Dissociation Constants of Weak Organic Acids in Protic Solvents Obtained from Their First Hyperpolarizabilities in Solution

Article abstract of DOI:10.1021/j100051a014

The first hyperpolarizabilities (β) of some weak aromatic organic acids have been measured in protic solvents by the hyper-Rayleigh scattering (HRS) technique at low concentrations.The measured hyperpolarizability (β_m) varies between the two extreme limits: the hyperpolarizability of the acid form (β_HA) at the lower side and that of the basic form (β_A-) at the higher side.The degree of dissociation (α) of the acid in a solvent is related to the measured hyperpolarizability, β_m, by the following relationship: β_m² = (1 - α)β_HA² + αβ_A-².The calculated β's including solvent effects in terms of an Onsager field do not reproduce the experimentally measured hyperpolarizabilities.Other solvent-induced effects like hydrogen bonding and van der Waals interactions seem to influence the first hyperpolarizability and, thus, indirectly the extent of dissociation of these weak acids in these protic solvents.