Solvation Dynamics in Water: 2. Energy Fluxes on Excited- and Ground-State Surfaces (original) (raw)
Related papers
Solvation Dynamics in Liquid Water. III. Energy Fluxes and Structural Changes
The journal of physical chemistry. B, 2017
In previous installments it has been shown how a detailed analysis of energy fluxes induced by electronic excitation of a solute can provide a quantitative understanding of the dominant molecular energy flow channels characterizing solvation-and in particular, hydration- relaxation dynamics. Here this work and power approach is complemented with a detailed characterization of the changes induced by such energy fluxes. We first examine the water solvent's spatial and orientational distributions and the assorted energy fluxes in the various hydration shells of the solute to provide a molecular picture of the relaxation. The latter analysis is also used to address the issue of a possible "inverse snowball" effect, an ansatz concerning the time scales of the different hydration shells to reach equilibrium. We then establish a link between the instantaneous torque, exerted on the water solvent neighbors' principal rotational axes immediately after excitation and the fin...
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...
The Journal of Chemical Physics, 2008
In a recent paper ͓Aschi et al., ChemPhysChem 6, 53 ͑2005͔͒, we characterized, by means of theoretical-computational procedures, the electronic excitation of water along the typical liquid state isochore ͑55.32 mol/ l͒ for a large range of temperature. In that paper we were able to accurately reproduce the experimental absorption maximum at room temperature and to provide a detailed description of the temperature dependence of the excitation spectrum along the isochore. In a recent experimental work by Marin et al. ͓J. Chem. Phys. 125, 104314 ͑2006͔͒, water electronic excitation energy was carefully analyzed in a broad range of density and temperature, finding a remarkable agreement of the temperature behavior of the experimental data with our theoretical results. Here, by means of the same theoretical-computational procedures ͑molecular dynamics simulations and the perturbed matrix method͒, we investigate water electronic absorption exactly in the same density-temperature range used in the experimental work, hence, now considering also the absorption density dependence. Our results point out that, ͑1͒ for all the densities and temperatures investigated, our calculated absorption spectra are in very good agreement with the experimental ones and ͑2͒ the gradual maxima redshift observed increasing the temperature or decreasing the density has to be ascribed to a real shift of the lowest X → A electronic transition, supporting the conclusions of Marin et al.
Practical computation of electronic excitation in solution: vertical excitation model
Chem. Sci., 2011
We present a unified treatment of solvatochromic shifts in liquid-phase absorption spectra, and we develop a self-consistent state-specific vertical excitation model (called VEM) for electronic excitation in solution. We discuss several other approaches to calculate vertical excitations in solution as an approximation to VEM. We illustrate these methods by presenting calculations of the solvatochromic shifts of the lowest excited states of several solutes (acetone, acrolein, coumarin 153, indolinedimethine-malononitrile, julolidine-malononitrile, methanal, methylenecyclopropene, and pyridine) in polar and nonpolar solvents (acetonitrile, cyclohexane, dimethyl sulfoxide, methanol, n-hexane, n-pentane, and water) using implicit solvation models combined with configuration interaction based on single excitations and with time-dependent density functional theory.
Theoretical Characterisation of the Electronic Excitation in Liquid Water
ChemPhysChem, 2005
Because of its central role in basically all aspects of science, water is certainly one of the most extensively investigated substances, from a theoretical point of view. Many properties have been, in fact, theoretically addressed both in the isolated and condensed phases. Nevertheless, many aspects are still not completely understood and represent the focus of active theoretical interest. Among them, one of the most appealing is certainly the understanding of the electronic properties, in particular the photoabsorption features, in condensed phase. Liquid water experimentally shows, under ambient conditions, the 0-1 absorption maximum at 147 nm, that is, 88 kJ mol À1 shifted toward the blue with respect to the corresponding absorption in vacuum. This blue-shift is known to be more pronounced in ice than in liquid water, and it is also present in small water clusters. From these observations, it is well-established that such a blue-shift is to be mainly ascribed to the short contacts of the excited molecule with its solvation shell (the water dipole moment undergoes an inversion upon 0-1 excitation ). However, only a few theoretical studies have been so far devoted to modelling water photoabsorption in the condensed phase. The computational methods available nowadays are, in fact, able to provide extremely accurate information about the photoexcitation of isolated molecules. However, there are still many difficulties in modelling the same phenomenon in the condensed phase. The inclusion of electronic degrees of freedom (necessary for studying an electronic excitation) into a simulation of a large number of molecules (necessary for a reliable modelling of a condensed phase) is, in fact, still challenging from a computational point of view. In this context, we recently proposed a theoretical computational approach, the perturbed matrix method (PMM), whose main computational feature is the possibility of including, into a classical simulation algorithm, electronic degrees of freedom. In a number [a] M.
