PARAMAGNETIC ENHANCED PROTON SPIN-LATTICE RELAXATION IN THE NI-2-AQUOCOMPLEX - A THEORETICAL AND MOLECULAR-DYNAMICS SIMULATION STUDY OF THE BLOEMBERGEN-MORGAN DECOMPOSITION APPROACH( HEXA)

A reinterpretation of the experimental NMR proton spin-lattice dispers ion curve of the Ni2+(H2O)6 complex is presented within a general slow -motion theory. The extended pseudo rotation (PR) model developed allo ws for crosscorrelation effects between the nuclear spin-electron spin dipole-dipole and zero field splitting (ZFS) interaction. It is shown that the decomposition approach, treating the electron spin relaxatio n and the reorientational dynamic of the dipole-dipole correlation fun ction as independent processes is not generally valid. For the Ni hexa -aquo complex the transient ZFS interaction and the reorientational co rrelation time change by about 20 per cent due to the correlation effe cts. Molecular dynamics (MD) simulation of a divalent ion in water pro vided the timescale of the dynamics present in the PR and the Smolucho wsky models. The structure and dynamics of the octahedral complex is d escribed. The transient ZFS interaction generated by the low frequency vibration modes n(M <-- --> OH2) is characterized by a correlation ti me tau(v) = 0.2 ps and the timescale of the orientational motion of th e PR model is in the range tau(v) = 1-10 ps. The fast E- and A-symmetr ic vibrations cause partial averaging of the ZFS interaction. The time scale of the ligand orientational modes, wag, twist and rock modes are comparable with the electron spin dynamics. The reinterpretation of t he NMRD curve suggests that the dynamics described by the single expon ential reorientational correlation time, tau(R) reflects the wag, twis t and rock modes rather than the overall reorientation of the whole me tal-aquo complex. The simulations also suggest that the relatively sho rt proton metal ion distance obtained from the interpretation of the N MRD curve reflects neglect of outer sphere contributions rather than o versimplification of electron spin dynamics.