Density Functional Theory Studies Of High-energetic Nitrogen Aromatic Compounds

Theoretical studies with density functional theory(DFT) method were performed on high-energy heterocyclic aromatic compounds.Computations were performed on a series of compounds including pyrazole derivatives,2,2'-bi-1H-imidazole derivatives and azido-triazine,azido-tetrazine.Fully optimized structures were obtained.The isodesmic reactions were designed to get the heats of formation and the disproportionation energies. And the bond dissociation energies(BDE) for the weaker bonds were computed to evaluate the sensitivities and stabilities of the title compounds.In the first section,computations were performed on a series of pyrazole derivatives using B3LYP and B3P86 functionals with aug-cc-pVDZ and 6-311++G^** basis sets.The general trend is that the HOFs increase with increasing number of-NH₂ and -N₃ groups.And the HOFs initially decrease then increase as the number-NO₂ and -NF₂ groups increasing. The HOF of 3,5-didifluoroaminopyrazole is the smallest(134.4 kJ/mol),and the HOF of 3,4,5-triazidopyrazole is the largest(1240.6 kJ/mol).The values of HOFs decrease as the -NO₂ groups being replaced by the -NH₂ and -NF₂,but increase dramatically as the -NO₂ groups being replaced by -N₃ with the average increment of 332.0 kJ/mol.The relative stability of the title compounds was evaluated based on the calculated HOFs and the energy gaps between the frontier orbits.The value of C-N₃ bond dissociation energy,524.5 kJ/mol in average,is relatively larger than other out-of-ring C-N bonds.The BDE_ZPE value of C-NO₂ bond is smaller than all the other C-N bonds,which is 270.7 kJ/mol in average.In the second section,calculations were performed for a series of 2,2'-bi-1H-imidazole derivatives using he B3LYP and B3P86 functionals with 6-311G^** basis set.Calculated results show that the HOFs decrease as the -NO₂ groups being replaced by -NF₂,but the HOFs increase as the -NO₂ groups being replaced by -N₃.When the -NO₂ groups are replaced by -NH₂,the HOFs initially decrease then increase.As for the isomers,the HOFs decrease following the increase of the number of hydrogen bonds for the different substituent position.The magnitudes of intramolecular group interactions were predicted through the disproportionation energies.The E_{disproportion} of 4,4'-diamino-5,5'-dinitro-2,2'-bi-1H-imidazole is -70.3 kJ/mol and is the smallest among the title compounds,and that of 4,4',5,5'-tetranitro-2,2'-bi-1H-imidazole is the largest,which is 128.1kJ/mol.Thermal stabilities were evaluated via bond dissociation energies(BDE) at the UB3LYP/6-311G^** level.The BDE_ZPE value for C-NO₂ bond,270.0 kJ/mol in average,is the smallest compared to other types of bonds.In the third section,density functional theory(DFT) calculations were performed for study the azido-tetrazolo tautomerizations.The geometries of reactant,products and transition states are initially optimized at the B3LYP/6-311G^** level.Frequency calculations were performed at the same level to verify that the tautomers and the transition state(with one and only one imaginary frequency) are local minimum and first saddle point on its potential energy surface.Intrinsic reaction coordinate(IRC) calculations were performed at the B3LYP/6-311G^** level to confirm that the transition state connects the designated intermediates.From the DiAT to DiAT(a) and DiAY(a) to DiAT(b) tautomerizations,the barriers are 100.5 kJ/mol and 117.4 kJ/mol,respectively.For the TAT to TAT(a),TAT(a) to TAT(b) and TAT(b) to TAT(c) conversions,the barriers are 101.8 kJ/mol,99.7 kJ/mol,108.7 kJ/mol,respectively.The heats of formation(HOFs) were predicted via designed isodesmic reactions.With the transformation from azido to tetrazolo,the HOF increase with the different increment.The solvent effects were theoretically investigated with the self consistent reaction field.