Number Of Solvent Molecules Research Articles

Solvation of electronically excited I2−

The interaction potentials between the six lowest electronic states of I−2 and an arbitrary discrete charge distribution are calculated approximately using a one-electron model. The model potentials are much easier to calculate than ab initio potentials, with the cost of a single energy point scaling linearly with the number of solvent molecules, enabling relatively large systems to be studied. Application of the model to simulation of electronically excited I−2 in liquids and CO2 clusters is discussed. In a preliminary application, solvent effects are approximated by a uniform electric field. If electronically excited (2Πg,1/2) I−2 undergoes dissociation in the presence of a strong electric field, the negative charge localizes so as to minimize the total potential energy. However, in a weak field the negative charge localizes in the opposite direction, maximizing the potential energy. Based on a study of the field-dependent potential surfaces, a solvent-transfer mechanism is proposed for the electronic relaxation of 2Πg,1/2I−2, in contrast to the conventional view of relaxation via electron transfer.

Femtosecond pH jump: dynamics of acid—base reactions in solvent cages

Studies are reported of the dynamics of proton-transfer reactions, with sub-picosecond time resolution, in solvent cages. The acid—base system studied in a molecular beam is 1-naphthol as a solute and ammonia (or water) as a solvent, with the number of solvent molecules (n) varying. At the threshold (n=3) for proton transfer we examine the accurate form of the decay, which has an apparent biexponential character, and we relate it to the nature of deprotonation and recombination. From studies of the effect of the total energy, isotope substitution and solvent number (n), we discuss the nature of the transfer and the interplay between the local structure of the base solvent and the dynamics.

Mechanism of ion ejection from a liquid beam following laser photoionization

An anisole—ethanol solution was introduced into vacuum as a continuous liquid flow (liquid beam), and the molecules in the liquid beam were ionized by laser two-photon ionization. Ions ejected from the liquid beam were extracted by applying a pulsed electric field for the measurement of time-of-flight mass spectra of the ions. The intensities and peak profiles of the ions were measured by varying the delay time from the laser ionization to the pulse extraction of the ions at different laser powers. All the ions observed have almost the same velocity (≈ 700 m s −1) and need ≈ 1 μs to leave the liquid beam after laser irradiation. This finding implies that each photoion forming a solvation structure with almost the same number of solvent molecules is expelled from the liquid surface by Coulomb ejection and is dissociated into a cluster ion outside the influence of the Coulomb potential. The rate constants for ion ejection were determined.

Ab initio theoretical study of the monomer-dimer equilibrium in lithium and sodium gem-difluoro allyl and methyl systems

Non-empirical calculations on lithium and sodium gem-difluoroallyl and difluoromethyl systems show that the monomer-dimer equilibrium is shifted in favour of the dimeric species. The two difluoromethyl systems show geometric and energetic features very close to those found in the larger systems. Difluoromethyl is then used as a model for difluoroallyl to allow the investigation of the structural and energetic effects of the interaction of a discrete number of water molecules (used to model ether molecules) with the counterion in both monomers and dimers. Two dimerization processes were investigated, in which the two cations present in the dimers are surrounded by a different number of solvent molecules. The dimerization energies obtained by taking into account the oxygen-cation interactions are significantly reduced, but the equilibrium is still estimated to lie in favour of the dimeric species.

1H and 13C NMR relaxation investigation of the copper(II) complex of amitraz, a formamidine pesticide

The copper(II) complex of amitraz, a formamidine pesticide, was investigated by measuring 1H NMR spin-lattice relaxation rate enhancements (PRRE) in [ 2H 6]-DMSO solution. The occurrence of fast exchange conditions was demonstrated by the temperature dependence of the PRRE. The PRRE was interpreted by the Solomon-Bloembergen-Morgan approach; the number of solvent molecules in the coordination shell was calculated at q = 2; the reorientational correlation time in the 1 : 1 metal complex was assumed to be the same as in the free ligand and calculated at τ c = 69 ps. The metal ion was shown to stabilize a cisoid conformation of the ligand.

