DETERMINATION OF PRIMARY AND SECONDARY SOLVATION NUMBERS AND BINDING CONSTANTS BASED ON THE SHIFT OF A PROTON-TRANSFER EQUILIBRIUM RESULTING FROM HYDROGEN BONDING SOLVATION

摘要

We show how the shift in the equilibrium constant K-PT for formation of a proton-transfer adduct in a non-interactive solvent, upon addition of a second, hydrogen-bonding solvent S reveals the nature of the hydrogen bonding solvation process. Data are analyzed for the pentachlorophenoltriethylamine proton-transfer equilibrium in cyclohexane solvent, undergoing solvation by the acidic alcohols, 2,2,2-trichloroethnnoi and 1,1,1,3,3,3-hexafluoro-2-propanol. K-PT vs.[S] data are fitted to a binding isotherm corresponding to two-stage solvation of both the adduct and the free amine. Stoichiometries and binding constants for both primary and secondary solvation ofboth solvated species are determined as adjustable parameters. Best fits correspond to both the adduct andfree amine undergoing primary solvation by one alcohol molecule (presumably at the oxygen and nitrogen lone-pairs, respectively) followed by secondary solvation by one to nine additional alcohol nrolecules, with binding constants ranging from 2100 M(-1), for primary solvation of the adduct by hexafluoro-2-propanol, down to 7 M(-1), for secondary solvation of the amine by trichloroethanol. We speculate that the secondary solvation numbers represent average sizes of hydrogen-bonded alcohol chains, nucleated by the enhanced basicity of the primary-solvation alcohol. [References: 35]