Theoretical investigation of the reaction mechanisms and kinetics of CFCl₂CH₂O₂ and ClO in the atmosphere

Abstract

The reaction between CFCl₂CH₂O₂ radicals and ClO was studied using the B3LYP and CCSD(T) methods associated with the 6-311++G(d,p) and cc-pVTZ basis sets, and subsequently RRKM-TST theory was used to predict the thermal rate constants and product distributions. On the singlet PES, the dominant reaction is the addition of the ClO oxygen atom to the terminal-O of CFCl₂CH₂O₂ to generate adduct IM1 (CFCl₂CH₂OOOCl), and then dissociation to final products P1 (CFCl₂CHO + HO₂ + Cl) occurs. RRKM theory is employed to calculate the overall and individual rate constants over a wide range of temperatures and pressures. It is predicted that the collision-stabilized IM1 (CFCl₂CH₂OOOCl) dominates the reaction at 200–500 K (accounting for about 60–100%) and the dominant products are P1 (CFCl₂CHO + HO₂ + Cl). The yields of the other products are very low and insignificant for the title reaction. The total rate constants exhibit typical “falloff” behavior. The pathways on the triplet PES are less competitive than that on the singlet PES. The calculated overall rate constants are in good agreement with the experimental data. The atmospheric lifetime of CFCl₂CH₂O₂ in ClO is around 2.04 h. TD-DFT calculations imply that IM1 (CFCl₂CH₂OOOCl), IM2 (CFCl₂CH₂OOClO) and IM3 (CFCl₂CH₂OClO₂) will photolyze under sunlight.

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1. Introduction

The detrimental influence of CFCs (chlorofluorocarbons) on the ozonosphere and stratosphere resulted in international agreement that the manufacturing of these chemical compounds would be discontinued by January 1996.^1,2 HCFCs (hydrochlorofluorocarbons) are the suggested alternatives to CFCs since they are relatively environmentally benign. When HCFCs are released into the troposphere, they could react with OH radicals to generate haloalkyl radicals, which subsequently react with O₂ to form peroxy radicals in low altitude atmosphere.^1,2 With HCFC-14lb (CFCl₂CH₃) as a substitute for CFC, the reactions are as follows:

CFCl₂CH₃ + OH → CFCl₂CH₂ + H₂O

CFCl₂CH₂ + O₂ + M → CFCl₂CH₂O₂ + M

To assess the atmospheric effects of CFCl₂CH₃, it is necessary to study the peroxy radical (CFCl₂CH₂O₂) generated by the atmospheric reaction of CFCl₂CH₃. The possible degradation mechanism of the peroxy radicals includes self-reactions^3–5 and reactions with free radicals, i.e. Cl, ClO, NO, HO₂ and CH₃O₂.^6–11 It is well-known that the ClO radical is the most abundant reactive halogen species in the atmosphere. It is of great atmospheric importance due to its ability to destroy ozone. Based on the catalytic cycle, ClO plays an important role in the generation of the Antarctic “ozone hole”, especially in the production of the South ozone hole.¹² The generation and photolysis for the dipolymer of ClO (ClOOCl) are critical for the chemical reaction. According to the statistics, this cycle contributed to about 70% of the damage to the Antarctic ozone.¹³ However, previous research indicated that this gas phase chemistry alone does not result in ozone depletion due to the ozone depletion by chlorine-catalyzed reactions.¹⁴ Therefore, the reaction of ClO with CFCl₂CH₂O₂ is a quite significant chemical reaction, which was only studied by one earlier experiment. In 1977, Wu and Carr⁷ studied the kinetics of the reaction between ClO and CFCl₂CH₂O₂ by employing time-resolved mass spectrometry and a UV flash photolysis technique and measured the rate constant at 253–321 K and 4–60 torr. The obtained rate constant was (6.0 ± 0.7) × 10⁻¹² cm³ per molecule per s at 298 K. Moreover, although studies have involved the CFCl₂CH₂O₂ + ClO reaction,¹⁵ there have been no theoretical investigations into the ClO + CFCl₂CH₂O₂ reaction, which may indicate the degradation of ClO with CFCl₂CH₂O₂, affecting the stratospheric ozone consumption, which may undergo the same pathways as that of the CFCl₂CH₂O₂ + Cl reaction. Therefore, the aim of the present theoretical investigation is to provide the mechanism and kinetics of the reaction between ClO and CFCl₂CH₂O₂ through a description of the potential energy surfaces by means of density functional theory (DFT) theory and RRKM theory,¹⁶ which has been employed to address the complex reactions successfully,^17–22 and shed light on future experimental research.

