Molecular motion of the dimethylthallium(III) cation in aqueous solution

Abstract

Thallium-205 spin-lattice relaxation times in the dimethylthallium cation have been measured over a range of temperature at frequencies of 21·96, 34·73 and 231·6 MHz. Although the relaxation is typically dominated by the chemical shift anisotropy (CSA) mechanism a contribution from the spin rotation (SR) mechanism has also been quantified. A value of the chemical shift anisotropy for this linear ion from other work (5550 ppm) enabled calculation of the reorientational correlation time τ⊥. An Arrhenius temperature dependence was found with a τ⊥ value of 39·1 ± 0·5 ps at 298 K and an activation energy of 19·7 ± 0·7 kJ mol-1. The axial symmetry, the linear thallium environment, a knowledge of Δσ, and of the isotropic shift have allowed an absolute shift scale to be determined which is in close agreement with previous estimates by other workers. Assignment of the shift zero allowed calculation of the spin rotation constant C ⊥, as 73·4 kHz. Hence values for the angular momentum correlation time τ J⊥ could be determined. An Arrhenius temperature dependence was found with a τ J⊥ value of 5·05 × 10-15 s at 298 K and an activation energy of 17·7 ± 0·7 kJ mol-1. The inadequacy of diffusion models to explain these data is demonstrated. The data are consistent with a quasi-lattice motional model in which librational motions and large angle jumps are present. In particular the short value for τ J⊥ is indicative of librational motions and consistent with molecular dynamics studies of aqueous systems.