Onsager Reaction Field Research Articles

Ab initio and dielectric studies of succinic acid and maleic acid in 1,4-dioxane

In a continuing effort to understand the hydrogen bond through the study of dielectric and computational conformal studies of dilute solutions, succinic acid and maleic acid are studied in solutions of 1,4-dioxane solvent. Dielectric studies give an account of the net dipole moment of the system under study, which is then compared with the values obtained from conformal analysis. The dielectric measurements were made at 303 K at a frequency of 9.83 GHz using a X-band microwave test bench in order to determine the relaxation times and the dipole moments. The static dielectric permittivity and the high frequency dielectric permittivity were measured using a LCR meter and an Abbe's refractometer, respectively. The results are inspected in comparison with the dipole moment results of ab initio calculations of some of the conformers in gas phase and in liquid phase. Gaussian-03 software package with 6–31G(d) basis set optimisation was used for this purpose. Onsager's reaction field model is used to study the solvation of the dicarboxylic acids in 1,4-dioxane. The results are interpreted in terms of the intermolecular and intramolecular hydrogen bond interactions in the dilute systems.

Theoretical Studies on the Intramolecular Hydrogen Bond and Tautomerism of 8-Mercaptoquinoline in the Gaseous Phase and in Solution Using Modern DFT Methods

A theoretical quantum chemical study of the intramolecular hydrogen bonding interactions in 8-mercaptoquinoline has been carried out. Special attention has been paid to the rotation of S-H bond and intramolecular proton-transfer reactions. Therewith, the B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,2p), MPW1K/6-311++G(d,p), MPW1K/6-31+G(2d,2p), BH&HLYP/6-311++G(d,p), and G96LYP/6-311++G(d,p) methods have been used. By means of the Onsager and PCM reaction field methods, the effects of solvent on hydrogen-bond energies, conformational equilibria, rotational barriers, and tautomerism in aqueous solution have been studied. These simulations were done at the MPW1K/6-311++G(d,p) and B3LYP/6-311++G(d,p) levels. Natural-bond orbital analysis has been performed to study the intramolecular hydrogen bond (IHB) in the gaseous phase and in aqueous medium. The stability of forms under consideration in solution does not coincide with that in the gaseous phase, underlining a great importance of the electrostatic influence of solvent. Double-proton transfer in the prototropic tautomerization of 8-mercaptoquinoline, one water molecule complex in the gaseous phase and in solution, has been systematically studied. The double-proton transfer occurs concertedly and synchronously. The water-assisted tautomerization is kinetically less, but thermodynamically more favorable, compared to that of the single-proton transfer. As in the case with single-proton transfer, for water-assisted reaction, the tautomerization energies and barrier heights decrease with the increase in dielectric constant, which implies faster and more complete tautomerization of 8-mercaptoquinoline in a polar solvent.

Solvent effect on the adduct formation of methyltrioxorhenium (MTO) and pyridine: enthalpy and entropy contributions

1:1 adduct formation between methyltrioxorhenium (MTO) and pyridine in different solvents (n-hexane, benzene, chloroform, ethyl acetate, dichloromethane and acetone) was studied using spectrophotometric techniques. The formation constants were determined from the absorbance change of the adduct versus pyridine concentration. The values of the formation constants vary from 114.5 to 752.5 L mol(-1) at T= 20 degrees C depending on the dielectric constant of the solvent (epsilon(r) = 1.89-20.7). Enthalpy and entropy changes during the adduct formation reactions were determined from van't Hoff plots. The measured enthalpy change of -37.0 to -22.2 kJ mol(-1) depends on epsilon(r), which is explained by Onsager's reaction field theory. The measured entropy change ranges from -71.2 to -36.6 J K(-1) mol(-1), and the dependence on the solvent is discussed in terms of the solvation effect.

