Role of the triplet state in gas-phase photolysis of aliphatic ketones and aldehydes

Abstract

The effect of cis-butene-2 and biacetyl on the gas-phase photolysis of selected carbonyl compounds has been investigated briefly. It is concluded that both α- and γ-bond cleavages occur from excited triplet states of aldehydes and ketones, the type II dissociation being exclusively triplet in character. Triplet state yields are in the sequence: acetone>n-butanone>n-pentanone-2>acetaldehyde>n-butyraldehyde. Attention is drawn to the influence of vibrational energy possessed by the sensitizer on efficiency of energy transfer. For example, it is suggested that n-butyraldehyde may act as triplet energy acceptor, from acetone, and as donor, to cis-butene-2, with greater efficiency than acetaldehyde as a consequence of greater vibrational freedom. The rearrangement of crotonaldehyde to 3-butenal is discussed as an example of a Norrish type II process involving an intermediate triplet state.