205Tl longitudinal relaxation rate measurements were performed on several thallium(III) complexes with the composition Tl(OH) n(H2O)6-n(3-n)+ (n = 1,2), Tl(Cl)n(H2O)m-n(3-n)+, Tl(Br) n(H2O)m-n(3-n)+ (m = 6 for n = 1-2, m = 5 for n = 3, m = 4 for n = 4), Tl(CN)n(H2O) m-n(3-n)+ (m = 6 for n = 1-2, m = 4 for n = 3-4) in aqueous solution, at different magnetic fields and temperatures. 13C and 2D isotopic labelling and 1H decoupling experiments showed that the contribution of the dipolar relaxation path is negligible. The less symmetric lower complexes (n < 4) had faster relaxation rate dominantly via chemical shift anisotropy contribution which depended on the applied magnetic field: T1 values are between 20 and 100 ms at 9.4 T and the shift anisotropy is Δσ = 1000-2000 ppm. The tetrahedral complexes, n = 4, relax slower; their T1 is longer than 1 s and the spin-rotation mechanism is probably the dominant relaxation path as showed by a temperature dependence study. In the case of the TlCl4- complex, presumably a trace amount of TlCl52- causes a large CSA contribution, 300 ppm. Since the geometry and the bond length for the complexes in solution are known from EXAFS data, it was possible to establish a correlation between the CSA parameter and the symmetry of the complexes. The relaxation behaviour of the Tl-bromo complexes is not in accordance with any known relaxation mechanism.
ASJC Scopus subject areas
- Materials Science(all)