MOLECULAR-DYNAMICS SIMULATION STUDY OF THE NMR RELAXATION OF XE-131 DISSOLVED IN 1,3-DIOXANE AND 1,4-DIOXANE

The experimental NMR relaxation study of Xe-131 dissolved in 1,3-dioxa ne and 1,4-dioxane indicates that the intermolecular quadrupole relaxa tion mechanism is equally as efficient in both solvents even if 1,3-di oxane is a dipolar molecule while 1,4-dioxane is not. In order to inte rpret this observation, molecular-dynamics simulations were performed for model systems of xenon gas dissolved in 1,3-dioxane and 1,4-dioxan e. The simulations were able to satisfactorily reproduce various exper imental data for each system and, in perfect agreement with the experi ment, yielded the same Xe-131 quadrupole relaxation rate in both solve nts. This result was obtained assuming an electrostatic origin of the electric-field gradient, and therefore validates this explanation. In 1,4-dioxane, the overwhelming part of the fluctuating electric-field g radient experienced by the xenon nucleus is due to the quadrupole mome nt of the solvent molecules. In 1,3-dioxane, the dipole moment is resp onsible for approximately half the value of the amplitude of the elect ric-field-gradient fluctuations only. Contributions at least up to the octopole moment are important and, consequently, the correlation time characterizing the electric-field-gradient fluctuations in 1,3-dioxan e is significantly shorter than the dipolar correlation time and is fo und to be similar to the correlation time value in 1,4-dioxane. The re laxation rate of Xe-131 in dioxanes is compared to the value in other solvents including cyclohexane, and comments are made on the general c oncept of polarity. (C) 1995 Academic Press, Inc.