Kinetic Study for the Reactions of Several Hydrofluoroethers with Chlorine Atoms

Abstract

The reaction kinetics of chlorine atoms with the hydrofluoroethers CHF2CF2OCH3 (1), CHF2CF2OCH2CH3 (2), CHF2CF2OCH2CF3 (3), CF3CHFCF2OCH3 (4), and CH2CHCH2OCF2CHF2 (5) were studied in the gas phase over the temperature range 273−363 K. The experiments were performed at very low pressures (<3 mTorr) in a molecular flow system equipped with a quadrupole mass spectrometer by monitoring both reactants. The proposed Arrhenius expressions for the studied reactions are (in cm3 molecule-1 s-1, 2σ uncertainties): k 1 = (5.49 ± 1.51) × 10-12 exp(−928 ± 155/T), k 2 = (1.99 ± 0.12) × 10-12 exp(−36 ± 33/ T), k 3 = (1.95 ± 0.72) × 10-12 exp(−1616 ± 202/T), k 4 = (8.72 ± 2.93) × 10-12 exp(−1112 ± 186/T), and k 5 = (5.54 ± 1.96) × 10-13 exp(821 ± 214/T). The title reactions proceed via hydrogen atom metathesis, with the exception of the unsaturated CH2CHCH2OCF2CHF2 ether, where the Cl-atom addition pathway leading to CH2CHCl is also present. The C−H bond strengths in several hydrofluoroethers were calculated at the B3P86/6-311++G(3df,2p) level of theory, and they show an increase in the order −OCH(CH3)−H < −OCH(CF3)−H < −CH2OCH2−H < −CF2OCH2−H < −OCH2CH2−H < −OCF2CF2−H < −OCF2−H. Furthermore, their room-temperature rate coefficients correlate with the weakest C−H bond strengths in each molecule, according to the expression log(k, in cm3 molecule-1 s-1) = (23.6 ± 7.4) − (0.09 ± 0.02) × [C−H bond strength (in kJ mol-1)]. In addition, their vertical ionization potentials were calculated at the B3LYP/6-311++G(3df,2p) level of theory, and they also correlate with the room-temperature rate coefficients according to the expression log(k, in cm3 molecule-1 s-1) = (4.0 ± 1.7) − (1.42 ± 0.15) × [ionization potential (in eV)].

Direct ab initio dynamics calculations of the rate constants for the reaction of CHF2CF2OCH3 with Cl

“…From Fig. 5, we can find that the calculated rate constants are in good agreement with the available experimental values [16], with the maximal deviation factor of 1.3 in the temperature range 273-363 K. The calculated k 1 /k ratios are about 99.9% at 200 K, 91.3% at 500 K, and 69.3% at 800 K, which indicate that channel Rl is the dominant reaction route for the title reaction. However, the contribution of channel R2 to the total rate constant increases with the increase in the temperature, for example, the k 2 /k ratios are 43.4% and 70.8% at 1000 and 2000 K, respectively.…”

“…Maybe, it is because the variational effect is significant for the channel Rl, which has a nearly zero barrier. Moreover, the corresponding theoretical activation energy based on the CVT/SCT method is 0.9 kcal/mol at 273-363 K, which is slightly lower than the experimental value of 1.8 kcal/mol obtained from linear least-squares fit of the temperaturedependent rates by Papadimitriou et al [16]. Therefore, by the comparison between theoretical and experimental rate constants and between theoretical and experimental activation energy, one may expect that the rate constants obtained at the G3(MP2)//B3LYP/6-31lG(d,p) level can provide reliable prediction of the higher temperature data, where no measurements are performed.…”

“…The individual CVT/SCT rate constants of each reaction channel, i.e., H-abstraction from CH 3 group (k 1 , including the contribution of the three equivalent hydrogen to the rate constants) and CHF 2 group (k 2 ), the theoretical total rate constants obtained from the sum of the individual rate constants associated with the two reaction channels (k = k 1 + k 2 ) and the available experimental values [16] are presented in Fig. 5, and the temperature dependence of branching ratios k 1 /k and k 2 is exhibited in Fig.…”

Contribution of the Atmospheric Chlorine Reactions to the Degradation of Greenhouse Gases: CFCs Substitutes

“…In general, the strength of the C-H bonds in hydrofluoroethers depends on the interplay of two counteracting electronic effects: a) the strengthening due to the electronwithdrawing inductive effects of F and/or O atoms through -bonds, and b) the weakening of the adjacent C-H bonds due to the -electron transfer from F or O atom to the central C atom (Papadimitriou et al, 2004). On the other hand, the inductive effect caused by an F atom is usually more important than their conjugative effect, contrary to the O atom.…”

“…Bowen & Maccoll (1984) and Molder et al (1983). The rest of IP is from Papadimitriou et al (2004). (2003))] are the lifetimes of a given compound and MCF, respectively, due to the reactions with hydroxyl radical in the troposphere only.…”