From quantum chemistry and the classical theory of polar liquids to continuum approximations in molecular mechanics calculations

AbstractBiological macromolecules and other polymers belong to the class of mesoscopic systems, with characteristic length scale of the order of a nanometer. Although microscopic models would be the preferred choice in theoretical calculations, their use in computer simulations becomes prohibitive for large systems or long simulation times. On the other hand, the use of purely macroscopic models in the mesoscopic domain may introduce artifacts, with effects that are difficult to assess and that may compromise the reliability of the calculations. Here is proposed an approach with the aim of minimizing the empirical nature of continuum approximations of solvent effects within the scope of molecular mechanics (MM) approximations in mesoscopic systems. Using quantum chemical methods, the potential generated by the molecular electron density is first decomposed in a multicenter‐multipole expansion around predetermined centers. The monopole and dipole terms of the expansion at each site create electric fields that polarize the surrounding aqueous medium whose dielectric properties can be described by the classical theory of polar liquids. Debye's theory allows a derivation of the dielectric profiles created around isolated point charges and dipoles that can incorporate Onsager reaction field corrections. A superposition of screened Coulomb potentials obtained from this theory makes possible a simple derivation of a formal expression for the total electrostatic energy and the polar component of the solvation energy of the system. A discussion is presented on the physical meaning of the model parameters, their transferability, and their convergence to calculable quantities in the limit of simple systems. The performance of this continuum approximation in computer calculations of amino acids in the context of an atomistic force field is discussed. Applications of a continuum model based on screened Coulomb potentials in multinanosecond simulations of peptides and proteins are briefly reviewed. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Infrared and Raman spectra, conformational stability, ab initio calculations of structure, and vibrational assignment of α and β glucose

α and β Glucose anomers were studied by using density functional/ab initio calculations in vacuum and in the presence of a solvent (water). Calculations in a continuum medium, using the Onsager reaction field model, suggest that α glucose is energetically most stable than β anomer in aqueous solution according with the experimentally reported. We also calculated the vibrational frequencies and optimal geometries for both glucose anomers, showing differences between them, depending on the medium around the molecule. The vibrational frequencies calculated are compared with experimental results obtaining very good agreement.

Non-convertional hydrogen bonding interaction of BH 3NH 3 complexes: a comparative theoretical study

A new type of hydrogen bond, called a dihydrogen bond, has recently been introduced. In this bond hydrogen is donated to (hydridic) hydrogen. In this paper, ab initio HF, MP2 and DFT(B3LYP) levels of theory with different basis sets in combination with counterpoise procedure for basis set superposition error correction have been applied to BH 3NH 3 dimer and BH 3NH 3 complexes of methane, hydrogen cyanide, ammonia, water, methanol and hydrogen fluoride to understand the features of dihydrogen bond. The optimized geometric parameters and interaction energies for various isomers at different levels are estimated. The structures obtained at different computational levels are in agreement with each other. Dihydrogen bond does not occur in both BH 3NH 3⋯CH 4 and BH 3NH 3⋯NH 3 complexes. Apart from the B–H⋯H–N dihydrogen bond found in the BH 3NH 3 crystal and dimmer, the B–H⋯H–X (X C, O, F) dihydrogen bonds have been observed in the BH 3NH 3⋯HCN, BH 3NH 3⋯H 2O, BH 3NH 3⋯CH 3OH and BH 3NH 3⋯HF complexes, while the classic H bonds also exist in the last three complexes. As for the complexes in which only dihydrogen bonds appear the strength of dihydrogen bonds ranges from 17.9 to 18.9 kJ mol −1 at B3LYP/6-311++g(d,p) level. Binding energies obtained from the MP2 and B3LYP optimized structures are more sensitive to basis sets than those from the HF method. Larger basis functions generally tend to produce slightly longer intermolecular distances, and the B3LYP and MP2 methods generate shorter intermolecular distances though they usually produce longer bond lengths compared with those at the HF level. The infrared spectrum frequencies, IR intensities and the vibrational frequency shifts are reported. Finally the solution phase studies on BH 3NH 3⋯HF complex are also carried out using the Onsager reaction field model with a range of dielectric constants from 2 to 80 at B3LYP/6-311++g(d,p) level.

