Two of the more fundamental ways in which molecules change their behavior when they are dissolved are that they can begin to exchange energy with the surrounding liquid and they can induce their surroundings to rearrange so as to provide a significant stabilizing influence. The first of these is typified by the process of vibrational population relaxation of a vibrationally hot species. The second conceptcritical to solution chemistryis what is known as solvation. Both of these processes are sufficiently fundamental that one would really like to know, at the most mechanical and molecular level possible, just what events are required in order to make them happen. But how difficult is it going to be to extract such molecular detail from the complicated many-body dynamics? The most microscopic level of understanding one could ever hope to possess might seem far removed from the finely detailed dynamical information which is available routinely for individual isolated molecules and for molecule−molecule collisions in molecular beams. It might even seem that the broad, almost featureless character of typical solute spectra would condemn us to never being able to measure anything more than a few paultry tidbits of highly averaged data caricaturing the intriguing processes that can take place in liquids. However, spectroscopists have for some time been able to infer at least some aspects of the dynamics of liquids from the spectroscopy of dissolved molecules, and with the advent of novel ultrafast time-resolved spectroscopies and new theoretical perspectives, the likelihood of resolving solution dynamics into genuinely molecular components has increased dramatically. We discuss a few of these recent developments here for the special, but nonetheless illuminating, cases of solvation dynamics and vibrational relaxation and note a few of the more promising directions that future work might take.

We present an instantaneous-normal-mode analysis of liquid water at room temperature based on a computer simulated set of liquid configurations and we compare the results to analogous inherent-structure calculations. The separate translational and rotational contributions to each instantaneous normal mode are first obtained by computing the appropriate projectors from the eigenvectors. The extent of localization of the different kinds of modes is then quantified with the aid of the inverse participation ratio-roughly the reciprocal of the number of degrees of freedom involved in each mode. The instantaneous normal modes also carry along with them an implicit picture of how the topography of the potential surface changes as one moves from point to point in the very-high dimensional configuration space of a liquid. To help us understand this topography, we use the instantaneous normal modes to compute the predicted heights and locations of the nearest extrema of the potential. The net result is that in liquid water, at least, it is the low frequency modes that seem to reflect the largest-scale structural transitions. The detailed dynamics of such transitions are probably outside of the instantaneous-normal-mode formalism, but we do find that short-time dynamical quantities, such as the angular velocity autocorrelation functions, are described extraordinarily well by the instantaneous modes. 6672

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The ease with which optical Kerr effect (OKE) spectroscopy manages to sample dynamics in the far-infrared would seem to makes it a rather attractive way of doing intermolecular spectroscopy on liquids. However, molecular-level calculations of such spectra are still far less common than microscopically ill-defined phenomenological fits. As a result, there are comparatively few liquids whose OKE spectra have ever been interpreted in any genuinely molecular fashion. In this paper we explore the OKE spectrum of an experimentally well-studied liquid, liquid benzene, at a fully microscopic level by making use of molecular dynamics simulation and an instantaneous-normal-mode analysis. As has often been noted, the long-time tail of the OKE signal is quantitatively accounted for by rotational diffusion (albeit a collective diffusion). Moreover, consistent with the usual expectations, the interaction-induced portion of the remaining signal (which we show to arise almost entirely from center-of-mass translation in this example) appears only at low frequency. However, contrary to the common assumptions, rotational dynamics, often strongly coupled with translational dynamics, contributes over the entire spectral range. The unusual shape of the experimental benzene OKE spectra is shown to arise from the presence of an especially large ratio of rotational to translational bandwidths, an explanation that may account for the similar spectra seen with numerous other planar molecules. Having a molecular-level picture also allows us to point out that there is no need to invoke any kind of hypothetical benzene aggregates in order to explain benzene's OKE spectrum. Benzene and related aromatics are simple liquids whose dynamics, as well as structure, depend largely on repulsive forces and molecular shape.

