Hydrolysis of Formyl Fluoride Catalyzed by Sulfuric Acid and Formic Acid in the Atmosphere.

Formyl fluoride (HFCO) is an important atmospheric molecule, and its reaction with the OH radical is an important pathway when degradation of HFCO is considered in earth's troposphere. Here, we study the hydrolysis of formyl fluoride (HFCO + H2O) with sulfuric acid (H2SO4) and formic acid (HCOOH) acting as catalysts by utilizing M06-2X, CCSD(T)-F12a, and conventional transitional state theory with Eckart tunneling to explore the atmospheric impact of the above-said hydrolysis reactions. Our calculated results show that H2SO4 has a remarkably catalytic role in the gas-phase hydrolysis of HFCO, as the energy barriers of the HFCO + H2O reaction are reduced from 39.22 and 41.19 to 0.26 and -0.63 kcal/mol with respect to the separate reactants, respectively. In addition, we also find that H2SO4 can significantly accelerate the decomposition of FCH(OH)2 into hydrogen fluoride (HF) and HCOOH. This is because while the barrier height for the unimolecular decomposition of FCH(OH)2 into HF and HCOOH is 31.63 kcal/mol, the barrier height for the FCH(OH)2 + H2SO4 reaction is predicted to be -5.99 kcal/mol with respect to separate reactants. Nevertheless, the comparative relative rate analysis shows that the reaction between HFCO and the OH radical is still the most dominant pathway when the tropospheric degradation of HFCO is taken into account and that the gas-phase hydrolysis of HFCO may only occur with the help of H2SO4 when the atmospheric concentration of OH is about 101 molecules cm-3 or less. Having an understanding from the present study that the gas-phase hydrolysis of HFCO in the presence of H2SO4 has very limited role possibly in the absence of sunlight, we also prefer here to emphasize that the HFCO + H2O + H2SO4 reaction may occur on the surface of secondary organic aerosols for the formation of HCOOH.

Introduction

Formyl fluoride (HFCO) is a key molecule, produced via atmospheric degradation of hydrofluorocarbons (HFCs)[1−7] and hydrofluoroolefins (HFOs).[8−10] HFCs have been used as a refrigerant instead of CFCs and HCFCs to protect the ozone layer.[6] Here, HFC-134a is one of the highest concentrations of HFCs in the atmosphere,[11] which mainly reacts with the OH radical; this leads to the formation of the dominant intermediate product HFCO.[12−15] In recent years, experimental results have shown that the concentration of HCF-134a has been increasing year by year due to anthropogenic emission;[11,16−23] this leads to the increase of the concentration of HFCO in the atmosphere. Therefore, it is particularly necessary and important for clarifying the atmospheric lifetime of HFCO to fully estimate atmospheric environmental effects of HFCs and HFOs.

The unimolecular reaction of HFCO and its bimolecular reactions with atmospheric radicals have been investigated both experimental and theoretical methods.[24−31,33,36] HFCO undergoes unimolecular decomposition and rearrangement reactions responsible for the formation of hydrogen fluoride (HF), CO, F, HCO, H, FCO, FCOH, and HCOF. Francisco et al. studied the ground-state potential energy surface of decomposition and rearrangement reactions of HFCO by the PMP4SDTQ/6-311++G(d,p)//MP2/6-311G(d,p) method. They reported a high barrier height of 43.2 kcal/mol for the HFCO → HF + CO reaction.[24] The reaction (HFCO → HF + CO) was also calculated by the MP4(SDTQ)/6-311G**//MP2/6-31G* method with a high barrier height of 46.9 kcal/mol.[25,26] In addition, the calculated results have also shown that the barrier of HFCO decomposition into HF and CO is about 30 kcal/mol lower than that of the unimolecular rearrangement of HFCO into t-FCOH.[27] It is worth noting that the high barrier heights of HFCO unimolecular decomposition and rearrangement reactions make these processes negligible in the atmosphere.