Theory of vibrational energy relaxation in liquids: Vibrational–vibrational energy transfer

A theoretical treatment of the vibrational–vibrational (VV) contribution to the vibrational energy relaxation time T1 of a solute normal mode in a molecular solvent, which extends a previous treatment [S. A. Adelman, R. H. Stote, and R. Muralidhar, J. Chem. Phys. 99, 1320 (1993), henceforth called Paper I] of the vibrational–translational–rotational (VTR) contribution to T1, is outlined and expressions for this VV contribution, valid for the infinitely dilute diatomic solution, are presented. The treatment is based on the formula T1=β−1(ωl), where β(ω) is the friction kernel of the relaxing solute mode and where ωl is its liquid phase frequency. β(ω) is evaluated as the cosine transform of the autocorrelation function 〈ℱ̃(t)ℱĩ〉0v of the fluctuating generalized force exerted by the vibrating solvent on the solute normal mode coordinate conditional that this coordinate is fixed at its equilibrium value. 〈ℱ̃(t)ℱ̃〉0v is expressed as a superposition of the rigid solvent autocorrelation function 〈ℱ̃(t)ℱ̃〉0 and a correction which accounts for solvent vibrational motion. For diatomic solvents one has 〈ℱ̃(t)ℱ̃〉0v= 〈ℱ̃(t)ℱ̃〉0+NSMD(t) cos ωDt F(ΩD), where NS=number of solvent molecules, MD(t) is the vibrational force gradient autocorrelation function, ωD and ΩD are solvent molecule liquid phase frequencies, and F(Ω)=1/2ℏΩ−1 coth[ℏΩ/2kBT]. The Gaussian model is assumed for 〈ℱ̃(t)ℱ̃〉0 and MD(t) yielding β(ω) as a superposition of a Gaussian centered at ω=0 which mediates VTR processes and a Gaussian centered at ω=ωD which mediates VV processes. Vector integral expressions for MD(t), ωD, and ΩD are presented which are similar to the expressions for ωl and 〈ℱ̃(t)ℱ̃〉0 given in Paper I. These expressions permit the evaluation of the VV contribution to T1 from the atomic masses, bondlengths, vibrational frequencies, and site–site interaction potentials of the solute and solvent molecules and from specified rigid solvent equilibrium site–site pair correlation functions of the liquid solution.

Statistical-Mechanical Theory of Osmotic Pressure of One-Dimensional Multicomponent Systems. III. Solution Immersed in a Very Large Bathof Pure Solvent

The osmotic pressure is discussed for a one-dimensional multicomponent system (solution) contained in a vessel made of a semipermeable membrane (permitting the passsage of the solvent only) and immersed in a very large bath of pure solvent with known density. The system consists of solvent molecules (species 1) and solute molecules (species 2, 3,···, r ) having hard cores and infinite-range attractions given by the potentials -2 a i j / L [ a i j =positive constant characteristic of the pair of species i , j , L =length (volume) of the system]. The number of solvent molecules spontaneously flowing through the semipermeable membrane (till the equilibrium is established) is also obtained.

Proton Transfer in Mixed Molecular Clusters After Resonant Two Photon Ionization

AbstractProton transfer reactions in clusters of aromatic cations and polar molecules have been studied by resonant two‐photon ionization spectroscopy. Two types of protonation reactions have been observed. While cations of methylated derivatives of benzene may act as an acid, inducing a direct proton transfer to the solvent, cations of non‐methylated derivatives may be neutralised by electron transfer with the solvent moiety, followed by intrasolvent proton transfer. If the derivative contains different substituents (o‐chlorotoluene), proton transfer competes with nucleophilic substitution. The branching ratio of both channels depends on the number of solvent molecules in the cluster.

Simulations of solvation in a Brownian dipole lattice

We have simulated the solvation of ions in a solvent consisting of point dipoles which undergo diffusive reorientation while translationally fixed to the sites of a cubic lattice. The simplicity of this model allows us to thoroughly explore how the energetics and dynamics of solvation depend on factors such as solute charge, solvent polarity, and number of solvent molecules. Some of the main features observed are as follows. The orientational response of first solvation shell dipoles saturates for moderate solute charges, resulting in a nonlinear dependence of the reaction potential on solute charge. This nonlinearity is to a good approximation independent of solvent polarity and can be rationalized on the basis of a simple phenomenological model. One effect of the nonlinear solvent response is to cause solvation free energy wells of the sort considered in electron transfer theories to be significantly anharmonic. Surprisingly, this deviation from harmonic behavior has little apparent impact on solvation barriers to charge transfer. The time dependence of the solvation response deviates substantially from exponential behavior in the more polar systems studied. Solvation times (1/e times of the solvation response) are directly related to the magnitudes of fluctuations in the solvation potential. The dynamics of solvation for times ≤t1/e can therefore be understood in terms of purely static correlations between solvent molecules. Dynamical interparticle correlations are only important in determining the longer time behavior of the solvation response. In contrast to the long-ranged character of the solvation energy, only 20–30 solvent molecules are required to produce solvation times characteristic of bulk solvent.

