The MNDO-PM3 study of the mechanism of nucleophilic substitution of the phenoxide anion for the nitro group in 1,3,5-trinitrobenzene and 2,4,6-trinitrotoluene in the gas phase and in polar solvents

The semi-empirical quantum chemical MNDO-PM3 calculations of the enthalpies of formation of Meisenheimerortho- andipso--complexes of 1,3,5-trinitrobenzene (TNB) and 2,4,6-trinitrotoluene (TNT) with the phenoxide anion in the gas phase and in water are performed within the framework of the point dipole model. Based on the calculated heats and activation barriers to substitution of the nitro group by the phenoxyl group in TNB and TNT, the possibility of the reactions of TNB and TNT with the phenoxide anion in water is shown. These reactions in water occurvia the S_NAr mechanism involving the correspondingipso--complex as an intermediate. In the gas phase, the S_NAr mechanism is impossible, because the reaction is strongly endothermic. In the case of TNT, the exothermic reaction of elimination of a proton from the methyl group by the phenoxide anion competes with nucleophilic substitution in a polar solvent. The activation energy calculated for this exothermic reaction is 8 kcal mol^–1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 624–628, April, 1995.     

Efficient electrochemical synthesis of pyrido[1,2-a]benzimidazoles

Electrochemical reduction of N-(2-nitroaryl)pyridinium chlorides in alcohol—dilute HCl mixtures gave pyrido[1,2-a]benzimidazoles in high yields in both divided and undivided cells. According to cyclic voltammetry measurements and DFT calculations (B3LYP/6-31+G(d)), the reaction involves intermediate formation of the corresponding hydroxylamine derivative followed by its heterocyclization.     

Activation of molecular oxygen in trifluoroacetic acid

The interaction of molecular oxygen with aqueous trifluoroacetic acid (TFA) led to an increase in pH. This effect was explained by a decrease in the concentration of the protonated CF₃CO₂H₂⁺ and H₃O⁺ species after oxygen was fed in the reactor. Quantum-chemical calculations show that a radical pair can be formed in an activation-free exothermal reaction involving the radical residue of the acid, the CF₃CO₂H₂⁺...³O₂...CF₃CO₂⁻ peroxide radical, and the acid molecule in the CF₃C₂^·...HOO^· collision complex. It was assumed that the activation of molecular oxygen in aqueous TFA solutions, providing the activity of the system in oxidations of various organic and inorganic substrates, is related to the formation of peroxide radicals in them.     

Quantum-chemical calculations by the MNDO method of the pre-exponential factor for the homolytic decomposition of aliphatic nitro and α-fluoronitro compounds in the gas phase

The MNDO method has been used to calculate the pre-exponential factor A for the homolytic decomposition of aliphatic nitro and -fluoronitro compounds. It is shown that the high value of A is due to the decrease in the frequencies of the deformation vibrations in the activated complexes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 339–344, February, 1990.The authors are grateful to K. Ya. Burshtein for his helpful criticism in discussing this work.     

Proton spin coupling constants in methylcyclopentadienes

A detailed procedure for analysing the ¹H n.m.r. spectra of two isomeric methylcyclopentadienes, two dimethylcyclopentadienes and 3-methylindene is reported. Finite perturbation INDO calculations are used to discuss the conformational features of the long range proton coupling constants.     

Proton spin coupling constants in cycloalkenes

The proton-proton coupling constants in cyclopentadiene, cyclohexadiene, cycloheptatriene, cyclooctatetraene, cyclopentene and cyclohexene have been calculated using the finite perturbation INDO method. The carbon geometry was taken from the corresponding electron diffraction studies. The theory gives satisfactory agreement with experiment. In terms of the many factor structural model, distance and angle distortions have been studied in ethylene, cis-1,3-butadiene and cis-2-butene. It has been established that these distortions produce noticeable changes only for those protons whose co-ordinates are changed. The coupling constants across three, four and five bonds are discussed separately in connection with the structural factors and the σ–π contribution. Finally, variations in the proton co-ordinates in cyclopentadiene are used to optimize the proton couplings.     

Quantum-Chemical Study of Alkyl Carbenium Ions in 100% Sulfuric Acid

A quantum-chemical study of neutral and protonated monoalkyl sulfates RHSO₄and [RH₂SO₄]⁺(where R = CH₃, C₂H₅, iso-C₃H₇, and tert-C₄H₉) is carried out. Calculations are performed using the Hartree–Fock method in the 6-31G** and 6-31++G** basis sets taking into account electron correlation according to the Müller–Plesset perturbation theory MP2/6-31+G*//6-31+G*. Protonated tert-butyl sulfate was also calculated by the DFT B3LYP/6-31++G** method. It was found that monoalkyl sulfates are covalent compounds, and the complete abstraction of alkyl carbenium ions from them has substantial energy cost: 196.4, 161.7, 150.8 and 136.0 kcal/mol, respectively. Protonated methyl and ethyl sulfates are also covalent compounds according to the calculation. They have lower but still high energies of heterolytic dissociation (65.0 and 33.5 kcal/mol, respectively). The energy of R⁺abstraction from protonated isopropyl sulfate is much lower: 23.6 kcal/mol. The main covalent state and the ion–molecular pair, which is a carbenium ion [C(CH₃)₂H]⁺solvated by the H₂SO₄molecule, have about the same energy. The ground state of protonated tert-butyl sulfate corresponds to the ion–molecular complex [C(CH₃)⁺ ₃H₂SO₄] with still lower energy of carbenium ion [C(CH₃)₃]⁺abstraction, which is equal to 10.0 kcal/mol. Calculation according to the DFT B3LYP/6-31++G** method shows the absence of a minimum for the protonated tert-butyl sulfate with a covalent structure on the potential energy surface.