Nuclear and electron spin relaxation in paramagnetic complexes in solution: effects of the quantum nature of molecular vibrations.

A model of the paramagnetic relaxation enhancement is developed in terms of electron-spin relaxation caused by the zero-field splitting (ZFS) fluctuating in time due to a coupling between the electron-spin variables and quantum vibrations. The ZFS interaction provides a coupling between the electron-spin variables and vibrational degrees of freedom, and is represented as a Taylor series expansion in a set of vibrational modes (normal coordinates). A two-level harmonic oscillator subsystem is assumed, and the electron-spin relaxation associated with T2V and T1V vibrational relaxation is considered. The description of vibrationally induced electron-spin dynamics is incorporated into the calculations of the paramagnetic relaxation enhancement by the Solomon-Bloembergen-Morgan approach as well as in the framework of the general slow-motion theory. The theoretical predictions are compared with the experimental paramagnetic relaxation enhancement values for the Ni(H2O)6(2+) complex in aqueous solution. The parameters required by the model are obtained from quantum chemical and molecular dynamics studies. Comparison is made between the current model and its recently published classical counterpart.