Supersonic jet studies of fluorene clustered with water, ammonia, and piperidine

Abstract : Mass resolved excitation spectroscopy and dispersed emission spectroscopy are employed to study van der Waals (vdW) clusters of jet cooled fluorene with ammonia, water and piperidine. For fluorene(H20)1 and fluorene(NH3)1 clusters, cluster geometries and binding energies can be suggested based on the experimental results and Lennard-Jones (LJ) potential (6- 12-1) energy calculations. As the number of solvent molecules in the cluster is increased, spectra of the clusters become more complex and broad probably due to the many possible stable configurations for these vdW clusters. Although the pKa for fluorene in its first excited singlet state (Forster cycle calculations) is quite acidic (-8.6), and solvent molecules can coordinate to the aliphatic hydrogens of the fluorene molecule in at least some cluster configurations, no direct evidence is found for the occurrence of proton transfer in S1 in these systems.

An ab-initio SCF-MO study of the solvent effect in α-substituted carbanions

This study reports an investigation at an ab-initio SCF-MO level of a supermolecule which involves a carbanion ( −CH 2-F), a counterion (Li +) and a certain number of solvent molecules. It was found that only the solvation of the counterion is very large, while that of the carbanion is quite small. Thus we have considered solvent molecules only around the counterion; in particular, three water molecules and two methanol molecules, to simulate the effect of an ether, the solvent most commonly used in reactions involving carbanions. We have found that for this supermolecule two distinct minima exist; the one with the lowest energy shows the feature of a tight ion pair and the other those of a solvent-separated ion pair. It was also found that the geometry of the carbanion is almost the same in both ion pairs, and more similar to that of the isolated carbanion than to that of the H 2CFLi species.

Glass Transition of the Solvent in a Polystyrene Gel

The mechanism of the glass transition of solvent quinoline in a polystyrene (quinoline) gel is studied by measurements of get length and depolarized Rayleigh scattering. It is found that the effect of intermolecular force between quinoline molecules and the polystyrene network is important in the glass transition of the solvent. Due to the force, fluctuations in orientations of solvent molecules are frozen and the molecules are bound to the network. With nearing the glass transition temperature, the intensity of a broad Rayleigh component decreases due to decrease of the number of solvent molecules fluctuating in the network and that of a narrow Rayleigh component increases due to increase of inhomogeneity of the gel density.

Preferential solvation and assisted substitution of chloropentaamminecobalt(III) ion in water-dimethyl sulfoxide media

The composition of the solvent cage of chloropentaamminechromium(III) ion was determined in water—dimethyl sulfoxide media using proton NMR line-broadening methods and the approach of Covington and coworkers. The number of solvent molecules in the solvent cage was found to be 10. The stepwise formation constants for the substitution of 10 water molecules by 10 dimethylsulfoxide molecules in this solvent cage were calculated. After the first such substitution each successive substitution becomes 1027 J mol −1 more difficult, exclusive of statistical factors, than the preceding substitution. The solvent cage composition was assumed to apply to the chloropentaamminecobalt(III) ion. Mercury(II)-assisted removal of chloride ion from the latter complex gave [Co(NH 3) 5{OSMe 2}] 3+/[Co(NH 3) 5(H 2O)] 3+ product ratios which did not correlate with either the solvent cage composition or the activity ratio of the two solvent components in the bulk phase of the solvent.

Preferential solvation and the composition of the solvation shell of (dimethyl sulphoxide)penta-amminechromium(III) ion in dimethyl sulphoxide–water mixtures

The number of solvent molecules in the solvation shell of the title complex ion has been found to be 10, the stepwise equilibrium constants for the replacement of the 10 water molecules by 10 dimethyl sulphoxide molecules have been obtained, and the free energy increment per replacement step has been found to be ∼+1.42 kJ mol–1.

Studies on the solvation of amino acids in ethanol and water mixtures

In order to look at the effects of different mixed solvents on amino acids, the compressibility values and the solvation numbers of different amino acids (glycine, alanine, proline, valine, histidine and arginine) in different ethanol—water mixtures have been determined using an ultrasonic interferometer. The changes in the compressibility values and solvation numbers of the amino acids with decreasing water concentration indicate some interesting features and can be explained on the basis of a decrease in the number of solvent molecules, structural rearrangement and preferential solvation of Zwitterions.

A new model of solvated-electron systems. Spectral behaviour

A new adiabatic model of infinitely dilute solvated-electron systems is described in which the solvent is accorded a dual role suggested by the experimental behaviour of the optical absorption bands attributed to solvated electrons. A small, finite number of solvent molecules and an excess electron, designated as a solvent–anionic-complex, are treated as adiabatic particles; the remaining solvent molecules, comprising a macroscopic solvating system, are treated as averageable particles. As a result, the solvated-electron absorption spectrum is determined by the intrinsic properties of the solvated solvent–anionic-complex.A basic consequence of the present model is its implication of spectral shape stability of the solvated-electron absorption band when the latter results only from a single band of discrete–continuum transitions of the electron. Under such circumstances, a linear solvated-electron–solvent energy relationship is implied.

