + hydrocarbons, respectively, originate from the lowest triplet state. Moreover, probing the triplet state (T 1) of cycloheptanone by triplet—triplet energy transfer to biacetyl revealed that the T 1 population is strongly dependent on the excitation energy. Whereas with excitation near the vibrationless S 1 level the quantum yield φ(T 1) = 0.90, this yield decreases to a value close to zero when the excess vibrational energy ( E vib) /s> 3500 cm −1. Since the photochemistry is most efficient with E vib /s> 4000 cm −1 it is concluded that photodissociation occurs mainly from a short-lived intermediate triplet state T 1 + which is not subject to energy transfer with biacetyl at the pressures employed. The highly efficient intersystem crossing process leads to the metastable T 1 + state which may be interpreted as a vibrationally non-randomized triplet or a conformational isomer of the lowest triplet state, T 1. The various deactivation and chemical decay channels available to T 1 + and T 1 are considered in detail.