HFCO can also react with atmospheric radicals such as OH, Cl, NO3, and HO2. The reaction of HFCO with OH has been both theoretically and experimentally investigated.[28−30] The reported rate constant for the HFCO + OH reaction is 4 × 10–15 cm3 molecule–1 s–1 at 298 K.[31] Hence, the estimated atmospheric lifetime of HFCO is ∼8 years when the OH radical concentration is 1 × 106 molecule cm–3 in the atmosphere.[32] Similarly, the rate constant of the HFCO + Cl reaction is 1.95 × 10–15 cm3 molecule–1 s–1 at 298 K[33] and the atmospheric lifetime of HFCO is estimated to be ∼81 years when the concentration of Cl is 2 × 105 molecule cm–3 (Arctic atmosphere at noon).[34,35] When the concentration of the OH radical is decreased at night, HFCO may be eliminated by another strong oxidant NO3.[36,37] The rate constant of the HFCO + NO3 reaction is 2.09 × 10–23 cm3 molecule–1 s–1,[37] and the corresponding atmospheric lifetime of HFCO is about 1.52 × 105 years at 298 K when nighttime NO3 concentration is 1 × 1010 molecule cm–3.[38] Additionally, the rate constant of the HFCO + HO2 reaction is 2.19 × 10–18 cm3 molecule–1 s–1 at 298 K[39] and the corresponding atmospheric lifetime of HFCO is expected to be ∼13 years when the concentration of HO2 is 1.1 × 109 molecule cm–3.[39] The previous results have shown that the HFCO + OH reaction dominates the sink of HFCO. However, the reaction of HFCO with OH is still quite slow. Thus, it is expected to further investigate the potential atmospheric reaction processes for the HFCO sink.

The reaction of HFCO + H2O has been considered in the atmosphere. However, the direct reaction of formyl fluoride with H2O has a quite high energy barrier of 43.1 kcal/mol calculated by MP4/6-311++G**//MP2/6-311G**;[40] this shows that this process is of negligible importance in the atmosphere. It is seen from both experimental and theoretical investigations that the atmospheric water and acids can assist the hydrolysis reactions of various atmospheric molecules in the gas phase,[41−55] while hydrolysis reactions play an important role in atmospheric chemistry. Experimental studies by Vaida and colleagues have shown that the gas-phase hydrolysis of aldehydes takes place in the presence of water.[41,42] Similarly, theoretical methods have been used to study the reaction mechanisms and kinetics of the gas-phase hydrolysis of some species catalyzed by acids in the atmosphere. For example, theoretical methods were used to investigate the gas-phase reactions of sulfuric acid-catalyzed hydrolysis of acetaldehyde,[43] gas-phase hydrolysis of HCHO assisted by H2SO4, HNO3, and CH3COOH,[44,45] and the gas-phase hydrolysis of SO3 catalyzed via HCOOH, H2SO4, and HNO3.[46−49] Thus, further investigation is expected to show whether the hydrolysis of formyl fluoride can be feasible in the atmosphere.

In this investigation, the mechanisms and kinetics of HFCO hydrolysis reaction catalyzed by sulfuric acid and formic acid have been studied by quantum chemical methods and conventional transition state theory with Eckart tunneling. The reactions involved are as follows.

Here, we have found that sulfuric acid exerts a significantly catalytic role in the hydrolysis of HFCO and a mechanistic pathway for the formation of formic acid. However, the comparative relative rate analysis shows that the gas-phase hydrolysis of HFCO may only occur with the assistance of H2SO4 when the concentration of OH is about 101 molecules cm–3 or less and that the HFCO + OH reaction is still the most important dominant pathway for the sink of HFCO in the atmosphere. Therefore, the present results should have an understanding of the sink of HFCO and stimulate one to study the gas-phase hydrolysis of other carbonyl compounds in the atmosphere.

Results and Discussion

We used the M06-2X/6-311++G(3df,3pd) theoretical method to investigate the HFCO gas-phase hydrolysis reactions. We report the energy barriers of HFCO + H2O catalyzed by H2SO4/HCOOH with zero-point vibrational energies corrected at the CCSD(T)-F12a/cc-pVTZ-F12//M06-2X/6-311++G(3df,3pd) level of theory in Table S1 (Supporting Information).

Gas-Phase Hydrolysis of HFCO

We reinvestigate the HFCO + H2O reaction to further show the catalytic abilities of sulfuric acid and formic acid. There are two transition state structures (TS1A and TS2A) in the HFCO + H2O reaction, as listed in Figure . The reaction starts with the formation of the prereactive complexes C1A and C2A and undergoes the corresponding transition states TS1A and TS2A responsible for the formation of FCH(OH)2 in Figure , respectively.

