No longer a complex, not yet a molecule: a challenging case of nitrosyl O-hydroxide, HOON.

HOON might be an elusive intermediate of atmospheric photochemical reactions of HONO or recombination of the parent nitrene HN and molecular oxygen. However, no reliable data on HOON structure and stability are available, and the nature of the O-O bond is not well understood. In this study, we used high-level single- [CCSD(T) and, CCSDTQ] and multireference [CASPT2, MR-AQCC] ab initio calculations to determine properties of HOON: geometry, harmonic and anharmonic vibrational frequencies, thermodynamic stability, and electronic structure. HOON has bonding minima only in the (1)A' electronic state that correspond to cis- and trans-conformers; trans-HOON is more stable by 6.4-8.5 kJ/mol. The O-O bond in trans-HOON is unusually long, R(O-O) = 1.89 Å, and weak, D(O-O) = 33.3 kJ/mol; however, trans-HOON might be stable enough to be identified in cryogenic matrices. Though the electronic structure of the NO moiety in HOON most resembles nitric oxide, some nitrene character as well nitrosyl cation character are also important; therefore, the current name of HOON, hydroperoxynitrene, is misleading; instead, we propose the name "nitrosyl O-hydroxide" or "isonitrosyl hydroxide".