Nucleophilic substitution reactions at liquid/liquid interfaces: molecular dynamics simulation of a model S(N)1 dissociation reaction at the water/carbon tetrachloride interface

The ionic dissociation step of the nucleophilic substitution reaction t-BuCl --> t-Bu(+) + Cl(-) is studied at the water/carbon tetrachloride interface using molecular dynamics computer simulations. The empirical valence bond approach is used to couple two diabatic states, covalent and ionic, in the electronically adiabatic limit. The umbrella sampling technique is used to calculate the potential of mean force along the reaction coordinate (defined as the t-Bu to Cl distance) at several interface regions of varying distances from the Gibbs dividing surface. We find a significant increase of the ionic dissociation barrier height and of the reaction free energy at the interface relative to bulk water. This is shown to be due to the reduced polarity of the interface which causes a destabilization of the pure ionic state. However, deformation to the neat interface structure in the form of water protrusions into the organic phase may provide partial stabilization of the ionic species. The importance of these structural effects is examined by repeating the calculations with an artificially smooth interface. The destabilization of the ionic state at the interface also manifests itself with a rapid (picosecond time scale) recombination dynamics of the ions to form the parent molecule followed by a slow vibrational relaxation.     

Isomerization reaction dynamics and equilibrium at the liquid-vapor interface of water. A molecular-dynamics study

The gauche-trans isomerization reaction of 1,2-dichloroethane at the liquid-vapor interface of water is studied using molecular-dynamics computer simulations. The solvent bulk and surface effects on the torsional potential of mean force and on barrier recrossing dynamics are computed. The isomerization reaction involves a large change in the electric dipole moment, and as a result the trans/gauche ratio is considerably affected by the transition from the bulk solvent to the surface. Reactive flux correlation function calculations of the reaction rate reveal that deviation from the transition-state theory due to barrier recrossing is greater at the surface than in the bulk water. This suggests that the system exhibits non-Rice-Ramsperger-Kassel-Marcus behavior due to the weak solvent-solute coupling at the water liquid-vapor interface.     

A computational study on metal ion/PADA complexation in water and in glycerol

Modelling of the metal ion/ligand complexation reaction between divalent metal cation (M²⁺) and bidentate ligand pyridine-2-azo-p-dimethylaniline (PADA) has been carried out in water and in glycerol. Using ZINDO/1 method, semi-empirical molecular orbital calculations were carried out on all the species involved. The results have been compared with the experimental data obtained previously in bulk and dispersed systems. Kinetic studies have shown that the rate and equilibrium constants of the reaction in bulk water and in dispersed water and/or glycerol is very dependent on the metal ion, with Zn²⁺>Ni²⁺, but not so in bulk glycerol. Theoretical calculations have been seen to be supportive of the experimental observations.     

A negative temperature dependence of the electron self-exchange rates of zinc porphyrin pi radical cations

The one-electron oxidation of ZnT(t-Bu)PP (T(t-Bu)PP2- = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin dianion) by one equivalent of Ru(bpy)33+ (bpy = 2,2'-bipyridine) results in quantitative formation of ZnT(t-Bu)PP*+ which is detected by ESR. In the presence of excess ZnT(t-Bu)PP, the ESR line width becomes broader with increasing ZnT(t-Bu)PP concentration due to the electron self-exchange between ZnT(t-Bu)PP*+ and ZnT(t-Bu)PP. The line width of the ESR signal of ZnT(t-Bu)PP*+ becomes broader as the temperature is decreased from 313 to 233 K. This indicates that the electron self-exchange reaction becomes faster at a lower temperature. The substituent and solvent effects on such a negative temperature dependence of the electron self-exchange rates are reported.     

