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

We show how the shift in the equilibrium constant K-PT for formation o f a proton-transfer adduct in a non-interactive solvent, upon addition of a second, hydrogen-bonding solvent S reveals the nature of the hyd rogen bonding solvation process. Data are analyzed for the pentachloro phenoltriethylamine 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 ad duct and the free amine. Stoichiometries and binding constants for bot h primary and secondary solvation ofboth solvated species are determin ed as adjustable parameters. Best fits correspond to both the adduct a ndfree amine undergoing primary solvation by one alcohol molecule (pre sumably at the oxygen and nitrogen lone-pairs, respectively) followed by secondary solvation by one to nine additional alcohol nrolecules, w ith binding constants ranging from 2100 M(-1), for primary solvation o f the adduct by hexafluoro-2-propanol, down to 7 M(-1), for secondary solvation of the amine by trichloroethanol. We speculate that the seco ndary solvation numbers represent average sizes of hydrogen-bonded alc ohol chains, nucleated by the enhanced basicity of the primary-solvati on alcohol.