Onsager Reaction Field Model Research Articles

Can Simple Quantum-Chemical Continuum Models Explain the Gauche Effect in Poly(ethylene oxide)?

We report ab initio quantum-chemical calculations at the level of second-order many-body perturbation theory aimed at the equilibrium between the all-trans (ttt) and the trans-gauche-trans (tgt) conformations of dimethoxyethane, which is the smallest analog of poly(ethylene oxide). Solvent effects are accounted for by two continuum models: one assumes statistically fluctuating electric fields around the dimethoxyethane molecule caused by the solvent, and the other is an Onsager reaction-field model. We find the ttt conformer to be more stable in the gas phase. Use of the stochastic field model has no effect on the relative stabilities, whereas the reaction-field method brings the energy of the tgt conformer down very close to the ttt energy

Experimental probes of excited state character in supercritical fluids

Abstract

The TiBe 2 magnetic susceptibility in the framework of the Onsager reaction-field model

We show that the high temperature part of the TiBe 2 low field magnetic susceptibility is much more suitable, at present, for the understanding of its physical nature than the low temperature part. The necessity for electron-electron correlations is demonstrated and their role in the susceptibility is estimated via the Onsager reaction-field model. The need to calculate the Lindhard function based on the band structure is emphasized.

Permanent Dipole Moment and Polarizability of a Gass of Merocyanine Dyes in Their Ground and First Excited Singlet State

Abstract The permanent dipole moment of a class of merocyanine dyes in their ground and first excited singlet state has been determined studying the influence of an external electric field on the absorption of the compounds in various solvents. Evaluation of the experimental results on the basis of the Onsager reaction field model also gives values for the polarizability in the ground and excited state, the latter approaches the value of a respective conducting ellipsoid with the longer chainlength molecules.

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.

Medium dependence of the proton chemical shift and H—H, H—F, F—F coupling constants in 1,1-difluoroethylene

The solvent dependence of the proton chemical shift, the geminal proton–proton, the cis and trans proton–fluorine, and the geminal fluorine–fluorine coupling constants in 1,1-difluoroethylene is reported for 25 solvents. The shift and the couplings involving protons depend primarily on the dielectric constant of the medium, but the Onsager reaction field model is inadequate to describe the changes observed. The fluorine–fluorine coupling depends also on dispersion interactions with the solvent molecules. A comparison with the solvent behavior of the spectral parameters of vinyl fluoride, 1-chloro-1-fluoroethylene, and trifluoroethylene indicates the importance, in varying degrees, of reaction field effects, hydrogen bonding, dispersion interactions, the orientation of the solute dipole moments, and the polarizability of specific bonds in the solute molecule in determining their solvent-induced changes.

Solvents effects on the proton chemical shift and H—H, H—F coupling constants in 1-chloro-1-fluoroethylene. Reaction field and dispersion interactions

The proton chemical shift and the geminal proton–proton and cis proton–fluorine coupling constants in 1-chloro-1-fluoroethylene depend primarily on the dielectric constant of the medium, although this dependence is not given by the Onsager reaction field model. A roughly linear relationship is found with the square root of the dielectric constant. The proton coupling constant decreases algebraically while the cis proton–fluorine coupling constant increases as the dielectric constant of the medium increases. The trans proton–fluorine coupling depends on dispersion interactions with the solvent molecules and increases as these interactions increase. The geminal coupling constant varies between − 3.8 and − 4.8 c.p.s., the cis coupling between 7.6 and 12.3 c.p.s., and the trans coupling between 36.9 and 38.6 c.p.s.