Isodensity Polarized Continuum Model Research Articles

The strong influence of the solvent on the electron spin resonance spectra of semiquinone radical anions I. A theoretical investigation of the hyperfine constants of 1,2- and 1,4-benzosemiquinone by using density functional theory and polarizable continuum solvation models

The influence of the solvent medium on the isotropic hyperfine constants of 1,2- and 1,4-benzosemiquinone is investigated by theoretical calculations, using B3LYP density functional theory and electrostatic continuum solvation models. The computed hyperfine constants are highly sensitive to the impact of the solvent field, particularly in the case of 1,2-benzosemiquinone where the relative values are completely rearranged when the dielectric constant of the solvent, ϵ, is increased above unity. Consistency with experimental hyperfine data observed in aprotic as well as protic solvents is obtained by appropriate adjustment of the surface isodensity parameter in the isodensity polarizable continuum model (IPCM) of Foresman et al.

Protonation enthalpies in fluorosulfonic acid usingab initio self-consistent reaction field theory

Electrostatic solvation free energies were computed for several small neutral bases and their conjugate acids using a continuum solvation model called the self-consistent isodensity polarizable continuum model (SCIPCM). The solvation energies were computed at the restricted Hartree–Fock (RHF) and second-order Moller–Plesset (MP2) levels of theory, as well as with the Becke3–Lee–Yang–Parr (B3LYP) density functional theory, using the standard 6–31G** Gaussian basis set. The RHF solvation energies are similar to those computed at the correlated MP2 and B3LYP theoretical levels. A model for computing protonation enthalpies for neutral bases in fluorosulfonic acid solvent leads to the equation ΔH(B)=−PA(B)+ΔEt(BH+)−ΔEt(B)+β, where PA(B) is the gas phase proton affinity for base B, ΔEt(BH+) is the SCIPCM solvation energy for the conjugate acid, and ΔEt(B) is the solvation energy for the base. A fit to experimental values of ΔH(B) for 10 neutral bases (H2O, MeOH, Me2O, H2S, MeSH, Me2S, NH3, MeNH2, Me2NH, and PH3) gives β=238.4±2.9 kcal/mol when ΔΔEt is computed using the 0.0004 e⋅bohr−3 isodensity surface for defining the solute cavity at the RHF/6–31G** level. The model predicts that for carbon monoxide ΔH(CO)=10 kcal/mol. Thus, protonation of CO is endothermic, and the conjugate acid HCO+ (formyl cation) behaves as a strong acid in fluorosulfonic acid. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 250–257, 1998

A Theoretical Study of Homogeneous Ziegler−Natta Catalysis

In this paper we have investigated, using a DFT (B3LYP) computational approach, the insertion process of ethylene in the titanium−carbon bond, which represents a fundamental step in the Ziegler−Natta polymerization reaction. The DFT results have been validated by a comparison with the results obtained at the MP2 and CASPT2 levels. The two following models have been considered: (i) the cationic species Cl2TiCH3+ reacting with an ethylene molecule which emulates the positive part of a solvent-separated ion pair (CH3)2AlCl2- || Cl2TiCH3+); (ii) the bimetallic species H2Al(μ-Cl)2TiCl2CH3 also reacting with ethylene and which mimics the possible bimetallic complexes or tight ion pairs that can originate from the catalyst−cocatalyst interaction. In the former case the process is highly exothermic (−45.5 kcal mol-1) and is characterized by an insertion energy barrier of about 5 kcal mol-1. In the latter case the energetically most favored channel is a two-step reaction path that requires the overcoming of a first barrier of about 5.6 kcal mol-1 to form an intermediate and of a second barrier of 5.8 kcal mol-1 to reach the insertion transition state. We suggest that in the real conditions used to carry out the reactions both reaction channels (bimetallic complex and separated ion pair) are simultaneously available and that their relative importance and the resulting reaction rate are determined by the solvent polarity: the more polar the solvent, the more important the reaction path involving the cationic species. Self-consistent isodensity polarized continuum model (SCI-PCM) computations have shown that the insertion barriers decrease with the increasing polarity of the solvent.

