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

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 geom etries 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 mode l the solvated species. The presence of the associated water molecules (and hydronium ions) account for the stabilization due to hydrogen bo nding as well as for a part of the effect of interaction of these mole cules with the respective reaction fields in an aqueous medium. To sim ulate the first solvation shell in a more complete manner, four more w ater molecules have been considered to be placed above and below the q uinonoid ring and the optimized geometries of the resulting hydrated s pecies, QH(2) (.) 10H(2)O, QH(- .) (H3O+) . 9H(2)O, and QH(- .) (H3O+) (.) 8H(2)O, have been determined. The standard free-energy changes ca lculated for the dissociation of QH(2) into QH(-) and H+ is 0.0251 Har tree (65.9 kJ mol(-1)) and that of QH(-) into Q(2-) and H+ is 0.0285 H artree (74.8 kJ mol(-1)). Experimentally observed dissociation constan ts for these two steps correspond to free-energy changes of 0.0214 Har tree (56.2 kJ mol(-1)) and 0.0248 Hartree (65.1 kJ mol(-1)), respectiv ely. (C) 1995 John Wiley and Sons, Inc.