Theoretical investigations on mechanisms and kinetics of the CH3CFClO2· with ClO· reaction in the atmosphere

The singlet and triplet potential energy surfaces of the ClO• radical reaction with the CH3CFClO2• radical have been investigated at the CCSD(T)/cc-pVTZ level based on the optimized geometries at the B3LYP/6–311++G(d,p) level. On the singlet potential energy surfaces (PES), the possible reaction involves association-dissociation, direct H-abstraction and Nucleophilic Substitution 2 (SN2) mechanisms. On the triplet PES, SN2 displacement and direct H-abstraction reaction pathways have been investigated, which are less competitive compared with the reaction pathways on the singlet PES. The rate constants have been calculated at 10–10 to 1010 atm and 200–3,000 K by Rice–Ramsperger–Kassel–Marcus (RRKM) theory for the important product pathways. At 200–800 K, IM1 produced (CH3CFClOOOCl) by collisonal deactivation is dominant; at high temperatures, the production P1 (CH3CFO + ClOOCl) becomes dominate. The calculated rate constants for CH3CFClO2• + ClO• are good agreement with the available experimental value. The atmospheric lifetime of CH3CFClO2• in ClO• is around 3.27 h. TD-DFT computations imply that IM1 (CH3CFClOOOCl), IM2 (CH3CFClOOClO) and IM3 (CH3CFClOClO2) will photolyze under the sunlight.


Results and discussion
The optimized geometries of the intermediates and transition stats involved on the triplet and singlet PESs in the title reaction at the B3LYP/6-311++G(d,p) level are depicted in Fig. 1. The optimized geometries for the reactants and products are shown in Fig. 2, along with the available experimental values 21 . All possible pathways involved in the CH 3 CFClO 2 • + ClO• reaction are presented in Fig. 3. Table 1 summarizes ZPE corrections, relative energies, reaction enthalpies and Gibbs free energy. The harmonic vibrational frequencies, moment of inertia and the Z-matrix Cartesian coordinate of all species found on the PESs as supplementary materials are shown in Tables S1, S2, respectively. The frequencies of CH 3 CFO, CH 3 CClO, OClO, HOCl, ClO•, HO 2 , O 3 and O 2 ( 3 Σ) are in agreement with experimental data 21 . The energies obtained at CCSD(T)/cc-pVTZ//B3LYP/6-311++G(d,p) level are employed to the following discussion.
The formation of adducts on the singlet PES. The ClO• + CH 3 CFClO 2 • reaction initiated by the oxygen or chlorine atom of ClO• radical addition to the terminal-O atom of CH 3 CFClO 2 • without a barrier and produced the original adduct IM1 (CH 3 CFClOOOCl) or IM2 (CH 3 CFClOOClO) with exothermicity of 18.44 kcal/ mol or 1.76 kcal/mol. The formed O-O and O-Cl bonds are 1.302 and 1.888 Å in IM1 and IM2, respectively. IM1 could isomerize to IM2 via a ClOO triangular structure TS1, in which the O-Cl bond that will be formed is 2.246 Å, while the distance of breaking O-O bond is 2.278 Å. TS1 lies 26.28 and 7.84 kcal/mol above IM1 and reactants, respectively. In addition, IM2 can isomerize to IM3 (CH 3  In a word, three adducts IM1 (CH 3 CFClOOOCl), IM2 (CH 3 CFClOOClO) and IM3 (CH 3 CFClOClO 2 ) are generated on the singlet PES with the energy of − 18.01, − 1.53 and − 11.42 kcal/mol, which could further dissociate to various products, and will be discussed as bellow.
