Rotational Relaxation in Polar Solvents. Molecular Dynamics Study of Solute-Solvent Interaction

Spectroscopic and molecular dynamics (MD) studies of organic dye molecules in polar solvents are performed to investigate the nature of solute-solvent interactions. Experimental and MD data demonstrate that positively charged molecules rotate more slowly than neutral and negatively charged solutes in polar aprotic solvents. MD simulations of the resorufin, resorufamine, and thionine molecules in DMSO solvent are analyzed to reveal differences in the origins of the frictional forces experienced by each solute molecule. It is demonstrated that specific associations (hydrogen bonds) are formed between the DMSO molecules and both the neutral and the cation solute molecules. No specific solute-solvent association is observed for the negative solute molecule. According to a force separation analysis, the calculated friction on the cation is mostly Coulombic in nature, while it is mostly collisional (mechanical) for the anion case.