RESUMEN

A method based on molecular dynamics simulations which employ two distinct levels of theory is proposed and tested for the prediction of Gibbs free energies of solvation for non-ionic solutes in water. The method consists of two additive contributions: (i) an evaluation of the free energy of solvation predicted by a computationally efficient molecular mechanics (MM) method; and (ii) an evaluation of the free energy difference between the potential energy surface of the MM method and that of a more computationally intensive first-principles quantum-mechanical (QM) method. The latter is computed by a thermodynamic integration method based on a series of shorter molecular dynamics simulations that employ weighted averages of the QM and MM force evaluations. The combined computational approach is tested against the experimental free energies of aqueous solvation for four solutes. For solute-solvent interactions that are found to be described qualitatively well by the MM method, the QM correction makes a modest improvement in the predicted free energy of aqueous solvation. However, for solutes that are found to not be adequately described by the MM method, the QM correction does not improve agreement with experiment. These preliminary results provide valuable insights into the novel concept of implementing thermodynamic integration between two model chemistries, suggesting that it is possible to use QM methods to improve upon the MM predictions of free energies of aqueous solvation.

RESUMEN

Nucleophilic aromatic substitution has been implicated as a mechanism for both the biotic and abiotic hydrodehalogenation of aromatics. Two mechanisms for the aqueous dehalogenation of aromatics involving nucleophilic aromatic substitution with hydride as a nucleophile are investigated using a validated density functional and continuum solvation protocol. For chlorinated and brominated aromatics, nucleophilic addition ortho to carbon-halogen bonds via an anionic intermediate is predicted to be the preferred mechanism in the majority of cases, while concerted substitution is predicted to be preferred for most fluorinated aromatics. Nucleophilic aromatic substitution reactions with the hydroxide and hydrosulfide anions as nucleophiles are also investigated and compared.

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RESUMEN

We have characterized the structural and energetic properties of CH3CN-BCl3via computations and matrix-IR spectroscopy. We find two equilibrium structures of the complex via computations. At the MP2/aug-cc-pVTZ level, the global minimum energy structure has a B-N distance of 1.601 Å, and a binding energy of 12.0 kcal mol(-1). The secondary structure lies 7.1 kcal mol(-1) higher in energy with a B-N distance of 2.687 Å and a binding energy of 4.9 kcal mol(-1). Computational scans of the B-N potential curve using both DFT and post-HF methods indicate that a significant barrier exists between these structures, and that it lies 1 to 2 kcal mol(-1) above the secondary minimum at a B-N distance of about 2.2 Å. We also observed several key, structurally-sensitive IR bands for six isotopic forms of the complex in neon matrices, including: the B-Cl asymmetric stretching band (ν) at 792 cm(-1) and the C-N stretching band (νCN) at 2380 cm(-1) (for the primary isotopomer, CH3C(14)N-(11)BCl3). These frequencies are consistent with computational predictions for the minimum-energy form of the complex. Energy decomposition analyses were conducted for CH3CN-BCl3 and also two related complexes, CH3CN-BF3 and CH3CN-BH3. These provide insight into the trend in Lewis acidity of the BX3 acceptors toward nitriles. Furthermore, these analyses indicate that the barrier along the B-N potential of CH3CN-BCl3 results from Pauli repulsion between the π electrons on the nitrile moiety and the chlorine atoms in BCl3, which is significant at relatively long distances where attractive bonding interactions fail to overcome it.

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Halogenated aromatics are one of the largest chemical classes of environmental contaminants, and dehalogenation remains one of the most important processes by which these compounds are degraded and detoxified. The thermodynamic constraints of aromatic dehalogenation reactions are thus important for understanding the feasibility of such reactions and the redox conditions necessary for promoting them. Accordingly, the thermochemical properties of the (poly)fluoro-, (poly)chloro-, and (poly)bromobenzenes, including standard enthalpies of formation, bond dissociation enthalpies, free energies of reaction, and the redox potentials of Ar-X/Ar-H couples, were investigated using a validated density functional protocol combined with continuum solvation calculations when appropriate. The results highlight the fact that fluorinated aromatics stand distinct from their chloro- and bromo- counterparts in terms of both their relative thermodynamic stability toward dehalogenation and how different substitution patterns give rise to relevant properties, such as bond strengths and reduction potentials.

