Pc. Ray et Pk. Das, DISSOCIATION-CONSTANTS OF WEAK ORGANIC-ACIDS IN PROTIC SOLVENTS OBTAINED FROM THEIR FIRST HYPERPOLARIZABILITIES IN SOLUTION, Journal of physical chemistry, 99(51), 1995, pp. 17891-17895
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.