Deuterium isotope effect in concentrated aqueous solutions. A potentiometric and 13C nuclear magnetic resonance study of acid dissociation constants

Abstract

The acid dissociation constants have been determined by potentiometric titration in H2O and D2O for the zinc(II) complexes of 2,2′-ethylenedi-iminobis(ethylamine)(trien), 2,2′-trimethylenedi-iminobis(ethylamine)(2,3,2-tet) and 2,2′,2″-nitrilotris(ethylamine)(tren) and by 13C n.m.r. spectroscopy in D2O for ethylenediamine (en;2), glycine (gly;2), L-alanine (ala), acetic acid (ace) and N-(2-aminoethyl)propane-1,3-diamine (2,3-tri;3). The equilibrium deuterium isotope effects are similar, pKa(D2O)–pKa(H2O)= 0.63±0.07. This result suports the treatment of the protonated forms of polyamines as ‘hybrid’ ions in respect of their contribution to ionic strength and indicates that the coordination number of ZnII in the low-pH forms of the three complexes is at least five.