Understanding nonequilibrium solute and solvent motions through molecular projections: Computer simulations of solvation dynamics in liquid tetrahydrofuran (THF) (original) (raw)
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The Journal of Physical Chemistry B, 2000
The fact that the motion of solvent molecules defines the reaction coordinate for electron-transfer and other chemical reactions has generated great interest in solvation dynamics, the study of how the solvent responds to changes in a solute's electronic state. In the limit of linear response (LR), when the perturbation caused by the solute is "small", the relaxation of the excited solute's energy gap should behave identically to the relaxation dynamics of the unperturbed solute following a natural fluctuation of the gap away from equilibrium. Despite the fact that the addition of a fundamental unit of charge to a small solute results in a solvation energy that is tens or hundreds of kT, computer simulations of solvation dynamics have found, with only a few exceptions, that LR is obeyed for changes in solute charge. Essentially none of this work, however, accounts for the fact that the solutes in real chemical reactions undergo changes in size and shape as well as in charge distribution. In this paper, we compare the results of molecular simulations of polar and nonpolar solvation dynamics for a simple Lennard-Jones solute in a flexible-water solution to explore the validity of LR. We find that, when short-range forces are involved, LR breaks down dramatically: both the inertial and diffusive components of the relaxation differ from those predicted by LR. For increases in solute size, expansion of the solute drives the first-shell solvent molecules into the second shell. The resulting nonequilibrium relaxation takes advantage of translation-rotation coupling that does not occur at equilibrium, resulting in faster solvation than that predicted by LR. Decreases in solute size, on the other hand, result in inward translational motions of solvent molecules that affect the solute's energy gap by destabilizing the energy of the (unoccupied) ground state. The inward motions involved in the nonequilibrium relaxation are not present at equilibrium because the destabilization of the ground state is much larger than kT. Because the energetically most important solvent molecules, those closest to the solute, are just as likely to be moving away from the solute as toward it at the time of excitation, solvation for decreases in size is much slower than predicted by LR. In the most realistic cases, when both the size and the charge of the solute change, the solvent translational motions resulting from the size change and those resulting from electrostriction, the net ion-dipole attraction between the charged solute and the polar solvent, combine in an additive fashion. When the solute both gains a charge and expands, the translational motions resulting from electrostriction nearly cancel those from the outward solute expansion so that rotational motions dominate the solvent response; the small net expansion that remains results in only a minor breakdown of LR. The additional inward solvent translations beyond those required by electrostriction, which are necessary when the solute becomes charged and its size decreases, on the other hand, result in a severe breakdown of LR. All of the results are compared with previous experimental and theoretical studies of solvation dynamics, and the implications for solvent-driven chemical reactions are discussed.
Rotational Relaxation in Polar Solvents. Molecular Dynamics Study of SoluteāSolvent Interaction
Journal of the American Chemical Society, 1998
Spectroscopic and molecular dynamics (MD) studies of organic dye molecules in polar solvents are performed to investigate the nature of solute-solvent interactions. Experimental and MD data demonstrate that positively charged molecules rotate more slowly than neutral and negatively charged solutes in polar aprotic solvents. MD simulations of the resorufin, resorufamine, and thionine molecules in DMSO solvent are analyzed to reveal differences in the origins of the frictional forces experienced by each solute molecule. It is demonstrated that specific associations (hydrogen bonds) are formed between the DMSO molecules and both the neutral and the cation solute molecules. No specific solute-solvent association is observed for the negative solute molecule. According to a force separation analysis, the calculated friction on the cation is mostly Coulombic in nature, while it is mostly collisional (mechanical) for the anion case.