Figure 1

Calculated potential energy profile for the HFCO + H2O reaction with zero-point vibrational energies corrected at the CCSD(T)-F12a/cc-pVTZ-F12//M06-2X/6-311++G(3df,3pd) level of theory (in kcal/mol).

The calculated binding energies of C1A and C2A are −2.21 and −2.06 kcal/mol with respect to the separate reactants, respectively. The values of the binding energies of C1A and C2A are close to the value (−1.62 kcal/mol) of the corresponding prereactive complex between HCHO and H2O calculated by the CCSD(T)/aug-cc-pv(T+d)z//M06-2X/6-311++G(3df,3pd) level of theory.[44] The energy barriers of TS1A and TS2A are computed to be 41.43 and 43.25 kcal/mol, respectively, which compare well with the previously theoretical results of 43.1 kcal/mol calculated by the MP4SDTQ/6-311G**//MP2/6-311G** level[40] and 42.1 kcal/mol calculated by the MP2/6-311++G(d,p) level,[56] respectively. The calculated results again show that the reaction of HFCO + H2O is negligible in the atmosphere.

Gas-Phase Hydrolysis of HFCO Catalyzed by H2SO4 and HCOOH

When H2SO4 and HCOOH are acted as catalysts, the probability of the three molecules colliding simultaneously is quite low for the HFCO + H2O + H2SO4/HCOOH reaction in the atmosphere. Therefore, we consider the three possible bimolecular collisions responsible for the formation of the three possible dimer species; this leads to the three different entrance channels in each reaction as listed in eqs –R9 and Figures –5.

Figure 2

Calculated potential energy profile for the HFCO + H2O + H2SO4 reaction by sulfuric acid approaching the side of F atom in HFCO with zero-point vibrational energies corrected at the CCSD(T)-F12a/cc-pVTZ-F12//M06-2X/6-311++G(3df,3pd) level of theory (in kcal/mol).

Figure 5

Calculated potential energy profile for the HFCO + H2O + HCOOH reaction by sulfuric acid approaching the side of F atom in HFCO with zero-point vibrational energies corrected at the CCSD(T)-F12a/cc-pVTZ-F12//M06-2X/6-311++G(3df,3pd) level of theory (in kcal/mol).

The mechanism for the gas-phase hydrolysis of HFCO catalyzed by H2SO4 is similar to the mechanism for the reaction of HCHO + H2O catalyzed by H2SO4.[43,44] Similarly, the HFCO + H2O + H2SO4 reaction proceeds by stepwise mechanistic pathways. However, a fluorine atom substitutes one hydrogen atom in HCHO, which results in the loss of C2 symmetry in HFCO. Thus, there are two different pathways for the gas-phase hydrolysis of HFCO catalyzed by H2SO4. One pathway starts with the formation of the C3A complex before the reaction, goes through the transition state TS3A to form C3B, and undergoes the subsequent transition state TS3B responsible for the formation of P3B, as shown in Figure . The other pathway undergoes a similar process, as shown in Figure .

Figure 3

Calculated potential energy profile for the HFCO + H2O + H2SO4 reaction by sulfuric acid approaching the side of H atom in HFCO with zero-point vibrational energies corrected at the CCSD(T)-F12a/cc-pVTZ-F12//M06-2X/6-311++G(3df,3pd) level of theory (in kcal/mol).

The calculated binding energy of H2O···H2SO4 is −10.45 kcal/mol, which is consistent with the value of −10.64 kcal/mol by CCSD(T)/aug-cc-pv(T+d)z//M06-2X/6-311++G(3df,3pd) reported in the literature.[44] The binding energies of the prereactive complexes C3A and C4A are −10.98 and −13.27 kcal/mol, respectively, which are 7.60 and 5.31 kcal/mol lower than those of the corresponding prereactive complexes (C21) in the HCHO + H2O + H2SO4 reaction at the CCSD(T)/aug-cc-pv(T+d)z//M06-2X/6-311++G(3df,3pd) level, respectively.[44] C3A and C4A undergo almost a barrierless process, leading to the formation of the C3B and C4B complexes.

