Substituent effects on homolytic versus heterolytic photocleavage of (1-naphthylmethyl)trimethylammonium chlorides

Abstract

Singlet excited state rate constants have been measured for both the heterolytic and homolytic photocleavage of 3- and 4-methoxy and 3- and 4-cyano (1-naphthylmethyl)trimethylammonium chlorides, 6–10. The results are interpreted in terms of the meta effect or changes in charge distribution upon excitation and the competition between bond cleavage, electron transfer, and hydrogen atom transfer in the contact pairs resulting from the two types of cleavage.

An acidity scale, [H+]hv, for proton quenching of excited states

“…This result was rationalized as a clear example of the photochemical "meta" effect as outlined by Zimmerman (34,35) in his pioneering work on photosolvolysis. In contrast to this observation, we had not observed an enhanced rate of photosolvolysis for the 3-methoxy versus the 4-methoxy substituent in a study of the photosolvolysis of (l-naphthylmethy1)trimethylammonium chlorides, 17 (19). In fact, the 4-methoxy compound reacted about 3.4 times as fast as the 3-methoxy one.…”

“…[19] is the only term that could vary with acid concentration. The fact that linear plots are obtained, i.e.…”

“…Therefore the term in brackets in eq. [19], which must be a constant, could be close to unity for all substrates. Results in aqueous perchloric acid, a medium in which measurement of these ratios is possible, indicate that this suggestion is very nearly true for most substrates (29).…”

“…Although only a working hypothesis, the simplification of eq. [19] to eq. [20] seems justified on this basis.…”

“…method is probably determined mainly by the cancellation of the activity coefficient term in eq. [19].…”