Rotation of large molecular ions and transient dielectric relaxation effects

Abstract

The rotation rates of charged molecules show dramatic changes as a function of solvent environment. The hydrodynamic models of rotational diffusion in the slip limit are useful estimates of the rotation rate for large, symmetrically charged molecules, if there are no strong solvent interactions. For cases of strong interactions, the rotation rate reduces by a factor of 5–6. For the symmetric, anionic dye molecule resorufin, we successfully modeled the rotation times in alcohols and water as a larger, solvent coordinated resorufin molecule in the slip limit. This model of average coordination was not successful in solvents of ethylene glycol or monosubstituted amides. These solvents have strong solvent-solvent interactions which probably reduced the average coordination to a point where the rotational rate is more like the hydrodynamic stick limit. Dielectric friction reduction of the rotation rate is possible for molecules which have an asymmetric charge distribution. Experimental evidence for this contribution exists in limited form; although more experiments are needed to separate dielectric friction from local coordination or solvent torques due to selective solvent interactions.

Several avenues for additional work were examined in this manuscript. We encourage developing a quantitative model of solvent torque to encompass the intermediate solvent interaction case, which is between coordination behavior and the slip limit. Dielectric friction theory should examine the conditions under which a symmetric placement of two charges can be treated in a “local friction” model. Models of pressure effects and solvent size effects, along with more data, will be helpful in experimental interpretations. Unusual rotational behaviour in mixtures of solvents represent a new frontier, although liquid volume and solvent interactions undoubtedly play a large role in such solvents.

The measurement of rotational motions can be useful in characterizing solvent interactions in a long term and transient sense. Interactions of solvent with ionic transition states include dielectric relaxation and motional effects. We briefly demonstrated these effects in the ionic photodissociation of malachite green leucocyanide to cyanide ion and the malachite green carbonium ion.