13C NMR, 15N NMR and quantum-chemical study of the tautomerism of 2-substituted 5-Me-7-OH-1,2,4-triazolo[1,5-a]pyrimidines

The 13C and 15N NMR spectra of a series of differently 2-substituted 5-Me-7-OH-1,2,4-triazolo[1,5-a]pyrimidines were recorded and the chemical shifts assigned by employing the whole arsenal of 1D and 2D NMR spectroscopic methods. Both the 13C chemical shifts of the C-7 carbon atoms and the line shape of the 15N resonances of N-3 and N-4, respectively, were not sufficiently characteristic to differentiate the lactim and the three lactam tautomers of the compounds studied. However, by calculating both the formation energies and the 13C as well as 15N chemical shifts of the various tautomers by quantumchemical ab-initio calculations at the 6-31G∗ level, the preferred tautomer(s) could be unequivocally assigned. The solvent effect on the present tautomerism, which proved very strong, could be studied quantitatively by including the present NMR solvent in the accompanying calculations employing a self-consistent isodensity polarized continuum model (SCIPCM).

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.

Ab initio study, semiempirical calculation and NMR spectroscopy of keto-enol tautomerism of triazolopyrimidines

The tautomerism of triazolopyrimidines in the gas phase as well as in DMSO has been studied with ab inito and semiempirical methods. The self-consistent reaction field method SCI-PCM (self-consistent isodensity polarized continuum model) has been used to represent solvent effects in the ab inito HF/6–31G ∗ calculations. Electron correlation was included at the second-order Møller-Plesset perturbation level (MP2). The calculated equilibria of tautomerism, taking solvent effects into account, are in good agreement with the 15N NMR spectroscopy data.

Density Functional Study of the Radical Reactions of 3-Methyl-1-phenyl-2-pyrazolin-5-one (MCI-186): Implication for the Biological Function of MCI-186 as a Highly Potent Antioxidative Radical Scavenger

The radical reaction intermediates and products from the antioxidative radical scavenger MCI-186, 3-methyl-1-phenyl-2-pyrazolin-5-one (1), and its model system 1,3-dimethyl-2-pyrazolin-5-one (2) are investigated by a density functional method (B3LYP/6-31G*) with the self-consistent isodensity polarized continuum model (SCI-PCM). It is shown that a radical intermediate, denoted 4, is produced as a result of one-electron oxidation of 2 by active oxygen species. The subsequent reactions are studied along the scheme proposed previously by an in vitro experimental study of 1. For 4, the isotropic hyperfine coupling constants are calculated using the B3LYP/EPR-II basis set, and the results are compared with the literature values given by ESR measurements at room and body temperatures. Consequently, it is confirmed that the calculation reproduces well the experimental results. One of the most important findings is that the radical intermediate is stabilized by delocalization of spin population over the pyrazolin...

An ab Initio Study of the Trimethylamine−Formic Acid and the Trimethylammonium Ion−Formate Anion Complexes, Their Monohydrates, and Continuum Solvation

High-level ab initio calculations carried out up to an effective MP4SDQ/6-311+G(3df,2p)//HF/6-31+G* level predict that the trimethylamine−formic acid complex in vacuo is favored by 7.0 kcal/mol (ΔG298) relative to the trimethylammonium−formate complex. Interaction with a single water molecule is, according to calculated results, not sufficient to make the ion-pair complex the predominant one, the lowest energy monohydrated neutral complex being favored by 4.7 kcal/mol (ΔG298) in comparison to the lowest energy monohydrated ion-pair. Calculations on the effect of a dielectric continuum on the binary and monohydrated ternary complexes using the Self-Consistent Isodensity Polarized Continuum Model (SCI-PCM) indicate that a strong dielectric continuum with a dielectric constant larger than ca. 9 is required to make the binary ion-pair complex predominant. However, only a relatively weak dielectric continuum with a dielectric constant in the range of 4−6 is required in order to favor the monohydrated ion-pair complex.