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Preferential solvation dynamics in liquids: How geodesic pathways through the potential energy landscape reveal mechanistic details about solute relaxation in liquids
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Image of FIG. 1.
FIG. 1.

The dependence of solvation dynamics on the solvent composition for the model used in this paper. The nonequilibrium solvation response function S(t) is shown for solutions consisting of 0%, 10%, 50%, 80%, and 100% S solvents. The larger scale figure depicts the longer-time-scale diffusive relaxation; the insert shows the initial (subpicosecond) inertial relaxation for the same solutions.

Image of FIG. 2.
FIG. 2.

The time evolution of solvent structure around the excited-state solute for the 10% S case. What we plot here are both the solute/strong solvent radial distribution function and the solute/weak solvent radial distribution function following a excitation of the solute from the ground to the excited-state. The insert in the lower panel highlights the nonmonotonic weak solvent behavior seen in the first shell peak: although the peak height eventually ends up being smaller than its initial value, during the initial (solvent compression) phase, this height actually grows. It is not until after the first 1–2 ps that the peak height begins to shrink.

Image of FIG. 3.
FIG. 3.

The time evolution of the first solvation shell population for the 10% S case. The curves drawn here are derived by integrating the radial distributions functions shown in Fig. 2 to the first minimum.

Image of FIG. 4.
FIG. 4.

Relationship between the average net configuration space (3N-dimensional) distance traveled and the average extent of solvation accomplished for the 10% S case. Both the average net distance (plotted in units of ) and the average solvation progress measure depend parametrically on the elapsed time t; the graph shown combines the two averages to determine their time-independent relation.

Image of FIG. 5.
FIG. 5.

The growth of the most efficient (geodesic) path length during preferential solvation. Unlike the earlier figures, which reported molecular dynamics, this figure looks solely at the geometry of the solution potential energy landscape. A set of geodesic paths is found, and the average conductivity ratios (the square of the direct-to-geodesic-path-lengths quotient, ) are shown as a function of average solvation progress, s, for 10%, 50%, 80%, and 100% S solvents. (In addition, for the 10% S case, we plot the results for both atom and atom solutions, demonstrating that finite size effects are negligible.) The color-coded arrows drawn above the figure indicate the locations we identify (from left to right) as the onsets of diffusive pathways for the 10%, 50%, and 80% S solvents, respectively.

Image of FIG. 6.
FIG. 6.

The evolution of the first-solvation-shell populations plotted as a function of the solvation progress variable S(t) for the three solvents shown. The figure, which simply replots the kinds of molecular dynamics results portrayed in Figs. 1 and 3 on a single graph, emphasizes that the first shell numbers of strong solvent (S), weak solvent (W), and total solvent begin to exhibit noticeable changes at different points in the solvation process. Vertical lines, drawn at the same locations as the arrows in Fig. 5, indicate the points at which the geodesic analysis predicts that the dynamics is becoming strongly diffusive.

Image of FIG. 7.
FIG. 7.

Extracting mechanistic information from the number of degrees of freedom waiting to participate. Whether we would say that the hypothetical chemical reaction illustrated here proceeds in a single concerted step, or as a sequence of two steps, depends on whether the shortening of the BC distance r(BC) takes place at the same time as (upper panel), or after (lower panel), the lengthening of the AB distance r(AB).

Image of FIG. 8.
FIG. 8.

The number of solvent atoms waiting to move, , in preferential and ordinary solvation, as determined from the participation numbers associated with the landscape geodesics. The three panels present the waiting numbers of weak, strong, and total solvent atoms for the 10%, 50%, and 80% S solutions, each as a function of the solvation progress variable s. For comparison, each panel also shows (as dots practically indistinguishable from the horizontal axes) the results from the pure solvent (100% S) case. As in Fig. 6, we draw vertical lines where the growing lengths of the geodesic paths indicate that the dynamics is starting to become strongly diffusive.


Generic image for table
Table I.

Potential energy parameters (Lennard-Jones well depths between species and ).

Generic image for table
Table II.

Thermodynamics properties of solutions with an excited solute. [For each of the excited-state-solute/solvent mixtures studied in this paper, we give the temperature T, landscape energy , and average solute energy gap , all reported in units of the reference well depth . The mixtures with a ground-state solute all have , , and (independently of the solvent composition). To help calibrate the differences in the solvation thermodynamics, we also report the equilibrium Stokes shift for each mixture.]


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Preferential solvation dynamics in liquids: How geodesic pathways through the potential energy landscape reveal mechanistic details about solute relaxation in liquids