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La-139 NMR kinetic study of lanthanum(III) complexation with acetohydroxamic acid

The application of La-139 NMR spectroscopy to kinetic and equilibrium studies of the substitution of coordinated water by acetohydroxamic acid (HA) is described. The reaction of La3+ with HA can be represented by the following equation (with equilibrium constant K-1): La3+ + HA reversible arrow La(A)(2+) + H+ (1). The following equilibrium parameters were calculated from the La-139 NMR chemical shift of the equimolar reactants measured as a function of acidity and temperature: K-1(298) = 2.36(8)(.)10(-4), Delta(r)H(1) = -28(l) kJ(.)mol(-1), Delta(r)S(1) = -162(4) J(.)K(-1.)mol(-1). The kinetics of the reaction between La3+ and HA can be represented by the following equations: La3+ HA reversible arrow La(A)(2+) + H+ (11) and La3+ + A(-) reversible arrow La(A)(2+) (12). The complexation rate constants were calculated from the line widths of the corresponding signals as a function of acidity and temperature: k(11)(298K) = 1.54(4)(.)10(6) M(-1.)s(-1), DeltaH(11)(double dagger) = 70(14) kJ(.)mol(-1) and DeltaS(11)(dagger) = +108(47) J(.)K(-1.)mol(-1), k(12) (298 K) = 1.6 10(10) M(-1.)s(-1), DeltaH(12)(double dagger) = 35(6) kJ(.)mol(-1) and DeltaS(12)(double dagger) = +68(20) J(.)K(-1.)mol(-1). The spin-spin relaxation times of La-III in the aqua ion and the hydroxamato complex are 4.36(4) ms and 54(l) ps at 298 K, respectively.

La-139 NMR; kinetics; equilibrium; acetohydroxamic acid;

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