Theoretical study on the hydrogen bond interaction between diacetamide and methanol

The hydrogen bonding of 1:1 complexes formed between diacetamide and methanol molecule have been completely investigated in the present study using density functional theory and second order Moller–Plesset perturbation (MP2) method. Various basis sets from 6-31G to 6-311++G** have been employed to determine the equilibrium structure of the interacting complexes. Three reasonable geometries on the potential energy hypersurface of diacetamide with methanol system are considered with the global minimum being a cyclic double-hydrogen bonded structure. The optimized geometric parameters and interaction energies for various isomers at different levels are estimated showing that the two carbonyl groups have about the same proton acceptor ability. Special attention was paid to the basis set superposition error and zero-point energy corrections that were not given appropriate attention in previous studies. Finally, the solution phase studies are also carried out using the Onsager reaction field model at B3LYP/6-311++G** level for the hydrogen-bonded complex of diacetamide with methanol. The results indicate the polarity of the solvent has played an important role on the structures and relative stabilities of different isomers or the proton acceptor ability of two carbonyl groups.

Temperature and Solvent Effects on the Reactions of o-Positronium with Nitroso Spin Traps in Methanol−Water Mixed Solvents

Positron annihilation lifetime spectroscopy (PALS) is used to study the reactions of the ortho-positronium (o-Ps) atom with the following nitroso spin traps: nitrosobenzene (NOB), N,N-dimethyl-4-nitroso-aniline (DMNA), and tert-nitroso-butane (t-NOBu). The reaction rate constants (k) are close to the range that corresponds to that of diffusion-controlled reactions and exhibit a similar trend to those reported in the literature for the analogous nitro compounds. The differences between the aromatic and aliphatic nitroso and nitro compounds and the observed substituent effect are in good correlation with the calculated LUMO energies of the molecules. The nucleophilic o-Ps attacks the molecules at their electron deficient aromatic ring. Solvent effects on the reaction rates in methanol−water solvent mixtures exhibit a complex and solute-dependent nature. The reactions are strongly affected by solvent viscosity; however, the expected 1/η versus k variation is valid only for limited ranges of solvent compositions. The dielectric permittivity effect on the reaction rate constants does not follow the expected trend; k values in water are significantly greater than in the less polar methanol. The average activation enthalpies (ΔH⧧ and entropies (ΔS⧧) evaluated for the reaction of o-Ps with DMNA exhibit maxima at a solvent composition which corresponds to the most rigid solvent structure, where the activation energies of the transport processes such as viscosity and self-diffusion have extremes too. The validity of the isokinetic relation between ΔH⧧ and ΔS⧧ shows that the mechanism of the reaction of o-Ps does not depend on the solvent composition. ΔH⧧ exhibits an “Onsager reaction field” type (ε − 1)/(ε + 2) dependence on solvent permittivity. The results are compared to literature data for the reactions of o-Ps with stable nitroxide free radicals and reaction of solvated electron with nitrobenzene (NO2B) in methanol−water mixtures. The observed similarities and differences are discussed.

Theoretical Analysis of the Excited State Properties of Wybutine: A Natural Probe for Transfer RNA Dynamics

We have theoretically characterized the ground state and the excited state properties of wybutine, a naturally fluorescent modified base occuring in tRNAs, using configuration interaction singles (CIS) and time dependent density functional (TDDFT) methods. Both gas phase excited state properties and solvent effects, modelled through Onsager reaction field method, were considered. In addition to vertical excitation energies, the fluorescence transitions were calculated, based on S1 equilibrium geometry optimized at CIS level. Our computations show encouraging agreement with known experimental data either directly (TDDFT) or after applying empirical scaling (CIS). The fluorescence Stokes’ shift for the S0 Ã S1 transition is computed taking into account the contributions from both intramolecular and solvent reorganization processes. The results suggest that intramolecular relaxation of the S1 state accounts for the major part of the magnitude of the Stokes’ shift, while the role of solvent reorganization seems to be of less importance.

