Separation of cross-relaxation and chemical exchange effects in magnetization transfer experiments: application to a trialkylaluminium-dialkylamine complex

In general, quantitative analysis of two-dimensional magnetization transfer experiments (e.g. NOESY) on labile molecular systems yields values for the differences between exchange rate coefficients and cross-relaxation rates (i.e. k-sigma). Such a system is the diastereotopic methylene proton pair i n the amine moiety of the triisobutylaluminium-diethylamine complex. It is shown how quantification of the C-13-H-1 dipole-dipole interaction may be u sed to estimate a value for the interproton cross-relaxation rate, sigma, w hich after analysis of the NOESY spectra, gave a value for the pseudo-first -order exchange rate coefficient, k, of 0.9(7) s(-1) at 300 K. Bandshape an alysis of the spectra of the same complex in the presence of excess amine g ave a pseudo-first-order rate coefficient, k = 128.6 s(-1), which may be a true second-order rate coefficient k(c) = 1.3 x 10(3) mol(-1) dm(3) s(-1). The possibility of a duality of mechanism is proposed for the exchange proc ess, viz a unimolecular (cf. S(N)1) and a bimolecular (cf. S(N)2). Eyring a nalysis of the temperature dependence of the derived rate coefficient from the sample with excess amine gave values for the activation parameters of D elta H* = 22.4 +/- 0.6 kJ mol(-1) and Delta S* = -130 +/- 3 J K-1 mol(-1). Copyright (C) 2000 John Wiley & Sons, Ltd.