Free energy of solvation of simple ions: molecular-dynamics study of solvation of Cl- and Na+ in the ice/water interface

Molecular-dynamics simulations of Cl(-) and Na(+) ions are performed to calculate ionic solvation free energies in both bulk simple point-charge/extended water and ice 1 h at several different temperatures, and at the basal ice 1 h/water interface. For the interface we calculate the free energy of "transfer" of the ions across the ice/water interface. For the ions in bulk water in the NPT ensemble at 298 K and 1 atm, results are found to be in good agreement with experiments, and with other simulation results. Simulations performed in the NVT ensemble are shown to give equivalent solvation free energies, and this ensemble is used for the interfacial simulations. Solvation free energies of Cl(-) and Na(+) ions in ice at 150 K are found to be approximately 30 and approximately 20 kcal mol(-1), respectively, less favorable than for water at room temperature. Near the melting point of the model the solvation of the ions in water is the same (within statistical error) as that measured at room temperature, and in the ice is equivalent and approximately 10 kcal mol(-1) less favorable than the liquid. The free energy of transfer for each ion across ice/water interface is calculated and is in good agreement with the bulk observations for the Cl(-) ion. However, for the model of Na(+) the long-range electrostatic contribution to the free energy was more negative in the ice than the liquid, in contrast with the results observed in the bulk calculations.     

Reconsideration of second-harmonic generation from isotropic liquid interface: broken Kleinman symmetry of neat air/water interface from dipolar contribution

It has been generally accepted that there are significant quadrupolar and bulk contributions to the second-harmonic generation (SHG) reflected from the neat air/water interface, as well as common liquid interfaces. Because there has been no general methodology to determine the quadrupolar and bulk contributions to the SHG signal from a liquid interface, this conclusion was reached based on the following two experimental phenomena: the breaking of the macroscopic Kleinman symmetry and the significant temperature dependence of the SHG signal from the neat air/water interface. However, because the sum frequency generation vibrational spectroscopy (SFG-VS) measurement of the neat air/water interface observed no apparent temperature dependence, the temperature dependence in the SHG measurement has been reexamined and proven to be an experimental artifact. Here we present a complete microscopic analysis of the susceptibility tensors of the air/water interface, and show that dipolar contribution alone can be used to address the issue of the breaking of the macroscopic Kleinman symmetry at the neat air/water interface. Using this analysis, the orientation of the water molecules at the interface can be obtained, and it is consistent with the measurement from SFG-VS. Therefore, the key rationales to conclude significantly quadrupolar and bulk contributions to the SHG signal of the neat air/water interface can no longer be considered as valid as before. This new understanding of the air/water interface can shed light on our understanding of the nonlinear optical responses from other molecular interfaces as well.     

A computational study of the role of hydrogen bonds in SN1 and E1 reactions

The reaction between tertiary butyl chloride and water clusters was examined by applying density functional theory calculations. The carbonium ion t-Bu(+) that is normally sandwiched between the water clusters was found to be absent, such that a Cbond;O covalent bond was formed in the intermediate (Int1) after heterolysis. An (H(2)O)(4) cluster is able to bridge the front and rear of the central carbon and promotes heterolysis. A correlation between bond interchanges at the central carbon and proton relays is presented. Stereochemical scrambling in the solvolysis products is discussed in terms of this correlation. In addition, an E1 pathway for the elimination product, iso-butene, is found from Int1.     

A theoretical study of the mechanism of selective fluorination of saturated hydrocarbons by molecular fluorine. Participation of CHCl3 solvent molecules in the ionic process

The fluorination reaction of methane and isobutane by molecular F(2) with and without CHCl(3) solvent was studied with the ab initio and ONIOM theoretical calculations. The electrophilic pathway for the RH + F(2) reaction in general is a two-step process, consisting of hydride abstraction leading to an intermediate of the type R(+)(delta)...HF...F(-)(delta), followed by complicated rearrangement to give the electrophilic substitution product, RF + HF. In the case of methane, the overall barrier for this reaction is too high for the reaction to take place under mild conditions even in the presence of CHCl(3) solvent molecules. In the case of isobutane without CHCl(3), the electrophilic pathway has a high rate-determining barrier of 25 kcal/mol, and is not likely to take place; the radical process forming t-Bu* + HF + F* may be preferred. However, the t-Bu(+)(delta)...HF...F(-)(delta) intermediate and, in particular, the transition state TS2 for rearrangement of the intermediate are highly ionic, and are stabilized dramatically when a few CHCl(3) solvent molecules form a solvation cage. The electrophilic reaction for isobutane + F(2) has a low overall barrier when at least three CHCl(3) molecules are present and can take place under mild conditions with full retention of configuration.     