Chemical Physics, 1994
We present a more formal development of our recent theory of solvation dynamics, allowing a more detailed examination of the mechanism of solvent response to the sudden change of the charge distribution of an immersed solute. A distinctive feature of the theory is that both the solute and the solvent molecules are described in terms of relatively realistic interaction site models. Furthermore, in the theory the solute-solvent coupling is formulated in terms of the solute-solvent site-site interactions, rather than by appealing to some sort of cavity construction. Applying the general formulation of nonequilibrium statistical mechanics we derive an approximate nonequilibrium distribution functionf i( I', I) for a "surrogate" Hamiltonian description of solvation dynamics. The surrogate Hamiltonian is expressed in terms of renormalized solute-solvent interactions, a feature that allows us to introduce a simple reduction scheme in the many body dynamic problem, without losing the important solute-solvent static correlations that are important for the equilibrium solvation. With f %( r, t) we can calculate not only the solvation time correlation function but also other observables of interest such as the evolution of the solvent polarization charge density around the solute. We also consider the spatial resolution of some of the measures of the solvent response to the sudden transition, namely the reaction potential and the electrostatic energy gap. Applications of the theory are illustrated with calculations of the solvation dynamics of a cation in acetonitrile and the solvation'dynamics of formaldehyde in water. which is measured in time-resolved fluorescence Stokes shift experiments [ l-3,5,6]. Here v(t) is the fluorescence frequency at time t of a solute molecule that was suddenly excited at c = 0. The frequency v(t) changes as the solvent adjusts to the excited state charge 0301-0104/94/$07.00 Q 1994 Elsevier Science B.V. All rights reserved SSDIO301-0104 (94)00026-7
Physical Review E, 2002
In the density functional theory formulation of molecular solvents, the solvation free energy of a solute can be obtained directly by minimization of a functional, instead of the thermodynamic integration scheme necessary when using atomistic simulations. In the homogeneous reference fluid approximation, the expression of the free-energy functional relies on the direct correlation function of the pure solvent. To obtain that function as exactly as possible for a given atomistic solvent model, we propose the following approach: first to perform molecular simulations of the homogeneous solvent and compute the position and angle-dependent two-body distribution functions, and then to invert the Ornstein-Zernike relation using a finite rotational invariant basis set to get the corresponding direct correlation function. This rather natural scheme is proved, for the first time to our knowledge, to be valuable for a dipolar solvent involving long range interactions. The resulting solvent free-energy functional can then be minimized on a three-dimensional grid around a solute to get the solvent particle and polarization density profiles and solvation free energies. The viability of this approach is proven in a comparison with ''exact'' molecular dynamics calculations for the simple test case of spherical ions in a dipolar solvent.
The Journal of Physical Chemistry B, 1999
The coupling between solvent fluctuations and the electronic states of solutes is critically important in charge transfer and other chemical reactions. This has piqued enormous interest in solvation dynamicssthe study of how solvent motions relax changes in a solute's charge distribution. In nearly every computer simulation of solvation dynamics, the system is modeled by an atomic or molecular solute whose charge (or higher multipole moment) is suddenly changed, and the motions of the solvent molecules that relax the new charge distribution are monitored. Almost none of this work, however, accounts for the fact that most reacting solutes also undergo significant changes in size and shape as well as charge distribution. For the excited states of dye molecules typically used as probes in solvation experiments or for the atoms and molecules that change oxidation state in charge transfer reactions, we expect changes in reactant size on the order of 5-20%. In this paper, we use computer simulation to explore the differences between dielectric solvation, due to changes in charge distribution, and mechanical solvation, due to changes in size and shape, for a Lennard-Jones sphere in flexible water. The solvation energy for the size changes expected in typical reactions is on the same order as that for the appearance of a fundamental unit of charge, indicating that dielectric and mechanical solvation dynamics should participate at comparable levels. For dielectric solvation, solvent librations dominate the influence spectrum, but we also find a significant contribution from the water bending motion as well as low-frequency translations. The influence spectrum for mechanical solvation, on the other hand, consists solely of low-frequency intermolecular translational motions, leading to mechanical solvation dynamics that are significantly slower than their dielectric counterparts. The spectrum of couplings for various mechanical perturbations (size, shape, or polarizability) depends somewhat on the magnitude of the change, but all types of mechanical relaxation dynamics appear qualitatively similar. This is due to the steepness of the solutesolvent interaction potential, which dictates that the majority of the solvation energy for mechanical changes comes from the translational motion of the closest one or two solvent molecules. Finally, we explore the solvation dynamics for combined changes in both size and charge and find that the resulting dynamics depend sensitively on the sign and magnitude of both the size and charge changes. For some size/charge combinations, the translational and rotational motions that lead to relaxation work cooperatively, producing rapid solvation. For other combinations, the key translational and rotational solvent motions for relaxation are antagonistic, leading to a situation where mechanical solvation becomes rate limiting: solvent rotational motions are "frustrated" until after translational relaxation has occurred. All the results are compared with previous experimental and theoretical studies of solvation dynamics, and the implications for solvent-driven chemical reactions are discussed.