The decomposition pathways from IM1 (CH 3 CFClOOOCl), IM2 (CH 3 CFClOOClO) and IM3 (cH 3 cfcloclo 2 ). Starting  www.nature.com/scientificreports/ 2.41 kcal/mol, the barrier for IM1 → TS5 → P3 is 48.40 kcal/mol. To any extent, the high barrier restrains the dissociation pathways from proceeding. Besides the above three decomposition pathways from IM1 (CH 3 CFClOOOCl), the reaction resulting in P4 (CH 3 CFCl 2 + O 3 ) takes place by synchronously the migration of the terminal Cl atom to the carbon atom of -CFCl-group and breaking of the C-O bond through a COOOCl five-center structure TS6. The activation barrier of the IM1 → TS6 → P4 process is 75.81 kcal/mol. Apparently, this decomposition channel is not important to the overall reaction. IM2 (CH 3 CFClOOClO) could take place decomposition into the end product P5 (CH 3 CFO + Cl 2 O 2 ) or P6 (CH 3 CClO + FClO 2 ) through five-center structure TS7 or TS8, respectively. These two decomposition channels involve the Cl atom or F atom of the -CFCl-group migrating to the chlorine atom of the -OOClO skeleton, accompanied by the O-O bond splitting, respectively. In TS7, the C-Cl bond (  Direct H-abstraction pathways on the singlet PES. One direct H-abstraction pathway is found for the CH 3 CFClO 2 • + ClO• reaction. One of the H atoms in CH 3 CFClO 2 • is abstracted by the O atom in ClO• via TS12 to form P10 (CH 2 CFClO 2 + HOCl). The distance of the breaking C-H bond is 1,254 Å, and forming O-H bond is 1.312 Å. We can define a parameter which represents the reactant-or product-like character of the forming transition state. The L parameter could be computed with the expression [22][23][24][25] : are the corresponding bond distance variations between the TS12 structure and the reactant CH 3 CFClO 2 • for the C-H bond and between the TS12 structure and the product HOCl for the O-H bond. The L parameter denotes if a transition state structure is reactant-like (L < 1) or product-like (L > 1) and also quantifies the corresponding trend. The value of L parameter for TS12 is 0.47, indicating that TS12 is a reactant-like transition state. The barrier for CH 3 CFClO 2 • + ClO• → TS12 → P10 (CH 2 CFClO 2 + HOCl) pathway is 12.05 kcal/mol, which may be important for the reaction. Based on our results, three S N 2 displacement and one direct H-abstraction channels were found. Figure 1 displays that surmounting T-TS1, T-TS2, T-TS3 and T-TS4, P12 (CH 3 CFClOCl + O 2 ( 3 ∑)), P13 (CH 3 CFClO + OClO), P14 (CH 3 CFClOOCl + O( 3 P)) and P10 (CH 2 CFClO 2 + HOCl) are produced, and the corresponding relative Kinetics. As discussed above, for the reaction pathways producing P1, P5 and P8 (Scheme 1), the reaction energy barriers are lower and the reactions are exothermic, so these reaction pathways are involved in the kinetics calculations. However, the reaction pathways producing P2, P3, P4, P5, P7, P9, P10, P11, P12, P13 and P14 with higher energy barrier are less competitive in energy, and their contribution to the total reaction is negligible. Temperature-or pressure-dependent rate constants for the important pathways (Scheme 1) of the CH 3 CFClO 2 • + ClO• reaction were computed at 200-3,000 K, 10 -10 -10 10 atm utilizing RRKM theory. The kinetics calculations based on the optimized geometries, moment of inertia and frequencies obtained at B3LYP/6-311 + + G(d,p) level, and the energies obtained at the CCSD(T)/cc-pVTZ level.