2. Computational methods

All the geometries were fully optimized using the B3LYP^23,24/6-311++G(d,p) method. All stationary points were identified for local minima and transition states by vibrational analysis, and connections of the transition states between designated reactants and products were confirmed by intrinsic reaction coordinate (IRC) calculations.^25,26 The energies for the singlet and triplet potential energy surfaces (PES) were refined by the single point calculations using the CCSD(T)²⁷//cc-pVTZ method. Initially obtained PES information, involving optimum geometries, frequencies, moment of inertia and energies of the dominant reaction pathways, were ready for dynamic calculations. RRKM-TST theory was employed to gain the rate constants over a wide temperature and pressure region (200–3000 K and 10⁻¹⁴ to 10¹⁴ torr). The Gaussian 09 program²⁸ was used to perform the density functional calculations, and Fortran code was used for the RRKM calculations based on the density functional data.

3. Results and discussion

The optimized geometries of all the stationary points 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 and 2, along associated with the available experimental values.²⁹ The frequencies of HOCl, OClO, HO₂, ClO, O₃ and O₂(³Σ) are in agreement with the experimental data (Table S1†). The reaction processes on the singlet and triplet PESs were described clearly, which are represented in Fig. 3. Table 1 summarizes the relative energies (ΔE), relative enthalpies (ΔH), Gibbs free energies (ΔG), and the ZPE for all the stationary points. Table 2 lists the excitation energy (T_V), wavelength (λ) and oscillator strength (f) of the obtained intermediates. The harmonic vibrational frequencies of all the intermediates and transition states found on the PESs are listed in Table S1 as ESI.†

Fig. 1 Optimized geometries for all the intermediates and transition states at the B3LYP/6-311++G(d,p) level for the reaction between CFCl₂CH₂O₂ and ClO. Bond distances are given in Å.

Fig. 2 Optimized geometries (length in Å and angle in degree) for all the reactants and products at the B3LYP/6-311++G(d,p) level for the reaction between CFCl₂CH₂O₂ and ClO. Angles are given in °, and bond distances are given in Å. The values in italics are experimental data from ref. 29.

Fig. 3 Potential energy surface obtained at the CCSD(T)//B3LYP level for the CFCl₂CH₂O₂ + ClO reaction.

Table 1 Zero point energies (ZPE), relative energies (ΔE), relative enthalpies (ΔH) and Gibbs free energy (ΔG) for the species involved in the CFCl₂CH₂O₂ + ClO reaction (energies in kcal mol⁻¹)

Species	ZPE^a	ΔE^b	ΔH^b	ΔG^b
a At the B3LYP/6-311++G(d,p) level.b The relative energies are calculated at the CCSD(T)/cc-pVTZ + ZPE level.
R: (CFCl2CH2O2+ClO)	29.16	0.00	0.00	0.00
IM1: (CFCl2CH2OOOCl)	30.87	−16.28	−16.80	−5.03
IM2: (CFCl2CH2OOClO)	30.57	2.05	1.78	12.98
IM3: (CFCl2CH2OClO2)	31.21	−9.21	−9.86	2.58
TS1	30.47	10.55	9.87	21.75
TS2	29.48	18.92	18.28	30.16
TS3	27.56	−0.86	−1.52	10.86
TS4	28.18	11.83	11.32	23.16
TS5	27.62	10.14	9.53	21.59
TS6	28.32	57.61	57.44	68.94
TS7	28.85	60.36	60.08	71.35
TS8	26.91	62.12	62.34	72.17
TS9	26.64	71.16	71.22	82.02
TS10	29.08	47.83	47.45	59.43
TS11	28.36	1.31	0.40	13.64
TS12	26.28	65.73	66.12	75.21
TS13	25.94	83.48	83.50	94.69
TS14	28.57	65.35	65.18	76.82
TS15	28.09	70.47	70.65	80.62
T-h-TS1	25.54	15.73	15.68	25.48
T-TS1	27.95	30.48	30.76	39.18
T-TS2	28.36	43.44	43.33	53.43
T-TS3	28.47	77.65	77.58	87.59
P1: (CFCl2CHO + HO2 + Cl)	27.38	−30.48	−29.60	−38.33
P2: (or P2T): (CFCl2CHO2 + HOCl)	28.62	−16.54	−16.42	−16.14
P3: (CFCl2CHO + HOOCl)	29.14	−57.22	−57.30	−57.48
P4: (CFCl2CH2OCl + O2(^1Δg))	29.55	−15.86	−15.76	−13.84
P5: (CFCl2CH2Cl + O3)	29.56	−12.43	−12.79	−11.94
P6: (CHFClCHO + Cl2O2)	29.64	−5.61	−5.84	−6.37
P7: (CHCl2CHO + FClO2)	29.31	2.55	2.26	1.98
P8: (CFCl2CHO + HOClO)	28.53	−43.06	−43.04	−43.41
P9: (CHFClCHO + ClOClO)	28.48	−2.86	−2.75	−4.16
P10: (CHCl2CHO + FOClO)	27.98	32.26	32.36	31.13
P11: (CFCl2CH2ClO + O2(^1Δg))	28.33	32.58	32.94	34.01
P12: (CFCl2CH2OCl + O2(^3Σ))	29.57	−46.01	−45.92	−44.64
P13: (CFCl2CH2O + OClO)	27.77	9.88	9.63	9.19
P14: (CFCl2CH2OOCl + O(^3P))	29.04	26.55	26.76	29.80

