A. G. Shamov

Theoretical Study of the Mechanism of the Nitro-Nitrite Rearrangement and Its Role in Gas-Phase Monomolecular Decomposition of C-Nitro Compounds

Semiempirical, ab initio, and density functional theory calculations were used to study the primary act of gas-phase monomolecular decomposition of certain C-nitro compounds and their radical cations, associated with the nitro-nitrile rearrangement. It was shown that the reaction fails to occur with all neutral molecules of aliphatic nitro compounds (except for fluoronitromethane and fluoronitroethene) and has a much lower barrier with the corresponding radical cations. An important role of the nitro-nitrile rearrangement in gas-phase decomposition of aromatic nitro compounds was demonstrated.

Effect of the molecular structure on the strength of the C-NO2 bond in a series of monofunctional nitrobenzene derivatives

Geometric parameters and formation enthalpy and the enthalpy of the radicals formed during the homolytic breakage of C-NO2 bond in 37 aromatic nitro compounds were calculated using different bases of the hybrid density functional method B3LYP, as well as the composite CBS-QB3 methods. On the basis of thermochemical data, were calculated the C-NO2 bond dissociation energy and the activation energy of the radical gas-phase decomposition. Donor substituents were shown to cause an increase in the C-NO2 bond dissociation energy, while the acceptors decrease it. The values of activation energies of gas-phase decomposition of aromatic nitro compounds calculated basing on the C-NO2 bond dissociation energy are in good agreement with experiment.

Theoretical Study of the Tautomeric Reactions of Dinitromethane and Its Radical Cation

Reactions of aci-form and diaci-form formation in dinitromethane and its radical cation have been theoretically studied at DFT B3LYP level of theory with 6-31G(d) basis set. The lowest energy structures of the dinitromethane aci-form and diaci-form were optimized. Analogous theoretical study was carried out for dinitromethane radical cation. In connection with observed conformation transitions in aci- and diaci-form, B3LYP level of theory with the 6-31G(d) basis set was used to investigate the relevant parts of dinitromethane and its radical cation ground state potential energy surfaces.

Secondary processes in the mechanism of gas-phase monomolecular destruction of o-nitrotoluene

A density functional theory method, B3LYP/6-31+G(2df,p), has been used to study the secondary processes of the most probable mechanism of gas-phase thermal decomposition of o-nitrotoluene (accompanied with formation of the aci-form at the first stage). The secondary processes of isomerization of aci-nitrotoluene have been recognized as the rate-limiting steps. This is consistent with results of earlier theoretical studies. The results on isomerization of the HONO group in the aci-form of o-nitrotoluene reported herein have substantially corrected the earlier suggested mechanism of thermal degradation of o-nitrotoluene.

Computational Study of Main Mechanisms for Gas-Phase Decomposition of 1,1- and 1,2-Dinitroethane

The gas-phase enthalpies of formation of 1,1- and 1,2-dinitroethane and corresponding radical products were calculated using G3B3, CBS-QB3 composite methods and DFT B3LYP level of theory with various basis sets. The enthalpies of the C–N, C–C bonds dissociation and activation enthalpies for HONO elimination were also calculated and compared with available experimental data. It was found that G3B3 calculations do provide a reasonable way to tackle the problem of the decomposition channels of 1,1- and 1,2-dinitroethane. Four main mechanisms for gas-phase decomposition of 1,1- and 1,2-dinitroethane were studied using G3B3 model chemistry. HONO elimination seems to be the most favorable mechanism for the decomposition of 1,2-dinitroethane. However, the difference in energies of the HONO elimination and C–N homolytic bond cleavage in 1,1-dinitroethane does not allow to favor any of these channels, especially at the working temperature. Gauche conformation of 1,2-dinitroethane is calculated to be the lowest-energy minimum.

