The association of neutral systems linked by hydrogen bond interactions: a quantitative electrochemical approach

The electrochemical characterization of the neutral-neutral association by hydrogen bonds was performed on the basis of voltammetric current measurements. The diffusion coefficient of the electroactive compound is modified by effect of association, this provoke important variations in the voltammetric current peak. As an example of the weak hydrogen bond between neutral complexes, it was determined that 1,4-benzoquinone (Q) and benzoic acid (HBz) can associate with a 1:1 stoichiometry with a conditional association constant between 10 and 15 M-1. The Q(HBz) complex geometry was optimized using density functional theory and Moller-Plesset perturbation theory. In both theories, the most stable geometry is flat and exhibits two hydrogen bond interactions: O-(HO)-O-... and C-(HO)-O-... interactions. The binding energy at our best level of theory was -7.7 kcal/mol, that supports the stability of the 1:1 Q-HBz complex and which is accord with the values of the conditional association constant obtained from the electrochemical method here described. (C) 2002 Elsevier Science B.V. All rights reserved.