The Onsager reaction field concept applied to the temperature dependent magneticsusceptibility of the enhanced paramagnets Pd and Pt

In this paper three different ways to calculate the temperature dependent susceptibility ofthe enhanced paramagnets Pd and Pt are presented and compared. Special attention ispaid to the relative importance of one-particle and many-particle excitations. One of thesemethods has never been applied before to real itinerant systems. It is based upon Onsager’sreaction field model, which seems to be a feasible and rigorous way to go beyond themean field approximation, and it allows one to make quantitative statements. Thebackbone of this theory is the calculation of a temperature dependent HubbardU due to thermal many-body excitations. In contrast to an earliercalculation we found a significant temperature dependence ofU for Pd. The theoretical results are in fairly reasonable agreement with the experimentalfindings for both Pd and Pt.

Study of the diacetamide–water dimer with ab initio and density functional theory methods

Abstract The hydrogen bonding of 1:1 complexes formed between diacetamide and water molecule have been completely investigated in the present study using density functional theory and second order Moller–Plesset perturbation (MP2) method. The large basis sets 6-311++g(d,p) and 6-311++g(2d,2p) have been employed to determine the equilibrium structure of the interacting complexes. All the results reveal a planar configuration of the amide groups and a tendency of the CH 3 group to eclipse the CO bond for the geometry of the isolated diacetamide molecule. Calculations at different theoretical levels indicate that cis – trans configuration is the most stable isomer in both gas phase and solution phase. Three reasonable geometries on the potential energy hypersurface of diacetamide with water system are considered with the global minimum being a cyclic double-hydrogen bonded structure. The optimized geometric parameters and interaction energies for various isomers at different levels are estimated and a result that two carbonyl groups have about the same proton acceptor ability has been obtained. Finally, the solution phase studies are also carried out using the onsager reaction field model at B3LYP/6-311++g** level for the isolated diacetamide molecule and the hydrogen-bonded complex of diacetamide with water. The results indicate the polarity of the solvent has played an important role on the structures and relative stabilities of different isomers.

Density functional theory study of formamide–formamidic acid tautomerization

AbstractThe mechanism of tautomerization reaction of formamide–formamidic acid in gas phase and in water has been completely investigated in the present study using, density functional theory (DFT) and the second‐order Møller–Plesset perturbation (MP2) method. Large basis sets 6‐311++G(d,p) have been employed to determine the equilibrium structure and vibrational frequencies. Activation energy of the non‐water F → FA reaction is 202.9 kJ/mol at B3LYP/6‐311++G(d,p) level. Extensions to situations in which tautomerization is assisted by neighboring solvent molecule(s) are considered. If a single H2O molecule is considered, three stable structures and transition states of formamide–water complexes are found. If the water locates between the HN group and the CO moiety, the activation energy decreases to 94.5 kJ/mol. If the water locates in other positions, it can cause the activation energy to increase to 2–4 kJ/mol. When two intimate H2O molecules locate between the HN group and the CO moiety, the activation energy decreases to 89.3 kJ/mol. Intimate solvent participation is thus found much more strongly affecting the activation energy than the overall thermodynamics in this case. Finally, the solution phase studies are also carried out using the Onsager reaction field model, with a range of dielectric constants from 2 to 80 at the B3LYP/6‐311++G(d,p) level. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Classical and quantum spin dynamics in the fcc antiferromagnetNiS2with frustration

The unusual coexistence of two antiferromagnetic (AF) long-range orderings (LRO) in single-crystal ${\mathrm{NiS}}_{2}$ is investigated through measurements of inelastic neutron scattering, specific heat, uniform magnetic susceptibility, and resistivity. Neutron scattering intensity reveals a honeycomb pattern of the intensity distribution in reciprocal lattice space (continuous-line structure along the Brillouin zone boundaries) in the extended critical temperature region ${(T}_{N1}=39.3\mathrm{K}<T<150\mathrm{K})$ providing direct evidence for nearly frustrated antiferromagnetism on the face centered cubic (fcc) lattice. The observed geometrical pattern of the critical scattering makes it possible to determine the exchange coupling constants between the first, second, and third nearest neighbors, which are found to be consistent with $\mathrm{LDA}+U$ band calculation. The system is found to be located very close to the phase boundary between the type-I AF state with ${\mathbf{Q}}_{\mathrm{AF}}=(1,0,0),$ and the type-II AF state with ${\mathbf{Q}}_{\mathrm{AF}}=(1/2,1/2,1/2).$ Theoretical analysis based on the Onsager reaction field reproduces the various features in experiments at a semiquantitative level. The type-I AF order appears the highest N\'eel temperature ${T}_{N1}=39.3\mathrm{K},$ while at ${T}_{N2}=30.6\mathrm{K},$ the type-I and type-II AF LRO coexist via a first-order phase transition accompanied by a small but finite lattice distortion. This coexistence is explained in terms of coupling between the spin and lattice in the nearly degenerate case, although the type-I and type-II AF LRO become mutually incompatible in the fcc symmetry at higher temperatures. The AF LRO and lattice distortion lift the degeneracy and the accumulation of spectral weight at low energy, recovering the quantum nature (spin-wave excitation) of the spin fluctuation.

