Theoretical mechanistic study on the radical-molecule reaction of CHCl ₂/CCl₃ with NO₂

The radical-molecule reaction mechanism of CHCl₂ and CCl ₃ with NO₂ have been explored theoretically at the B3LYP/6-311G(d,p) and MC-QCISD (single-point) levels. For the singlet potential energy surface (PES) of CHCl₂ + NO₂ reaction, the association of CHCl₂ with NO₂ was found to be a barrierless carbon-to-nitrogen approach forming an energy-rich adduct a (HCl₂CNO₂) followed by isomerization to b₁ (trans-cis-HCl₂CONO), which can easily interconvert to b₂, b₃, and b₄. Subsequently, the most feasible pathway is the 1,3-chlorine migration associated with N-O1 bond cleavage of b₁ leading to P₁ (CHClO + ClNO). The second competitive pathway is the 1,4-chlorine migration along with N-O1 bond rupture of b₄ giving rise to P₂ (CHClO + ClON). Moreover, some of P₁ and P ₂ can further dissociate to give P₆ (CHClO + Cl + NO). The lesser followed competitive channel is the 1,3-H-shift from C to N atom along with N-O1 bond rupture of b₁ to form P₃ (CCl₂O + HNO). The concerted 1,4-H-shift accompanied by N-O1 bond fission of b ₃ to product P₄ (CCl₂O + HON) is even much less feasible. For the singlet PES of CCl₃ + NO₂ reaction, the only primary product is found to be P₁ (CCl₂O + ClNO), which can lead to P₂ (CCl₂O + Cl + NO) via dissociation of ClNO. The obtained major products CHClO and CCl₂O for CHCl ₂ + NO₂ and CCl₃ + NO₂ reactions, respectively, are in good agreement with kinetic detection in experiment. Compared with the singlet pathways, the triplet pathways may have less contributions to both reactions. Because the rate-determining transition state involved in the feasible pathways lie above the reactants R, the title reactions may be important in high-temperature processes, The similarities and discrepancies among the CH_nCl_3-n + NO₂ (n == 0-2) reactions are discussed in terms of the substitution effect. The present study may be helpful for further experimental investigation of the title reactions.