Showing papers on "Solvation published in 1989"

Reduction potentials involving inorganic free radicals in aqueous solution

Abstract: Publisher Summary This chapter discusses the reduction potentials involving inorganic free radicals in aqueous solution. Because free radicals are usually transients, knowing their thermodynamic properties is primarily useful in mechanistic studies. Thus, the useful redox couples associated with a given free radical correspond to plausible elementary steps in reaction mechanisms. All potentials are expressed against the normal hydrogen electrode (NHE). Apart from the NHE, the standard state for all solutes is the unit molar solution at 25 °C. This violates the usual convention for species, such as O 2 that occur as gases, but because the rates of bimolecular reactions in solution are significant, the unit molar standard state is most convenient. The emphasis is on electron transfer reactions in which no bonds are formed or broken, electron transfer reactions in which concerted electron transfer and bond cleavage could occur, and certain atom transfer reactions. The chapter also presents a tabulation of ∆ f G ° values for all the radicals. A common approach in estimating the thermochemistry of aqueous free radicals is to use gas-phase data with approximations of solvation energies.

Polar Solvent Dynamics and Electron-Transfer Reactions

Abstract: 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.

A surface constrained all‐atom solvent model for effective simulations of polar solutions

Abstract: A consistent simulation of ionic or strongly polar solutes in polar solvents presents a major challenge from both fundamental and practical aspects. The frequently used method of periodic boundary conditions (PBC) does not correctly take into account the symmetry of the solute field. Instead of using PBC, it is natural to model this type of system as a sphere (with the solute at the origin), but the boundary conditions to be used in such a model are not obvious. Early calculations performed with our surface constrained soft sphere dipoles (SCSSD) model indicated that the dipoles near the surface of the sphere will show unusual orientational preferences (they will overpolarize) unless a corrective force is included in the model, and thus we implemented polarization constraints in this spherical model of polar solutions. More recent approaches that treated the surface with stochastic dynamics, but did not take into account the surface polarization effects, were also found to exhibit these nonphysical orientational preferences. The present work develops a surface constrained all‐atom solvent (SCAAS) model in order to consistently treat the surface polarization effects in all‐atom molecular dynamics simulations. The SCAAS model, which was presented in a preliminary way in previous works, introduces surface constraints as boundary conditions in order to make the necessarily finite system behave as if it was part of an infinite system. The performance of the model with regard to various properties of bulk water is examined by comparing its results to those obtained by PBC simulations. The results obtained from SCAAS models of different sizes are found to be similar to each other and to the corresponding PBC results. The performance of the model in simulations of solvated ions is emphasized and a comparison of the results obtained with spheres of different sizes demonstrates that the model does not possess significant size dependence. This indicates that the model can be used with a relatively small number of solvent molecules for convergent simulation of structure, energetics, and dynamics of polar solutions. The much simpler fixedcenter Langevin dipoles (FCLD) model is also examined and found to provide a powerful tool for estimating solvation free energies. Finally, a preliminary study of the dielectric properties of the SCAAS model is reported and the potential of this model for exploring the correct implementation of the solvent reaction field is discussed.

Dynamics of Solvation and Charge Transfer Reactions in Dipolar Liquids

Abstract: The effects of solvent and solvent dynamics on chemical reactions, especially on charge transfer processes, have long been a subject of great importance in physical chemistry. In the past, attention was focused pri­ marily on equilibrium solvent effects, such as the effect of solvent polarity on the reaction potential surface. Tn recent years it has become clear that in many fast reactions solvent dynamics can play a direct role and can affect both the rate and the outcome of a reaction profoundly. Thus, an understanding of the time-dependent response of a polar solvent to a changing charge distribution in a polar solute molecule is essential to understand the role of solvent in many important chemical and biological processes in liquids. Such understanding can be achieved by studying the dynamics of solvation of a newly created ion or of an instantaneously changed dipole in a polar liquid. This subject has undergone a renaissance in recent years because of the availability of ultra-short laser pulses that make it possible to study solvation dynamics directly with a time resolution hitherto impossible. An understanding of the details of solvent response to a sudden change in the charge distribution of a polar solute "probe" molecule is beginning to emerge. Experimental studies on the dynamics of solvation are usually carried out by instantaneously creating a charged species inside a polar solvent and subsequently monitoring the emission/absorption spectrum of this

