Onsager Reaction Research Articles

Tautomeric Rearrangements in Mono- and Dichalcogenide Analogs of Formic Acid, HC(X)YH (X, Y = O, S, Se, Te): A Theoretical Study

Theoretical calculations are reported at the Hartree−Fock (HF), MP2, and Becke3LYP (B3LYP) levels on a complete series of 16 chalcogenic derivatives of formic acid HC(X)YH (X, Y = O, S, Se, Te) using all-electron basis sets. The periodic variations observed on substituting the chalcogens are discussed. The transition structures for the tautomeric rearrangement of these formic acid derivatives are also characterized. The variations in relative energies corrected for zero-point vibrations show that the barrier for tautomerism is reduced as the electronegativity of chalcogens is decreased. The trends of natural charges on atoms of chalcogenides are described. At the correlated level of calculations both MP2 and B3LYP methods give comparable results. The solvent effects on tautomeric equilibrium are assessed by performing self-consistent reaction field (SCRF) calculations at the HF level. A comparative study is provided for two solvation models: the electrostatic solvation model based on Onsager's reaction field theory and the self-consistent isodensity polarized continuum model (SCI-PCM). The latter is shown to be a better model for solvation. The solvents with dielectric constants 2.0, 7.6, and 35.9 are shown to be less effective on the thermodynamic stabilities of these reactions. The dipole moments show significant variations between solvents of lower dielectric medium, while the variations are insignificant between solvents of higher dielectric media. A comparison of thermodynamic preferences for keto and enol forms in monochalcogeno acetc acids with the solvent model SCI-PCM at the HF and B3LYP levels is also provided. Chemical shifts calculated using the GIAO method (at the B3LYP/6-311+G(2D,P)//B3LYP/6-31G(D) level) correlate well with the experimental results. However we conclude from these results that the thion form of CH3C(S)OH is less predominant.

Structural and energetic relations between ? turns

A systematic quantum chemical study on the structure and stability of the major types of β-turn structures in peptides and proteins was performed at different levels of ab initio MO theory (MP2/6-31G*, HF/6-31G*, HF/3-21G) considering model turns of the general type Ac(SINGLE BOND)Xaa(SINGLE BOND)Yaa(SINGLE BOND)NHCH3 with the amino acids glycine, l- and d-alanine, aminoisobutyric acid, and l-proline. The influence of correlation effects, zero-point vibration energies, thermal energies, and entropies on the turn formation was examined. Solvent effects on the turn stabilities were estimated employing quantum chemical continuum approaches (Onsager's self-consistent reaction field and Tomasi's polarizable continuum models). The results provide insight into the geometry and stability relations between the various β-turn subtypes. They show some characteristic deviations from the widely accepted standard rotation angles of β turns. The stability order of the β-turn subtypes depends strongly on the amino acid type. Thus, the replacement of l-amino acids in the two conformation-determining turn positions by d- or α,α-disubstituted amino acid residues generally increases the turn formation tendency and can be used to favor distinct β-turn subtypes in peptide and protein design. The β-turn subtype preferences, depending on amino acid structure modifications, can be well illustrated by molecular dynamics simulations in the gas phase and in aqueous solution. © 1997 by John Wiley & Sons, Inc. J Comput Chem 18: 1415–1430, 1997

Theoretical and Experimental Study of Medium Effects on the Structure and Spectroscopy of the [Fe(CN)(5)NO](2)(-) Ion.

The influence of the solvent on the structure and IR spectrum of the [Fe(CN)(5)NO](2)(-) ion is investigated by using gradient corrected density functional theory. IR spectra are also measured on different solvents and the results obtained are compared with the predicted ones. We have treated the solvent effects with a continuum model, based on the Onsager's reaction field approach; in order to mimic strong specific interactions, calculations were also performed on the complex protonated at the cyanide trans to the nitrosyl group. The reaction field calculations predict only qualitatively the most important observed trends, e.g., the shifts in the nitrosyl stretching wavenumber, but fail in accounting quantitatively for the differences between the spectra in water and acetonitrile. The possible role of specific interactions is consistently accounted for by interpreting the experimental shifts of the NO stretching wavenumber nu(NO), as well as the visible absorption energies, when changing the Lewis acidity of the solvent, as measured by the Gutmann's acceptor number. Ligand population analysis was performed to relate the solvent effects with the sigma donor and pi acceptor behavior of cyanide and nitrosyl ligands. The significance of nu(NO) shifts as a result of changes in the medium is discussed in view of the physiological relevance of transition-metal nitrosyl chemistry.

Solvent effects in density functional calculations of uracil and cytosine tautomerism

AbstractThe effects of the solvent on the tautomeric equilibria of cytosine and uracil are studied using Onsager's reaction field model in the framework of density functional theory. In this model, the solute molecule is placed in a spherical cavity of radius ao immersed in a continuous medium with a dielectric constant e, treating the solute‐solvent electrostatic interactions at the dipole level. The cavity radius is evaluated by determining the molecular volume with a quantum mechanical approach. The solvent effect brings significant changes in the geometrical parameters of some cytosine tautomers, but only very small changes to those of the uracil tautomers. Our results are in good agreement with available experimental results and confirm that the polarization of the solute by the continuum has important effects on the absolute and relative solvation energies. Frequency shifts and intensity variations in the infrared spectra due to the presence of the solvent are also presented. © 1995 John Wiley & Sons, Inc.