Vinyl oxidative coupling as a synthetic route to catalytically active monovalent chromium

Reaction of the deprotonated form of cis-{(t-Bu)N(H)P[μ-N(t-Bu)](2)PN(H)(t-Bu)} with CrCl(3)(THF)(3) afforded the trivalent cis-{(t-Bu)NP[μ-N(t-Bu)](2)PN(t-Bu)}[Li (THF)])CrCl(2) (1). Subsequent reaction with 2 equiv of vinyl Grignard (CH(2)=CH)Mg Cl gave the butadiene derivative (cis-{(t-Bu)NP[μ-N(t-Bu)](2)PN(t-Bu)}[Li(THF)])Cr(cis-η(4)-butadiene) (3) formally containing the metal in its monovalent state. The presence of the monovalent state was thereafter confirmed by DFT calculations. The coordination of the butadiene unit appears to be rather robust since reaction with Me(3)P afforded cleavage of the dimeric ligand core but not its displacement. The reaction formed the new butadiene complex [(t-Bu)N-P-N(t-Bu)]Cr(cis-η(4)-butadiene)PMe(3) (4) containing a regular NPN monoanion. In agreement with the presence of monovalent chromium, complexes 3 and 4 act as single-component self-activating catalysts for selective ethylene trimerization and dimerization, respectively.     

Interpretation of uptake coefficient data obtained with flow tubes

The uptake of trace gases such as OH and HO2 radicals, NH3, ClONO2, N2O5, ozone, and many other gases by water, aqueous solutions of acids, and salts has been reported by numerous investigators using a variety of techniques. Reported uptake coefficients vary greatly, ranging from 10(-8) to 1. This paper describes a new analysis of uptake data obtained in flow tubes that consolidates data obtained for various flow rates and trace gas concentrations. Previous analyses, which have been often used, are shown to be limiting cases or special cases of the analysis outlined here. Of particular emphasis are results for wetted-wall columns and trace gas uptake by aerosol entrained in flow tubes. In the absence of aerosol, the analysis is shown to predict the decrease in trace gas concentration due to bulk chemical reaction and/or reaction at the tube wall or gas-liquid interface. Uptake coefficients for OH and HO2 radicals on water in wetted-wall tubes are shown to range from 0.01 to 1, and on sulfuric acid, they vary from 0.008 to 0.03. For O3 on a water film doped with a scavenger, the uptake coefficient is found to be 0.0008. Uptake coefficients determined by different techniques are compared.     

Surface reactions of carbon dioxide at the adsorbed water-iron oxide interface

Despite the fact that carbon dioxide is an abundant atmospheric gas with profound environmental implications, there is little information on the reaction of carbon dioxide at the adsorbed water-oxide interface. In this study, the chemistry of carbon dioxide at the adsorbed water-iron oxide interface is investigated with FTIR spectroscopy. As shown here, the thin water layer on the iron oxide surface plays an important role in the surface chemistry of carbon dioxide. In particular, adsorbed water enhances CO(2) uptake, undergoes isotope exchange with CO(2) in O(18)-labeled experiments, and influences the chemical nature of the predominant adsorbed product on the surface from bicarbonate to carbonate. The resultant thin water film is acidic in nature from the reaction of CO(2). The IR spectrum recorded of adsorbed carbonate at the adsorbed water-iron oxide interface is remarkably similar to that at the bulk liquid water-iron oxide interface. Since reactions in thin water films estimated to be approximately 2 layers will play a role in a number of environmental processes, it is essential to understand the chemistry of these "wet" interfaces with atmospheric gases.     