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Table 2 The excitation energy T_V (in eV), oscillator strength f (in atomic units) and wavelength λ (in nm) of the first five excited states of IM1 (CFCl₂CH₂OOOCl), IM2 (CFCl₂CH₂OOClO) and IM3 (CFCl₂CH₂OClO₂) at the TD-B3LYP level of theory

Excited states	IM1 (CFCl2CH2OOOCl)			IM2 (CFCl2CH2OOClO)			IM3 (CFCl2CH2OClO2)
	TV	f	λ	TV	f	λ	TV	f	λ
1	3.01	0.0001	411.6	1.98	0.0000	625.59	3.48	0.0016	356.4
2	4.07	0.0001	304.7	3.47	0.0015	357.27	4.31	0.0641	287.3
3	5.02	0.0371	247.1	4.42	0.2574	280.47	4.67	0.0015	265.4
4	5.12	0.0201	242.1	4.69	0.0004	263.91	5.61	0.0663	221.0
5	5.72	0.1630	216.8	5.57	0.0013	222.45	5.71	0.0611	217.0

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3.1. Generation of adducts on the singlet PES

The generation of the initial adduct involves the addition process of ClO to CFCl₂CH₂O₂, which could be described as the approach of the O atom of ClO to the terminal-O atom of CFCl₂CH₂O₂ along the O–O reaction coordinate, resulting in the IM1 (CFCl₂CH₂OOOCl) intermediate. Since ClO and CFCl₂CH₂O₂ are both radicals, the first association step is expected to be a barrierless process. Moreover, the relaxed scan along the reactive O–O bond confirmed that the first step is a barrierless process. The forming O–O bond is 1.320 Å, and the O–O bond energy is calculated to be 16.28 kcal mol⁻¹. The intermediate IM1 (CFCl₂CH₂OOOCl) can overcome the TS1 barrier, resulting in IM2 (CFCl₂CH₂OOClO), where the terminal Cl atom is shifted to the middle-O atom of the –OOO– skeleton, and the O–O bond is broken simultaneously. The breaking O–O bond in the triangle structure TS1 is elongated to 2.307 Å and the forming Cl–O bond is 2.253 Å. The energy barrier for the rearrangement IM1 (CFCl₂CH₂OOOCl) → TS1 → IM2 (CFCl₂CH₂OOClO) is 26.83 kcal mol⁻¹. The conformer IM2 (CFCl₂CH₂OOClO) can easily isomerize to a minimum energy IM3 (CFCl₂CH₂OClO₂), resulting from the –ClO group shifting to the middle-O atom of the –COO– skeleton, while the O–O bond is broken. The breaking O–O bond in the triangle structure TS2 is elongated to 2.168 Å and the forming Cl–O bond is 2.414 Å. The energy barrier for IM2 (CFCl₂CH₂OOClO) → TS2 → IM3 (CFCl₂CH₂OClO₂) is 16.87 kcal mol⁻¹. It should be noted that several conformers for IM1, IM2 and IM3 exist. We used the lowest-energy conformers in the following discussion. To summarize, three adducts IM1 (CFCl₂CH₂OOOCl), IM2 (CFCl₂CH₂OOClO) and IM3 (CFCl₂CH₂OClO₂) are generated on the singlet PES with the energy of −16.28, 2.05 and −9.21 kcal mol⁻¹, which can further generate many products by isomerization or dissociation before being quenched by collisions, as will be discussed below.

