Abnormally Long O-O Bond in trans-HOON: An Exemplary Charge-Shift Bond

Nitrous acid (HONO) plays a significant role in atmospheric and combustion chemistry. While extensive attention has been devoted to the study of HONO, its isomer (HOON) has remained relatively unexplored until recent experimental and theoretical analyses revealed its unusually long and weak O-O bond. In contrast, its sulfur-substituted analogue, HOSN, exhibits a normal O-S bond. Here, we explored the intriguing bonding nature of trans-HOXN (X = O, S) from the perspective of the ab initio valence bond (VB) theory in order to elucidate the different behaviors of the O-O and O-S bonds therein. Our results demonstrated that the bonding in trans-HOON can be described as a three-center four-electron charge-shift bond, where the ON moiety most closely resembles nitric oxide, with some nitrene characters. Since the π bond in ON is a dative bond resulting from one lone pair on the oxygen atom, the accumulated negative charge on N enhances the hyperconjugation from the nitrogen lone pair of σ symmetry to the σ*_O-O orbital. Ultimately, it is the enhanced hyperconjugative interaction that plays a dominating role in the elongation and weakening of the O-O bond. In contrast, trans-HOSN is characterized as a two-center two-electron charge-shift bond. Compared with H₂O₂ which takes a skew geometry, both trans-HOXN (X = O, S) prefer a planar geometry. While geometric relaxation provides the primary stabilizing force for trans-HOON, the planarity of trans-HOSN arises dominantly from the conjugation and hyperconjugation effects.