Theoretical study on the mechanism of reaction of CHF2 with NO2

The singlet potential-energy surface for the reaction of CHF2 radical with NO2 is explored at the density functional theory of B3LYP/6-311++G(d, p) as well as ab initio QCISD/6-311++G(d, p), CCSD(T)/6-311++G(3df, 2pd) and Gaussian-4 composite method levels. It is shown that the reaction mostly initiated with the association of NO2 with CHF2 by the barrierless nitrogen-to-carbon attack to the adduct 1 (HF2C–NO2) associated with all the reaction channels leading to six products (P1–P6). Among these, the H-migration product P1 (CF2 + HONO) may be the most competitive one due to that the barrier of rate-determining step (RDS) of TS1/P1 is just 0.9 kcal/mol above the reactants at the G4 level, whereas to give the remaining five products have to surmount the barrier of TS1/2 which is 5.6 kcal/mol above the reactants; P2 should be the second competitive product due to that it is 18.7 kcal/mol kinetically lower than P3–P6 at least; in view of P3 (−68.9 kcal/mol) < P4 (−46.2 kcal/mol) < P5 (−26.2 kcal/mol) energetically, P3 should be the third competitive one followed by P4 and P5 as the fourth feasible and the less competitive product; the formation of P6 (CHFO + FON) is far more impossible due to that the largest barrier encountered in the pathway is 81.9 kcal/mol above the reactants at CCSD(T)//B3LYP level. From the comparisons with analogous reactions of CH3−nXn + NO2 (n = 0, 1, 2 and 3; X = F and Cl), it is found that the most competitive channel for the title reaction corresponds to an H-migration process with low barrier from adduct 1 (HF2C–NO2) to P1 (CF2 + HONO), whereas that of the remaining reactions including CH3−nCln + NO2 (n = 0, 1, 2 and 3) and CH3−nFn + NO2 (n = 1 and 3) correspond to the isomerization processes from H3−nXnC–NO2 to H3−nXnCONO with high barriers. Based on simple transition state theory, the evaluated rate constant of the title reaction is very fast as 1.48 × 10−8 cm3 mol−1 s−1 with the sequence of k (CHF2 + NO2) > k (CH2F + NO2) > k (CF3 + NO2) for fluorinated methyl reactions at the G4 level, which is different from that of k (CH2Cl + NO2) > k (CHCl2 + NO2) > k (CCl3 + NO2) associated with the increment of substituted Cl atom with larger electronegativity for chlorinated methyl reactions. The present theoretical results are expected to be helpful for the identification of products experimentally and understanding the halogenated methyl chemistry.