3.2. Decomposition pathways from IM1 (CFCl₂CH₂OOOCl), IM2 (CFCl₂CH₂OOClO) and IM3 (CFCl₂CH₂OClO₂)

Starting from IM1 (CFCl₂CH₂OOOCl), five dissociation pathways were identified. Three intramolecular H-migration channels can be competitive as they possess low barriers and generate stable products. 1,4-H migration from the –CH₂ group to the middle-O atom of the –OOCl skeleton, respectively associated with breaking the O–O and O–Cl bonds or O–O bond through TS3 or TS4, gives rise to P1 (CFCl₂CHO + HO₂ + Cl) or P2 (CFCl₂CHO₂ + HOCl). In TS3, the breaking C–H bond (1.306 Å), O–O bond (2.121 Å) and the O–Cl (2.086 Å) bond are elongated by 0.217, 0.633 and 0.244 Å, compared to the intermediate IM1 (CFCl₂CH₂OOOCl) (1.089, 1.488 and 1.842 Å, respectively), and the forming O–H bond is 1.349 Å. In TS4, the breaking C–H bond (1.177 Å) and the O–O bond (2.580 Å) are elongated by 0.088 and 1.260 Å, and the forming O–H bond is 1.520 Å. Alternatively, 1,3-H migration from the –CH₂ group to the middle-O atom of the –OOO– skeleton in IM1, along with breakage of the O–O bond via TS5, results in P3 (CFCl₂CHO + HOOCl). The above three decomposition channels need to overcome 15.42, 28.11 and 26.42 kcal mol⁻¹ energy barriers, respectively. The overall exothermicities of generating the P1 (CFCl₂CHO + HO₂ + Cl), P2 (CFCl₂CHO₂ + HOCl) and P3 (CFCl₂CHO + HOOCl) channels are estimated to be 29.60, 16.42 and 57.30 kcal mol⁻¹, respectively, implying that the most significant reaction pathway is the generation of P1 (CFCl₂CHO + HO₂ + Cl), and the pathways to generate P2 (CFCl₂CHO₂ + HCl) and P3 (CFCl₂CHO + HOOCl) compete with each other.

When we considered the other direct dissociation channels from IM1 (CFCl₂CH₂OOOCl), one four-center and five-center transition state (TS6 and TS7) were identified. TS6 connects the IM1 (CFCl₂CH₂OOOCl) and P4 (CFCl₂CH₂OCl + O₂(¹Δ_g)) end products, whereas P5 (CFCl₂CH₂Cl + O₃) is generated from IM1 (CFCl₂CH₂OOOCl) passing through TS7. These two channels can be attributed to the –ClO group or terminal-Cl atom shifting to the carbon atom, and the O₂(¹Δ_g) or O₃ leaving simultaneously, respectively. TS6 and TS7, with imaginary frequencies of 386i and 439i cm⁻¹, respectively, are first-order saddle points, which was confirmed by vibrational frequency analysis. The energy barriers of IM1 (CFCl₂CH₂OOOCl) → TS6 → P4 (CFCl₂CH₂OCl + O₂(¹Δ_g)) and IM1 (CFCl₂CH₂OOOCl) → TS7 → P5 (CFCl₂CH₂Cl + O₃) are 73.89 and 76.64 kcal mol⁻¹. Thus, neither the O₂(¹Δ_g) or O₃ elimination channels from IM1 are favorable judging from the high barrier height.

Two elimination mechanisms were located from IM2 (CFCl₂CH₂OOClO) through a dicyclo-transition state TS8 or TS9 to generate P6 (CHFClCHO + Cl₂O₂) or P7 (CHCl₂CHO + FClO₂); this occurred through one of the H atoms in the –CH₂ group migrating to another carbon atom, and one of the Cl atoms or F atom in the -CFCl₂ group shifting to the Cl atom in the –OOClO skeleton, accompanied by the O–O bond breaking. The breaking C–H, C–Cl and O–O bonds are elongated to 1.207, 2.925 and 2.529 Å, and the forming C–H and Cl–Cl bond are 1.672 and 2.502 Å for TS8. The breaking C–H, C–F and O–O bonds are elongated to 1.202, 2.326 and 2.554 Å, and the forming C–H and F–Cl bonds are 1.676 and 1.976 Å for TS9. Vibrational frequency analysis of TS8 and TS9 reveals one imaginary frequency of 661i and 699i cm⁻¹, respectively. The activation barriers for IM2 (CFCl₂CH₂OOClO) → TS8 → P6 (CHFClCHO + Cl₂O₂) and IM2 (CFCl₂CH₂OOClO) → TS9 → P7 (CHCl₂CHO + FClO₂) are 60.07 and 69.11 kcal mol⁻¹. The high barriers restrain these two dissociation pathways from proceeding.