Evaluation of the dispersion contribution to the solvation energy. A simple computational model in the continuum approximation

Abstract: We present a simple computational method for the evaluation of solute-solvent dispersion energy contributions in dilute isotropic solutions, supplementing the method with an analysis of its sensitivity with respect to several parameters (or features of the solvation model) which are left free in the general formulation. The method is a natural complement of the electrostatic solvation procedure described in preceding articles.

Nonequilibrium solvation effects on reaction rates for model SN2 reactions in water

Abstract: Molecular dynamics (MD) simulations of the model SN2 reaction Cl−+CH3Cl→ClCH3+Cl− in water, and variants thereof, are presented. The resulting transmission coefficients κ, that measure the deviations of the rates from the transition state theory (TST) rate predictions due to solvent‐induced recrossings, are used to assess the validity of the generalized Langevin equation (GLE)‐based Grote–Hynes (GH) theory. The GH predictions are found to agree with the MD results to within the error bars of the calculations for each of the 12 cases examined. This agreement extends from the nonadiabatic regime, where solvent molecule motions are unimportant and κ is determined by static solvent configurations at the transition state, into the polarization caging regime, where solvent motion is critical in determining κ. In contrast, the Kramers theory predictions for κ fall well below the simulation results. The friction kernel in the GLE used to evaluate the GH κ values is determined, from MD simulation, by a fixed‐particle time correlation function of the force at the transition state. When this is expressed as a (Fourier) friction spectrum in frequency, marked similarities to the pure solvent spectrum are observed, and are used to identify the water solvent motions that determine the transmission coefficient κ. The deviations of κ from unity, the TST value, are dominated by solvent motions (translational and reorientational) which on the time scale of the recrossings are essentially static configurations. The deviations from the frozen solvent, nonadiabatic limit values κNA are dominated by the hinderd rotations (librations). Finally, the underlying assumptions of the GLE and the GH theory are discussed within the context of the simulation results.

Femtosecond resolved solvation dynamics in polar solvents

Abstract: The transient solvation of a polar fluorescent probe has been studied by the time resolved Stokes shift technique with roughly five times shorter time resolution than previously reported. New shorter time components in the solvation relaxation function C(t) have been discovered for methanol, propionitrile, and propylene carbonate; the C(t) function for acetonitrile is singly exponential within the limitations of the instrument. The observed C(t) has been compared to theoretical calculations using the dielectric continuum (DC) model for each solvent, with non‐Debye expressions for the solvent dielectric response. For methanol the DC model predictions agree closely with experiment. For the polar aprotic solvents propylene carbonate and propionitrile, the shape of the experimental decay is different from the DC predictions, but the average decay times 〈τs〉 are closer to the DC predictions than previously reported. The comparison of theory and experiment is clearly limited by the inconsistencies and limited f...

Solute–solvent interactions in infinitely dilute supercritical mixtures: A molecular dynamics investigation

Abstract: A molecular dynamics investigation of the environment surrounding an infinitely dilute Xe‐like Lennard‐Jones atom in a supercritical Ne‐like Lennard‐Jones fluid shows enhanced solute–solvent interactions (clustering) in the vicinity of the solvent’s critical point. Under near‐critical conditions, the solute is at all times surrounded by an environment which is greatly enriched in solvent with respect to bulk conditions, but the identity of the solvent molecules in the cluster changes continuously. The enhancement of solute–solvent interactions in the Xe‐in‐Ne near‐critical system is in contrast with the behavior exhibited by the symmetric Ne‐in‐Xe near‐critical system (i.e., infinitely dilute Ne in near‐critical Xe). In this case, the environment surrounding Ne atoms tends to be solvent lean with respect to bulk conditions. As Ne’s critical point is approached, Ne–Xe interactions become progressively irrelevant with respect to Ne–Ne interactions, giving rise to pronounced density fluctuations around the Xe atoms. Differences between Xe‐in‐Ne and Ne‐in‐Xe near‐critical systems are confirmed by cluster statistics and stability analysis.

