A density functional study of substituted pyrazole derivatives

Computations by density functional theory (DFT) method were performed on a series of pyrazole derivatives. The heats of formation (HOFs) were predicted using B3LYP and B3P86 functionals with aug-cc-pVDZ and 6-311++G∗∗ basis sets via designed isodesmic reactions. In the isodesmic reactions the pyrazole was chosen as a reference compound. 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) at the B3LYP/aug-cc-pVDZ level. The position of the group also affects the HOFs. Judged by the HOF values, the 4- and 3-monosubstituted isomers are the most stable monosubstituted isomers when the electron pulling and pushing groups are attached to the pyrazole ring, respectively. The 3,5-isomers are the most stable di-substituted isomers. 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. Thermal stabilities were evaluated via bond dissociation energies (BDE) at the UB3LYP/aug-cc-pVDZ level. 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 BDEZPE value of C-NO2 bond is smaller than all the other C-N bonds, which is 270.7 kJ/mol in average. These results provide basic information for the molecular design of novel high energetic density materials.