We also considered the dissociation pathway from IM2 (CFCl₂CH₂OOClO) resulting in the P4 (CFCl₂CH₂OCl + O₂(¹Δ_g)) products, and transition state TS10 was located for this channel. TS10 is a COOClO five-center transition state, that involves the C–O and Cl–O bonds breaking and forming another C–O bond, accompanied with the dissociation of O₂(¹Δ_g). The energy barrier is 45.78 kcal mol⁻¹, suggesting that the conversion of IM2 (CFCl₂CH₂OOClO) → TS10 → P4 (CFCl₂CH₂OCl + O₂(¹Δ_g)) was inefficient.

IM3 (CFCl₂CH₂OClO₂) involves a 1,4-H shift from the carbon atom of the –CH₂ group to the oxygen atom, along with breakage of the O–Cl bond leading to P8 (CFCl₂CHO + HOClO) via TS11. TS11 presents a non-planar and loose HCOClO five-membered ring structure with long C–H, O–Cl and O–H distances, that is, r(C–H) = 1.217 Å, r(O–Cl) = 2.238 Å, and r(O–H) = 1.498 Å. The IM3 (CFCl₂CH₂OClO₂) → TS11 → P8 (CFCl₂CHO + HOClO) transformation barrier is 10.52 kcal mol⁻¹ and TS11 is only 1.31 kcal mol⁻¹ higher than the reactants. Therefore, this dissociation pathway may be important for the reaction.

IM3 (CFCl₂CH₂OClO₂) could also undergo a respective 1,5-Cl shift or 1,5-F shift from the carbon atom to the oxygen atom, and a 1,2-H shift from the carbon atom of the –CH₂ group to another carbon atom as well, which is associated with breaking the O–Cl bond (i.e., TS12 and TS13 together generate product P9 (CHFClCHO + (ClO)₂) and P10 (CHCl₂CHO + FOClO)). The loose ClCCOClO and FCCOClO six-membered rings are nonplanar and found in TS12 and TS13, respectively. In TS12, the breaking C–H, C–Cl and O–Cl bonds and the forming C–H and Cl–O bonds are 1.191, 3.005, 2.720, 1.720 and 2.064 Å, respectively. In TS13, the C–H, C–F and O–Cl bonds that will be broken and the C–H and F–O bond that will be formed are 1.451, 1.837, 2.534, 1.330 and 2.312 Å, respectively. The decomposition barriers of IM3 (CFCl₂CH₂OClO₂) → TS12 → P9 (CHFClCHO + (ClO)₂) and IM3 (CFCl₂CH₂OClO₂) → TS13 → P10 (CHCl₂CHO + FOClO) are 74.94 and 92.69 kcal mol⁻¹, which are quite high, making the generation of P9 (CHFClCHO + (ClO)₂) and P10 (CHCl₂CHO + FOClO) highly impossible.

3.3. S_N2 displacement pathways on the singlet PES

Another type of mechanism (S_N2 displacement reaction) could take place for the CFCl₂CH₂O₂ + ClO reaction. Two different transition states were identified. The two S_N2 displacement channels can be attributed to the O atom or the Cl atom in ClO respectively attacking the C atom of the –CH₂ group in CH₃CFClO_2, along with the O₂(¹Δ_g) group leaving via TS14 or TS15, leading to P4 (CFCl₂CH₂OCl + O₂(¹Δ_g)) or P11 (CFCl₂CH₂ClO + O₂(¹Δ_g)). The forming and breaking C–O bonds in TS14 and the forming C–Cl bond and breaking C–O bond in TS15 are respectively 2.046, 1.794 Å, and 2.414, 2.004 Å. The processes of R → TS14 → P4 (CFCl₂CH₂OCl + O₂(¹Δ_g)) and R → TS15 → P11 (CFCl₂CH₂ClO + O₂(¹Δ_g)) require quite high energy barriers of 65.35 and 70.47 kcal mol⁻¹. Thus, these two S_N2 displacement channels are prohibited kinetically.

