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, pK_a(D₂O)–pK_a(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 Zn^II in the low-pH forms of the three complexes is at least five.