Steady-state assumption for all the excited (IMj) generates to the following expressions: for the second-order rate constants of diverse product pathways: Table 1. Zero Point Energies (ZPE) and relative Energies (ΔE), relative enthalpies (ΔH) and Gibbs free energy (ΔG) for the species involved in the CH 3 FClO 2 • with ClO• reaction (energies in kcal/mol). a At the B3LYP/6-311 + + G(d,p) level. b The relative energies are calculated at the CCSD(T)//B3LYP/6-311 + + G(d,p) level. The microcanonical rate constant is calculated using the RRKM theory as follows: In the above equations, α α is the statistical factor for the reaction path a, and α i is the statistical factor (degeneracy) for the ith reaction path; E a is the energy barrier for the reaction step a. Q ClO• and Q CH 3 CFClO 2 • are the total partition function of ClO• and CH 3 CFClO 2 •, respectively; Q = t and Q = r are the translational and rotational partition functions of entrance transition state, respectively; N a E = is the number of state for the association transition state with excess energy E = above the association barrier. k i (E) is the energy-specific rate constant for the ith channel and C i is the ratio of the overall rotational partition function of the TS i and IM j ; N i E = i is the number of states at the energy above the barrier height for transition state i; ρ j E j is the density of states at energy E j of the intermediate. The density of states and the number of states are calculated using the extended Beyer-Swinehart algorithm.
The rate constants of IM1 (CH 3 CFClOOOCl), IM2 (CH 3 CFClOOClO) and IM3 (CH 3 CFClOClO 2 ) collisional stabilization channels, and those for the P1 (CH3CFO + ClOOCl), P5 (CH3CFO + Cl 2 O 2 ) and P8 (CH3CFO + ClOClO) channels (denoted as k IM1 , k IM2 , k IM3 , k P1 , k P5 and k P8 ) and the total rate coefficient (k tot = k IM1 + k IM2 + k IM3 + k P1 + k P5 + k P8 ) at 200-3,000 K, 12 torr N 2 are presented in Fig. 4. k tot appears to reduce firstly and then increase with the temperature increasing. Meanwhile, k tot was in accord with the experimental data (e.g. k tot = 4.50 × 10 -12 cm 3 molecule −1 s −1 vs. k tot(exp) = 3.84 × 10 -12 cm 3 molecule -1 s -1 at 253 K). The branching ratios are listed Fig. 5. The generation of IM1 (CH 3 CFClOOOCl) is dominated at 200-800 K, and with the production of the P1 (CH 3 CFO + ClOOCl) becoming predominant quickly with the rise of temperature. The P5 (CH 3 CFO + Cl 2 O 2 ) or P8 (CH 3 CFO + ClOClO) product pathway generating from the IM2 www.nature.com/scientificreports/ (CH 3 CFClOOClO) or IM3 (CH 3 CFClOClO 2 ) and the collisional stabilization of the IM2 (CH 3 CFClOOClO) and IM3 (CFCl 2 CH 2 OClO 2) almost don't occur. The rate constants for formation of individual products and total rate constants of the CH 3 CFClO 2 • + ClO• reaction at 200-3,000 K and 10 -10 -10 10 atm are shown in Fig. 6. As seen from the figure, k P1 , k P5 , k P8 , k IM1 , k IM2 and k IM3 for the formation of IM1 (10 -10 -10 10 atm), IM2 (10 -10 -10 2 atm) and IM3 (10 -10 -10 2 atm) by collisional deactivation is strongly pressure dependent with a pattern opposite to that of the decomposition processes and ( )  www.nature.com/scientificreports/ IM2 (10 4 -10 10 atm) and IM3 (10 4 -10 10 atm) by collisional deactivation because of the competitive effect of the stabilization vs decomposition as alluded to above. k IM1 , k IM2 and k IM3 become smaller and less competitive at lower pressure; at pressure over 1 atm, k IM1 is approaching the high pressure limit at T ≤ 1,000 K. In addition, k IM1 displays negative dependence on temperature at 200-3,000 K owing to the reduction of collision inactivation rate, except at high-pressure limit pressure. The rate constants k P1 , k P5 and k P8 for the dissociation reactions display positive dependence on temperature and negative pressure dependent. At low temperatures and high pressures, k P1 become insignificantly small. k tot reflects positive pressure dependent. The high-pressure limit rate constants monotone increase firstly and then reduce monotonously with the temperatures increase, with a model contrary to the collisionless limit pressure, which may due to competition between addition and decomposition reaction. The branching ratios of the individual product pathways of the CH 3 CFClO 2 • + ClO• reaction at low-pressure limit (10 -10 atm), atmospheric pressure (1 atm) and high-pressure limit pressure (10 10 atm) are presented in Fig. 7. Six product channels dominant noticeably-the competitive deactivation and decomposition producing IM1, IM2, IM3, P1, P5 and P8, respectively. At high-pressure limit pressure, the formation of the stabilization product, IM1 (CH 3 CFClOOOCl) dominants the reaction at 200-3,000 K. At low-pressure limit and atmospheric pressure, the production of P1 (CH 3 CFO + ClOOCl) dominates the reaction at T ≥ 300 K and T ≥ 1,000 K, respectively; conversely at low temperatures, the collision inactivation of IM1 (CH 3 CFClOOOCl) dominants the reaction.
