Inter- and intramolecular energy transfer in gammaradiolysis of n-alkylbenzenes

Hydrogen production in γ-irradiated n-alkylbenzenes (toluene to heptadeoylbenzene), in isomeric n-alkyl-benzenes and n-alkylbenzenes-d5 was investigated. Radicals formed at 77 °K were identified by EPR-spectroscopy and their G-values measured. A mechanism for the formation of hydrogen, based on ion-molecule reactions and hot atom reactions, is proposed. The low G(H2) und G(R) values of the various n-alkylbenzenes indicate that the phenyl group protects the alkyl group, with respect to hydrogen and radical formation. Energy transfer was suggested to explain the results. A Markov chain approach shows that energy transfer from the aliphatic chain to the phenyl group is 13 times as probable as the dissociation of a C-H bond and the desactivation of the excited state involved. To separate inter- and intramolecular energy transfer, a similar investigation with alkane/benzene mixtures was carried out. The probability for energy transfer from the alkane chain to the benzene molecule was found to be 10 times as probable as the two other reactions. The results suggest that in n-alkylbenzenes besides intermolecular transfer also intramolecular transfer takes place, with comparable efficiency.