Longitudinal relaxation of protons in partially deuterated [Al(H2O)6]3+

Abstract

Chemical shifts, isotopic fined-sturcture patterns and relaxation times are reported for HOD and HOH protons in partially deuterated [Al (OH₂)₆]³⁺ dissolved in acetonitrile-d3 at –30 and –50 °C. The chemical and isotopic shifts are discussed in relation to different shifts is other solvents, and the find structure proves the Al^III cation to be hexahydrated. T₁ and T^*₂ for the HOH protons are much shorter than for the HOD protons, and both vary somewhat with the extent of deuteration. We account for all these data at –50 °C, plus ²H relaxation times, in an approximately quantitative way, using τ_c= 840 ps, Some allowance for internal motions is necessary. Rotational correlation times are also presented for dissolved water, and comparisons are made with other data, to show that outer-sphere association or ion-pairing slows both the cation and the dissolved water motions at –50 °C.