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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]

The primary photophysical and photochemical processes in the photochemistry of 1-acetoxy-2-methoxyanthraquinone (1a) were studied using femtosecond transient absorption spectroscopy. Excitation of 1aat 270 nm results in the population of a set of highly excited singlet states. Internal conversion to the lowest singlet n excited state, followed by an intramolecular vibrational energy redistribution (IVR) process, proceeds with a time constant of 150 ± 90 fs. The 1n excited state undergoes very fast intersystem crossing (ISC, 11 ± 1 ps) to form the lowest triplet excited state which contains excess vibrational energy. The vibrational cooling occurs somewhat faster (4 ± 1 ps) than ISC. The primary photochemical process, migration of acetoxy group, proceeds on the triplet potential energy surface with a time constant of 220 ± 30 ps. The transient absorption spectra of the lowest singlet and triplet excited states of 1a, as well as the triplet excited state of the product, 9-acetoxy-2-methoxy-1,10-anthraquinone (2a), were detected. The assignments of the transient absorption spectra were supported by time-dependent DFT calculations of the UV−vis spectra of the proposed intermediates. All of the stationary points for acyl group migration on the triplet and ground state singlet potential energy surfaces were localized, and the influence of the acyl group substitution on the rate constants of the photochemical and thermal processes was analyzed. [ABSTRACT FROM AUTHOR]

Photolysis of 1,3,2,4-benzodithiadiazine (1) at ambient temperature yields stable 1,2,3-benzodithiazolyl radicals. In order to reveal the mechanism of this unusual transformation, the photochemistry of 1was studied in argon matrices using IR and UV−vis spectroscopy. A series of intermediates, including four- and five-membered heterocyclic and o-quinoid acyclic species, were characterized spectroscopically with the help of quantum chemical calculations. With selective irradiation, these intermediates can be mutually interconverted as well as converted back to the starting compound 1. [ABSTRACT FROM AUTHOR]

The degenerate electron exchange (DEE) reaction involving radical cations (RCs) of n-nonane, n-dodecane, and n-hexadecane in n-hexane solution was studied over the temperature range 253-313 K using the method of time-resolved magnetic field effect in recombination fluorescence of spin-correlated radical ion pairs. In the dilute solutions the rate constant of DEE was found to be 200 times slower than the diffusion limit. Using n-nonane as an example, we showed that two reasons are responsible for the low value of the RC self-exchange rate:? (1) conformational variability of molecules and RCs and (2) the activation barrier of DEE reaction. The calculations of the reaction enthalpy performed by the B3LYP/6-31G(d) method indicated that electron transfer can be effective only upon collision of RC with a neutral molecule either in the all-trans conformation or in the conformation differing from the latter by rotation of the end ethyl fragment. The activation barrier of the DEE reaction was estimated using the reorganization energy of the internal degrees of freedom calculated at the B3LYP level and was found to be about 6 kcal/mol. A possible influence of the interaction between RC and a neutral molecule in an encounter complex on DEE rate constant is also discussed. [ABSTRACT FROM AUTHOR]