3.4. The pathways on the triplet PES

On the triplet surface, both H-abstraction and S_N2 displacement reaction mechanisms were identified. Surmounting T-h-TS1, T-TS1, T-TS2 and T-TS3, P2_T (CFCl₂CHO₂ + HCl), P12 (CFCl₂CH₂OCl + O₂(³Σ)), P13 (CFCl₂CH₂O + OClO) and P14 (CFCl₂CH₂OOCl + O(³P)) are produced, and the relative energy is −16.54, −46.01, 9.88 and 26.55 kcal mol⁻¹, respectively. The barriers of these four channels are 15.73, 30.48, 43.44 and 77.65 kcal mol⁻¹, respectively. Compared with the addition/elimination pathways on the singlet PES, the pathways on the triplet PES are energetically less convenient because of the higher barrier heights.

3.5. RRKM-TST calculations of the rate constants

Because the energy barriers are much higher than the obtained dominant pathways (Scheme 1), we ignore the pathways to produce P4 (CFCl₂CH₂OCl + O₂(¹Δ(_g))), P5 (CFCl₂CH₂Cl + O₃), P6 (CHFClCHO + Cl₂O₂), P7 (CHCl₂CHO + FClO₂), P9 (CHFClCHO + (ClO)₂), P10 (CHCl₂CHO + FOClO), P11 (CFCl₂CH₂ClO + O₂ (¹Δ_g)), P12 (CFCl₂CH₂OCl + O₂(³Σ)), P13 (CFCl₂CH₂O + OClO) and P14 (CFCl₂CH₂OOCl + O(³P)) in the RRKM calculations. The predicted rate constants of IM1 (CFCl₂CH₂OOOCl), IM2 (CFCl₂CH₂OOClO), IM3 (CFCl₂CH₂OClO₂), P1 (CFCl₂CHO + HO₂ + Cl), P2 (CFCl₂CHO₂ + HOCl), P3 (CFCl₂CHO + HOOCl), and P8 (CFCl₂CHO + HOClO) are denoted as k_IM1, k_IM2, k_IM3, k_P1, k_P2, k_P3 and k_P8, and the total rate constant is denoted as k_tot = k_IM1 + k_IM2 + k_IM3 + k_P1 + k_P2 + k_P3 + k_P8 at 200–3000 K, 12 torr N₂, which are presented in Fig. 4. The estimated values of k_tot are consistent with the experimental data (e.g. k_tot = 6.39 × 10⁻¹² cm³ per molecule per s vs. 6.70 × 10⁻¹² cm³ per molecule per s at 298 K). k_tot appears to decrease first and then increase as the temperature increases. The branching ratios are presented Fig. S1.† The low-temperature (200–500 K) association is dominated by the production of IM1 (CFCl₂CH₂OOOCl), and the production of P1 (CFCl₂CHO + HO₂ + Cl) quickly becomes dominant with the rise of temperature (500–3000 K). The P2 (CFCl₂CHO₂ + HOCl) product channel contributes to the reaction at T > 1200 K. The P3 (CFCl₂CHO + HOOCl) product pathway generating from IM1 (CFCl₂CH₂OOOCl), the P8 (CFCl₂CHO + HOClO) product pathway generating from IM3 (CFCl₂CH₂OClO₂) and the pathways of the IM2 (CFCl₂CH₂OOClO) and IM3 (CFCl₂CH₂OClO₂) collisional stabilization rarely occur at any temperature.

Scheme 1 The primary pathways of the CFCl₂CH₂O₂ + ClO reaction.

Fig. 4 Plots of the rate coefficients for total and primary reaction channels versus 1000/T (K⁻¹) at 200–1000 K associated with the available experimental value.

