The Nitrosyl Azide Potential Energy Hypersurface:  A High-Energy-Density Boom or Bust?

Motivated by the recent isolation and spectroscopic characterization of nitrosyl azide (N 4 O), we have undertaken an ab initio investigation of the originally reported structure as well as various structural isomers on the potential energy hypersurface. Geometries and harmonic vibrational frequencies have been predicted for the trans -chain isomer along with the 6 π electron potentially aromatic ring structure with various levels of theory up through the triple-ζ plus double polarization single and double excitation coupled cluster (TZ2P CCSD) method and the multireference configuration interaction method (MRCISD). In addition, estimates are made for extension to higher levels of theory, arriving at final predictions of r e (ON 1 ) = 1.176 Å, r e (N 1 N 2 ) = 1.472 Å, r e (N 2 N 3 ) = 1.272 Å, r e (N 3 N 4 ) < 1.130 Å, ϑ e (ON 1 N 2 ) = 110.5°, ϑ e (N 1 N 2 N 3 ) = 105.4°, ϑ e (N 2 N 3 N 4 ) = 174.5°, and r e (ON 1 ) < 1.389, r e (N 1 N 2 ) < 1.250, r e (N 3 N 4 ) > 1.423, ϑ e (N 1 ON 4 ) = 106.3° for the trans -chain and ring isomers, respectively. Energy relationships, bond lengths, vibrational frequencies, Mulliken bond indices, and molecular orbital arguments are used to elucidate nitrogen oxide bonding. While the ring isomer is predicted to be the most stable structure on the hypersurface, the barrier to dissociation is most likely between 1 and 2 kcal mol -1 (including zero-point vibrational energy [ZPVE], the existence of any barrier becomes questionable) making isolation theoretically possible but experimentally difficult. This small barrier also detracts from the attractiveness of the N 4 O ring structure as a high-energy-density material. The trans -chain isomer, however, lies in an energy valley with higher sides, consistent with its previous experimental observation.