MOLECULAR-DYNAMICS SIMULATION OF THE ZERO-FIELD SPLITTING FLUCTUATIONS IN AQUEOUS NI(II)

The fluctuations in the zero-held splitting (ZFS) of the electronic gr ound state of the Ni(II) ion in aqueous solution have been studied thr ough a combination of ab initio quantum chemistry calculations, includ ing spin-orbit coupling, and molecular dynamics (MD) simulations. The ab initio calculations for the hexa-aquo Ni(II) complex have been used to generate an expression for the ZFS as a function of the distortion s of the idealized T-h symmetry of the complex along the normal modes of E(g) and T-2g symmetries. The MD simulations provide a 200 ps traje ctory of motions in the system consisting of a Ni(II) ion and 255 wate r molecules, which is analyzed in detail in terms of both the structur e and the dynamics in the solvation sphere around the ion. The time co rrelation function (TCF) for the ZFS interaction has been computed and analyzed. It is found that the mean square amplitude of the ZFS is ab out 5.2 cm(-1), which is about twice the estimates based on the model- dependent analysis of the proton spin relaxation in the aqueous Ni(II) solution. The decay of the ZFS TCF is found to occur on a subpicoseco nd time scale, which is much faster than earlier proposals. It is also interesting to note, for comparison with theoretical models, that the ZFS tenser is far from cylindrical and that the normal modes of E(g) och T-2g symmetry both contribute to its fluctuations. (C) 1995 Americ an Institute of Physics.