Salt Effects on the Kinetics of the Proton-Transfer Reactions in the 3,5-Dinitrohydroxybenzoic Acid/Ammonia System

Abstract

A T-jump study of the 3,5-dinitrohydroxybenzoic acid (DNSA)/ammonia system has been undertaken, as a function of the nature and concentration of the added salt. Rate constants for the different possible proton-transfer reactions have been obtained from the experimental relaxation curves. The mechanism has been analyzed with the aid of a theoretical model based on Marcus- like free energy surfaces for each elementary step. Despite possible uncertainties in the estimations and approximations used, two conclusions seem to be well founded: first, the stepwise character of the proton-transfer reactions involving DNSA, and second, the possible change in the mechanism connected with the change in the final proton acceptor in the deprotonation of DNSA. This result, not previously assessed in the literature, is relevant for analyzing the salt effects, as is discussed in this work, and also for the hypothetical application of such a system in a molecular protonic device, an area of current research interest. Finally, the use of the well-known plot of log(k) versus ΔpKa for analyzing the negative or positive salt effects on the proton-transfer reactions is suggested.