DISSOCIATION-CONSTANTS OF WEAK ORGANIC-ACIDS IN PROTIC SOLVENTS OBTAINED FROM THEIR FIRST HYPERPOLARIZABILITIES IN SOLUTION

The first hyperpolarizabilities (beta) of some weak aromatic organic a cids have been measured in protic solvents by the hyper-Rayleigh scatt ering (HRS) technique at low concentrations. The measured hyperpolariz ability (beta(m)) varies between the two extreme limits: the hyperpola rizability of the acid form (beta(HA)) at the lower side and that of t he basic form (beta(A-)) at the higher side. The degree of dissociatio n (alpha) of the acid in a solvent is related to the measured hyperpol arizability, beta(m), by the following relationship: beta(m)(2)=(1-alp ha)beta(HA)(2)+alpha beta(A-)(2). The calculated beta's including solv ent effects in terms of an Onsager field do not reproduce the experime ntally measured hyperpolarizabilities. Other solvent-induced effects l ike hydrogen bonding and van der Waals interactions seem to influence the first hyperpolarizability and, thus, indirectly the extent of diss ociation of these weak acids in these protic solvents.