Peaks in absorption and fluorescence spectra of molecules in solution are generally broader than those in the gas phase because the energy of solute molecules in a solvent is fluctuating due to the orientational fluctuations of the surrounding solvent molecules. The fluctuating motion of the solvent molecules plays an important role in chemical reactions in the solution phase. For example, in an electron transfer reaction the energy fluctuation of the reactants caused by the fluctuating solute-solvent interaction is believed to be essential to reach the transition state of that reaction. One of the most fundamental and challenging problems regarding chemical reactivity in solution is to explain how the microscopic structure and dynamics of the solute-solvent interaction can assist or impede a chemical reaction. The dynamic aspects of solvation, however, are very complex, since the orientational and translational motions of the surrounding solvent molecules occur on the same time-scale as the reactant dynamics. The most important and characteristic features of the solution phase are the types of motional degrees of freedom which are possible for both solute and solvent molecules, and which include intra- and intermolecular rotational, vibrational, or librational motion, as well as translational diffusive motion under multidimensional interaction potentials.
KeywordsSolute Molecule Solvent Viscosity Generalize Langevin Equation Torsional Potential Electronic State Energy
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