Molecular–Continuum Model for the Cyanide Ion Adsorption from Aqueous Solutions on Copper Metals

Quantum-chemical calculations of the cyanide ion adsorption from aqueous solutions on copper metals are performed for the first time in a combined molecular–continuum model of polar solvent. The calculations use the cluster model of the surface and are carried out by the density functional in the B3LYP version. The effect of the adsorption system's polar dielectric environment is considered in a self-consistent reactive field model, namely, the SCIPCM model. The dielectric cavity is built in SCIPCM self-consistently with the particle's electron density distribution in solution. Calculations show that the CN^– adsorption energy decreases in the sequence Au > Cu > Ag. The calculated energy agrees best with the experimental data when the molecular–continuum model is used, rather than the simpler molecular and continuum models.     

Molecular dynamics study of ethanolamine as a pure liquid and in aqueous solution

Molecular dynamics simulations have been carried out for ethanolamine as a pure liquid and in aqueous solution at 298 and 333 K. The ethanolamine force field has been parametrized to reproduce intramolecular energies from quantum mechanical calculations and experimentally determined properties of the liquid. The results are presented for conformer distributions, density, enthalpy of vaporization, self-diffusion constant, dielectric constant, and radial distribution functions. The results strongly suggest that the main (O-C-C-N) dihedral tends to stay in its gauche conformers in solution and that the ethanolamine molecules populate conformers with a significant degree of intramolecular hydrogen bonding. This result is also supported by results from a continuum solvation model. Simulation of a 10 mol % aqueous ethanolamine system suggests that ethanolamine is preferentially solvated to by water molecules. The results suggest that ethanolamine dimer formation in aqueous solution is very limited. Simulations were also carried out for CO2 in an aqueous ethanolamine system. The results suggest that CO2 has a comparable level of attraction to ethanolamine and water. The degree of interaction between CO2 and the amine and alcohol functionalities in ethanolamine also appear to be of comparable strength.     

Hydration of l-tyrosine in aqueous medium. An experimental and theoretical study by mixed quantum mechanical/molecular mechanics methods

Quantum mechanical/molecular mechanics (QM/MM) calculations were carried out in order to study the theoretical structures of l-tyrosine in both gas phase and in aqueous solution and observe the changes that occur on the structural and vibrational properties in two phases. Therefore, the molecule was characterized by infrared and Raman spectroscopy in solid phase and aqueous solution. Optimized geometries and relative stabilities for the zwitterion l-tyrosine derivatives have been calculated taking into account the solvent effects by using the self-consistent reaction field (SCRF) theory. For a complete assignment of the IR and Raman spectra of l-tyrosine in solid and aqueous solution phases, density functional theory (DFT) calculations were combined with Pulay's scaled quantum mechanical force field (SQMFF) methodology in order to fit the theoretical wavenumber values to the experimental ones. A good agreement between theoretical and available experimental results is found.     

Nuclear magnetic shielding of the 113Cd(II) ion in aqua solution: a combined molecular dynamics/density functional theory study

We present a combined molecular dynamics simulation and density functional theory investigation of the nuclear magnetic shielding constant of the (113)Cd(II) ion solvated in aqueous solution. Molecular dynamics simulations are carried out for the cadmium-water system in order to produce instantaneous geometries for subsequent determination of the nuclear magnetic shielding constant at the density functional theory level. The nuclear magnetic shielding constant is computed using a perturbation theory formalism, which includes nonrelativistic and leading order relativistic contributions to the nuclear magnetic shielding tensor. Although the NMR shielding constant varies significantly with respect to simulation time, the value averaged over increasing number of snapshots remains almost constant. The paramagnetic nonrelativistic contribution is found to be most sensitive to dynamical changes in the system and is mainly responsible for the thermal and solvent effects in solution. The relativistic correction features very little sensitivity to the chemical environment, and can be disregarded in theoretical calculations when a Cd complex is used as reference compound in (113)Cd NMR experiments, due to the mutual cancelation between individual relativistic corrections.     

A DFT study of the degradation mechanism of anticancer drug carmustine in an aqueous medium

Structural Chemistry - Density functional theory calculations were carried out to study the degradation mechanism of anticancer drug carmustine in an aqueous medium. The calculations indicate that...     

