139La NMR as a tool for kineic studies

Abstract

AbstractThe application of 139La NMR to study the kinetics of substitution reactions is described. Rate constants associated with fast substitution reactions on La3+ complexes in the presence of excess of the aqua‐ion can be obtained by measuring the line widths of the corresponding signals. Measurements as a function of temperature allow the separation of the kinetic and quardrupolar relaxation effects. The rate of aquation of La(2,6‐dicarboxy‐4‐hydroxypyridine)+ was measured in aqueous solution. The dissociation rate constant and the activation parameters obtained for this process are k298r = 207 ± 51 s−1, ΔH = 38.3 ± 3.1 kJ mol−1 and ΔS = −72 ± 8 J K−1 mol−1. The spin–spin relaxation rates of the lanthanum(III) in the two environments are described by 1/T = 550 ± 83 s−1 and EA = 26.4 ± 5.8 kJ mol−1 for the aqua‐ion and by 1/T = (2.00 ± 0.08) × 104 s−1 and EB = 16.3 ± 1.6 kJ mol−1 for the mono‐complex.