Primary steps of the mechanism of gas-phase monomolecular decomposition of nitropropenes, according to results of quantum-chemical calculations (Review)

The mechanisms of gas-phase monomolecular decomposition of cis- and trans-nitropropenes, 2-nitro-1-propene, and 2-methyl-1-nitro-1-propene were examined by DFT B3LYP/6-31G(d) calculations using GAUSSIAN’98 program package. The most probable pathway of thermal decomposition of these compounds involves formation in the primary step of four-membered cyclic intermediates, substituted oxazetes. For cis-nitropropene and 2-methyl-1-nitro-1-propene, the mechanism whose primary step is 1,5-sigmatropic hydrogen shift from the CH3 group to the NO2 group is principally possible.

Mechanism of formation and monomolecular decomposition of aci-nitromethanes: a quantum-chemical study

A quantum-chemical study of the reactions of formation of aci-nitromethane (aci-NM) and aci-dinitromethane (aci-DNM) and their decomposition with elimination of water was carried out. The methods employed were the ab initio RHF method with inclusion of electron correlation at the MP2 level of theory and the Dunning—Hay double zeta basis set augmented with polarization d-functions on heavy-element atoms, the DFT approach at the B3LYP level, and the semiempirical PM3 method. The formation of aci-NM and aci-DNM was found to be the limiting stage of the mechanism under study. For DNM, the barrier to reaction is substantially lower than for NM. The estimates of the heights of the barriers to formation found from density functional calculations at the B3LYP/6-311++G(df,p) level (258 kJ mol–1 for aci-NM and 218.5 kJ mol–1 for aci-DNM) are thought to be the most reliable.

Some peculiarities of the influence of molecular structure on the activation energy of radical gas-phase decomposition of aliphatic nitro compounds

Geometric parameters of nitro and fluoronitro derivatives of nitromethane, nitroethane, I-nitropropane, and I-nitrobutane (for which experimental data on kinetics of radical gas-phase decomposition are available) have been determined by the MINDO/3 method. Correlations between changes in logarithm of the activation energy and those in the lengths of C-N and C-H bonds as well as in the ionization potentials have been found. The major changes in the C-N- and C-H- bonds occur when bulky atoms and groups (N02, Cl, Br, and 1) are introduced into the molecules. Nonempirical calculations of energies of dissociation of the C-N bonds in the molecules of nitromethane and its halogen derivatives have been carried out. An equation was proposed which allows one to perform a high-accuracy determination of the activation energy for radical gas-phase decomposition of nitroalkanes using the coefficients of steric interactions in the molecules calculated by the methods of molecular mechanics.

Quantum-chemical study of methane dehydrogenation on neutral, cationic, and anionic clusters Pt2–5 by DFT method

Gas phase reaction of platinum clusters with small organic molecules exhibit unusual dependence of reactivity on the size and charge of the cluster [1]. The experimental data [2, 3] indicate that the Pt2–5 clusters have the greatest activity in the reaction of the methane activation. The studies we carried out [4] have revealed the isomers of Pt2–5 clusters with a maximum binding energy and the corresponding multiplicity value. For the neutral Pt2–5 clusters, the lowest energy show the triplets, except for a trigonal bipyramid [Pt5(B)], whose multiplicity value is 5. The multiplicities 2 and 4 correspond to the optimal structures of the charged clusters Pt3 and Pt4, respectively. DOI: 10.1134/S1070363211040311

Alternative mechanisms of thermal decomposition of o-nitrotoluene in the gas phase

The density functional theory methods were used to demonstrate that during the thermal decomposition of o-nitrotoluene, with the formation of 5-methylene-6-aci-nitrocyclohexa- 1,3-diene (aci-form) being the primary event, the rotation of the =N(O)OH group around the CN double bond in the aci-form is of key importance. The activation enthalpy is lower for this step than for the alternative process of H atom transfer between the O atoms in this group. This accounts for the competitive formation of the experimentally observed products of о-nitrotoluene thermal decomposition, namely, the hydroxyl radical and water. The activation barriers of the reactions were estimated over a broad temperature range, which indicated the possible contribution of о-nitrotoluene thermal decomposition and other alternative primary event mechanisms (nitro—nitrite rearrangement, bicyclization) to the efficient rate constant. The results account for the differences between the activation parameters experimentally determined at various temperatures by different authors.