Calculation of the Microscopic and Macroscopic Linear and Nonlinear Optical Properties of Acetonitrile: I. Accurate Molecular Properties in the Gas Phase and Susceptibilities of the Liquid in Onsager's Reaction-Field Model

As part of a program to investigate the linear and nonlinear susceptibilities of acetonitrile in the condensed phase, we report on the accurate calculation of the molecular electric properties of acetonitrile, taking into account geometry and basis set effects, static and dynamic electronic correlation, vibrational contributions, and frequency dispersion. All correlated single reference state methods as well as the multireference SCF with a Møller−Plesset second-order perturbation correction (MRMP2) yield similar values for the electronic contribution to the polarizability α and the second hyperpolarizability γ. For the first hyperpolarizability, however, differences between the highly correlated methods CCSD(T) and MRMP2 remain. Vibrational contributions to the electric properties are calculated analytically and using two numerical finite difference methods at the Hartree−Fock level and at the correlated second-order Møller−Plesset level using finite field difference methods. Basis set convergence and convergence with the level of anharmonicity are examined. Computed values of the quantity μβ∥(−2ω; ω, ω)/(3kT) + γav(−2ω; ω, ω, 0) agree with temperature-dependent experimental values at two different frequencies within 10%. Using the highest correlated methods, liquid-phase susceptibilities are computed in the dipolar Onsager reaction-field approximation. Excellent agreement with experiment for the relative permittivity and the refractive indices is found as well as acceptable agreement for the nonlinear susceptibility.

Ab initio quantum chemical studies of cluster models for doped anatase and rutile TiO 2

Cluster models for the anatase and rutile structures of bulk TiO 2 were calculated by an ab initio Hartree–Fock method with the purpose of establishing qualitative trends for the size of the band gap upon transition metal doping. Different sizes and stabilization methods of the models (Onsager reaction field method, surrounded by water) were tested. The effect of doping cations on the size of the band gap was studied with the anatase model Ti 20M( n+)O 42 and the rutile model Ti 14M( n+)O 30. Doping the anatase model with Ti 3+, V 3+, Cr 3+, Mn 3+ and Fe 3+ narrowed the anatase band gap and the results were principally in agreement with the photocatalytic activity results of doped anatase. Doping of the rutile model with V 3+, Mn 3+, and Fe 3+ did not have a noticeable effect on the size of the rutile band gap. Chromium(3+), on the contrary, broadened the band gap of the rutile model.

Density functional theory study of the hydrogen bonding interaction of 1:1 complexes of formamide with water

The hydrogen bonding of 1:1 complexes formed between formamide and water molecule have been completely investigated in the present study using density functional theory and second-order Moller–Plesset Perturbation (MP2) method, the large basis sets 6-311++g(d,p) and 6-311++g(2d,2p) have been employed to determine the equilibrium structure and vibrational frequencies of the interacting complexes. Four reasonable geometries on the potential energy hypersurface of formamide and water system are considered, three stable structures and one transition state are found with the global minimum being a cyclic double-hydrogen bonded structure. The optimized geometric parameters and interaction energies for various isomers at different levels are estimated. The infrared spectrum frequencies, IR intensities and the vibrational frequency shifts are reported. Study of the blue shift of the n– π ∗ transition of formamide with water shows that the hydrogen bond has a great contribution to the shift. Finally the solution phase studies are also carried out using the Onsager reaction field model with a range of dielectric constants from 2 to 80 at B3LYP/6-311++g(d,p) level.