Solute Dependence of Polar Solvation Dynamics Studied by RISM/Mode-Coupling Theory
Journal of Solution Chemistry, 2000
We apply the combination of the reference interaction-site model (RISM) theory and mode-coupling theory (MCT) for the description of solvation dynamics in acetonitrile. The RISM theory is used for the static features of the solute-solvent system, whereas MCT is employed to capture solvent dynamics. We calculate the dynamic response function of the average-energy relaxation of the system, S S (t), a corresponding quantity which is experimentally measured through the dynamic Stokes shift of fluorescence spectra, for example. The RISM-MCT framework turns out to be very applicable for realistic estimation of S S (t). We vary the model solutes from a simple ion, to dumbbell dipoles, to quadrupole, to octapole. As a general feature, S S (t) begins to relax with the initial Gaussian decay, followed by the underdamped oscillation. Higher multiplicity of the pole or larger charges put on the solute-site causes S S (t) to relax more slowly. We suggest this behavior is related to an increased complexity of solventsolvent interactions just around the charged sites on the solute. On the other hand, the amplitude of the underdamped oscillation becomes smaller as the multiplicity of the pole increases.
Equilibrium and nonequilibrium solvation and solute electronic structure
International Journal of Quantum Chemistry, 1990
When a molecular solute is immersed in a polar and polarizable solvent, the electronic wave function of the solute system is altered compared to its vacuum value; the solute electronic structure is thus solvent-dependent. Further, the wave function will be altered depending upon whether the polarization of the solvent is or is not in equilibrium with the solute charge distribution. More precisely, while the solvent electronic polarization should be in equilibrium with the solute electronic wave function, the much more sluggish solvent orientational polarization need not be. We call this last situation "nonequilibrium solvation." We outline a nonlinear Schrodinger equation approach to these issues. The nonlinearity arises from the self-consistent aspect that the solute electronic Hamiltonian depends on the solvent electronic polarization which is induced by the solute charge distribution. We illustrate the predictions of the theory for electron transfer reactions, ionic dissociations, and solvation dynamics in polar solvents. Special features of interest include activation barriers that differ markedly from standard predictions, and novel solvent dynamical features.
Solvation dynamics in liquid water
2015
Solvation dynamics in liquid water is addressed via nonequilibrium energy transfer pathways activated after a neutral atomic solute acquires a unit charge, either positive or negative. It is shown that the well-known nonequilibrium frequency shift relaxation function can be expressed in a novel fashion in terms of energy fluxes, providing a clearcut and quantitative account of the processes involved. Roughly half of the initial excess energy is transferred into hindered rotations of first hydration shell water molecules, i.e. librational motions, specifically those rotations around the lowest moment of inertia principal axis. After integration over all water solvent molecules, rotations account for roughly 80 % of the energy transferred, while translations have a secondary role; transfer to intramolecular water stretch and bend vibrations is negligible. This picture is similar to that for relaxation of a single vibrationally or rotationally excited water molecule in neat liquid wate...