Enzymes work by solvation substitution rather than by desolvation.

Abstract: Considerable attention has recently been drawn to the hypothesis that enzymes catalyze their reactions by displacing solvent and creating an environment similar to the gas phase for the reacting substrates. This "desolvation hypothesis" is reexamined in this paper by defining a common reference energy for reactions in various environments. It is argued that consistent attempts to describe the actual energetics of enzymatic reactions, taking either gas phase or solution as a reference, would contradict the above hypothesis. That is, the enzyme does remove water molecules from its substrate, but substitutes these molecules for another polar environment (namely, its active site). By taking amide hydrolysis as an example, we use experimentally estimated solvation energies and analyze the reaction profile in the gas phase, in solution, and in enzyme active sites. We show that the gas-phase reaction is characterized by an enormous activation barrier (associated with forming the charged nucleophile from neutral fragments), although the nucleophilic attack is essentially barrierless. On the other hand, the enzyme and solution reactions are found to have similar reaction profiles, with a lower activation barrier for the enzymatic reaction. Presumably, the fact that previous analyses of this problem did not involve the construction of the relevant thermodynamic cycles (and quantitative estimates of the corresponding solvation energies) led to the desolvation hypothesis. Our conclusion is that enzyme active sites provide specific polar environments that do not resemble the gas phase but that are designed for electrostatic stabilization of ionic transition states and that "solvate" these states more than water does.

Solvent effects on optical absorption spectra: the 1A1 .fwdarw. 1A2 transition of formaldehyde in water

Abstract: We have examined solvation of the singlet ground ('Al) and excited (]A2) states of formaldehyde by water using a combination of classical molecular dynamics and ab initio quantum mechanics techniques. Molecular dynamics simulations were carried out for a formaldehyde solute molecule in a bath of 209 water molecules. The solute was represented by Lennard-Jones plus electrostatic terms with net atomic natural charges generated from ab initio Hartree-Fcck calculations using a 6-31G+d-type basis set. The SPC model was used to describe the water-water interaction potential. Radial distribution functions show structured binding by several water molecules at the oxygen end of formaldehyde in its ground state. This structure is largely, but not completely, destroyed for formaldehyde in its lowest excited singlet state. The 'A, 'A2 vertical transition energy of formaldehyde was calculated at the ab initio Hartree-Fock level including the electrostatic interactions with the solvent molecules for 70 configurations along the trajectory. No single water molecule or first solvation shell (cluster model) adequately describes the formaldehydesolvent interactions in both electronic states. When an ensemble of water configurations is considered, the calculated spectral blue shift and bandwidth of about 1900 and 4400 cm-', respectively, are in reasonable accord with the available experimental data.

