Atmospheric degradation of 2,3,7,8-tetrachlorinated dibenzo-p-dioxins in the presence of N[O.sub.3] at night

The density functional theory (DFT) has been applied to studies on the homogeneous gas-phase degradation of 2,3,7,8-tetrachlorinated dibenzo-p-dioxins (2,3,7,8-TeCDD) initiated by the N[O.sub.3] radical, which is an important atmospheric species at night. The geometrical parameters and vibrational frequencies of all the stationary points were calculated at the MPWB1K/6-31+G (d,p) level. Potential energies were calculated at the MPWB1K/6-311+G (3df,2p) level. Three sites on 2,3,7,8-TeCDD react with the N[O.sub.3] radical with different barriers and reaction heats. The addition of N[O.sub.3] to the carbon atom on the central C-O ring is the most appropriate pathway and with the lower barriers, and the central ring of polychlorinated dibenzo-p- dioxin is opened in the subsequent reactions. Some other pathways are stressed for the dechlorination mechanism. Canonical variational transition-state theory with small curvature tunneling contribution was used to calculate the rate constants of each elementary reaction over the temperature range of 200-400 K. The Arrhenius equations were fitted to show the relationship between rate constants and temperature. Key words: 2,3,7,8-TeCDD, ≥N[O.sub.3] radical, atmospheric reaction mechanism, rate constant. Resume : On a applique la theorie de la fonctionnelle de la densite a l'etude de degradations en phase gazeuse homogene de 2,3,7,8-tetrachlorodibenzo-p-dioxines (2,3,7,8-TecDD), initiees par le radical N[O.sub.3,] une espece atmospherique importante la nuit. Les parametres geometriques et les frequences vibrationnelles de tous les points stationnaires ont ete calcules au niveau MPWB1K/6-31+G(d,p) de la theorie. On a calcule les energies potentielles au niveau MPWB1K/6-311+G(3df,2p). Il y a trois sites sur les 2,3,7,8-TeCDD qui reagissent avec le radical N[O.sub.3] avec des barrieres et des chaleurs de reactions differentes. L'addition du N[O.sub.3] sur l'atome de carbone du cycle C-O central correspond aux voies reactionnelles les plus appropriees avec de faibles barrieres et un noyau central de la dibenzo-p-dioxine polychloree qui s'ouvre dans les reactions subsequentes. On propose aussi quelques autres voies reactionnelles pour le mecanisme de dechloruration. Pour calculer les constantes de vitesse de chaque reaction elementaire sur la plage de temperature allant de 200 a 400 K, on a fait appel a la theorie de l'etat de transition variationnelle canonique avec une contribution par effet tunnel recourbe. Les equations d'Arrhenius ont ete ajustees pour montrer la relation entre les constantes de vitesses et la temperature. [Traduit par la Redaction] Mots-cles: 2,3,7,8-TeCDD, radical N[O.sub.3,] mecanisme de reaction atmospherique, constante de vitesse.
Introduction Persistent organic pollutants have become a hot topic for their toxicity and widespread distribution in the environment. 2,3,7,8-Tetrachlorinated dibenzo-p-dioxin (2,3,7,8-TeCDD, or TeCDD) is the most toxic dioxin congener, owing [...]