Shape stability of solvated-electron optical absorption bands. Part 3.—Linear solvated-electron–solvent energies relationship

A new, fundamental linear relationship between the mean energy of a solvated electron and the mean energy of a representative solvent molecule of the system is derived for solvated-electron optical absorption bands which exhibit spectral shape stability. In simplified form, [ωav(T)+ 4Neff(T)text-decoration:overlineEsolv(T)] is independent of temperature T, where (in atomic units)ωav(T) is the mean absorption frequency of the band, text-decoration:overlineEsolv(T) is the equilibrium-averaged molecular energy of the solvent and Neff(T) is an effective number of solvent molecules about which localization of a solvated electron presumably occurs. Temperature-independent, possibly solvent-dependent values of Neff ranging from 0.4 to 1.0 are obtained from available data for a number of solvents. When spectral shape stability obtains it further follows that the energy of the solvent must decrease in the threshold absorption process occurring at the zero of absolute temperature.

Adsorption of adenine derivatives on a gold electrode studied by specular reflectivity measurement

The adsorption of adenine, deoxyadenosine, deoxyadenosine-5′-monophosphate,-diphosphate and-triphosphate on a gold electrode has been studied by specular reflectivity measurement in 0.1 M NaClO 4 solution. In the presence of these compounds, a marked decrease in reflectivity was found on reflectivity-potential curves in the potential region more positive than −0.8 V vs. Ag/AgCl, the decrease being ascribed to the adsorption of them. The magnitude of change in reflectivity was dependent on both the concentration and the electrode potential. The reflectivity change observed in the negative potential region was analyzed quantitatively according to the procedure previously described. The results were elucidated on the basis of the same isotherm as used by Green and Dahms in their adsorption study of aromatic hydrocarbons, and the number of solvent molecules being replaced through the adsorption of one organic molecule and the free energy change of adsorption were obtained. The former is suggestive of a flat orientation of the adsorbed molecule in contact with its adenine moiety on the electrode surface. It is also suggested from the latter that the presence of phosphate groups leads to a decrease in Δ G ad 0 resulting from their hydrophilic properties and a repulsive interaction between these groups and the negative charges on the surface.

Adsorption of adenine derivatives on a gold electrode studied by specular reflectivity measurement

The adsorption of adenine, deoxyadenosine, deoxyadenosine-5′-monophosphate,-diphosphate and-triphosphate on a gold electrode has been studied by specular reflectivity measurement in 0.1 M NaClO 4 solution. In the presence of these compounds, a marked decrease in reflectivity was found on reflectivity-potential curves in the potential region more positive than −0.8 V vs. Ag/AgCl, the decrease being ascribed to the adsorption of them. The magnitude of change in reflectivity was dependent on both the concentration and the electrode potential. The reflectivity change observed in the negative potential region was analyzed quantitatively according to the procedure previously described. The results were elucidated on the basis of the same isotherm as used by Green and Dahms in their adsorption study of aromatic hydrocarbons, and the number of solvent molecules being replaced through the adsorption of one organic molecule and the free energy change of adsorption were obtained. The former is suggestive of a flat orientation of the adsorbed molecule in contact with its adenine moiety on the electrode surface. It is also suggested from the latter that the presence of phosphate groups leads to a decrease in Δ G ad 0 resulting from their hydrophilic properties and a repulsive interaction between these groups and the negative charges on the surface.

Standard thermodynamic transfer functions from water to water + acetone mixtures for a n-nonylcarbon chain attached to different ionic groups

The standard enthalpy of transfer of trimethyldecylammonium bromide, tetramethylammonium bromide, methyl and decylsodium sulfate have been determined from water to water + acetone mixtures from calorimetric measurements at 298.15 K. The standard entropy function has been calculated using standard Gibbs free energy of transfer data for the same compounds. It is shown that the standard enthalpy and entropy of transfer of a n-nonylhydrocarbon chain attached to the sulfate or to the trimethylammonium groups are quite different whereas the standard Gibbs free energy functions are practically equal in the mixed solvents. It is concluded that the sign of the charge on the ionic groups is responsible for this behaviour and that the influence of this effect extends to a large number of solvent molecules. It is suggested that a similar effect may contribute to the standard enthalpy of so called reference ions: e.g. tetraphenylboron ion casting some doubt on the reliability of these extrathermodynamic approaches at least in mixed solvents, as far as the standard enthalpy function is concerned.