The thermodynamics of solutions

Abstract: This paper explores the relationship between thermodynamics and two other types of investigation. Structure determinations by neutron diffraction and computer simulation give a clear picture of first-shell solvation for a number of cations in water. Replacement of a water molecule in the complex M(H20): by an organic ligand S in a mixed aqueous solvegt S-H20 may result in distortion which raises the free energy of transfer(AtG ) of the cation M+ from water above that expected from an unhindered base-line. The sequence of these deviations is that predictable from the known geometries, i.2. Li+>Na'>Cs+>Ag+, H' . relative viscosities of solutions are examined by Transition-State Tneory. Electrolytes like CsCl lower the free energy, and markedly, the enthalpy and entropy of activation for viscous flow of water, but do not necessarily break down solvent structure as in the classical view. Enchanced co-ordination of solvent to the ions could occur in a transition-state solvent more weakly structured and bonded than the ground-state solvent. Enthalpies of transfer of the "hydrophobic" solute E-butanol in methanoluater mixtures suggest that TBA makes strong solute-solvent bonds, but breaks solvent-solvent bonds. The large positive activation parameters for viscous flow in highly aqueous mixtures suggest that water encages this type of solute in the ground state, thereby enhancing the solute-solvent interaction. The INTRODUCTION Our basic thermodynamic process is the transfer of a solute between standard states in two different solvents; it will be accompanied by changes in the free energy,&G enthalpy, Aty , and entropy, AtS , of the system, which reflect differences in the solvation of the solute in the two solvents (ref. I). Most of our solutes will be electrolytes. Most of the transfers will involve binary aqueous mixtures. We shall first show how recent structural studies (refs 2 & 3) help us to understand steric influences on the free energies of transfer, AtG , of some simple electrolytes from water to mixed aqueous solvents. We shall then discuss briefly a simple theoretical model (ref. 4 ) for the enthalpy of transfer, AtH*, in binary solvent systems. Finally, we shall consider viscous flow (ref. 5). This process played a crucial part in the development of the established models (refs. 6 & 7 ) for ions in solution. It can be treated quasi-thermodynamically, by Transition-State theory (ref. 8). Our transfer quantities now involve something we call the transitionstate solvent. This access to an unusual type of solvent helps us to a clearer understanding of the solvation process. 8 8 6

Range of the solvation pressure between lipid membranes: dependence on the packing density of solvent molecules

Abstract: Well-ordered multilamellar arrays of liquid-crystalline phosphatidylcholine and equimolar phosphatidylcholine-cholesterol bilayers have been formed in the nonaqueous solvents formamide and 1,3-propanediol. The organization of these bilayers and the interactions between apposing bilayer surfaces have been investigated by X-ray diffraction analysis of liposomes compressed by applied osmotic pressures up to 6 X 10(7) dyn/cm2 (60 atm). The structure of egg phosphatidylcholine (EPC) bilayers in these solvents is quite different than in water, with the bilayer thickness being largest in water, 3 A narrower in formamide, and 6 A narrower in 1,3-propanediol. The incorporation of equimolar cholesterol increases the thickness of EPC bilayers immersed in each solvent, by over 10 A in the case of 1,3-propanediol. The osmotic pressures of various concentrations of the neutral polymer poly(vinylpyrrolidone) dissolved in formamide or 1,3-propanediol have been measured with a custom-built membrane osmometer. These measurements are used to obtain the distance dependence of the repulsive solvation pressure between apposing bilayer surfaces. For each solvent, the solvation pressure decreases exponentially with distance between bilayer surfaces. However, for both EPC and EPC-cholesterol bilayers, the decay length and magnitude of this repulsive pressure strongly depend on the solvent. The decay length for EPC bilayers in water, formamide, and 1,3-propanediol is found to be 1.7, 2.4, and 2.6 A, respectively, whereas the decay length for equimolar EPC-cholesterol bilayers in water, formamide, and 1,3-propanediol is found to be 2.1, 2.9, and 3.1 A, respectively. These data indicate that the decay length is inversely proportional to the cube root of the number of solvent molecules per unit volume.(ABSTRACT TRUNCATED AT 250 WORDS)

Relaxation dynamics following transition of solvated electrons

Abstract: Relaxation dynamics following an electronic transition of an excess solvated electron in clusters and in bulk water is studied using an adiabatic simulation method. In this method the solvent evolves classically and the electron is constrained to a specified state. The coupling between the solvent and the excess electron is evaluated via the quantum expectation value of the electron–water molecule interaction potential. The relaxation following excitation (or deexcitation) is characterized by two time scales: (i) a very fast (∼20–30 fs) one associated with molecular rotations in the first solvation shell about the electron, and (ii) a slower stage (∼200 fs), which is of the order of the longitudinal dielectric relaxation time. The fast relaxation stage exhibits an isotope effect. The spectroscopical consequences of the relaxation dynamics are discussed.

