Reconsideration of solvent effects calculated by semiempirical quantum chemical methods

AM1 and PM3 semiempirical calculations are reported for the solvent effects on the tautomeric equilibria of 2-pyridone/2-hydroxypyridine and 4-pyridone/4-hydroxypyridine in the gas phase and solution. The solvent effects on the tautomeric equilibria were investigated by self-consistent reaction field (SCRF) theory implemented in the AMPAC and MOPAC program in two different ways: one in which all the solvent relaxation is included in the quantum mechanics and the total energy must be corrected for the solvent change in energy, method A; and a second in which the quantum mechanics directly includes this term, method B. The calculated (AM1, method A) tautomeric equilibrium constants (log K₁) for 2-pyridone in the gas phase, cyclohexane, chloroform, and acetonitrile are ?0.3, 0.3, 0.8, and 1.3, respectively, in good agreement with the experimental data (?0.4, 0.24, 0.78, and 2.17, respectively). For 4-pyridone/4-hydroxypyridine differences between calculated log K₁ for the gas phase, chloroform and acetonitrile (?6.0, ?2.6, and ?1.2, respectively) and experimental data (< ?1, 0.11, and 0.66, respectively) are larger but the experimental values are also less certain. The experimental acetonitrile data are disturbed by specific interactions. An extension of the SCRF for aqueous solutions is reviewed. © 1993 John Wiley & Sons, Inc.     

Some quantum chemical aspects of solvent effects on NMR parameters

The non-bonded interactions which occur between solute and solvent molecules are discussed and the various models which describe the interactions are presented. The models are employed, within a quantum chemical framework, to estimate the extent of medium effects on chemical shifts and spin-spin couplings. Comparison between the calculated and available experimental data is provided for some first row nuclei.     

A combined experimental and theoretical quantum chemical studies on 4-morpholinecarboxaldehyde

Extensive spectroscopic investigations have been carried out by recording the Fourier transform infrared (FTIR) and FT-Raman spectra and carrying out the theoretical quantum chemical studies on 4-morpholinecarboxaldehyde (4MC). From the ab initio and DFT analysis using HF, B3LYP and B3PW91 methods with 6-31G(d,p) and 6-311G++(d,p) basis sets the energies, structural, thermodynamical and vibrational characteristics of the compound were determined. The energy difference between the chair equatorial and chair axial conformers of 4MC have been calculated by density functional theory (DFT) method. The optimised geometrical parameters, theoretical wavenumbers and thermodynamic properties of the molecule were compared with the experimental values. The effect of carbonyl group on the characteristic frequencies of the morpholine ring has been analysed. The mixing of the fundamental modes with the help of potential energy distribution (PED) through normal co-ordinate analysis has been discussed.     

Evaluation of free-volume theories for solvent self-diffusion in polymer–solvent systems

An evaluation of free-volume theories for solvent self-diffusion is carried out using recent comprehensive data sets for penetrant self-diffusion in polymer solutions. Different theories are compared, and free-volume theories in the prediction of penetrant self-diffusion coefficients in glassy polymer systems is also evaluated. © 1993 John Wiley & Sons, Inc.     

Conformational analysis,tautomeric preference,intramolecular hydrogen bonding,and solvent effect on dinitrosamine: A quantum chemical study

HF, B3LYP, and MP2 methods with the standard basis set, 6‐311++G(d,p), were used to study various aspects of dinitrosamine. These results were compared with the outcomes of G2 and CBS‐QB3 methods. First, the conformational analysis and characterization of equilibrium conformations, especially global minima, were performed. On the basis of relative energies, we found that the dinitroso tautomers are more stable than the nitroso‐hydroxy (NH) ones. This preference is well‐interpreted in terms of tautomerization process and nitrosamine resonance. Furthermore, the nature of O? H···O intramolecular hydrogen bond (IMHB), in chelated forms of NH (NH‐11 and NH‐13) was comprehensively studied to evaluate the effect of hetero atoms (N) on the characteristic of IMHB systems. According to the results of isodesmic reaction method, the hydrogen bond energy of NH‐11 is greater than the malonaldehyde (MA) and NH‐13, whereas the electron density analysis and energy‐geometry correlation methods clearly predict that the hydrogen bond of NH‐11 is weaker than the MA. Additionally, the geometrical, atoms in molecules (AIM) and natural bond orbital's (NBO) parameters also emphasize on the MA as a chelated form with the strongest hydrogen bond. Finally, the solvent effects on the relative stability of selected dinitrosamine conformers are evaluated by different continuum (polarizable‐continuum model, isodensity polarizable continuum model, and self‐consistent isodensity polarizable continuum model), discrete and mixed solvent models. Theoretical results readily show that the potential energy surface of dinitrosamine, especially global minima, is strongly affected by the solvent. © 2012 Wiley Periodicals, Inc.     