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Highly energized molecules normally are rapidly equilibrated by a solvent; this finding is central to the conventional (linear-response) view of how chemical reactions occur in solution. However, when a reaction initiated by 33-femtosecond deep ultraviolet laser pulses is used to eject highly rotationally excited diatomic molecules into alcohols and water, rotational coherence persists for many rotational periods despite the solvent. Molecular dynamics simulations trace this slow development of molecular-scale friction to a clearly identifiable molecular event: an abrupt liquid-structure change triggered by the rapid rotation. This example shows that molecular relaxation can sometimes switch from linear to nonlinear response.

At long enough times, the idiosyncratic motions of individual solvent molecules have long since ceased to matter to the process of solvation; the fact that a real solvent is not a featureless continuum just has no bearing on the dynamics. However, at short times, typically times well under a picosecond, the situation is quite different. We show here that at least within the realm of classical mechanics, one can indeed talk about how specific molecular motions contribute to short-time solvation. Precisely how one should think about these motions depends on just how short a time interval one is considering. At the very shortest times, we use the fact that it is possible to express solvation time correlation functions rigorously as power series in time to confirm that the onset of solvation is unequivocally a matter of inertial (free-streaming) motion of individual solvent molecules. We allow for somewhat longer, but still short, time intervals by writing these same correlation functions in terms of the instanteous normal modes of the solvent. The instantaneous-normal-mode expressions allow us to decompose the solvent dynamics into separate, well-defined collective motions, each with its own characteristic abilities to foster solvation. As distinctive as they appear, these two complimentary short-time views are, in fact, equally correct in the inertial regime, a point we establish by proving that two are simply different mathematical representations of the same underlying behavior.

The microscopic details of how a solution responds to changes in a solute are now becoming experimentally accessible at the kinds of times that should allow us to follow even the earliest events in solvation. For time scales this short there is a genuine chance that one can identify actual elementary events in the solvation process, meaning that one can begin to think about explicit solvation mechanismsthe specific molecular motions that comprise the crucial steps in the process. Most of the current theories of solvation dynamics, however, try to resolve this early-time dynamics by looking at finer and finer details of the dielectric response of the bulk solvent, an approach which not only seems to be starting from the opposite extreme of the behavior one is trying to understand, but which erects an artificial conceptual barrier between the solvation processes of polar and nonpolar liquids. We suggest that, at least at the times of interest for questions of solvation mechanism, the distinction between polar and nonpolar solvents is superficial. The ultrafast dynamics of both kinds of solvents are more naturally regarded in terms of their instantaneous normal modeswhich can be further dissected into contributions from such mechanistic elements as solvent libration and solvent translation, and even into contributions from individual solvent shells surrounding the solute, if so desired. We show how, from the perspective of this kind of analysis, a simple scaling argument makes it clear why solvent libration is usually, but not always, the most efficient route to solvationand why the important distinctions are not between the different families of solvents, but between the differing symmetries of the various solute−solvent interactions that one can choose to monitor experimentally. We illustrate these ideas by performing an instantaneous-normal-mode analysis of the manifestly nonpolar situation of I2 dissolved in liquid CO2, an example deliberately chosen to contrast with our previous study of dipolar solvation in CH3CN. In accordance with the predictions of the simple scaling argument, the primary solvation mechanism shifts from libration to center-of-mass translation as the solute−solvent interaction being monitored is changed from being multipolar in character (dipolar or quadrupolar) to something more symmetric. We find, moreover, that the range of the solute−solvent interaction is of no more than secondary importance in understanding the solvation mechanism:  Coulombic (1/r) potentials behave little differently than dispersion (1/r 6) potentials in their libration/translation preferences, and both exhibit a prompt solvation process dominated by the first solvation shell. Much the same kind of analysis can be applied to the question of whether solute motion is an important part of solvation:  although unfreezing the solute will always allow for faster solvent response, we show that the extent of the effect can be quantitatively predicted by comparing the solute's mass and moment of inertia with that of the sol...