The rate constants for the generation of branched products of the ClO + CFCl₂CH₂O₂ reaction at different pressures are shown in Fig. 5. Due to the competition between decomposition and stabilization, k_IM1, k_IM2 and k_IM3 are for the generation of IM1, IM2 and IM3 by collisional deactivation, and k_P1, k_P2, k_P3 and k_P8 are for the generation of P1, P2, P3 and P8, which demonstrate strong dependence on pressure. The pattern of the pressure dependence of IM1 (10⁻¹⁰ to 10¹⁰ atm), IM2 (10⁻¹⁰ to 10² atm) and IM3 (10⁻¹⁰ to 10² atm) is contrary to the dissociation process and IM2 (10² to 10¹⁰ atm) and IM3 (10² to 10¹⁰ atm). k_IM1, k_IM2 and k_IM3 are very small and not competitive at lower pressure; when the pressure exceeds 10⁻⁴ atm, k_IM1 is near the high pressure limit at T > 1000 K. In addition, k_IM1 displays strong negative dependence on temperature at 200–3000 K, owing to the reduction of the collision inactivation rate. The rate constants for the dissociation procedure k_P1 display positive dependence on temperature and negative dependence on pressure. At low temperatures and high pressures, k_P1, k_P2, k_P3 and k_P8 become negligible.

Fig. 5 Predicted rate coefficients for the total reaction and each individual product pathway of the reaction between CFCl₂CH₂O₂ and ClO at 200–3000 K and 10⁻¹⁰ to 10¹⁰ atm.

The branching ratios of the individual product pathways of the CFCl₂CH₂O₂ + ClO reaction at the low pressure limit (10⁻¹⁰ atm), atmospheric pressure (1 atm) and high pressure limit (10¹⁰ atm) are presented in Fig. 6. Seven primary product pathways dominate noticeably – the competitive inactivation and dissociation that generate IM1, IM2, IM3, P1, P2, P3 and P8, respectively. At the low and high pressure limits, the production of P1 (CFCl₂CHO + HO₂ + Cl) and IM1 (CFCl₂CH₂OOOCl) predominate the reaction at 200–3000 K, respectively. At atmospheric pressure and high temperatures, the generation of P1 (CFCl₂CHO + HO₂ + Cl) dominates the reaction; conversely, at moderate and low temperatures, the collision inactivation of IM1 (CFCl₂CH₂OOOCl) takes over the reaction.

Fig. 6 Predicted branching ratios for the CFCl₂CH₂O₂ + ClO reaction at the low pressure limit, atmospheric pressure and high pressure limit.

The three-parameter Arrhenius equations for the rate constants of generating IM1 (CFCl₂CH₂OOOCl) (k_IM1) and P1 (CFCl₂CHO + HO₂ + Cl) (k_P1) at the low pressure limit, atmospheric pressure and high pressure limit of N₂ can be represented by:

k_IM1⁰(CFCl₂CH₂OOOCl)/(cm³ per molecule per s) = 4.23 × 10⁻²³T^−4.13 exp(7517.87/T) (200 ≤ T ≤ 3000 K)

k⁰_P1(CFCl₂CHO + HO₂ + Cl)/(cm³ per molecule per s) = 1.01 × 10⁻¹⁶T^0.85 exp(1001.83/T) (200 ≤ T ≤ 500 K) = 9.45 × 10⁻¹⁵T^0.69 exp(−1327.18/T) (500 < T ≤ 3000 K)

k_IM1(CFCl₂CH₂OOOCl)/(cm³ per molecule per s) = 1.84 × 10⁻⁷T^−1.72 exp(62.58/T) (200 ≤ T ≤ 3000 K)

k_P1(CFCl₂CHO + HO₂ + Cl)/(cm³ per molecule per s) = 2.38 × 10⁻¹⁵T^0.85 exp(−990.60/T) (200 ≤ T ≤ 3000 K)

k^∞_IM1(CFCl₂CH₂OOOCl)/(cm³ per molecule per s) = 5.43 × 10⁻¹⁸T^2.36 exp(47.22/T) (200 ≤ T ≤ 1800 K) = 1.08 × 10⁻⁹T^−0.33 exp(2682.26/T) (1800 < T ≤ 3000 K)

k^∞_P1(CFCl₂CHO + HO₂ + Cl)/(cm³ per molecule per s) = 1.14 × 10⁻¹⁰T^−1.30 exp(8395.50/T) (200 ≤ T ≤ 3000 K)