Solvent‐induced interactions: Hydrophobic and hydrophilic phenomena

Abstract: The solvent‐induced interactions between two kinds of molecules (or groups) are examined from the point of view of the solvation free energies of a pair of interacting molecules (or groups) The first interaction is that between two simple nonpolar solutes (or groups) in water and is known as the hydrophobic interaction Both theoretical as well as experimental evidence indicate that these interactions are quite weak The solvent‐induced interactions between two functional groups that can form hydrogen bonds, referred to as intramolecular hydrophilic interactions, are found to be much stronger than the corresponding hydrophobic interactions It is therefore argued that intramolecular hydrophilic interactions are probably more significant in biochemical processes than hydrophobic interactions

Magnitude of the solvation pressure depends on dipole potential.

Abstract: As polar surfaces in solvent are brought together, they experience a large repulsive interaction, termed the solvation pressure. The solvation pressure between rough surfaces, such as lipid bilayers, has been shown previously to decay exponentially with distance between surfaces. In this paper, we compare measured values of the solvation pressure between bilayers and the dipole potential for monolayers in equilibrium with bilayers. For a variety of polar solvents and lipid phases, we find a correlation between the measured solvation pressures and dipole potentials. Analysis of the data indicates that the magnitude of the solvation pressure is proportional to the square of the dipole potential. Our experiments also show that the oriented dipoles in the lipid head-group region, including those of both the lipid and solvent molecules, contribute to the dipole potential. We argue that (i) the field produced by these interfacial dipoles polarizes the interbilayer solvent molecules giving rise to the solvation pressure and (ii) both the solvation pressure and the dipole potential decay exponentially with distance from the bilayer surface, with a decay constant that depends on the packing density of the interbilayer solvent molecules (1-2 A in water). These results may have importance in cell adhesion, adsorption of proteins to membranes, characteristics of channel permeability, and the interpretation of electrokinetic experiments.

Spectroscopic studies of the solvation of amides with N—H groups. Part 1.—The carbonyl group

Abstract: Infrared shifts of the CO stretch (νco) band, and n.m.r. shifts for the 13CO carbon have been studied for formamide, acetamide, N-methyl formamide and N-methyl acetamide for dilute solutions in a range of pure and mixed solvents. The results are compared with those previously reported for N,N-dimethylamides in the same systems. There are good linear relationships between Δν(13C) and νco for the pure solvent systems, provided allowance is made for the presence of two types of solvate for methanol. For mixed methanol-aprotic solvents (B) the low-frequency (νco) component for pure methanol was lost as the concentration of B was increased. The high-frequency band initially gained intensity, but this was ultimately replaced by a third band characteristic of the amide in pure B. These results suggest that the CO group forms both one and two hydrogen bonds in methanol. Aqueous solutions have a single νco band close to that for the disolvate in methanol. As [B] was increased, this gave way to a band close to that for the mono-solvate, which again was steadily replaced by the non-hydrogen-bonded form. Hence it is concluded that for all the amides, the di-hydrogen-bonded species dominates in water. Reasons for the different behaviour in methanol and water are discussed.In all cases, as well as the gain and loss of i.r. bands, those assigned to the hydrogen-bonded units shifted considerably as [B] increased. For aqueous systems these shifts are assigned to changes in secondary solvation.We have looked for specific differences between the results for the dimethyl derivatives and the present compounds, in the expectation that N—H solvation would have a discernible affect on CO solvation. There are small differences, but these are not systematic on going from RCONMe2via RCONHMe to RCONH2, and it is concluded that cooperativity effects involving CO and N—H solvation are small compared with those for water and alcohols. The 13C resonance (13CO) shifted systematically for mixed protic–aprotic solvent systems. Using the i.r.–n.m.r. correlation and the intensity changes for the νCO bands, reasonable predictions of these n.m.r. shifts were obtained.Marked changes in the amide II band were also observed, but these are less readily interpreted. Studies of the overtone infrared spectra for aqueous solutions reveal the presence of (OH)free and (NH)free bands, showing that the N—H groups are not fully hydrogen-bonded in water despite the effect of two bonds to the carbonyl group.