Dynamics of Water Molecules in Aqueous Solvation Shells
Science, 2001
We report on the direct measurement of the dynamics of water molecules in the solvation shell of an ion in aqueous solution. The hydrogen-bond dynamics of water molecules solvating a Cl Ϫ , Br Ϫ , or I Ϫ anion is slow compared with neat liquid water, indicating that the aqueous solvation shells of these ions are rigid. This rigidity can play an important role in the overall dynamics of chemical reactions in aqueous solution. The experiments were performed with femtosecond midinfrared nonlinear spectroscopy, because this technique allows the spectral response of the water molecules in the solvation shell to be distinguished clearly from that of the other water molecules in the solution.
The Journal of Physical Chemistry B, 1997
The solvation free energy curves for electron transfer between several types of ions in aqueous solution are studied by molecular dynamics computer simulations and by simple theoretical models. By using models of increasing complexity, contributions of different physical effects are evaluated. The theoretical models are for two ions at infinite separation in a dielectric continuum: the first is based on the Born solvation free energy, which assumes a linear response of the solvent and thus is basically Marcus theory, and the second is based on a nonlinear response model of ionic solvation by Hyun, Babu, and Ichiye (HBI), which includes the effects of dielectric saturation. Finally, molecular dynamics simulations of ions at infinite and finite separations describe the molecular nature of the solvent, with the latter including the influence of the solutes on each other. The focus here is on the orientational rather than electronic polarization, although the latter also will contribute. Previously, comparison of HBI and Born free energy curves showed that nonlinearities are most pronounced in electron transfer reactions involving a neutral to charged species or vice versa and become much less evident as the magnitude of charges of the solutes increases. Here, a comparison of results for ions at infinite separation from the molecular dynamics simulations and the HBI and Born models shows that dielectric saturation greatly reduced the activation energy ∆G q mainly by shifting the free energy curves closer together (i.e., by reducing the polarization energy) but affected ∆G q to a lesser degree by the nonparabolic nature of the curve. Moreover, it shows that the contribution of the molecular structure of water such as density variations and hydrogen bonding was to shift the curves apart, resulting in a smaller increase in ∆G q. In addition, a comparison of molecular dynamics results for ions at infinite and finite separation shows that the effect of bringing the ions to a close separation was to reduce ∆G q mainly by the reduction of the solvent reorganization at large distances, thus shifting the curves together. The direct influence of one solute on the polarization of the other was to increase the nonparabolic nature of the curve, which affects ∆G q less.
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.
Previous work describes a computational solvation model called semi-explicit assembly (SEA). The SEA water model computes the free energies of solvation of nonpolar and polar solutes in water with good efficiency and accuracy. However, SEA gives systematic errors in the solvation free energies of ions and charged solutes. Here, we describe field-SEA, an improved treatment that gives accurate solvation free energies of charged solutes, including monatomic and polyatomic ions and model dipeptides, as well as nonpolar and polar molecules. Field-SEA is computationally inexpensive for a given solute because explicit-solvent model simulations are relegated to a precomputation step and because it represents solvating waters in terms of a solute’s free-energy field. In essence, field-SEA approximates the physics of explicit-model simulations within a computationally efficient framework. A key finding is that an atom’s solvation shell inherits characteristics of a neighboring atom, especially strongly charged neighbors. Field-SEA may be useful where there is a need for solvation free-energy computations that are faster than explicit-solvent simulations and more accurate than traditional implicit-solvent simulations for a wide range of solutes.
Molecular Physics, 2006
Carr-Parrinello MD calculations for a simplistic periodic model of liquid water are performed to probe temperature dependence of infrared activation lifetime. IR activation is classically simulated by adding an appropriate velocity to the proton in a tagged water molecule. The evolution of hydrogen bonding descriptors is monitored through consecutive simulations to spot the onset of qualitative changes in the hydrogen bonding network; they are related to vibrational energy relaxation. The applied ionic simulation temperature (elevated by 20%) decreases the tendency to overbinding characteristic for CP MD calculations. Qualitatively estimated stretch lifetimes are 280, 320 and 400 fs for temperatures of 298, 320 and 370 K, respectively. This work gives direct evidence of the parallel dependence of both the decay of OH activation and the hydrogen bond network on temperature, which offers a viable explanation for the experimentally observable unusual increase in OH excitation lifetime with temperature.