B3LYP /aug-cc-pvDZ level of theory is applied to study the geometric structures, hyperconjugation and exchange repulsion of 13 isoelectronic equivalents of benzene obtained by successive replacement of CH groups by nitrogen atoms in benzene. More attention is paid to discussing 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. G3 method is applied to calculate heats of formation. With the increase in the number of nitrogen atoms, heats of formation increase substantially. Hexazine and pentrazine have higher heats of formation in 13 kinds of compounds, which implies that they should be ap-plicable as potential of high nitrogen content high energy materials (HNC-HEMs). 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 rather than repulsive forces that plays a key role in the relative stabilities of isomers.

Key words: benzene and its nitrogen isoelectronic equivalent, density functional theory, energy, heat of formation