Density functional theory for efficient ab initio molecular dynamics simulations in solution

We present a density functional for first-principles molecular dynamics simulations that includes the electrostatic effects of a continuous dielectric medium. It allows for numerical simulations of molecules in solution in a model polar solvent. We propose a smooth dielectric model function to model solvation into water and demonstrate its good numerical properties for total energy calculations and constant energy molecular dynamics.     

Efficient evaluation of the effective dielectric function of a macromolecule in aqueous solution

We propose an analytical approach to calculate the effective dielectric function of proteins in aqueous solution. The screening effect if quantified by a measure of enclosure which is based on the distribution of solute atomic volumes around a pair of charges in a macromolecule. For protein conformations that vary significantly in size and shape, a comparison with finite difference Poisson calculations shows that pair interaction energies, their sums and solvation energies are well reproduced. The approach rivals the speed of simple distance dependent dielectric functions and the accuracy of the generalized Born model.     

A New Approach in the Theory of Spatially-Restricted Nonlocal Dielectric Media

A new method for the calculation of electric field distributions in the systems with spatiallyrestricted regions filled with polar media with nonlocal dielectric characteristics is proposed. The presence of regions with different dielectric properties in the system (as exemplified by the presence of an ion-filled cavity in which there is no polar medium surrounding the ion) is automatically taken into account within this procedure. The field distribution inside a uniform and isotropic nonlocal dielectric medium outside the spherical cavity containing a spherically symmetric charge distribution (the system represents the ion inside the polar solvent) has illustrated this new approach. In contrast to the previously suggested approach (dielectric approximation), where calculations of this characteristic required complex numerical and analytical calculations, the new method requires only single integration in order to determine the distribution of the potential of the electric field inside the polar medium with an arbitrary law of spatial dispersion of its dielectric permittivity (which can be specified eithter analytically or numerically).

     

Complexes of dichloro(ethylenediamine)palladium(II) observed from aqueous solutions by electrospray mass spectrometry

Aqueous solutions of dichloro(ethylenediamine)palladium(II) were investigated using electrospray mass spectrometry (ESMS). The most abundant peak (m/z 436.8) was attributed to the dimeric Pd(en)Cl2.Pd(en)Cl+ ion. We conjecture that the structures of the observed ions arise from the clustering of the hydrolysis products of the parent compound. This hypothesis was tested experimentally by carrying out a series of collision-induced dissociation (CID) experiments and deuterium exchange reactions. It was also assessed by performing density functional theory (DFT) calculations, from which optimized structures and reaction energetics were obtained. These results were compared with our earlier ESMS study of an aqueous Pd(en)Br2 solution. Calculations were also carried out on the Pd(en)Br2 system to facilitate the comparisons. Conclusions are drawn regarding the species present in the two aqueous solutions.     

Influence of environment on proton-transfer mechanisms in model triads from theoretical calculations

We have carried out theoretical calculations to analyze molecular interactions and proton transfer mechanisms in the formate–imidazole–water system, which may be considered the simplest model of catalytic triads in serine proteases. Computations were carried out at the density functional theory level. The effect of a dielectric environment on energy surfaces is considered using a polarizable continuum model and the self-consistent reaction field approach. The role played by inertial and noninertial polarization of this environment is emphasized. Nonequilibrium solvation effects have been estimated. The results show that there are different reaction mechanisms, concerted or stepwise, that may be competitive, depending on the nature of the molecular environment. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1675–1688, 1998     

Density-functional methods for the study of the ground-state vibrations of the guanidinium ion

Sixteen density functional methods (including four hybrid methods) with the 3–21G and the 6–31G(d, p) basis set of atomic functions are used to predict the structure and vibrations of the guanidinium ion [C(NH₂)₃]⁺. The study was done with the ion both in a vacuum and in an aqueous solution. To account for the solvation effect on the vibrating behavior of the ion, the solvent was modeled in two ways of increasing complexity: First, the guanidinium was inserted into a cavity of a continuous medium (dielectric constant ϵ = 78) and, second, six explicit water molecules were considered around the ion and the whole aggregate inserted into the cavity of the continuum. The conformation corresponding to the energy minimum is predicted to have D₃ symmetry rather than D_3h. The harmonic vibrational frequencies obtained have a mean absolute deviation from the experimental data of about one-half the value achieved by pure Hartree-Fock methods. Isotopic substitution calculations were also carried out and shifts obtained are in good agreement with experience and so are the assignments of the observed bands to the vibrational normal modes. The study of the solvent effect shows the existence of modes that are not affected by hydration and some improvement in the values predicted, especially for low-frequency vibrations. © 1997 John Wiley & Sons, Inc.     

