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Separation of cross‐relaxation and chemical exchange effects in magnetization transfer experiments: application to a trialkylaluminium–dialkylamine complex
Author(s) -
Bento Edson S.,
Bradley Donald C.,
Hawkes Geoffrey E.,
Maia Izaque A.,
Sales Keith D.
Publication year - 2000
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/1097-458x(200005)38:5<331::aid-mrc648>3.0.co;2-3
Subject(s) - chemistry , diethylamine , relaxation (psychology) , magnetization transfer , analytical chemistry (journal) , magnetization , amine gas treating , computational chemistry , organic chemistry , medicine , psychology , social psychology , physics , quantum mechanics , magnetic field , magnetic resonance imaging , radiology
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 13 C– 1 H 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 0.9 7 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 k c = 1.3 × 10 3 mol −1 dm 3 s −1 . The possibility of a duality of mechanism is proposed for the exchange process, viz a unimolecular (cf. S N 1) and a bimolecular (cf. S N 2). Eyring analysis of the temperature dependence of the derived rate coefficient from the sample with excess amine gave values for the activation parameters of Δ H * = 22.4± 0.6 kJ mol −1 and Δ S * = −130 ± 3 J K −1 mol −1 . Copyright © 2000 John Wiley & Sons, Ltd.