Preferential solvation in two-component systems

Abstract: Deux types de melanges binaires sont etudies: (alcool ou THF ou dioxanne) avec soit de l'eau, soit du tetrachlorure de methane

Solvation of anion radicals: gas-phase versus solution

Abstract: Reversible half-wave potentials for 38 neutral/anion radical couples have been measured at 298 K by cyclic voltammetry in five solvents: tetrahydrofuran, N,N-dimethylformamide (DMF), acetonitrile, dimethyl sulfoxide, and methanol. Among the compounds are 22 substituted nitrobenzenes and nine quinones. The potentials are referenced to the cobaltocenium/cobaltocene couple

Hydrophobic hydration: A free energy perturbation study

Abstract: Differences in free energy of hydration between molecules within a class of chemical compounds, such as normal alkanes, tetraalkylmethanes, alkyl- and tetraalkylammonium ions, amines and aromatic compounds, have been calculated by a coordinate coupled free energy perturbation method. The calculated free energy differences agree reasonably well with the experimental values. The patterns of variation of free energy change with coupling parameter {lambda} are found to differ for different classes of compounds. These results are interpreted in terms of the differing hydration processes of these molecules. Hydrophobic hydration of hydrocarbons and ammonium ions bearing large hydrocarbon groups seems to result from tightly bound water structure around the solute. In contrast, the water structure around amines and aromatic compounds bearing polar functional groups is dictated by the directional hydrogen bonding of the polar group with water. The direction of the free energy change seems to be dictated by solute-solvent interaction energy, which has major contribution to free energy change for alkyl-substituted ammonium ions.

Polarization relaxation, dielectric dispersion, and solvation dynamics in dense dipolar liquid

Abstract: A unified treatment of polarization relaxation, dielectric dispersion and solvation dynamics in a dense, dipolar liquid is presented. It is shown that the information of solvent polarization relaxation that is obtained by macroscopic dielectric dispersion experiments is not sufficient to understand dynamics of solvation of a newly created ion or dipole. In solvation, a significant contribution comes from intermediate wave vector processes which depend critically on the short range (nearest‐neighbor) spatial and orientational order that are present in a dense, dipolar liquid. An analytic expression is obtained for the time dependent solvation energy that depends, in addition to the translational and rotational diffusion coefficients of the liquid, on the ratio of solute–solvent molecular sizes and on the microscopic structure of the polar liquid. Mean spherical approximation (MSA) theory is used to obtain numerical results for polarization relaxation, for wave vector and frequency dependent dielectric function and for time dependent solvation energy. We find that in the absence of translational contribution, the solvation of an ion is, in general, nonexponential. In this case, the short time decay is dominated by the longitudinal relaxation time but the long time decay is dominated by much slower large wave vector processes involving nearest‐neighbor molecules. The presence of a significant translational contribution drastically alters the decay behavior. Now, the long‐time behavior is given by the longitudinal relaxation time constant and the short time dynamics is controlled by the large wave vector processes. Thus, although the continuum model itself is conceptually wrong, a continuum model like result is recovered in the presence of a sizeable translational contribution. The continuum model result is also recovered in the limit of large solute to solvent size ratio. In the opposite limit of small solute size, the decay is markedly nonexponential (if the translational contribution is not very large) and a complete breakdown of the continuum model takes place. The significance of these results is discussed.