Electron-stimulated production of molecular hydrogen at the interfaces of amorphous solid water films on Pt(111)

The electron-stimulated production of molecular hydrogen (D(2), HD, and H(2)) from amorphous solid water (ASW) deposited on Pt(111) is investigated. Experiments with isotopically layered films of H(2)O and D(2)O are used to profile the spatial distribution of the electron-stimulated reactions leading to hydrogen within the water films. The molecular hydrogen yield has two components that have distinct reaction kinetics due to reactions that occur at the ASW/Pt interface and the ASW/vacuum interface, but not in the bulk. However, the molecular hydrogen yield as a function of the ASW film thickness in both pure and isotopically layered films indicates that the energy for the reactions is absorbed in the bulk of the films and electronic excitations migrate to the interfaces where they drive the reactions.     

Aqueous interfaces with hydrophobic room-temperature ionic liquids: a molecular dynamics study

We report a molecular dynamics study of the interface between water and (macroscopically) water-immiscible room-temperature ionic liquids "ILs", composed of PF6(-) anions and butyl- versus octyl-substituted methylimidazolium+ cations (noted BMI+ and OMI+). Because the parameters used to simulate the pure ILs were found to exaggerate the water/IL mixing, they have been modified by scaling down the atomic charges, leading to better agreement with the experiment. The comparison of [OMI][PF6] versus [BMI][PF6] ILs demonstrates the importance of the N-alkyl substituent on the extent of solvent mixing and on the nature of the interface. With the most hydrophobic [OMI][PF6] liquid, the "bulk" IL phase is dryer than with the [BMI][PF6] liquid. At the interface, the OMI+ cations retain direct contacts with the bulk IL, whereas the more hydrophilic PF6(-) anions gradually dilute in the local water micro-environment and are thus isolated from the "bulk" IL. The interfacial OMI+ cations are ordered with their imidazolium moiety pointing toward the aqueous side and their octyl chains toward the IL side of the interface. With the [BMI][PF6] liquid, the system gradually evolves from an IL-rich to a water-rich medium, leading to an ill-defined interfacial domain with high intersolvent mixing. As a result, the BMI+ cations are isotropically oriented "at the interface". Because the imidazolium cations are more hydrophobic than the PF6(-) anions, the charge distribution at the interface is heterogeneous, leading to a positive electrostatic potential at the interface with the two studied ILs. Mixing-demixing simulations on [BMI][PF6]/water mixtures are also reported, comparing Ewald versus reaction field treatments of electrostatics. Phase separation is very slow (at least 30 ns), in marked contrast with mixtures involving classical organic liquids, which separate in less than 0.5 ns at the microscopic level. The results allow us to better understand the specificity of the aqueous interfaces with hydrophobic ionic liquids, compared with classical organic solvents, which has important implications as far as the mechanism of liquid-liquid ion extraction is concerned.     

Theoretical studies of solute vibrational energy relaxation at liquid interfaces

Recent advances in the theoretical understanding of solute vibrational energy relaxation at liquid interfaces and surfaces are described. Non-equilibrium molecular dynamics simulations of the relaxation of an initially excited solute molecule are combined with equilibrium force autocorrelation calculations to gain insight into the factors that influence the vibrational relaxation rate. Diatomic and triatomic nonpolar, polar, and ionic solute molecules adsorbed at the liquid/vapor interface of several liquids as well as at the water/CCl(4) liquid/liquid interface are considered. In general, the vibrational relaxation rate is significantly slower (a factor of 3 to 4) at the liquid/vapor and liquid/liquid interface than in the bulk due to the reduced density, which gives rise to a reduced contribution of the repulsive solvent-solute forces on the vibrational mode. The surface effects on the ionic solutes are much smaller (50% or less slower relaxation relative to the bulk). This is due to the fact that ionic solutes at the interface are able to keep part of their solvation shell to a degree that depends on their size. Thus, a significant portion of the repulsive forces is maintained. A high degree of correlation is found between the peak height of the solvent-solute radial distribution function and the vibrational relaxation rate. The relaxation rate at the liquid/liquid interface strongly depends on the location of the solute across the interface and correlates with the change in the density and polarity profile of the interface.     

