Translational versus rotational energy flow in water solvation dynamics

Abstract

Early molecular dynamics simulations discovered an important asymmetry in the speed of water solvation dynamics for charge extinction and charge creation for an immersed solute, a feature representing a first demonstration of the breakdown of linear response theory. The molecular level mechanism of this asymmetry is examined here via a novel energy flux theoretical approach coupled to geometric probes. The results identify the effect as arising from the translational motions of the solute-hydrating water molecules rather than their rotational/librational motions, even though the latter are more rapid and dominate the energy flow.Postprint (author's final draft

Solvation Dynamics in Water. 4. On the Initial Regime of Solvation Relaxation

“…The computations in the present work are similar to those in our previous contributions of water solvation dynamics [32][33][34][35] to which we refer for a more detailed account. The results to be presented correspond to the well known SPC/E rigid water model.…”

“…Our choice of NED is due to its character as a direct approach that most closely (though certainly not perfectly) relates to experiments, and avoids the inevitable approximations necessary to achieve a TCF description; we do not however enter into any general discussion of the "linear response theory" issues concerning the disparities between NED and TCF predictions. 10,[12][13][14][15][16][17]39,40,45,81 Within the NED approach, we recently [32][33][34][35] presented an alternative perspective for solvation dynamics founded on the computation of the nonequilibrium energy fluxes ensuing as a result of the initial excitation. This perspective exploited a power/work energy flow methodology previously implemented for vibrational/rotational relaxation.…”

Ultrafast Rotational and Translational Energy Relaxation in Neat Liquids

“…As noted within, the work/power formulation for energy flow has not been fully exploited in the current effort, devoted to the comparison between examples of hydrogen-bonded and nonpolar solvents. More can be done for these solvents, and solvent classes, ,− as well as mixed solutions; clearly, polar nonhydroxylic solvents such as acetonitrile (see, e.g., ref ) merit comparable attention in the future as well. As for the present liquids, most especially water for which most experiments referred to in the Introduction have been carried out, extension of the energy flow formulation to, e.g., specific spatial and structural variables directly related to measured spectra (e.g., transition dipole orientational angles as in ref ) or configurational variables (e.g., as probed in the X-ray studies of ref ) could be carried out in the future in direct connection with the present kinetic energy studies.…”

“…We will start by expanding our previous study of the relaxation of an initial rotational excitation in water, thus broadening its scope, and further, we extend this approach to other neat liquids listed above, namely, methanol, carbon tetrachloride, and methane; one aim here is to assess the generality of the findings for water, by means of a comparative study with progressively less associated molecular liquids. The present analysis certainly does not exploit all aspects of the energy flow for these liquids, which would require considerably further discussion, left for the future (e.g., elucidation of the electronic excitation-induced energy flow has required a number of publications ,− ). Rather, we focus on exposing a number of principal features of the energy flows in these liquids, including their main similarities and contrasts.…”

“…This ranges from the polar and hydrogen-bonded (H-bonded) liquids water (H 2 O) and methanol (MeOH) to nonpolar liquids carbon tetrachloride CCl 4 and (high density) supercritical methane CH 4 (the latter chosen to reduce the intermolecular interactions). This work follows the contributions of the corresponding energy flow studies of relaxation dynamics subsequent to molecular vibrational, , rotational, and electronic ,− excitations in aqueous solution or pure water by several of us, and to vibrational studies by others. − The analysis of energy flows has several attractive features, − the most important of which is the ability to follow at a molecular level the pathways and time scales of the flow of energy, both directly in a nonequilibrium framework such that short-lived motions can be followed, and in a fashion devoid of cross terms which greatly complicate analysis in equilibrium time correlation function approaches.…”