KINETIC METHOD BASED ON NUCLEAR-MAGNETIC-RESONANCE MEASUREMENTS

A new kinetic method to study redox reactions in solution based on the magnetic susceptibility measurements has been developed. The resonance frequency of the H-O-D signal of an aqueous solution with 1-5% D2O containing paramagnetic species depends on the concentration and number of unpaired electrons; the frequency-time traces constitute the kinetic curves. These rate profiles can be solved by applying conventional rate laws. In addition to rate constants, these curves readily yield molar susceptibilities of intermediates and products and therefore the oxidation states of the same can be determined by monitoring the time domain H-O-D signal. For consecutive first-order reactions, the sequence of rate constants can be assigned. Furthermore, these traces deal with the redox reactions only and complications due the secondary substitution reactions can be avoided. This method has been applied to four known redox reactions. The frequency-time traces for the oxidation of homocysteine by the dichromate ion yielded a rate constant of 1.56 x 10(-3) s-1 using a 10-fold excess of reductant (20.0 mM), which is in good agreement with 1.63 x 10(-3) s-1 obtained by the visible spectroscopic method. The molar susceptibility, 6.23 x 10(-3) cm3 mol-1, and a magnetic moment, 3.84-mu-B, were also evaluated for the Cr(III) product from the kinetic traces. The oxidation of cysteine by the dichromate ion yielded a rate constant of 1.2 x 10(-2) s-1 using a 10-fold excess of reductant (21.0 mM), which is in excellent agreement with the value reported by Kwong and Pennington. The molar susceptibility (6.98 x 10(-3) cm3 mol-1) and magnetic moment (4.08-mu-B) are in keeping with the formation of chromlum(III) products. The second-order rate constant, 8.5 x 10(-2) M-1 s-1 obtained for the permanganate ion oxidation of uracil is also in excellent agreement with the literature values (8.8 x 10(-2) M-1 s-1). The magnetic moment, 3.80-mu-B supports a predominant Mn(IV) product, in keeping with the report by Freeman and co-workers. Finally, the oxalic acid oxidation by the dichromate ion exhibits biphasic kinetic profiles and the kinetic traces yielded a magnetic moment, 1.9-mu-B, for the intermediate in accord with the formation of a transient Cr(V) species during the reaction. The rate constants for the formation of intermediate and its decomposition were calculated to be 1.8 x 10(-3) and 3.2 x 10(-4) s-1 utilizing 5.0 mM Cr(VI) and 50.0 mM oxalic acid. These rate constants are also in good agreement with the values (1.7 x 10(-3) and 3.6 x 10(-4) s-1 for the formation and decomposition of the intermediate) obtained from conventional absorbance-time curves. These traces also demonstrate that the rate constant for the formation of the intermediate is larger than that for its decay.