N.M.R. relaxation studies of thallium compounds

Abstract

The thallium-205 spin relaxation behaviour of several thallium compounds have been investigated as afunction of temperature and frequency. For the aqueous dimethylthallium(III) cation there were significant contributions from both the chemical shift anisotropy (CSA) and spin rotation (SR) mechanisms at low fields but at high fields the former was dominant. Reorientational and angular momentum correlation times were obtained and their values were rationalised in terms of a structure limited motional model. Thallium-205 spin rotation constants and an absolute shielding scale were also determined.In the aqueous glycerol medium CSA was found to be the only significant relaxation mechanism for the dimethylthallium(III) cation at both high and low field. At high field the reorientational motion was outside extreme narrowing and an R1 maximum was observed. This allowed simultaneous determination of the motional parameters and of the shielding anisotropy as 5550 +/- 32 ppm. Again a structure limited reorientational model was appropriate.The motional dependence of the ratio R2/R1 was as expected for a single correlation time model, extrapolating to 7/6 at extreme narrowing. Dimethylthallium(III) chloride in DMSO-d, dineopentylthallium(III) chloride in pyridine-d and neat thallium(I) ethoxide all showed significant CSA relaxation. Low field thallium(I) ethoxide spectra were dominated by exchange phenomena but at high field severe line broadening of undetermined origin was observed. The observed effects of spin-lattice relaxation on the spectra of coupled nuclei were discussed. The proton spectra of aqeuous dimethylthallium(III) cation at 400 MHz and dineopentylthallium(III) chloride at 360 MHz were reproduced using a chemical exchange formalism. Preliminary measurements on aqueous thallium(I) ion in the presence of dissolved oxygen taken with previous studies showed an inverse dependence of on frequency.This was interpreted in terms of scalar relaxation and enabled estimation of the electronic relaxation times.