Dispersion self-free energies and interaction free energies of finite-sized ions in salt solutions

The role for many-body dipolar (dispersion) potentials in ion-solvent and ion-solvent-interface interactions is explored. Such many-body potentials, accessible in principle from measured dielectric data, are necessary in accounting for Hofmeister specific ion effects. Dispersion self-energy is the quantum electrodynamic analogue of the Born electrostatic self-energy of an ion. We here describe calculations of dispersion self-free energies of four different anions (OH-, Cl-, Br-, and I-) that take finite ion size into account. Three different examples of self-free energy calculations are presented. These are the self-free energy of transfer of an ion to bulk solution, which influences solubility; the dispersion potential acting between one ion and an air-water interface (important for surface tension calculations); and the dispersion potential acting between two ions (relevant to activity coefficient calculations). To illustrate the importance of dispersion self-free energies, we compare the Born and dispersion contributions to the free energy of ion transfer from water to air (oil). We have also calculated the change in interfacial tension with added salt for air (oil)-water interfaces. A new model is used that includes dispersion potentials acting on the ions near the interface, image potentials, and ions of finite size that are allowed to spill over the solution-air interface. It is shown that interfacial free energies require a knowledge of solvent profiles at the interface.     

Effect of complexation of cadmium ions with N-methylpyrrolidone on kinetics of their electroreduction in sulfate electrolyte

Calculation of coordination numbers showed that N-methylpyrrolidone (N-MP) forms stable 1: 4 metal-ligand complexes in sulfate cadmium-plating electrolytes. Methods of computer simulation and quantum-chemical calculations allowed establishing that thermodynamically stable Cd(II) complexes formed in the bulk of the sulfate electrolyte contain, apart from four N-MP molecules, four water molecules that provide additional stabilization of the complex due to formation of hydrogen bonds between the ligands. Electrochemical studies indicate the predominant participation of Cd(II) complexes in the electrode reaction. Their discharge is preceded by their slow dissociation. Hindrance of the process of Cd²⁺ ion electrore-duction from complex electrolytes results in improvement of the structure and quality of cathodic cadmium deposits.     

Electron-stimulated sputtering of thin amorphous solid water films on Pt(111)

The electron-stimulated sputtering of thin amorphous solid water films deposited on Pt(111) is investigated. The sputtering appears to be dominated by two processes: (1) electron-stimulated desorption of water molecules and (2) electron-stimulated reactions leading to the production of molecular hydrogen and molecular oxygen. The electron-stimulated desorption of water increases monotonically with increasing film thickness. In contrast, the total sputtering--which includes all electron-stimulated reaction channels--is maximized for films of intermediate thickness. The sputtering yield versus thickness indicates that erosion of the film occurs due to reactions at both the water/vacuum interface and the Pt/water interface. Experiments with layered films of D2O and H2O demonstrate significant loss of hydrogen due to reactions at the Pt/water interface. The electron-stimulated sputtering is independent of temperature below approximately 80 K and increases rapidly at higher temperatures.     

O-H bond dissociation enthalpies in oximes: order restored

The O-H bond dissociation enthalpies (BDEs) of 13 oximes, RR'C=NOH, having R and/or R' = H, alkyl, and aryl are reported. Experimental anchor points used to validate the results of theoretical calculations include (1) the O-H BDEs of (t-Bu)2C=NOH, t-Bu(i-Pr)C=NOH, and t-Bu(1-Ad)C=NOH determined earlier from the heat released in the reaction of (t-Bu)2C=NO* with (PhNH)2 in benzene and EPR spectroscopy (Mahoney, L. R.; Mendenhall, G. D.; Ingold, K. U. J. Am. Chem. Soc. 1973, 95, 8610), all of which were decreased by 1.7 kcal/mol to reflect a revision to the heat of formation of (E)-azobenzene (which has significant ramifications for other BDEs) and to correct for the heat of hydrogen bonding of (t-Bu)2C=NOH (alphaH2 = 0.43 measured in this work) to benzene, and (2) the measured rates of thermal decomposition of six RR'C=NOCH2Ph at 423 or 443 K, which were used to derive O-H BDEs for the corresponding RR'C=NOH. Claims (Bordwell, F. G.; Ji, G. Z. J. Org. Chem. 1992, 57, 3019; Bordwell, F. G.; Zhang, S. J. Am. Chem. Soc. 1995, 117, 4858; and Bordwell, F. G.; Liu, W.-Z. J. Am. Chem. Soc. 1996, 118, 10819) that the O-H BDEs in mono- and diaryloximes are significantly lower than those for alkyloximes due to delocalization of the unpaired electron into the aromatic ring have always been inconsistent with the known structures of iminoxyl radicals as are the purported perpendicular structures, i.e., phi(Calpha-C=N-O*) = 90 degrees, for sterically hindered dialkyl iminoxyl radicals. The present results confirm the 1973 conclusion that simple steric effects, not electron delocalization or dramatic geometric changes, are responsible for the rather small differences in oxime O-H BDEs.     

