−OH-Induced shift from carbon to oxygen acidity in the side-chain deprotonation of 2-, 3- and 4-methoxybenzyl alcohol radical cations in aqueous solution: results from pulse radiolysis and DFT calculations

DFT calculations have been carried out for 2-, 3- and 4-methoxybenzyl alcohol radical cations (radical dot+, radical dot+ and radical dot+, respectively) and the α-methyl derivatives radical dot+ and radical dot+ using the UB3LYP/6-31G(d) method. The theoretical results have been compared with the experimental rate constants for deprotonation of radical dot+–radical dot+ under acidic and basic conditions. In acidic solution, the decay of radical dot+–radical dot+ proceeds by cleavage of the C–H bond, while in the presence of −OH all the radical cations undergo deprotonation from the α-OH group. This pH-dependent change in mechanism has been interpreted qualitatively in terms of simple frontier molecular orbital theory. The −OH induced α-O–H deprotonation is consistent with a charge controlled reaction, whereas the C–H deprotonation, observed when the base is H2O, appears to be affected by frontier orbital interactions.