Theoretical characterization of tetrahedral N4

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Abstract
The tetrahedral (Td) form of the N4 molecule has been investigated via ab initio quantum mechanical methods. The lowest triplet state of N4 in the tetrahedral region of the potential energy surface, and the barrier separating TdN4 from two N2 molecules have also been examined. Both the transition state and 3A‘ state have Cs symmetry. The singlet state of tetrahedral N4 is predicted to lie 13 kcal/mol below the 3A‘ state. The energy difference between TdN4 and two N2 molecules is accurately determined to be 186 kcal/mol while the barrier separating these species is found to be about 61 kcal/mol. The possibility that TdN4 may be used as a high energy density fuel is investigated and discussed. In addition, a highly accurate determination of the equilibrium structure, harmonic vibrational frequencies, and infrared intensities of TdN4 has been performed using large atomic natural orbital (ANO) basis sets in conjunction with the coupled‐cluster single and double (CCSD) excitation level of theory and the CCSD(T) extension. The largest ANO basis sets used for N4 contain f‐type functions. Analogous studies of the N2 molecule are presented in order to judge the reliability of the theoretical predictions for the experimentally unknown TdN4.