A molecular approach to quadrupole relaxation. Monte Carlo simulations of dilute Li +, Na +, and Cl − aqueous solutions

Abstract

The electric field gradient fluctuation at the Li +, Na +, and Cl − ions in dilute aqueous solutions is calculated by means of Monte Carlo simulations. The potential energy functions and the field gradient expressions were obtained from ab initio quantum chemical calculations. The simulations reveal that the fluctuations at the ions are dominated by contributions from the water molecules in the first hydration shell. For the cations Li + and Na +, translations of water molecules within the first shell cause the main part of the fluctuation, while rotations of individual water molecules contribute to a lesser extent. For the anion Cl −, translations and rotations give about the same fluctuation. An analysis of the time scales associated with different kinds of motions of the water molecules in the first shell strongly supports a relaxation model where the decay of the field gradient time correlation function is determined by two different time scales. One fast process is associated with the translations within the first shell, with a correlation time well below 1 picosecond, and another slower process is composed of the rotations of individual water molecules and the hydration complex as a whole. The correlation time for the latter process is about 10 psec. A comparison is also made between the field gradients obtained from a classical description (point dipole) of a water molecule versus a quantum chemical treatment. Both qualitative and quantitative differences occur, which are especially pronounced for Li +. Finally, a polarization factor, often used in a classical treatment of quadrupolar relaxation of ions, is examined in some detail, and the derivation of a more general form of such a factor is given.