Theoretical Studies On Energetic Properties Of Nitrogen Heterocyclic Compounds And Tautomerism Of Cyameluric Acid

At first, B3LYP /aug-cc-pvdz level of theory is applied to study the geometric structures, electronic topologies, heats of formation, hyperconjugation and exchange repulsion of 27 isoelectronic equivalents obtained by successive replacement of CH groups by nitrogen atoms in cyclobutadiene, cyclopentadiene and benzene. It is paid more attention to discuss how the energetic properties change upon the number of N atoms. The results reveal that the total energy linearly decreases along with the replacement of CH group by nitrogen atom for the three systems. G3 method is applied to calculate heats of formation. With the increase in the number of nitrogen atoms, heats of formation increase substantially, the greatest degree of increase is the four-membered nitrogen heterocyclic ring, followed by the six-membered ring, and then with the five-membered ring. Hexazine, tetraazacyclobutadiene, pentrazine and triazacyclobutadiene have higher heats of formation in 27 kinds of compounds, which implies that they should be applicable as potential of high nitrogen content high energy materials (HNC–HEMs). In addition, our studies indicate that the relationship between the total energy or heats of formation of isomers and the position of N atoms is (ortho) meta < (ortho) para < ortho. In addition, NBO analysis shows that it is hyperconjugation, not repulsive forces, play a key role in the relative stabilities of isomers.In the second part, B3LYP /aug-cc-pvdz level of theory, combined with NBO, AIM and G3 methods, is applied to study substituent effects on the geometries, electric structures, bond dissociation energies and high energetic density material...