Nitrogen clusters have been intensively studied for their potential application as high-energy density materials,
but a six-membered nitrogen ring (N6) was not found to be stable and aromatic. To explore the possibility of
inducing an aromatic N6 ring via cation-e interaction, quantum chemistry calculations were performed on
the systems of Ca2N6, CaN6, CaN6
2-, N6, and N6
4- at the B3LYP/6-311+G* level. The optimized geometries
reveal that the planar structure of the N6 ring is stable only in the Ca2N6 complex. The computed NBO and
CHelpG charges demonstrate that the planar N6 moiety in the Ca2N6 complex is almost a 10e-electron system.
The predicted nucleus-independent chemical shift (NICS) values demonstrate that the N6 moiety is aromatic
in comparison with the NICS values of benzene. The estimated enthalpy of formation for the Ca2N6 complex
is 100.4 kcal/mol for the reaction of 2Ca and 3N2. The binding energy between the Ca2+ cation and the N6
4-
moiety is -1928.8 kcal/mol, with electrostatic interaction serving as the predominant component. When all
the calculated results are taken into account, including the planar structure, 10e-electron system, identical
bond length, and negative NICS value of the N6
4- moiety in the Ca2N6 complex, it is deduced that the alkaline
earth metal Ca is capable of inducing an aromatic N6 ring through the cation-e interaction formed by electron
transfer from the Ca atom to the N6 ring.