G. van der Zwan

Nonequilibrium solvation dynamics in solution reactions

We construct a theoretical framework for the description of nonequilibrium solvation and solvent participation in the reaction coordinate for solution reactions. The framework is illustrated by a model of reactive dipole isomerization. We show that a multidimensional reaction coordinate picture is equivalent to a one dimensional description in which a generalized friction characterizes and quantifies nonequilibrium solvation effects on the reaction rate. The adiabatic regime where equilibrium solvation and mean potential ideas are correct is identified. Several distinct regimes of nonequilibrium solvation are identified and described in molecular terms. In the effective mass regime, equilibrium solvation ideas give the reaction barrier curvature correctly, but solvent inertia modifies the barrier passage rate. In the nonadiabatic regime, the solvent is ‘‘frozen’’ during the barrier passage and cannot provide equilibrium solvation. In the polarization caging regime, the reacting species adjust to the moving solvent, rather than vice versa, and the solvent is heavily involved in the reaction coordinate. The rate constant in each of these regimes is related to reactive and solvent dynamics.

Dynamical polar solvent effects on solution reactions: A simple continuum model

The influence of polar solvent dynamics in solution reactions is investigated for a simple model. In this model, a charge is subject to a chemical free energy barrier, the successful crossing of which constitutes reaction. The retarding influence of the time dependent solvent polarization is described at the continuum Debye relaxation level. The reaction rate constant k is determined as a function of the barrier curvature, the charge‐solvent interaction strength, and the solvent polarization relaxation time. The reduction of k with increasingly slower solvent relaxation is found to depend sensitively on the magnitude of charge‐solvent interactions. When the latter are weak, the behavior of k differs qualitatively from a standard Kramers prediction. The analogous reaction problem for a dipole is discussed briefly. Directions for a more realistic treatment of polar solvent effects on reaction rates are described.

A simple dipole isomerization model for non-equilibrium solvation dynamics in reactions in polar solvents

Abstract A simple model for dipole isomerization reactions in polar solvents is studied. A full dynamical treatment at the generalized Langevin equation level reveals several non-equilibrium solvation regimes in which the rate constant differs considerably from the standard equilibrium solvation, transition state theory predictions. In addition, the solvent is found to be heavily involved in the reaction coordinate, in contrast to the standard equilibrium solvation view. The marked differences from a Kramers Langevin equation level description are also described. In each regime, the rate constant and reaction coordinate are found analytically, and typical reaction system-solvent trajectories are displayed. A related normal-mode description convenient for strong reaction system-nearest-neighbor solvent dipole interactions is constructed. In this perspective, non-equilibrium solvation effects on rates and reaction coordinates are interpreted in terms of a bias of the reactive motion towards the direction of least friction.

Chemical reaction rates and solvation dynamics in electrolyte solutions: ion atmosphere friction

Abstract We consider the influence of ion atmosphere dynamics on a model dipolar isomerization in an electrolyte solution, as well as on the time dependent fluorescence (TDF) ofa dipolar solute. The impact of the resulting ion atmosphere friction on reducing the reaction rate below the equilibrium solration transition state theory value can sometimes be pronounced. Connections of this friction to the TDF dynamics are indicated for solvents both of low and high polarity.

Polarization diffusion effects on reaction rates in polar solvents

Abstract The influence of solvent dipole translational relaxation via the mechanism of polarization diffusion is examined for simple models of dipole isomerization and charge transfer reactions in polar solvents. In certain solvents, e.g. methanol, polarization diffusion has marked effects on reaction rate constants when compared to the Debye orientational solvent relaxation picture.

Reactive paths in the diffusion limit

The character of the reactive motion in multidimensional passage over a chemical barrier in the diffusion limit is examined. Frictional coupling between the reactive and nonreactive modes that are defined by the potential forces leads to an enhancement of the rate constant. This effect arises from a bias of the reactive motion towards the direction of least friction, and is modulated by the potential forces. A detailed explanation of this effect is presented and an application to a simple model of isomerization in polymers is given.

Polarization diffusion and dielectric friction

Relaxation of nonequilibrium polarization fields in dielectrics is usually theoretically treated solely in terms of the reorientational motion of the polar molecules. If, however, the translational diffusion of these molecules is also taken into account, dielectric relaxation is accelerated. This results in the lowering of the dielectric friction that retards the motion of an ion or dipole in a dielectric. This reduction in dielectric friction is here calculated for two models: a time-dependent dipole at the center of a sphere, and a moving charge on the axis of a cylinder, each immersed in a dielectric. A parameter measuring the relative importance of rotation and diffusion is identified, and a substantial lowering of the dielectric friction is implied in many situations. It is shown that the boundary conditions play a decisive role in the properties of the fields. Directions for a further development of the theory of polarization diffusion are indicated.

Functional electric field changes in photoactivated proteins revealed by ultrafast Stark spectroscopy of the Trp residues

Ultrafast transient absorption spectroscopy of wild-type bacteriorhodopsin (WT bR) and 2 tryptophan mutants (W86F and W182F) is performed with visible light excitation (pump) and UV probe. The aim is to investigate the photoinduced change in the charge distribution with 50-fs time resolution by probing the effects on the tryptophan absorption bands. A systematic, quantitative comparison of the transient absorption of the 3 samples is carried out. The main result is the absence in the W86F mutant of a transient induced absorption band observed at ≈300–310 nm in WT bR and W182F. A simple model describing the dipolar interaction of the retinal moiety with the 2 tryptophan residues of interest allows us to reproduce the dominant features of the transient signals observed in the 3 samples at ultrashort pump-probe delays. In particular, we show that Trp86 undergoes a significant Stark shift induced by the transient retinal dipole moment. The corresponding transient signal can be isolated by direct subtraction of experimental data obtained for WT bR and W86F. It shows an instantaneous rise, followed by a decay over ≈500 fs corresponding to the isomerization time. Interestingly, it does not decay back to zero, thus revealing a change in the local electrostatic environment that remains long after isomerization, in the K intermediate state of the protein cycle. The comparison of WT bR and W86F also leads to a revised interpretation of the overall transient UV absorption of bR.

LIGHT SCATTERING FROM A FLUID WITH A STATIONARY TEMPERATURE GRADIENT

The spectrum of light, scattered from a fluid with a stationary temperature gradient, is calculated on the basis of fluctuating hydrodynamics. Explicit expressions are obtained for the spectrum of the scattered light which is no longer symmetric around the frequency of the incident light. In particular the difference in height and intensity of the Brillouin lines is given. Furthermore the shift in the position of the maximum of the Rayleigh line is calculated.

Spectroscopy and structure of bacteriochlorophyll dimers. I. Structural consequences of nonconservative circular dichroism spectra.

The origin of the nonconservative nature of the circular dichroism (CD) spectrum of bacteriochlorophyll dimers is investigated. It is shown that coupling between the Qy and Qx transitions can, under rather restricting circumstances, lead to an asymmetrical CD spectrum: only for a limited set of relative orientations of the monomers within the dimer is the spectrum found to be asymmetrical. The relation between intensity and asymmetry of the CD spectrum is elucidated. The results are applied to the B820 subunit of the LH1 antenna system and subsequently to the antenna system LH1 itself. Differences in the geometry of the BChls in LH1 versus the LH2 structure are discussed.