Influence of anionic and cationic reverse micelles on nucleophilic aromatic substitution reaction between 1-fluoro-2,4-dinitrobenzene and piperidine

The nucleophilic aromatic substitution (S(N)Ar) reaction between 1-fluoro-2,4-dinitrobenzene and piperidine (PIP) were studied in two different reverse micellar interfaces: benzene/sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (AOT)/water and benzene/benzyl-n-hexadecyl dimethylammonium chloride (BHDC)/water reverse micellar media. The kinetic profiles of the reactions were investigated as a function of variables such as surfactant and amine concentration and the amount of water dispersed in the reverse micelles, W0 = [H2O]/[surfactant]. In the AOT system at W0 = 0, no micellar effect was observed and the reaction takes place almost entirely in the benzene pseudophase, at every AOT and PIP concentration. At W0 = 10, a slight increment of the reaction rate was observed at low [PIP] with AOT concentration, probably due to the increase of micropolarity of the medium. However, at [PIP] > or = 0.07 M the reaction rates are always higher in pure benzene than in the micellar medium because the catalytic effect of the amine predominates in the organic solvent. In the BHDC system the reaction is faster in the micellar medium than in the pure solvent. Increasing the BHDC concentration accelerates the overall reaction, and the saturation of the micellar interface is never reached. In addition, the reaction is not base-catalyzed in this micellar medium. Thus, despite the partition of the reactants in both pseudophases the reactions effectively take place at the interface of the aggregates. The kinetic behavior can be quantitatively explained taking into account the distribution of the substrate and the nucleophile between the bulk solvent and the micelle interface. The results were used to evaluate the amine distribution constant between the micellar pseudophase and organic solvent and the second-order rate coefficient of S(N)Ar reaction in the interface. A mechanism to rationalize the kinetic results in both interfaces is proposed.     

Structural and spectroscopic characterization of a charge-separated uranium benzophenone ketyl radical complex

The reaction of [((t-Bu)ArO) 3tacn)U (III)] ( 1) with 4,4'-di- tert-butylbenzophenone affords a unique isolable U(IV) ketyl radical species [((t-Bu)ArO) 3tacn)U (IV)(OC* (t-Bu)Ph 2)] (2) supported by XRD data, magnetization measurements, and DFT calculations. Isolation and full characterization of the corresponding diphenyl methoxide complex [((t-Bu)ArO) 3tacn)U (IV)(OCH ( t-Bu )Ph 2)] (3) is also presented. The one-electron reduction of benzophenone by [((Ad)ArO) 3tacn)U (III)] (4) leads to a purple U(IV) ketyl radical intermediate [((Ad)ArO) 3tacn)U (IV)(OC*Ph 2)] (5). This species is highly reactive, and attempts at isolation were unsuccessful and resulted in methoxide complex [((Ad)ArO) 3tacn)U (IV)(OCHPh 2)] (6) from H abstraction and dinuclear para-coupled complex [((Ad)ArO) 3tacn)U (IV)(OCPhPhCPh 2O)U (IV)((Ad)ArO) 3tacn)] (7).