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

TL;DR

This paper explains why the O–O bond in trans-HOON is unusually long and weak using valence bond theory, comparing it to the more stable O–S bond in trans-HOSN.

Contribution

The study reveals that trans-HOON's O–O bond is a charge-shift bond with enhanced hyperconjugation, unlike trans-HOSN's simpler bond structure.

Findings

Trans-HOON's O–O bond is a three-center four-electron charge-shift bond with nitrene-like characteristics.

Hyperconjugation from the nitrogen lone pair to the σ*O–O orbital weakens and elongates the O–O bond in trans-HOON.

Trans-HOSN has a two-center two-electron charge-shift bond and is stabilized by conjugation and hyperconjugation.

Abstract

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.

Figures