Effects of zero-field splitting interactions on NMR relaxation in paramagnetic solutions.

Abstract

The influence of ZFS interactions on the NMR paramagnetic relaxation enhancement (NMR-PRE) produced by S $\ge$ 1 transition metal ions was studied theoretically using the spin dynamics (SD) simulation method and experimentally through T$\sb1$ and T$\sb2$ measurements. Three of the central predictions of theory were examined. First, the influence of zfs-rhombicity on the relaxation efficiency produced by an S=1 spin system was studied using (Ni(acac)$\rm\sb2(H\sb2O)\sb2\rbrack$ and (Ni(II)Cl$\sb2$(pda)$\sb2\rbrack$. A theoretical prediction that zfs-rhombicity (E/D$\not=$0) strongly depresses the relaxation enhancement in the zfs-limit, by an order of magnitude or more, relative to the uniaxial condition (E=0) has been confirmed experimentally. This was attributed to the oscillatory motion induced in $\ S\sb{Z}$ of the ZFS-PAS, which decouples the spin motion from the zfs-interaction. The T$\sb1$ and T$\sb2$ field dispersion profiles (fdp's) increased several-fold with increasing magnetic field strength in the intermediate regime for the S=1 spin systems. The effects of zfs-rhombicity on the NMR-PRE were investigated for an S=3/2 spin system, (CO(acac)$\rm\sb2(H\sb2O)\sb2\rbrack$. This complex is isostructural with (Ni(acac)$\rm\sb2(H\sb2O)\sb2\rbrack$. The T$\sb1$ and T$\sb2$ relaxation of (CO(acac)$\rm\sb2(H\sb2O)\sb2\rbrack$ were found to be field-independent in the zfs-limit and intermediate regime. This is due to Kramers two-fold degeneracy that is characteristic of half-integer spin systems in the zfs-limit. The dependence of NMR-PRE on the positional coordinates (r$\Theta\Phi$) of the resonant nucleus in the molecule-fixed coordinate frame was examined. The T$\sb1$ and T$\sb2$ relaxation rates of the near axial and near equatorial protons of (Ni(acac)$\rm\sb2(H\sb2O)\sb2\rbrack$ and (CO(acac)$\rm\sb2(H\sb2O)\sb2\rbrack$ were measured, and the (axial/equatorial) R${\sb1\sb{\rm p}}$ ratios were calculated. Theory predicted that the (axial/equatorial) R${\sb1\sb{\rm p}}$ ratio is about 4:1 and 2:1 in the zfs-limit for S=1 and S=3/2 spin systems respectively. The experimental analysis indicate that the (axial/equatorial) R${\sb1\sb{\rm p}}$ ratio for spin S=1 is about 20:1 in the zfs-limit. The experimental ratio is approximately 10:1 for spin S=3/2 in the zfs-limit. These finding confirms a qualitative prediction of theory, although the experimental ratios are substantially larger than theoretically predicted. This is the first study undertaken to measure the magnetic-field dependence of T$\sb1$ and T$\sb2$ NMR-PRE of specific solvent and ligand 1H resonances for S $\ge$ 1 complex ions.