Determination of Effective Potentials for Non-polar Liquids Using the Continuum Dielectric Model

Abstract The “effective potential” for each system, nitrogen, argon, krypton, xenon, methane, benzene, carbon tetrachloride, carbon disulfide, n-hexane and cyclohexane was calculated by using the Clausius-Mossotti, Onsager and Kirkwood reaction fields with Lennard-Jones and Kihara potential parameters assumed. It was found that the Kirkwood reaction field with an assumed Kihara potential is a useful technique to study interactions arising due to physical association and chemical bonding.

Solute-solvent interaction in nonpolar supercritical fluid: a clustering model and size distribution

Solubility and solvatochromic shift of 4-(dimethylamino) benzonitrile (DMABN) in supercritical ethane were measured. The bathochromic shifts observed at various densities were much larger than expected from Onsager's reaction field theory. The discrepancy can be attributed to the solvent aggregation around DMABN even in nonpolar ethane

Onsager's reaction field for the Potts model from the path integral

A new path integral formulation for theq-state Potts model is proposed. This formulation reproduces known results for the Ising model (q=2) and naturally extends these results for arbitraryq. The mean field results for both the Ising and the Potts models are obtained as a leading saddle point contribution to the corresponding functional integrals, while the systematic computation of corrections to the saddle point contribution produces the Onsager reaction field terms, which forq=2 coincide with results already known for the Ising model.

Ultrathin magnetic layers in semiconductor superlattices

We present a calculation of the magnetization of thin ferromagnetic layers, based on Onsager's reaction field approximation, as a function of temperature and applied magnetic field. We show that this approximation satisfies the Mermin-Wagner theorem and allows for a numerically efficient treatment of the problem. This is necessary step in the study of the electronic-magnetic properties of semimagnetic supperlattices.

Solvent effect on thermodynamics of molecular equilibria

The solvent effect on changes in enthalpy and entropy of molecular equilibria is discussed. General relations, describing enthalpy and entropy changes of molecular equilibria, are proposed after assuming a form for the free-energy dependence on solvent polarity. A special case of the relations is derived from Onsager's dipole reaction field model. These considerations show that the enthalpy and entropy changes of molecular equilibria can be defined in various ways and two extreme approaches are examined.

Influence of the reaction field on the nature of the phase transition in the Potts model

It is shown how Onsager's reaction field may be incorporated in mean-field-like calculations for the q-state Potts model. Its influence on the nature of the phase transition is considered both in a one-site and a two-site cluster approximation. Considerable improvement is obtained for qc(d), the value which separates first- and second-order transitions. The way in which qc(d) is modified when a second-neighbour interaction is added to the ferromagnetic nearest-neighbour one is also studied.

Far-infrared absorption intensities of H 2O and D 2O in C 6H 6

Far-i.r. absorption intensities have been measured for H 2O and D 2O in C 6H 6 solutions. Beer's law plots were found to be nonlinear. From the plots, the equilibrium constants of dimer formation in benzene were estimated to be 2.4M -1 and 3.3M -1 for H 2O and D 2O at 20°C, respectively. Based on Onsager's reaction field and including explicitly the effect of differences in molecular size between solute and solvent molecules, the internal moment of a water molecule and the volume ratio of a molecule to a cavity were estimated from the integrated absorption intensity of a D 2O monomer in C 6H 6 as 1.98 D and 0.7, respectively

Theoretical Studies of Solvent Effects on Nuclear Spin–Spin Coupling Constants. I. The Reaction Field Model

A theoretical study of the solvent dependence of nuclear spin–spin coupling constants is presented in terms of Onsager's reaction field model and self-consistent perturbation theory in the semiempirical INDO (intermediate neglect of differential overlap) approximation of self-consistent-field molecular orbital theory. A possible mechanism for reaction field induced changes in coupling constants is discussed in terms of the sum over virtual orbital method, corresponding to the uncoupled Hartree–Fock approximation. Although the latter method does not give satisfactory values for coupling constants, the reaction field induced shifts are reasonable. Finite perturbation theory, which corresponds to the coupled Hartree–Fock approximation, gives much better coupling constant results. Calculated reaction field induced shifts for a variety of H–H and H–F coupling constants are compared with the experimental results in a variety of solvents. Although the calculated results are linear functions for very large values of the reaction field, best agreement with experiment is obtained in solvents of low dielectric constant. The importance of solute dipole orientation in a series of fluoroethylenes is investigated.

Influence of Solvent upon the Energy Difference between Rotational Isomers

The energy difference between the rotational isomers of the gauche- and trans-forms of 1,2-dichloroethane in various solutions have been calculated by use of a modified Onsager's reaction field. The following assumptions have been employed in the calculation: first, the cavity of the solute molecule is spheroidal; second, the dipole moment of the molecule is composed of two point dipoles which are placed at the foci of the spheroidal cavity, the focal distance being taken as equal to the distance between the contact points of the carbon and chlorine atoms in the dichloroethane molecule. The change of energy difference between rotational isomers by solvents thus computed has been found to be in close agreement with the experimental observations, that is, those obtained by infrared absorption measurements as well as dipole moment.