Solvation dynamics in dielectric solvents with restricted molecular rotations: Polyethers

Abstract

A molecular dynamics study of macromolecules in good solvents: Comparison with dielectric spectroscopy experiments Molecular dynamics simulations are used to study solvation and solvation dynamics of a classic charge in a series of ethers of increasing molecular weights, CH 3 ͑CH 2 OCH 2 ͒ n H with nϭ1, 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 ͑Stockmayer͒ fluid. For a typical positive ion, Na ϩ , 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 ͑tϽ0.5 ns͒ 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 ether͒ the short time component is not Gaussian ͑i.e., its Gaussian part is limited to insignificantly short times͒ 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 fluorescence͒ can be used as a probe for the dynamics of such segmental motions in polymer electrolytes.

Searching for solvent cavities via electron photodetachment: The ultrafast charge-transfer-to-solvent dynamics of sodide in a series of ether solvents

“…69 We note that the time scale observed for Na + solvation in the classical simulations in Ref. 29 was faster than that seen experimentally for the TCP in Fig. 10.…”

“…Thus, the observation of similar long-time solvation time scales here strongly suggests that the linear response approximation would fail to describe the solvation of the TCP in the entire series of ethers; 51 such a breakdown of linear response also has been observed in the classical MD simulations of Olender and Nitzan. 29 We close by commenting on the fact that essentially all of the short-time dynamics we observe following CTTS photodetachment from sodide are identical in the different ethers, including the time scales for physical processes as varied as electron ejection, electron relocalization, TCP formation and geminate recombination. The fact that these time scales are so similar across our solvent series reinforces the idea that all of these processes are driven by local solvent motions, and that bulk solvent properties such as viscosity ͑with the exception of polarity in the case of the longer-time geminate recombination͒ play only a secondary role.…”

“…The similar polarity of these solvents is perhaps not surprising given that molecular dynamics simulations show that CH 3 ͑CH 2 OCH 2 ͒ n H with n = 1 and 2 give nearly identical solvent structures around both neutral and cationic solutes. 29 Thus, we expect the local solvation structure to vary minimally among the straight-chain ethers, so that the only significant variation in bulk property across our solvent series is the factor of ϳ4 variation in viscosity.…”

“…11 Although solvent cavities play a direct role in electron photodetachment reactions, characterizing such cavities, and indeed even probing their existence, is challenging. For example, many groups have performed molecular dynamics simulations of liquid THF [24][25][26][27][28] and other ethers [29][30][31] over the years, but the presence of cavities in such simulations was noticed only fairly recently. 10,11 In addition, Zhong and Fourkas 32 used the optical Kerr effect to examine the lowfrequency motions of a series of organic solvents, but they were unable to find any obvious experimental signatures that could be associated with the presence of cavities.…”

Nonlinear effects of number density of solvent molecules on solvation dynamics

“…This is because solvation dynamics is a typical example of nonequilibrium relaxation. Thus, many authors have studied solvation dynamics using experiments, 2-4 numerical simulations, [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] and theoretical models. [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] Linear response theories are successful in solvation dynamics.…”