Facile S(N)2 reaction in protic solvent: quantum chemical analysis

We study the effects of protic solvent (water, methanol, ethanol, and tert-butyl alcohol) and cation (Na+, K+, Cs+) on the unsymmetrical SN2 reaction X- + RY --> RX + Y- (X = F, Br; R = CH3,C3H7;Y = Cl, OMs). We describe a series of calculations for the S(N)2 reaction mechanism under the influence of cation and protic solvent, presenting the structures of pre- and postreaction complexes and transition states and the magnitude of the activation barrier. An interesting mechanism is proposed, in which the protic solvent molecules that are shielded from the nucleophile by the intervening cation act as a Lewis base to reduce the unfavorable Coulombic influence of the cation on the nucleophile. We predict that the reaction barrier for the S(N)2 reaction is significantly lowered by the cooperative effects of cation and protic solvent. We show that the cation and protic solvent, each of which has been considered to retard the SN2 reactivity of the nucleophile, can accelerate the reaction tremendously when they interact with the fluoride ion in an intricate, combined fashion. This alternative S(N)2 mechanism is discussed in relation to the recently observed phenomenal efficiency of fluorination in tert-alcohol media [Kim, D. W.; et al. J. Am. Chem. Soc. 2006, 128, 16394].     

A theoretical study of solvent effects on the 13C chemical shifts of some polar molecules

Calculations of the dependence of ¹³C chemical shifts on the dielectric constant, ?, of the medium are reported using the INDO/S parameterized version of Pople's method together with the solvaton model. Satisfactory agreement with the available experimental data is obtained. The rather sensitive dependence of chemical shift upon ? suggests that polar solute—solvent interactions in aprotic media should be taken into account when considering ¹³C chemical shifts.     

Evaluation of theories for diffusion in polymer–solvent systems

A method is proposed for examining differences in the predictive capabilities of three versions of the free-volume theory of polymer—solvent diffusion using only a small amount of diffusivity data. The utility of the method is illustrated using a limited diffusivity data set for the methanol-poly(vinyl acetate) system.     

Valence bond theoretical study for chemical reactivity

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Free-volume theories for self-diffusion in polymer–solvent systems. I. Conceptual differences in theories

Conceptual differences in two recent versions of the free-volume theory for self-diffusion in polymer–solvent systems are identified and discussed. The validity of the assumptions of these two theories is tested using experimental data. In all cases, the experimental evidence favors the Vrentas–Duda free-volume theory over the Paul version of this theory.     

Vibrational spectroscopic study on the quantum chemical model and the X-ray structure of gallic acid,solvent effect on the structure and spectra

Infrared, Raman and far-IR spectra of gallic acid were recorded both in crystalline and in its dry form. Solvent effect of water was studied. The molecular and crystal structures were determined by single crystal X-ray diffraction. A complex system of intermolecular interactions was revealed. Optimized geometries, vibrational frequencies and infrared intensities were calculated utilizing the post-HF DFT method with the Becke3LYP functional and the 6-31G* basis set. Normal coordinate analysis was carried out. The results of the calculations were applied to simulate the infrared and Raman spectra and the full assignment of the acquired spectra is presented.     

A theoretical study of the dielectric solvent effect on vicinal spin coupling constants

The dielectric solvent effect on HCCH, HCNH and HCNC spin coupling constants in ethane, tetrachloroethane and trans N-methylformamide has been calculated by finite perturbation theory based on the INDO and CNDO/2 approximations incorporating solvaton theory. The available experimental data are interpreted using the calculated variations of spin coupling constants. The effect of dielectric constant on the general form of the Karplus relation is included in the finite perturbation calculations.     

