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

Abstract

It is shown, by means of numerical and analytic work, that initial molecular momenta play little significant role in the initial fast solvation relaxation that follows electronic excitation of, and charge creation for, a standard model system of a solute in water. Instead, the nonequilibrium dynamics are predominantly described by non-inertial "steering" by the torques directly generated by the newly created charge distribution. It is this process that largely overcomes inertia, and drives the relaxation dynamics on a time scale of a few tens of femtoseconds in the key initial regime of the dynamics. These results are discussed in the context of commonly employed descriptions such as inertial, Gaussian, and underdamped dynamical behavior.

The Role of Hydrogen Bonding in the Preferential Solvation of 5-Aminoquinoline in Binary Solvent Mixtures

“…Solvation is a key process that modulates the dynamics of excited states, − modifies energy barriers, and determines the reaction pathways , of solutes. The study of solvation dynamics has been an area of intense activity over the last few decades and has provided insight into solute–solvent interactions at the molecular level.…”

Ultrafast Rotational and Translational Energy Relaxation in Neat Liquids

“…The remarkable applicability of linear response has been observed for various relaxation processes (e.g., refs , , , , , , , − , see ref for a more extensive reference list), though some important exceptions exist. ,,,− …”

“…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.…”

“…Typically, the ultrafast relaxation dynamics of liquids are experimentally and/or computationally/theoretically studied via electronic or vibrational pumping of a foreign solute dissolved in the liquid, and the subsequent spectroscopic probing of that solute, or the surrounding liquid itself. The former, typically involving an electronically excited dye molecule or reaction system (see, e.g., refs and and the extensive list in ref ), largely falls under the rubric of “solvation dynamics”. The latter (see, e.g., refs and ) is often (though not exclusively) focused on vibrational energy transfer and relaxation of the solute, i.e., (in quantum language) the population dynamics of the solute’s vibrational state.…”

“…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.…”