DETERMINATION OF THE ACID DISSOCIATION-CONSTANTS OF P-BENZOHYDROQUINONE BY THE INDO METHOD

Abstract

The stepwise acid dissociation constants for p-benzohydroquinone (QH(2)) in aqueous media have been explicitly calculated for the first time, with the INDO parametrized SCF-MO method. We have optimized the geometries of QH(2), QH(-), and Q(2-) and of the QH(2) (.) 6H(2)O, QH(- .) (H3O+) (.) 5H(2)O, and Q(2- .) (H3O+)(2) (.) 4H(2)O systems that model the solvated species. The presence of the associated water molecules (and hydronium ions) account for the stabilization due to hydrogen bonding as well as for a part of the effect of interaction of these molecules with the respective reaction fields in an aqueous medium. To simulate the first solvation shell in a more complete manner, four more water molecules have been considered to be placed above and below the quinonoid ring and the optimized geometries of the resulting hydrated species, QH(2) (.) 10H(2)O, QH(- .) (H3O+) . 9H(2)O, and QH(- .) (H3O+) (.) 8H(2)O, have been determined. The standard free-energy changes calculated for the dissociation of QH(2) into QH(-) and H+ is 0.0251 Hartree (65.9 kJ mol(-1)) and that of QH(-) into Q(2-) and H+ is 0.0285 Hartree (74.8 kJ mol(-1)). Experimentally observed dissociation constants for these two steps correspond to free-energy changes of 0.0214 Hartree (56.2 kJ mol(-1)) and 0.0248 Hartree (65.1 kJ mol(-1)), respectively. (C) 1995 John Wiley and Sons, Inc.