The three-parameter Arrhenius equations for the rate constants of generation of IM1 (CH 3 CFClOOOCl) (k IM1 ) and P1 (CH 3 CFO + ClOOCl) (k P1 ) at low-pressure limit, 1 atm and high-pressure limit N 2 can be represented by: . The calculated average daytime atmospheric concentrations of chlorine monoxide radical (ClO•) are 1 × 10 7 molecules per cm 3 26 , and k ClO = 8.49 × 10 -12 molecules per cm 3 at 298 K 760 Torr was considered. The atmospheric lifetime of CH 3 CFClO 2 • is approximately 3.27 h, which suggests that ClO-initiated reaction of CH 3 CFClO 2 • plays an important role in some special areas and the marine boundary layer.

Vertical excitation energy of IM1 (CH 3 CFClOOOCl), IM2 (CH 3 CFClOOClO) and IM3
(cH 3 cfcloclo 2 ). The photo-oxidation of compounds containing chlorine is significant for Cl atmospheric chemistry. As source of Cl, the photolysis might influence the stratosphere and troposphere. In order to get new insights of photolytic stability of the chlorinated compounds, the vertical excitation energy (T V ) of the first five excited states for IM1 (CH 3 CFClOOOCl), IM2 (CH 3 CFClOOClO) and IM3 (CH 3 CFClOClO 2 ) were calculated by employing TDDFT method based on the B3LYP/6-311 + + G(d,p) optimized geometries, and the calculation results including wavelength (λ), excitation energy (T V ) and oscillator strength (f) are listed in Table 2. Compounds will be considered to photolyze if T V value is smaller than 4.13 eV or wavelength is longer than 300 nm. From Table 2 it is seen that the T V (wavelength) value of the first two excited states of IM1 (CH 3   . Forecasted branching ratios for the CH 3 CFClO 2 • with ClO• reaction at low-pressure limit pressure, atmospheric pressure and high-pressure limit pressure.

conclusions
The reaction mechanisms, kinetics, and products distribution for the CH 3 CFClO 2 • + ClO• reaction in atmosphere were investigated by using the CCSD(T)//B3LYP method. Addition-elimination, direct H-abstraction and S N 2 displacement mechanisms are located on the singlet PES, and direct H-abstraction and S N 2 displacement mechanisms are located on the triplet PES. The result suggests that major product is P1 (CH 3 CFO + ClOOCl) on the singlet PES produced by the addition-elimination reaction, which proceeds the addition of the O in ClO to the terminal-O atom in CH 3 CFClO 2 • and then the ClOOCl-elimination forming the products. Owing to the higher barrier heights, other products contribute less to the title reaction. The rate constants and branch ratio of products are estimated by means of RRKM theory at extensive temperature and pressure range. The rate constants at 200-3,000 K show stronger temperature dependence. The stabilization of the adduct IM1 (CH 3 CFClOOOCl) is dominant at 200-800 K, while the generation of P1 (CH 3 CFO + ClOOCl) is the primary channel at high temperature. The lifetime of CH 3 CFClO 2 • in the presence of ClO• was predicted to 3.27 h. IM1 (CH 3 CFClOOOCl), IM2 (CH 3 CFClOOClO) and IM3 (CH 3 CFClOClO 2 ) will photolyze under the sunlight.