Ortho effect in fluorobenzenes: cross-correlated relaxation and quantum chemical studies

An early solid-state NMR study of the shielding tensors in substituted fluorobenzenes had indicated the presence of the 'ortho effect'. This was confirmed recently in the liquid state from a study of cross-correlated relaxation, which gives a handle on the shielding tensor. We report here a combined experimental and computational study on substituted fluorobenzenes where the ortho substituent is varied systematically. Experimental measurements of the longitudinal relaxation of 19F indicate the cross-correlation between the chemical shift anisotropy (CSA) of fluorine and its dipolar interaction with the ortho proton, and provide a measure of the CSA orientation parameter. This parameter is obtained also from quantum chemical calculations of the 19F CSA tensor. We establish a correlation between the CSA orientation parameter and linear free energy parameters by resorting to a multi-parameter regression analysis. Excellent correlation is obtained for most of these substituents only when a parameter for the ortho effect is included.     

Non-adiabatic molecular dynamics with quantum solvent effects

Three novel approaches extending quantum-classical non-adiabatic (NA) molecular dynamics (MD) to include quantum effects of solvent environments are described. In a standard NA-MD the solute subsystem is treated quantum mechanically, while the larger solvent part of a system is treated classically. The three novel approaches presented here are based on the Bohmian formulation of quantum mechanics, the stochastic Schrödinger equation for the evolution of open quantum systems and the quantized Hamilton dynamics generalization of classical mechanics. The approaches extend the standard NA-MD to incorporate the following quantum effects of the solvent. (1) Branching, i.e. the ability of solvent quantum wave packets to split and follow asymptotically diverging trajectories correlated with different quantum states of the solute. (2) Decoherence, i.e. loss of quantum interference within the solute subsystem induced by the diverging solvent trajectories. (3) Zero point energy that contributes to NA coupling and must be preserved during the energy exchange between solvent and solute degrees of freedom. The Bohmian quantum-classical mechanics, stochastic mean-field and quantized mean-field approximations incorporate the quantum solvent effects into the standard quantum-classical NA-MD in a straightforward and efficient way that can be easily applied to quantum dynamics of condensed phase chemical systems.     

A theoretical analysis of the effect of solvent on the Kerr constant of dilute polymer solutions

A new model in which solvent molecules are regarded as discrete particles is used for investigating the influence of solvent orientation near a polymer chain and of the anisotropy of the internal electric field on the Kerr effect in a polymer solution. The relation derived for the Kerr constant consists of two parts containing nine terms each in the general case (the physical meaning of the terms is briefly discussed). Relations for the above effect in a nonpolar isotropic solvent, and explicit calculations of the Kerr constant for a simple model of a rigid polymer molecule are given.     

Correlation analysis of quantum chemical data and the polarizability effect in H-complexes

The study is concerned with analysis of the energies of formation (E), frequency shifts (Δν) in IR spectra, ionization potentials (IP) of H-complexes, hydrogen bond lengths (r), and spin densities (sd) in H-complexes involving radical cations, obtained from quantum chemical calculations for 20 series of H-complexes. It was for the first time established that the E, IP, r, and sd values and the changes in enthalpy (δH) depend not only on the inductive and resonance effects but also on the polarizability effect of the substituents bound to the donor and acceptor centers in the H-complexes. Interrelations between the polarizability effect and the molecular structure of H-complexes are considered. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 602–608, April, 2006.     

A framework for the evaluation of chemical structure databases

Access to desk-top structure and reaction databases through applications such as Chemical Abstracts' SciFinder, MDL's Beilstein CrossFire, and ISIS Reaction Browser has led to changes in information seeking habits of research chemists, the impact of which has implications when database purchasing decisions are made. A semiquantitative assessment is proposed which takes into account key aspects of structure and reaction databases. Assessment criteria are identified which can be weighted according to an organization's information needs. Values are then assigned to criteria for each data source, after which a formula is applied which leads to an indication of the relative value of systems under consideration. The formula takes into account the cost of database products and also the incremental benefit of adding a new system to an existing collection. This work is presented as a generic approach to the evaluation of databases and is not limited in scope to only structure and reactions databases.