Proton magnetic relaxation in acidified aqueous solutions containing Cr(III)

Abstract

Nuclear magnetic relaxation rates of protons in acidic solutions of Cr (III) have been measured as a function of hydrogen ion concentration at 28°C and 60.00Mhz in HClO4 from 0.001 M to 10 M and in Hcl and H2SO4 from 0.5 M to 10 M. The scalar coupling constant was determined for each acid from experimental proton chemical shifts. The proton relaxation rates are controlled by both relaxation processes, T2b' and proton-exchange rates.

In weak acid solutions that proton-exchange is due to an uncatalyzed hydrolysis reaction. In solutions of moderate acidity (0.5 M to 6 M) the increase in the observed proton relaxation is due to an acid-catalyzed proton-exchange between the Cr(III) ions and the solvent. The increase in proton relaxation rates in that region is controlled by the proton-exchange correlation time given by Tb= (kba±)-l where a± is the mean acid activity. The rate constant for acid-catalyzed proton-exchange, kb, was determined for each acid. The relative magnitudes of those rate constants were used as evidence of a specific acid-catalysis of proton-exchange in HCl04 , HCl and H2SO4. In addition to the specific acid-catalyzed proton-exchange, a general acid catalyzed exchange was proposed for H2SO4 where the general acid-catalyst is presumable HSO4-. At higher acid concentrations the proton relaxation rates become relaxation controlled (T2b) for HCl. At high concentrations (above 6 M) of HClO4 and H2SO4 the relaxation rates are controlled by the T2b and some other process(es). We have rationalized that process(es) by proposing that the electron-spin relaxation time and/or the proton residence time decreased at the highest acid concentrations.