The proton relaxation of methyl cyanide in the presence of Ni(II) ions, as studied by spin-echo techniques

The proton relaxation times (T 1 and T 2 ) of methyl cyanide, in the presence of Ni(II) ions, have been measured as a function of temperature and frequency by spin-echo techniques. The observed decay times (T 2 †) have also been measured as a function of the separation, t ep , between the pulses in a Carr-Purcell train of echos. The results obtained in the limit of long pulse separation have been fitted to existing theories. The rotational correlation time of the Ni/MeCN complex, τ R , the electron spin relaxation time, τ S , and the distance of closest approach of the protons to the nickel have been estimated from the results. In addition the scalar coupling constant, the exchange rate τ B and the activation energies for τ B , τ S and τ R have been evaluated. The chemical exchange parameters obtained differ significantly from those obtained by previous workers. There is evidence from the pulse dependence experiments that the complex is a distorted octahedron.