Permanganate oxidation of alkenes. Substituent and solvent effects. Difficulties with MP2 calculations

The permanganate oxidation of alkenes has been studied both experimentally and computationally. Transition state structures were located for the reaction of permanganate ion with a variety of monosubstituted alkenes at the B3LYP/6-311++G** level. Although the calculated activation energy for the reaction with ethene was reasonable, the calculated effect of substituents, based on the energies of the reactants, was much larger than that experimentally found. This was shown to be due to the formation of an intermediate charge-dipole complex which led to the transition state. Reaction field calculations found the complex to disappear in a high dielectric constant medium, and the range of activation energies for the reaction in solution became quite small. MP2 calculations were carried out in order to have a comparison with the DFT results. MP2-MP4 gave unusual results for calculations on permanganate ion as well as chromate ion and iron tetraoxide. They also gave markedly unreasonable results for the activation energy of the reaction of permanganate with ethane. CCSD/6-311++G** calculations gave satisfactory results for permanganate ion and chromate ion. At this level of theory, the reaction of permanganate with ethene was found to have a very early transition state, when the bond lengths of the reactants just began to change. The reaction was calculated to be very exothermic (-69 kcal/mol), and this was confirmed via calorimetry. The rates of permanganate oxidation of allyl alcohol and acrylonitrile were determined, and they had similar reactivities. The kinetics and the products of the reaction of permanganate with crotonate ion were examined in some detail.     

Reaction of the 4-biphenylnitrenium ion with 4-biphenyl azide to produce a 4,4'-azobisbiphenyl stable product: a time-resolved resonance Raman and density functional theory study

A time-resolved resonance Raman (TR(3)) and density functional theory (DFT) study of the reaction of the 4-biphenylnitrenium ion with 4-biphenyl azide in a mixed aqueous solution is reported. The reaction of the 4-biphenylnitrenium ion with its unphotolyzed precursor 4-biphenyl azide in a mixed aqueous solution generates a 4,4'-azobisbiphenyl stable product via an intermediate species. With the aid of DFT calculations for likely transient species, this intermediate was tentatively assigned to a 4,4'-azobisbiphenyl cation. The DFT calculations predict this reaction can take place via two pathways that compete with one another to produce the trans and cis 4,4'-azobisbiphenyl product. The observation of the 4,4'-azobisbiphenyl cation intermediate demonstrates that the reaction of the arylnitrenium ion with its aryl azide to produce a stable azo product occurs via a stepwise mechanism.     

Theoretical study of the uranyl complexation by hydroxamic and carboxylic acid groups

A theoretical study on the complexation of uranyl cation (UO2(2+)) by three different functional groups of a calix[6]arene cage, that is, two hydroxamic and a carboxylic acid function, has been carried out using density functional theory calculations. In particular, interaction energies between the uranyl and the functional groups have been used to determine their affinity toward uranyl, whereas pKa calculations give some information on the availability of the functional groups in the extraction conditions. On the one hand, calculations of the interaction energies have pointed out clearly a better affinity with the hydroxamic groups. The stabilization of this complex was rationalized in terms of a stronger electrostatic interaction between the uranyl cation and the hydroxamic groups. The presence of a water molecule in the first coordination sphere of uranyl does not destabilize the complex, and the most stable complex is obtained with two functional groups and two water molecules, leading to a coordination number of 8 for the central uranium atom. On the other hand, pKa theoretical evaluation shows that both hydroxamic (deprotonated on the oxygen site) and carboxylic groups are potential extractants in aqueous medium with a preference for carboxylic functions at low pH. Moreover, these data allowed to unambiguously identify the oxygen of the alcohol function as the favored deprotonation site on the hydroxamic function.     

A density functional and quantum Monte Carlo study of glutamic acid in vacuo and in a dielectric continuum medium

We present density functional theory (DFT) and quantum Monte Carlo (QMC) calculations of the glutamic acid and glutamate ion in vacuo and in various dielectric continuum media within the polarizable continuum model (PCM). In DFT, we employ the integral equation formalism variant of PCM while, in QMC, we use a PCM scheme we have developed to include both surface and volume polarization. We investigate the gas-phase protonation thermochemistry of the glutamic acid using a large set of structural conformations, and find that QMC is in excellent agreement with the best available theoretical and experimental results. For the solvated glutamic acid and glutamate ion, we perform DFT calculations for dielectric constants, ε, between 4 and 78. We find that the glutamate ion in the zwitterionic form is more stable than the non-zwitterionic form over the whole range of dielectric constants, while the glutamic acid is more stable in its non-zwitterionic form at ε = 4. The dielectric constant at which the two glutamic acid species have the same energy depends on the cavity size and lies between 5 and 12.5. We validate these results with QMC for the two limiting values of the dielectric constant, and find qualitative agreement with DFT even though the solvent polarization is less pronounced at the QMC level.     

