-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-metboxybenzyl alcohol radical cations (1(+), 3(.+) and 4(.+), respectively) and the alpha-methyl derivatives 2(.+) and 5(.+) using the U33LYP/6-31G(d) method. The theoretical results have been compared with the experimental rate constants for deprotonation of 1 1(.+)-5(.+) under acidic and basic conditions. In acidic solution, the decay of 1(.+)-5(.+) proceeds by cleavage of the C-H bond, while in the presence of -OH all the radical cations undergo deprotonation from the alpha-OH group. This pH-dependent change in mechanism has been interpreted qualitatively in terms of simple frontier molecular orbital theory. The -OH induced alpha-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. (C) 2003 Elsevier Science Ltd. All rights reserved.