Rotational dynamics of nondipolar probes in electrolyte solutions: Can specific interactions be modeled as dielectric friction?

In a bid to explore how the presence of electrolyte ions influence the friction experienced by hydrogen bonding and nonhydrogen bonding solute molecules, rotational dynamics of two structurally similar nondipolar probes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP), has been investigated in dimethylsulfoxide (DMSO) at several concentrations of LiNO[sub 3]. The reorientation times of DMDPP, which does not strongly interact with the solvent, follow solution viscosity and dielectric parameters as the electrolyte concentration is increased. However, for DPP, which forms hydrogen bonds with DMSO, there is a 30% decrease in the viscosity-normalized reorientation times upon the addition of 2M LiNO[sub 3] due to the presence of electrolyte ions that shield the hydrogen-bonding interactions between the solute and the solvent. However, the reorientation times correlate well with the solution dielectric parameters with an increase in the electrolyte concentration as in the case of DMDPP. An attempt has been made to model the specific interactions between DPP and DMSO as dielectric friction using the extended charge distribution model of Alavi and Waldeck since both are electrostatic in nature. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]