Thermodynamics of titanium and vanadium reduction in non-aqueous environment calculated at various levels of theory

Reduction of titanium and vanadium compounds is a process accompanying the activation of coordinative olefin polymerization catalysts. Four density functional theory (DFT) functionals, coupled cluster with single, double, and perturbative triple excitations method CCSD(T) as well as complete active-space second-order perturbation theory method CASPT2 with a complete active-space self-consistent field CASSCF reference wave function were applied to investigate the thermodynamics of titanium and vanadium reduction. The performance of these theoretical methods was assessed and compared with experimental values. The calculations indicate that vanadium(IV) chloride is more easily reduced by trimethylaluminum than the corresponding titanium compound; the energies of reaction calculated at the CCSD(T) level are equal -57.21 and -33.10 kcal/mol, respectively. The calculations deal with the redox reactions of metal chlorides in the gas phase, rather than solvated ions in the aqueous solution. This approach may be more appropriate for olefin polymerization, usually carried out in nonpolar solvents.     

Solvation and ion-pairing properties of the aqueous sulfate anion: explicit versus effective electronic polarization

We assessed the relative merits of two approaches for including polarization effects in classical force fields for the sulfate anion. One of the approaches is the explicit shell model for atomic polarization and the other is an implicit dielectric continuum representation of the electronic polarization, wherein the polarizability density is spatially uniform. Both the solvation and ion association properties of sulfate were considered. We carried out an ab initio molecular dynamics simulation for a single sulfate anion in aqueous solution to obtain a benchmark for the solvation structure. For the ion-pairing properties, the models were compared to experimental thermodynamic data through Kirkwood-Buff theory, which relates the integrals of the pair correlation functions to measurable properties. While deficiencies were found for both of the approaches, the continuum polarization model was not systematically worse than the shell model. The shell model was found to give a more structured solution than the continuum polarization model, both with respect to solvation and ion pairing.     

Spectroscopic and theoretical study on the interaction between diperoxovanadate and oxazole

Detailed investigations were carried out to explore the interaction systems of NH(4)VO(3)/H(2)O(2)/oxazole in aqueous solution under physiological conditions by a combined use of multinuclear NMR ((1)H, (13)C, (14)N and (51)V), diffusion ordered spectroscopy (DOSY), variable temperature NMR, electrospray ionization mass spectrometry (ESI-MS), spin-lattice relaxation and density functional calculations. The results indicated the formation of a new peroxovanadate species [OV(O(2))(2)(oxazole)](-) with oxazole coordinating to vanadium through nitrogen atom. The solution structure of the new species was predicted from theoretical calculations.     

Theoretical insights on the electronic properties of eosin Y, an organic dye for photovoltaic applications

A theoretical analysis, based on density functional theory, has been carried out on the electronic properties of eosin yellowish (2',4',5',7'-tetrabromofluorescein), a dye used for photovoltaic applications. In particular different oxidation states of the bare molecule as well as its complexes with a Zn dication have been considered, with the aim to gain some insights on its electro- and photochemical behavior. The calculations have been carried out both in the gas phase and in solution, this last modeled by a continuum model. Besides the agreement with the experimental data, our results allow for a better interpretation of the spectroscopic properties of this dye and their tuning by interaction with the solvent or metal atom in aqueous solution.     

Dielectric permittivity in a dense layer of the hydration shell of an ion in a strong-electrolyte solution

Based on the experimental data on the dielectric dispersion and the static dielectric permittivity of solutions of strong electrolytes, the effective value of the latter in the dense layer of the hydration shell of an ion has been calculated. The calculations have been carried out in terms of the three-layer model of the hydration complex. The calculations have shown that for the solutions of strong electrolytes the value of the static dielectric permittivity (dielectric constant) in the dense layer of the hydration shell of an ion proves to be close to 2 and is almost independent of the concentration and temperature.