We also examined the pressure effect of the total rate constants at selected low (298 K), moderate (500 K and 1000 K), and high (3000 K) temperatures (Fig. 7), and established the branching ratios of the individual product pathways of the CFCl₂CH₂O₂ + ClO reaction (Fig. 8). Fig. 7 displays the typical “S” behavior, and the “S” region moves to the high pressure range as the temperature increases. The “S” regions are 10⁻⁴ to 10⁴, 10⁻¹ to 10⁶, 10²–10⁹ and 10⁵–10¹⁰ torr at selected temperatures, respectively. At each respective temperature, the branching ratio for P1 (CFCl₂CHO + HO₂ + Cl) (k_P1/k_tot) possesses the largest values in the pressure ranges of 10⁻¹⁴ to 10⁻², 10⁻¹⁴ to 10¹, 10⁻¹⁴ to 10⁴, and10⁻¹⁴ to 10⁷ torr, respectively. But at higher pressures, the branching ratio for the energized IM1 (CFCl₂CH₂OOOCl) intermediate, which is stabilized by collisions, becomes dominant.

Fig. 7 Pressure dependence of the total rate coefficients for the CFCl₂CH₂O₂ + ClO reaction at 298, 1000 and 3000 K.

Fig. 8 Branching ratios for the CFCl₂CH₂O₂ + ClO reaction at 10⁻¹⁴ to 10¹⁴ torr at 298, 1000 and 3000 K.

3.6. Atmospheric lifetimes of CFCl₂CH₂O₂

The atmospheric lifetime of CFCl₂CH₂O₂ can be deduced by the following formula: The calculated average daytime atmospheric concentration of chlorine monoxide radical (ClO) is 1 × 10⁷ molecules per cm³ (Tang, 2004),³⁰ and k_ClO = 1.36 × 10⁻¹¹ cm³ per molecule per s was estimated. The atmospheric lifetime of CFCl₂CH₂O₂ is approximately 2.04 h, which suggests that the ClO-initiated reaction with CFCl₂CH₂O₂ plays an important role in some special areas and the marine boundary layer.

3.7. Vertical excitation energy of IM1 (CFCl₂CH₂OOOCl), IM2 (CFCl₂CH₂OOClO) and IM3 (CFCl₂CH₂OClO₂)

The photo-oxidation of compounds containing chlorine is significant for Cl atmospheric chemistry and might influence the stratosphere and troposphere. To gain new insight into the photolytic stability of the Cl-containing compounds, the vertical excitation energy (T_V) of the first five excited states for IM1 (CFCl₂CH₂OOOCl), IM2 (CFCl₂CH₂OOClO) and IM3 (CFCl₂CH₂OClO₂) were calculated by TDDFT employing B3LYP/6-311++G(d,p), and the results, including wavelength (λ), excitation energy (T_V) and oscillator strength (f), are listed in Table 2. Compounds will typically photolyze if the T_V value is smaller than 4.13 eV or the wavelength is longer than 300 nm. It is seen from Table 2 that the T_V value of the first two excited states of IM1 (CFCl₂CH₂OOOCl) are 3.01 eV (411.6 nm) and 4.07 eV (304.7 nm) and the oscillator strengths are 0.0001 and 0.0001; the second excited state of IM2 (CFCl₂CH₂OOClO) is 3.47 eV (357.27 nm) and the oscillator strength is 0.0015; the T_V value and oscillator strength of the first excited state of IM3 (CFCl₂CH₂OClO₂) are 3.48 eV (356.4 nm) and 0.0016, respectively, indicating that the photolysis of IM1 (CFCl₂CH₂OOOCl), IM2 (CFCl₂CH₂OOClO) and IM3 (CFCl₂CH₂OClO₂) could occur under sunlight.

4. Conclusions

The ClO-initiated oxidation reaction of CFCl₂CH₂O₂ was researched by means of density functional theory (DFT) and RRKM theory to understand the mechanism and product distribution. On the singlet PES, the addition of the O atom of ClO to the terminal-O of CFCl₂CH₂O₂ to generate IM1 (CFCl₂CH₂OOOCl) is more favorable at 200–500 K, followed by intramolecular 1,4-H shifts along with breakage of the O–Cl bond to generate P1 (CFCl₂CHO + HO₂ + Cl), which dominates the reaction at high temperatures. The kinetics simulations revealed that the total rate constants exhibit typical “falloff” behavior. The pathways on the triplet PES are less preferred over the pathways on the singlet PES. The ClO-determined lifetime of CFCl₂CH₂O₂ is 2.04 h. IM1 (CFCl₂CH₂OOOCl), IM2 (CFCl₂CH₂OOClO) and IM3 (CFCl₂CH₂OClO₂) will photolyze under the sunlight.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Natural Science Foundations of China (No. 21707062).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra04707d

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