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 geometries of QH 2 , QH − , and Q 2− and of the QH 2 · 6H 2 O, QH − · (H 3 O + ) · 5H 2 O, and Q 2− · (H 3 O + ) 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 − · (H 3 O + ) · 9H 2 O, and QH − · (H 3 O + ) · 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. © 1995 John Wiley & Sons, Inc.