MOLECULAR PARAMETERIZATION AND THE THEORETICAL CALCULATION OF ELECTRODE-POTENTIALS

The difference in reduction potentials between ortho and para-benzoqui nones has been calculated. The employs gas phase ab initio and semi-em pirical computations in combination with free energy perturbation theo ry applied to gas and solution phase Monte Carlo simulations. The effe cts on calculated results of altering solute electrostatic parameteris ation in solution phase simulations is examined. Atom centred charges derived from the molecular electrostatic potentials, MEPs, from optimi sed 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 realist ic manner. The coupled torsional system of the ortho-hydrobenzoquinone molecule is described by a potential energy surface calculated using gas phase AM1 semiempirical computations rather than the simple torsio nal energy functions frequently employed in such calculations. Calcula ted differences in electrode potentials show that the electrostatic in teractions of quinone and hydroquinone molecules in aqueous solution a re not wed described by atom centred charges derived from ab initio ca lculated MEPs. Moreover, results in good agreement with the experiment al reduction potential difference can be obtained by employing high le vel ab initio calculations and solution phase electrostatic parameters developed by consideration of hydrogen bonded complexes.