Solvation dynamics in dielectric solvents with restricted molecular rotations: Polyethers

Molecular dynamics simulations are used to study solvation and solvation dynamics of a classic charge in a series of ethers of increasing molecular weights, CH₃(CH₂OCH₂)_nH with n=1, 2, and 4. Equilibrium structures of the solvated species, ion mobility, linear response solvation functions, and nonequilibrium solvation are studied and compared with the corresponding results for a simple (Stockmayer) fluid. For a typical positive ion, Na⁺, solvation in these systems is found to belong to the nonlinear response regime; the nonlinear behavior is associated with the specific binding of the cation to the negative oxygen sites. Solvation dynamics in the timescale studied (t<0.5 ns) is found to be essentially bimodal, with a short component similar in duration and magnitude to that found in simpler solvents. However, except for the simplest system studied (ethyl methyl ether) the short time component is not Gaussian (i.e., its Gaussian part is limited to insignificantly short times) and cannot be interpreted as inertial free streaming of solvent molecules in the potential field of the solute. Instead we suggest that it originates from damped solvent vibrations about solvent inherent structures. The character of the solvent motions that drive the solvation process changes as the molecular size increases: From overall molecular rotations in the monoether, to intramolecular segmental motions in the larger solvents. It is suggested that solvation dynamics (studied, e.g., by laser induced fluorescence) can be used as a probe for the dynamics of such segmental motions in polymer electrolytes.