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The structural and energetic properties of CH(3)CN-BH(3), HCN-BH(3), FCH(2)CN-BH(3), and F(3)CCN-BH(3) have been examined via density functional theory and post-Hartree-Fock calculations. The B-N distances in these systems are notably short, less than 1.6 Å, and the binding energies are substantial, about 20 kcal/mol. The properties of these systems do vary as a result of the nitrile substituent, but surprisingly, more electronegative substituents result in shorter B-N distances. For example, the B-N distance for F(3)CCN-BH(3) is 1.576 Å via MP2/aug-cc-pVTZ, while that for CH(3)CN-BH(3) is 1.584 Å. However, the binding energies vary as expected, from 17.4 kcal/mol in the case of F(3)CCN-BH(3) to 22.6 kcal/mol for CH(3)CN-BH(3) (via MP2/aug-cc-pVTZ). The extent of charge transfer and the degree of covalent character in the B-N bonds were explored by a natural bond orbital analysis, and the atoms in molecules formalism, respectively, and do provide some rationale for the substituent effects. Frequency calculations indicate that BH(3)-localized vibrational modes do shift appreciably upon complex formation, especially the BH(3) asymmetric stretch. For CH(3)CN-BH(3), experimental and calculated frequency shifts compare well for the asymmetric BH(3) bending mode, but the observed shift for the BH(3) asymmetric stretch, the most structurally sensitive mode, is about 40 cm(-1) larger than the predictions. While this may suggest a very slight contraction of the B-N bond upon formation of solid CH(3)CN-BH(3) (for which experimental data are available) the balance of evidence indicates that no significant medium effects occur in these complexes. We also discuss the distinct differences between these complexes and their BF(3) analogs. The underlying reasons for the markedly different structural properties are illustrated through an energy decomposition analysis applied to HCN-BH(3) and HCN-BF(3). These data indicate that less Pauli repulsion of the electrons on each respective subunit is the most significant factor that favors the overall stability of the BH(3) complex.

RESUMEN

The structure, bonding, and energetic properties of the N(2)-BH(3) complex are reported as characterized by density functional theory (DFT) and post-Hartree-Fock (HF) calculations. The equilibrium structure of the complex exhibits a short B-N distance near 1.6 A, comparable to that of a strong acid-base complex like H(3)N-BH(3). However, the binding energy is only 5.7 kcal/mol at the CCSD(T)/6-311+G(2df,2dp) level of theory, which is reminiscent of a weak, nonbonded complex. Natural bond orbital (NBO) and atoms in molecules (AIM) analyses of the electron density from both DFT and post-HF calculations do indicate that the extent of charge transfer and covalent character in the B-N dative bond is only somewhat less than in comparable systems with fairly large binding energies (e.g., H(3)N-BH(3) and OC-BH(3)). Energy decomposition analysis indicates key differences between the N(2), CO, and NH(3) complexes, primarily associated with the natures of the lone pairs involved (sp vs sp(3)) and the donor/acceptor characteristics of the relevant occupied and virtual orbitals, both sigma and pi. Also, CCSD/6-311+G(2df,2dp) calculations indicate that the B-N distance potential is rather anharmonic and exhibits a flat, shelf-like region ranging from 2.1 to 2.5 A that lies about 1.5 kcal/mol above the minimum at 1.67 A. However, this region is more sloped and lies about 2.5 kcal/mol above the equilibrium region according to the CCSD(T)/6-311+G(2df,2dp)//CCSD/6-311+G(2df,2dp) potential. A 1D analysis of the vibrational motion along the B-N stretching coordinate in the CCSD/6-311+G(2df,2dp) potential indicates that the average B-N distance in the ground vibrational state is 1.71 A, about 0.04 A longer than the equilibrium distance. Furthermore, the vibrationally averaged distance obtained via an analysis of the CCSD(T)/6-311+G(2df,2dp)//CCSD/6-311+G(2df,2dp) potential was found to be 0.03 A longer than the CCSD(T)/6-311+G(2df,2dp) minimum.