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–σ). Such a system is the diastereotopic methylene proton pair in the amine moiety of the triisobutylaluminium–diethylamine complex. It is shown how quantification of the 13C–1H dipole–dipole interaction may be used to estimate a value for the interproton cross-relaxation rate, σ, which after analysis of the NOESY spectra, gave a value for the pseudo-first-order exchange rate coefficient, k, of 0.97 s−1 at 300 K. Bandshape analysis of the spectra of the same complex in the presence of excess amine gave a pseudo-first-order rate coefficient, k = 128.6 s−1, which may be a true second-order rate coefficient kc = 1.3 × 103 mol−1 dm3 s−1. The possibility of a duality of mechanism is proposed for the exchange process, viz a unimolecular (cf. SN1) and a bimolecular (cf. SN2). Eyring analysis of the temperature dependence of the derived rate coefficient from the sample with excess amine gave values for the activation parameters of ΔH* = 22.4± 0.6 kJ mol−1 and ΔS* = −130 ± 3 J K−1 mol−1. Copyright © 2000 John Wiley & Sons, Ltd.