Ab Initio Investigation of Structures and Energies of Low-Lying Electronic States of AlN3, Al3N, and Al2N2

The structures and energies of low-lying electronic states of AlN 3, Al 3 N, and Al 2 N 2 have been evaluated at the HF, MP2, QCISD(T), CCSD, and CCSD(T) levels of theory, using the several basis sets of 6-31G* (for HF), cc-pVDZ (for MP2 and QCISD(T)), and cc-pVTZ (for CCSD and CCSD(T)). The ground state of AlN 3 is predicted to be a 1 Σ + state with a linear Al−N−N−N structure. The most stable species of Al 3 N is found, however, to have D 3 h symmetry and 1 A‘ 1 ground state. For Al 2 N 2, various isomers are found to be energetically favorable. A rhombic isomer with the nitrogen atoms along the short diagonal and with a 1 A g electronic state is the lowest in energy at the MP2/cc-pVDZ, QCISD(T)/cc-pVDZ, CCSD/cc-pVTZ, and CCSD(T)/cc-pVTZ levels. A linear structure Al−N−N−Al with a 3 electronic state is the second lowest. The third stable isomer with the aluminum atoms bonded directly to the N 2 π orbital seems to be one of the model species for the sake of the nitrogen fixation. Our results suggest that the formation of a variety of the configuration of Al 2 N 2 is energetically plausible under the reaction conditions employed since the energy differences in the Al 2 N 2 species are relatively small.