Magnetic interactions and Onsager reaction field from paramagnetic diffuse neutron scattering

Magnetic interaction parameters can be deduced from the diffuse scattering distribution using a mean-field theory. However, this theory is only valid at quite high temperatures. We introduce into the theory an additional parameter for the Onsager reaction field. Diffuse scattering data of the antiferromagnetic model system MnF2 are used to refine the additional parameter as a function of temperature. The Onsager parameter is also calculated by integrating the diffuse scattering in the Brillouin zone. We show that this parameter correctly describes the deviation from the mean-field theory over a large temperature range, from 1.06 to 2.4 TN.

Structure, bonding, and spectra of cyclic dithia radical cations: a theoretical study.

Ab initio molecular orbital and hybrid density functional theory methods are employed to characterize the structure, bonding and properties of several cyclic dithia radical cation systems, particularly in the context of intra molecular two-center three-electron (2c-3e) bonding between two sulfur atoms. The calculated results are able to interpret the time-resolved transient optical spectra obtained from pulse radiolysis technique for these positively charged dithia systems in aqueous solution. Visualization of the appropriate molecular orbital (MO) in the systems is able to depict the presence of a 2c-3e bond between two sulfur atoms and its sigma character. Geometry optimizations of these doublet systems are carried out at restricted open shell Becke's half-and-half (BHH) nonlocal exchange and Lee-Yang-Parr (LYP) nonlocal correlation functionals (BHHLYP) with 6-311+G(d,p) basis set including solvent effects adopting Onsager's reaction field model. Hessian calculations are done at the same level to check the nature of the equilibrium geometry. Energy data are further improved by performing MP2/6-311+G(d,p) calculations on these radical cation systems. Excited-state calculations are done following configuration interaction with single-electron excitation (CIS) method and the optical transition wavelength from the highest doubly occupied molecular orbital (HDOMO) to the lowest singly occupied molecular orbital (LSOMO) is seen to correspond and match to the position of the absorption maxima (lambda(max)) obtained from the experimental spectra for all these radical cation systems in aqueous solution. These calculations are able to resolve a long-standing ambiguity in the assignment of intra molecular 2c-3e bonding in the case of the 3-methyl-2,4-dithiapentane radical cation system and to provide new insights into bonding features of this odd electron system as well as of other cyclic dithia systems studied.

Vibrational Spectroscopy of Hydrogen Bonding: Origin of the Different Behavior of the C−H···O Hydrogen Bond

The spectroscopic behavior of the C−H···O hydrogen bond is generally opposite to that of typical (e.g., O−H···O and N−H···O) hydrogen bonds: the C−H stretching frequency is blue-shifted and the infrared band intensity decreases on hydrogen bonding. We show that this can be understood on the basis of the dynamic properties of the donor group, in particular the force on the bond resulting from the interaction of the external electric field created by the acceptor atom with the permanent and induced dipole derivatives of the X−H bond: when the field and dipole derivative are parallel, as in the case of O−H···O, the bond lengthens, and red shift and intensity increases result; when the field and dipole derivative are antiparallel, as in the case of C−H···O, the bond shortens and blue shift results with the possibility of intensity decrease. We demonstrate these properties in an ab initio and perturbation study of cis formic acid in an Onsager reaction field. This analysis provides the basis for a more gener...

Theoretical molecular structures for partially bonded complexes of trimethylamine with SO 2 and SO 3: ab initio and density functional theory calculations

The self-consistent reaction field (SCRF) method based on Onsager's reaction field theory is applied to investigate the effect of polar media on molecular structures of complexes of trimethylamime (TMA) with SO x ( x=2,3). The calculated SCRF N–S bond lengths at the MPW1PW91/6-311+G(3df) level are in satisfactory agreement with the experimental N–S bond lengths for the TMA–SO x upon crystallization. The results are enough to demonstrate the usefulness of the reaction field theory in providing qualitative understanding of the medium effect on the partially bonded system such as TMA–SO x .