The density functional theory (DFT) and semi-empirical MO-PM3 methods were employed to study the reaction mechanism of the nitration of adamantane with NO₂. The calculated results showed that the H atom in adamantane can not be directly substituted with NO₂. By comparing the potential barriers E_a of the three possible reaction mechanisms at the B3LYP/6-311＋＋G(3df,2pd)//B3LYP/6-31G* level, it was found that the process of N to attack H competed with that of O to attack H during the crucial step, and 1-nitroadamantane was considered to be the main product. In addition, the changes of molecular geometry, atomic natural charge and IR spectra of the reaction system during the crucial step of O to attack H showed that the rupture of C—H bond and the formation of O—H bond were concerted stepwise. Obvious changes of atomic natural charge happened to atoms C(1), H(11), O(28), O(29) and N(27). The bond lengths and bond angles related to these atoms were also varied greatly. The dipole moment changes predicted that polar solvent could decrease reaction potential barrier E_a to facilitate the reaction.

Key words: adamantane, density functional theory, nitration mechanism, potential barrier