I. D. Nesterov

Quantum chemical studies of azoles 2. Thermodynamic stability of neutral molecules and intermediates formed during electrophilic substitution of 1,2- and 1,3-azoles

The thermodynamic stability of 1,2- and 1,3-azole molecules, as well as of cationic and bipolar (carbenoid) intermediates in the gas phase and in aqueous solution formed by electrophilic substitution (proton as a model electrophile) was compared based on the analysis of quantum chemical calculations performed at the DFT/B3LYP/6-31G(D) level of theory with zero-point energy corrections. The differences in the chemical behavior of the isomeric 1,2- and 1,3-azole pairs, viz., pyrazole—imidazole, isoxazole—oxazole, and isothiazole—thiazole, were considered.

Quantum-Chemical Investigation of Azoles 1. Alternative Electrophilic Substitution Mechanisms in 1,2- and 1,3-Azoles*

Quantum-chemical calculations were performed for the molecular structures of 1,2-azoles (pyrazole, isoxazole, isothiazole), 1,3-azoles (imidazole, oxazole, thiazole), and the corresponding intermediates of electrophilic substitution reactions (with protons as the model electrophiles): azolium ions, bipolar ions (ylides/carbenes), cationic σ-complexes, as well as activation energy values were calculated for the decomposition of ylides. The calculations were performed for gas phase and aqueous solutions according to the B3LYP method in a 6-31G(d) basis set, with corrections for the zero-point vibration energy. The solvation effects were taken into account by using the overlapping spheres model (IEFPCM). The results of the calculations explained some features of electrophilic substitution in azoles according to two alternative mechanisms: the classical addition-elimination with cationic σ-complex intermediates, and the mechanism of elimination-addition that involves ylides (carbenes) as key intermediates.

Evolution of cluster models in quantum chemical analysis of spectroscopic characteristics of surface structures on oxides at the IOC RAS

The author’s review integrates and analyzes the results of quantum chemical studies on a common topic carried out at the Institute of Organic Chemistry of the RAS (IOC RAS). The cluster approaches to quantum chemical analysis of adsorption and heterogeneous catalysis processes on various oxides proposed and developed at the IOC RAS are characterized. The approaches comprise the construction of covalent and also neutral and partially charged ionic cluster models of surface active sites and their complexes with molecules and free radicals. Examples of chemically acceptable cluster models specially designed for the review and calculated by density functional theory (DFT) with the B3LYP functional and the 6-311G(d) basis set clearly demonstrate the efficiency of using such models to retrieve structure-chemical information from radiospectroscopic, ultraviolet, and infrared spectra of simple molecules and free radicals adsorbed on oxide systems that are widely used in practice.

Quantum chemical studies of sulfonation of pyrrole with pyridine sulfur trioxide

In the framework of the density functional theory (B3LYP/6-31G(d) method), a detailed quantum chemical study of sulfonation of pyrrole with pyridine sulfur trioxide in a dichloroethane medium was performed with allowance for its influence on the energy characteristics of the reaction at the IEFPCM level. A direct transfer of the electrophile SO3 from pyridine to pyrrole was studied as an example of the formation of the intermediate σ-complexes from the starting compounds. A possibility of preliminary dissociation of pyridine sulfur trioxide was considered. The similarity of energy barriers on the way to isomeric σ-complexes was due to the closeness of early transition states of the SO3 molecule addition to the α- or β-C atom of the pyrrole molecule to the starting compounds on the reaction coordinate. At the first step of the reaction, the latter should form with approximately equal rates and then be subjected to rapid deprotonation involving pyridine, which leads to the reaction products. Comparatively slow interconversions are also possible. Under these conditions, the formation of the pyridinium salts of α- and β-pyrrolesulfonic acids can be reversible. The α-isomer of the product is kinetically less stable, which is favorable for the predominance of the β-isomer in the reaction mixture.