Search

The mutual interconversion and decomposition reactions of four tetrazole isomers (1H-TZ, 2H-TZ, 5H-TZ, and an N-heterocyclic carbene 14H) have been studied theoretically using the W1 high-level procedure. Computations allowed resolution of the existing discrepancies in the mechanism and key intermediates of TZ thermolysis. The tautomeric equilibria between 1H-TZ, 2H-TZ, and 14H turned out to play a very important role in the mechanism of thermal decomposition. Although the barriers of monomolecular tautomeric transformations were found to be high (∼50−70 kcal/mol), the concerted double H atom transfer reactions in the H-bonded complexes of TZ tautomers have profoundly lower barriers (∼18−28 kcal/mol). These reactions lead to fast interconversion between 1H-TZ, 2H-TZ, and 14H. The carbene 14H has never been considered before; however, it was predicted to be a key intermediate in the mechanism of thermal decomposition of TZ. For all species considered, the unimolecular reactions of N2elimination were predicted to dominate over the elimination of hydrazoic acid. In agreement with existing experimental data, the effective activation energy of thermolysis was calculated to be 36.2 kcal/mol. [ABSTRACT FROM AUTHOR]

The primary reactions of the thermal decomposition of hydrazinium nitroformate (HNF) as well as ammonium nitroformate (ANF) were investigated theoretically using the G3 multilevel procedure. Calculations were performed for the reactions in the gas phase and in the melt using a simplified model of the latter. The influence of the melt on the reaction barriers was taken into account by calculation of the solvation free energies using a PCM model. In contrast with many other energetic salts, the ionic salt structures of the HNF and ANF were found to be minima on the PES. However, the most energetically favorable structures of the HNF and ANF in the gas phase are H-bonded complexes. In the melt, on the contrary, the ionic structure is lowest in free energy because of solvation effects. In both the gas phase and melt, the HNF decomposes preferably to nitroform and hydrazine. This fact agrees well with the experimentally observed absence of HNF among the gas-phase decomposition products. Thermolysis of HNF occurs mainly through the intermediacy of nitroform; computations do not support the suggestion that the aci-nitroform is an important intermediate of HNF thermolysis. The primary dissociation reactions of ANF resemble those of the HNF. [ABSTRACT FROM AUTHOR]

The thermal decomposition of 5-aminotetrazole was studied theoretically using the G3 multilevel procedure and DFT B3LYP technique. The unimolecular primary decomposition reactions of the three most stable isomers of 5-ATZ were studied in the gas phase and in the melt using a simplified model of the latter. The influence of the melt on the elementary reaction barrier was taken into account by the calculation of the solvation free energies using the PCM model. In contrast to all previous publications, we considered the bimolecular reactions of 5-ATZ and demonstrated that they are very important especially in the condensed phase. It was found that the imino form undergoes fast isomerization to the amino form in the H-bonded dimers and does not participate in the 5-ATZ thermolysis. On the contrary, amino and, probably, the 2H isomer are the main isomers of 5-ATZ in the melt and gas phase. The N2elimination reaction was found to be the dominant unimolecular channel of the amino and 2H isomer decomposition in both the gas phase and melt. The significant lowering of the activation barriers of decomposition reactions in H-bonded dimers was found. In agreement with the existing experimental data, HN3elimination dominates for some of the considered complexes. It was concluded that the initial stages of thermolysis of 5-ATZ cannot be satisfactory described by the simple unimolecular reactions proposed in the literature. [ABSTRACT FROM AUTHOR]

The gas phase enthalpies of formation of mono-, di-, tri-, tetranitromethane and nitroethane, as well as of their nitrite and aci-form isomers were calculated using different multilevel (G2, G3, G2M(CC5)) and density functional theory (DFT)-based (B3LYP, MPW1B95 and MPWB1K) techniques. The enthalpies of the C−N bond dissociation and isomerization of these nitroalkanes were also calculated. The calculated values of the formation and reaction enthalpies were compared with the experimental data when these data were available. It was found that only the G3 procedure gave accurate (within 1 kcal/mol) results for the formation enthalpy of nitroalkanes, their isomers, and radical products. The G3 procedure and two new hybrid meta DFT methods proposed by Truhlar’s group ( Zhao, Y.; Truhlar, D.J. Phys. Chem. A2004, 108, 6908) showed good results for the reaction enthalpies of the nitromethane isomerization and the C−N bond dissociation. Our calculation results were used to analyze thermodynamics of the dissociation and isomerization reactions of the poly nitro-substituted methanes. [ABSTRACT FROM AUTHOR]

ThepH-dependent kinetics of C–ON bond homolysis reactionsof the four alkoxyamines (viz., the derivatives of 2-(4-(dimethylamino)-2-ethyl-5,5-dimethyl-2-(pyridin-4-yl)-2,5-dihydro-1H-imidazol-1-oxyl and 2-(2-carboxyethyl)-5,5-diethyl-2,4-dimethyl-2,5-dihydro-1H-imidazol-1-oxyl) in water solution have been scrutinizedusing DFT calculations (M06-2X/6-311++G(2df,p) level of theory withthe PCM model). On the basis of computations, the experimental resultsobtained before (J. Org. Chem.2011, 76, 5558) have been rationalized. The concentration dependenceof all possible protonated forms of the four alkoxyamines was obtainedfrom pH measurements. The contributions of particular protonated formsinto the overall rate constants of thermolysis were estimated usingthe DFT calculated Gibbs free energies Δr0Gof C–ON bond homolysis reactions. The computations revealedthat the observed rate constants of thermolysis of the two speciesat several pH values are dominated by decomposition reactions of twoor even three protonated forms. The observed trends in reactivityof the alkoxyamines were mainly attributed to destabilization of theradical products of C–ON bond scission reactions. A linearcorrelation between the sum of radical stabilization energies (RSEs)of products of thermolysis and the calculated Gibbs free energiesof reactions was found for various protonated forms of the speciesstudied. Apart from this, the linear correlation exists between therelative RSE and nitrogen hyperfine constants aNof various protonated forms of the nitroxide radical products. [ABSTRACT FROM AUTHOR]