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Theoretical Study of the Hydrogen Abstraction of Substituted Phenols by Nitrogen Dioxide as a Source of HONO
The
mild yet promiscuous reactions of nitrogen dioxide (NO<sub>2</sub>) and phenolic derivatives to produce nitrous acid (HONO)
have been explored with density functional theory calculations. The
reaction is found to occur via four distinct pathways with both proton
coupled electron transfer (PCET) and hydrogen atom transfer (HAT)
mechanisms available. While the parent reaction with phenol may not
be significant in the gas phase, electron donating groups in the ortho
and para positions facilitate the reduction of nitrogen dioxide by
electronically stabilizing the product phenoxy radical. Hydrogen bonding
groups in the ortho position may additionally stabilize the nascent
resonantly stabilized radical product, thus enhancing the reaction.
Catechol (<i>ortho</i>-hydroxy phenol) has a predicted overall
free energy change Ξ<i>G</i><sup>0</sup> = β0.8
kcal mol<sup>β1</sup> and electronic activation energy <i>E</i><sub><i>a</i></sub> = 7.0 kcal mol<sup>β1</sup>. Free amines at the ortho and para positions have Ξ<i>G</i><sup>0</sup> = β3.8 and β1.5 kcal mol<sup>β1</sup>; <i>E</i><sub>a</sub> = 2.3 and 2.1 kcal
mol<sup>β1</sup>, respectively. The results indicate that the
hydrogen abstraction reactions of these substituted phenols by NO<sub>2</sub> are fast and spontaneous. Hammett constants produce a linear
correlation with bond dissociation energy (BDE) demonstrating that
the BDE is the main parameter controlling the dark abstraction reaction.
The implications for atmospheric chemistry and ground-level nitrous
acid production are discussed