Mark Maroncelli

Picosecond solvation dynamics of coumarin 153: The importance of molecular aspects of solvation

Solvation dynamics in polar liquids have been examined using the probe molecule coumarin 153 (Cu153) and picosecond spectroscopic techniques. Steady‐state absorption and fluorescence spectra of Cu153 as a function of solvent show that the frequency of the electronic spectrum of this probe provides a convenient measure of solvation energetics. Both nonspecific dipolar and to a smaller degree H‐bonding solute–solvent interactions are involved. Time‐correlated single photon counting was used to observe time‐dependent shifts of the fluorescence spectrum of Cu153 in a variety of alcohols, propylene carbonate, and N‐methylpropionamide solvents as a function of temperature. These time‐dependent spectral shifts provide a direct measure of the time dependence of the solvation process. Theoretical models that treat the solvent as a dielectric continuum do not adequately account for the observed solvation dynamics. In the solvents studied, such theories predict a single exponential shift of the fluorescence spectrum...

Polar Solvent Dynamics and Electron-Transfer Reactions

Polar solvents often exert a dramatic influence on reactions in solution. Equilibrium aspects of this influence involve differential solvation of reactants compared to the transition state that lead to alteration of the free-energy barrier to reaction. Such effects are well known, and often give rise changes in reaction rates of many orders of magnitude. Less well understood are effects arising from non-equilibrium, dynamical aspects of solvation. During the course of reaction, charge is rapidly redistributed among reactants. How the reaction couples to its solvent environment depends critically on how fast the solvent can respond to these changes in reactant charge distribution. In this article the dynamics of solvation in polar liquids and the influence of this dynamics on electron-transfer reactions are discussed. A molecular picture suggests that polar solvation occurs on multiple time scales as a result of the involvement of different types of solvent motion. A hierarchy of models from a homogeneous continuum model to one incorporating molecular aspects of solvation, combined with computer simulations, gives insight into the underlying dynamics. Experimental measures of solvation dynamics from picosecond and subpicosecond time-dependent Stokes shift studies are compared with the predictions of theoretical models. The implication of these results for electron-transfer reactions in solution are then briefly considered.

The dynamics of solvation in polar liquids

Abstract Recent experiments, theories, and computer simulations aimed at elucidating the dynamics of solvation in polar liquids are critically reviewed.

Computer simulation of the dynamics of aqueous solvation

Equilibrium and nonequilibrium molecular dynamics computer simulations have been used to study the time dependence of solvation in water. The systems investigated consisted of monatomic ions immersed in large spherical clusters of ST2 water. Relaxation of the solvation energy following step junction jumps in the solute’s charge, dipole moment, and quadrupole moment have been determined from equilibrium molecular dynamics (MD) simulations under the assumption of a linear solvation response. The relaxation times observed differ substantially depending on the type of multipole jump and the charge/size ratio of the solute. These results could not be quantitatively understood on the basis either of continuum or molecular theories of solvation dynamics currently available. Even the qualitative picture of a distribution of relaxation times which monatonically increases with distance away from the solute is not correct for the systems studied. This lack of agreement is partially explained in terms of the structur...

Computer simulations of solvation dynamics in acetonitrile

Computer simulations of the solvation of monatomic ions in acetonitrile are used to investigate dynamical aspects of solvation in polar aprotic solvents. The observed dynamics depend significantly on solute charge and on which multipole moment of the solute is perturbed. In all cases, the solvation response has a two‐part character. One part consists of a fast initial relaxation and attendant oscillations, both of which occur on a time scale of 0.1–0.2 ps. The initial response is well fit by a Gaussian function and accounts for ∼80% of the total relaxation. The second dynamical component occurs on a much slower, ∼1 ps time scale, and accounts for the remainder of the relaxation. The fast response results from small amplitude inertial dynamics of solvent molecules within the confines of their instantaneous environment. The slow component reflects larger amplitude motions involving the breakup and reorganization of these local environments, especially in the first solvation shell of the solute. Comparison o...

Subpicosecond resolution studies of solvation dynamics in polar aprotic and alcohol solvents

Subpicosecond resolution measurements of the kinetics of dipolar solvation have been made. The time resolved Stokes shift of a dye molecule, LDS‐750 was measured using the fluorescence upconversion technique in the solvents acetonitrile, DMSO, nitrobenzene, methanol, and n‐butanol. The solvation dynamics in both aprotic and alcohol solvents occur on a time scale roughly given by the longitudinal relaxation time as predicted by simple continuum theories. The relaxation in nitrobenzene and butanol is nonexponential and the relaxation in methanol is significantly faster than the calculated time. These deviations from simple theory are discussed in the context of (i) the significance of high frequency dispersions in the dielectric response, (ii) translational contributions to the solvent relaxation, and (iii) molecular aspects of the solvation not accounted in the continuum description.

Ionic liquids: structure and photochemical reactions.

Ionic liquids are subjects of intense current interest within the physical chemistry community. A great deal of progress has been made in just the past five years toward identifying the factors that cause these salts to have low melting points and other useful properties. Supramolecular structure and organization have emerged as important and complicated topics that may be key to understanding how chemical reactions and other processes are affected by ionic liquids. New questions are posed, and an active debate is ongoing regarding the nature of nanoscale ordering in ionic liquids. The topic of reactivity in ionic liquids is still relatively unexplored; however, the results that have been obtained indicate that distributed kinetics and dynamical heterogeneity may sometimes, but not always, be influencing factors.

Polar solvation dynamics of polyatomic solutes: Simulation studies in acetonitrile and methanol

This paper describes results of simulations of solvation dynamics of a variety of solutes in two reference solvents, acetonitrile and methanol. Part of these studies involve attempts to realistically model the solvation dynamics observed experimentally with the fluorescence probe coumarin 153 (C153). After showing that linear response simulations afford a reliable route to the dynamics of interest, experimental and simulation results for C153 are compared. Agreement between the observed and calculated dynamics is found to be satisfactory in the case of acetonitrile but poor in the case of methanol. The latter failure is traced to a lack of realism in the dielectric properties of the methanol model employed. A number of further simulations are then reported for solvation of a number of atomic, diatomic, and benzenelike solutes which are used to elucidate what features of the solute are important for determining the time dependence of the solvation response. As far as large polyatomic solutes like C153 are ...

The distribution of conformational disorder in the high‐temperature phases of the crystalline n‐alkanes

The distributions of conformational defects that exist in the high‐temperature phase II (also referred to as the hexagonal or rotator phase) of the crystalline n‐alkanes C21 and C29 have been measured by an infrared CD2‐substitution technique and have been accounted for in terms of a lattice model that provides freedom for longitudinal displacement of the chains. The defects consist almost entirely of gtg’ kinks distributed nonuniformly along the chain. The uneven distribution is indicated in the variation in the concentration of gauche bonds measured at various sites along the chain. The highest concentration is at the chain ends, and the concentrations at interior sites decrease exponentially in going toward the middle. To explain the distribution we used a modification of a lattice model that had been successfully applied to the lipid bilayer. Comparison of observed distributions with those computed from the model indicates that the factors that determine the shape of the distribution are quite differe...

Phase Transitions and Nonplanar Conformers in Crystalline n-Alkanes

Crystals of n-alkanes show a remarkable series of solid-solid phase transitions. In the odd n-alkanes C25, C27, and C29 a previously unknown transition is found by both calorimetry and infrared spectroscopy. The ubiquitous presence of nonplanar conformations of the chains is shown by infrared spectroscopy. The nonplanar conformers constitute approximately half the molecules in the highest temperature solid phase of C29.