Theoretical insight into dihydrogen activation with β-diketiminato ligand supported Group 13 and 14 elements: mechanism and activity difference

On the basis of density functional theory (DFT) calculations within the framework of the ONIOM method, here we report a new bimolecular mechanism for understating dihydrogen activation using the β-diketiminate...

Substituição nucleofílica alifática: qual o mecanismo preferencial? Estudo computacional dos efeitos da estrutura do substrato e solvente

Nucleophilic aliphatic substitution reactions constitute important steps in the synthesis of substances with biological activity and industrial appeal, beyond to participating in steps in biosynthetic routes of natural products. Unimolecular (SN1) and bimolecular (SN2) pathways can be understood as limiting cases of a mechanistic continuum. In between them, borderline mechanisms are proposed. The preference for one path over another depends on several factors, such as the structure of the substrate, the nucleophile and the solvent used. This plurality is still a topic of discussion and needs further understanding. In this context, the present work aims to rationalize the preferential reaction pathway for nucleophilic aliphatic substitutions, whose substrates do not fit only in the uni- and bimolecular models, by identifying lower energy reaction pathways due to the structural and electronic characteristics. The evaluation was carried out by molecular modeling at the Density Functional Theory (DFT) level, simulating substrates with the nucleofuge (Cl and NH3 + ) connected to secondary carbon atoms, with the computational method M06-2X/aug-cc-pVTZ, previously validated according to geometrical and energetic parameters. Besides, we checked the effect of a polar solvent with high dielectric constant in the reaction pathways. The analyzed substrates demonstrated preference for the bimolecular mechanism and the influence of a solvent in these reactions was evident.

Thermokinetic study of the formation of carbon monoxide in the dimerization of isocyanic acid

<p>The mechanism of the dimerization of HNCO was investigated using the ab initio method. The barrier heights, reaction energies and geometrical parameters of the reactants, products, intermediates and transition state were studied. It was found that there are possibly three reaction channels. The B3LYP 6-311++G(3df,2p)//CCSD(T) 6-311++G(d,p) calculated barriers show that reaction of formation of four-membered ring O=CNHNHC=O is more favorable than formations of CO₂ or CO. The reaction pathway of the formation of carbon monoxide from the dimerization of isocyanic acid is a new pathway. Its rate constant is very small, k = 2.62´ 10^-19 cm³/molecule/s at 2500K, 1atm, which is about 2000 times as small as rate constant of formation CO from a bimolecular mechanism of HNCO with Ar.</p>

Toxicophores and Quantitative Structure -Toxicity Relationships for Some Environmental Pollutants

The electron-conformational (EC) method is employed to reveal the toxicophore and to predict aquatic toxicity quantitatively using as a training set a series of 51 compounds that have aquatic toxicity to fish. By performing conformational analysis (optimization of geometries of the low-energy conformers by the PM3 method) and electronic structure calculations (by ab initio method corrected within the SM54/PM3 solvatation model), the Electron-Conformational Matrix of Congruity (ECMC) was constructed for each conformation of these compounds. The toxicophore defined as the EC sub-matrix of activity (ECSA), a sub-matrix with matrix elements common to all the active compounds under consideration within minimal tolerances, is determined by an iterative procedure of comparison of their ECMC’s, gradually minimizing the tolerances. Starting with only the four most toxic compounds, their ECSA (toxicophore) was found to consists of a 4x4 matrix (four sites with certain electronic and topologic characteristics) which was shown to be present in 17 most active compounds. A structure-toxicity correlation between three toxicophore parameters and the activities of these 17 compounds with R2=0.94 was found. It is shown that the same toxicophore with larger tolerances satisfies the compounds with les activity, thus explicitly demonstrating how the activity is controlled by the tolerances quantitatively and which atoms (sites) are most flexible in this respect. This allows for getting slightly different toxicophores for different levels of activity. For some active compounds that have no toxicophore a bimolecular mechanism of activity is suggested. Distinguished from other QSAR methods, no arbitrary descriptors and no statistics are involved in this EC structure-activity investigation.

Kinetics of macroradicals propagation and decay in polymeric matrix of 1,6-hexanediol diacrylate

Dependence of acrylate macroradicals concentration in polymeric matrix of 1,6-hexanediol diacrylate on time within temperature range of 45-80С has been investigated by means of ESR-spectroscopy. It has been established that experimental data do not linearize in coordinates of equation of the second order reaction, what coordinates with bimolecular mechanism of macroradicals decay. It has been suggested, that the effect of decreasing of the rate constant of bimolecular macroradicals decay in time is related with their propagation in chain propagation reaction. Kinetic equation obtained on this basis satisfactorily describes experimental data, what allowed to estimate quantitatively such parameters as the rate constants of chain propagation an termination in polymeric matrix, diffusion coefficient of macroradicals and characteristic time of segmental movement.