Molecular Parameterisation and the Theoretical Calculation of Electrode Potentials

The difference in reduction potentials between ortho and para-benzoquinones has been calculated. The employs gas phase ab initio and semi-empirical computations in combination with free energy perturbation theory applied to gas and solution phase Monte Carlo simulations. The effects on calculated results of altering solute electrostatic parameterisation in solution phase simulations is examined. Atom centred charges derived from the molecular electrostatic potentials, MEPs, from optimised ab initio wavefunctions and charges generated by consideration of hydrogen bonded complexes are considered. Parameterisation of hydroxyl torsions in hydroquinone molecules is treated in a physically realistic manner. The coupled torsional system of the ortho-hydrobenzoquinone molecule is described by a potential energy surface calculated using gas phase AM1 semi-empirical computations rather than the simple torsional energy functions frequently employed in such calculations. Calculated differences in electrode potentials show that the electrostatic interactions of quinone and hydroquinone molecules in aqueous solution are not well described by atom centred charges derived from ab initio calculated MEPs. Moreover, results in good agreement with the experimental reduction potential difference can be obtained by employing high level ab initio calculations and solution phase electrostatic parameters developed by consideration of hydrogen bonded complexes.