A. N. Subbotin

Quantum chemical studies of azoles 4. A novel elimination–addition mechanism of tetrazole and 1,2,4-triazole electrophilic substitution

Thermodynamic parameters of the addition–elimination and elimination–addition electrophilic substitution reactions of 1H-tetrazole and 1,2,4-1H-triazole obtained from DFT B3LYP/ 6-31G(d,p) quantum chemical calculations with proton as model electrophile are compared. According to calculations, the elimination–addition reactions can proceed without preliminary formation of N-protonated azolium salts.

Quantum chemical studies of azoles 5. Electrophilic substitution in tetrazole and 1, 2, 4-triazole: the influence of basis set on calculated thermodynamic parameters of the elimination—addition mechanism without preliminary formation of N-protonated azolium salts

Thermodynamic parameters of electrophilic substitution reactions of 1H-tetrazole and 1H-1, 2, 4-triazole proceeding by the addition–elimination and elimination–addition mechanisms were calculated by the DFT/B3LYP/6-31G(2df, p) method using proton as model electrophile and compared. The results obtained substantiate that the elimination–addition mechanism may not involve preliminary formation of N-protonated azolium salts, as was shown earlier in our DFT/B3LYP/6-31G(d, p) calculations.

Quantum chemical studies of azoles 3. Thermodynamic stabilities of neutral molecules and intermediates formed in electrophilic substitution of azoles containing three or four heteroatoms

Density functional theory quantum chemical calculations of thermodynamic stabilities in the gas phase and in water were carried out for 1,3,4-oxadiazole and 1,3,4-thiadiazole, 1,2,4-1H-triazole and 1,2,4-4H-triazole, 1,2,3,4-1H-tetrazole and 1,2,3,4-2H-tetrazole molecules, and for cationic and bipolar (carbenoid) intermediates formed by these molecules in electrophilic substitution reactions (with proton as model electrophile) and the results obtained are compared. Differences in the chemical behavior of pairs of isomeric 1H- and 4H-1,2,4-triazoles and 1H- and 2H-tetrazoles are analyzed.

Quantum chemical studies of azoles 6. The effect of specific solvation on the calculated thermodynamic parameters of electrophilic substitution in tetrazole according to the elimination—addition scheme without preliminary formation of N -protonated azolium salts

Quantum chemical calculations (DFT/B3LYP/6-31G(d)) considering specific solvation effects were used to compare the thermodynamic parameters of electrophilic substitution reactions (with the hydroxonium ion as a model electrophile) in 1H-tetrazole according to the addition—elimination and elimination—addition schemes. The latter scheme can proceed without preliminary formation of N-protonated azolium salts, as demonstrated earlier by the DFT/ B3LYP/6-31G(d,p) and DFT/B3LYP/6-31G(2df,p) calculations considering the solvation effects in aqueous solution in terms of the polarizable continuum model (PCM) with a proton as a model electrophile.

Quantum chemical studies of azoles 8. Effect of N(2)-methyl substituent on the calculated thermodynamic parameters of electrophilic substitution in tetrazole according to the elimination–addition scheme without preceding formation of N-protonated azolium salts

Thermodynamic characteristics of electrophilic substitution reactions of 2-methyltetrazole and 2H-tetrazole proceeding according to the elimination–addition pathway (with the HO– anion and hydroxonium ion as model agents) were compared on the basis of analysis of the results obtained by quantum chemical calculation performed using the DFT/B3LYP/6-31G(d,p) method and taking account of the specific solvation effects. Comparison with previous results for 1-methyltetrazole obtained by the same authors using the same method revealed probable reasons for the known lack of ability of 2-methyltetrazole to undergo electrophilic substitution reactions, particularly, the substantially lower polarity and HOMO and LUMO energies, that is, higher Mulliken electronegativity of 2-methyltetrazole compared with the 1-isomer.

Quantum chemical studies of azoles 7. N-alkyl substituent effect on calculated thermodynamic parameters of the electrophilic substitution mechanism in 1H-tetrazole via elimination–addition scheme without preceding formation of N-protonated azolium salts

Thermodynamic characteristics of electrophilic substitution reactions in 1-methyltetrazole and 1H-tetrazole via the elimination–addition scheme (HO– anion and hydroxonium ion as model agents) were compared by analysis of the quantum chemical calculation results performed using the DFT/B3LYP/6-31G(d,p) method taking into account specific solvation effects. The possibility for both reactions to occur without the preceding formation of N-protonated azolium salts was shown. This possibility has earlier been demonstrated for 1H-tetrazole using the DFT/B3LYP/6-31G(d,p) and DFT/B3LYP/6-31G(2df,p) methods taking into account the solvation effects in an aqueous solution in terms of polarized continuum model when proton was chosen as a model electrophile.

On the phenomena of temperature hysteresis in hydrogenation reactions over heterogeneous catalysts

For the first time the phenomenon of a temperature hysteresis in the exothermic heterogeneous catalytic reactions of CO hydrogenation (methanation) and propene is discussed; the assumption is made, that the reason of hysteresis is the local overheating of the active centers caused by the complicated dissipation of the released energy through the dispersed particles of the catalyst.

Quantum chemical studies of azoles 9. Tetrazole halogenation according to the elimination—addition scheme without preliminary formation of N-protonated azolium salts

Thermodynamic characteristics of electrophilic substitution reactions of 1H-tetrazole and 2H-tetrazole proceeding by the elimination—addition scheme with F+, Cl+, and Br+ as model cations were compared using the results of DFT/B3LYP/6-31G(d,p) quantum chemical calculations carried out with inclusion of specific solvation effects. Possible reasons for lower reactivity (based on the results of calculations) of 2H-tetrazole compared to that of 1H-isomer are discussed.

Temperature Hysteresis in CO Oxidation on Copper Oxide Catalyst Applied to a Steel Gauze

The temperature hysteresis during the heterogeneous catalysis was studied by an example of model reaction of CO oxidation on copper oxide catalyst applied to a stainless steel gauze.

Quantum chemical studies of azoles 10. Transition states in the routes of electrophilic substitution in 1H-tetrazole via the elimination—addition mechanism without preliminary formation of N-protonated azolium salts

The results of theoretical search for model transition states of electrophilic substitution in 1H-tetrazole (1) without preceding formation of N-protonated azolium salts are presented. Two routes of the reaction were proposed: A, attack of molecule 1 by the nucleophile HO–(aq)) to form the anion to which the electrophile H3O+(aq)) is added further; and B, attack of molecule 1 by the same electrophile with the subsequent addition of the same nucleophile to the specifically solvated protonated species. The thermodynamic parameters were calculated earlier at the indicated routes. In this article, the kinetic characteristics of the reactions were estimated by the DFT/B3LYP/6-31G(d) method using the scanning pro-cedure of the potential energy surface. Both steps of route A turned out to be barrier-less, while in route B only its first step is barrier-less and the second step is conjugated with surmounting an activation barrier of ~35 kcal mol–1 between the formed prereaction complex and the products of electrophilic substitution.