Hydroxyl Radical Initiated Oxidation of s-Triazine: Hydrogen Abstraction Is Faster than Hydroxyl Addition

Reaction with the hydroxyl radical (HO(*)) is the primary removal mechanism for organic compounds in the atmosphere, and an important process in combustion. Molecules with unsaturated carbon sites are thought to react with HO(*) via a rapid addition mechanism, with little or no barrier; this results in short lifetimes relative to the saturated alkanes, which undergo slower abstraction reactions. Computational chemistry and reaction rate theory are used in this study to investigate the s-triazine + HO(*) reaction. We report that HO(*) addition at a carbon ring site proceeds with the largest known barrier for addition to an unsaturated carbon (9.8 kcal mol(-1) at the G3X level of theory). Abstraction of a hydrogen atom in s-triazine by HO(*), forming the s-triazinyl radical + H(2)O, proceeds with a barrier of only 3.3 kcal mol(-1), and this process dominates over HO(*) addition. Our results are in contrast to those for the analogous reactions in benzene, where the abstraction reaction to phenyl + H(2)O is slower than the HO(*) addition, which forms a radical adduct that can further react with O(2) or dissociate to phenol + H(*). The lifetime of s-triazine toward the hydroxyl radical in the troposphere is estimated at 6.4 years, potentially making it a long-lived pollutant. The aromatic s-triazine (1,3,5-triazine) molecule is a structural feature in herbicides such as atrazine and is a decomposition product of the common energetic material cyclotrimethylenetrinitramine (RDX). While the abstraction reaction dominates for the parent s-triazine, the addition mechanism may be of importance in the atmospheric degradation of substituted triazines, like atrazine, where ring H atoms are not available for abstraction. The high-barrier addition mechanism forms an activated hydroxy-triazinyl adduct which predominantly dissociates to 2-hydroxy-1,3,5-triazine (OST) + H(*). This OST species is a known intermediate of RDX decomposition. Results are also presented for isomerization of the less-stable 1,3,5-triazine-N-oxide OST species (which may form via unimolecular pathways in the liquid-phase decomposition of RDX) to 2-hydroxy-1,3,5-triazine. A reaction mechanism is proposed for further oxidation of the s-triazinyl radical, where an OST isomer is also a potential product.