ROTATIONAL RELAXATION IN POLAR-SOLVENTS - MOLECULAR-DYNAMICS STUDY OFSOLUTE-SOLVENT INTERACTION

Spectroscopic and molecular dynamics (MD) studies of organic dye molec ules in polar solvents are performed to investigate the nature of solu te-solvent interactions. Experimental and MD data demonstrate that pos itively charged molecules rotate more slowly than neutral and negative ly charged solutes in polar aprotic solvents. MD simulations of the re sorufin, resorufamine, and thionine molecules in DMSO solvent are anal yzed to reveal differences in the origins of the frictional forces exp erienced by each solute molecule. It is demonstrated that specific ass ociations (hydrogen bonds) are formed between the DMSO molecules and b oth the neutral and the cation solute molecules. No specific solute-so lvent association is observed for the negative solute molecule. Accord ing to a force separation analysis, the calculated friction on the cat ion is mostly Coulombic in nature, while it is mostly collisional (mec hanical) for the anion case.