Compared with the HFCO hydrolysis reaction without the catalyst, the energy barriers of TS3B and TS4B with respect to C3B and C4B are reduced to 13.36 and 13.77 kcal/mol from 41.43 and 43.25 kcal/mol, respectively. However, it is still higher than the energy barrier of the corresponding transition state in the HCHO + H2O + H2SO4 reaction because the energy barrier of the corresponding transition state in the HCHO + H2O + H2SO4 reaction (C22 → TS22) is ∼8.50 kcal/mol.[44] Obviously, H2SO4 has a positive effect on reducing the reaction energy barrier of the HFCO + H2O reaction. It is also shown that H2SO4 has different catalytic effects with different reactants.

The reaction mechanism for the gas-phase hydrolysis of HFCO catalyzed by HCOOH is similar to that of the gas-phase hydrolysis of HFCO catalyzed by H2SO4 as provided in Figures and 5. Similarly, there are still two different pathways for the HFCO + H2O + HCOOH reaction. The binding energy (−7.88 kcal/mol) of the calculated dimer complex H2O···HCOOH is consistent with the values obtained in the literature, −7.97 kcal/mol by the CCSD(T)/6-311++G(3df,3pd)//MP2/6-311++G(3df,3pd) level of theory.[57] The rate-determining step of the reaction (HFCO + H2O + HCOOH) is still the gas-phase hydrolysis of HFCO with a concerted mechanism that H2O molecule is decomposed into OH group and H atom, the H atom of H2O is transferred to O atom of C=O group in HCOOH, and the H atom of OH group in HCOOH is migrated to O atom in HFCO.

Figure 4

Calculated potential energy profile for the HFCO + H2O + HCOOH reaction by sulfuric acid approaching the side of H atom in HFCO with zero-point vibrational energies corrected at the CCSD(T)-F12a/cc-pVTZ-F12//M06-2X/6-311++G(3df,3pd) level of theory (in kcal/mol).

Regarding to the transition states, TS5B and TS6B have energy barriers of 16.0 and 16.49 kcal/mol to C5B and C6B, respectively. Compared with the HFCO + H2O reaction without catalyst, the energy barriers of the HFCO + H2O + HCOOH reaction are decreased by 25.43 and 26.76 kcal/mol, respectively. However, it is still higher than the energy barrier of the corresponding transition state in the HCHO + H2O + HCOOH reaction because the energy barrier of the corresponding transition state in HCHO + H2O + HCOOH reaction is ∼9.84 kcal/mol at the MP2/6-311++G(3df,3pd) level of theory.[53]

Unimolecular Decomposition of FCH(OH)2 Catalyzed by H2SO4 and HCOOH

FCH(OH)2 dominantly decomposes into HF and HCOOH reported by Francisco and Williams.[40] However, the energy barrier for the FCH(OH)2 → HF + HCOOH reaction has been reported to be about 28.6 kcal/mol.[40] Thus, the direct decomposition of FCH(OH)2 into HF and HCOOH is negligible in the atmosphere. Here, the calculated results indicate that the energy barrier via TS7A (FCH(OH)2 → HF + HCOOH) is estimated to be 31.63 kcal/mol (see Figure ).

Figure 6

Calculated potential energy profile for the decomposition of FCH(OH)2 catalyzed by H2SO4 and HCOOH with zero-point vibrational energies corrected at the CCSD(T)-F12a/cc-pVTZ-F12//M06-2X/6-311++G(3df,3pd) level of theory (in kcal/mol).

As can be seen from Figure , the energy barriers for H2SO4 and HCOOH to catalyze the unimolecular decomposition of FCH(OH)2 into HF and HCOOH are calculated to be −5.99 and 1.18 kcal/mol, which are reduced by 37.59 and 30.45 kcal/mol, respectively, with regard to the respectively separate reactants. Therefore, H2SO4 plays a better catalytic role in the unimolecular decomposition of FCH(OH)2 into HF and HCOOH.

Reaction Kinetics

The rate constants of the reactions investigated here were calculated using conventional transitional state theory with Eckart tunneling at temperatures between 200 and 298 K. We consider different temperatures because there are different temperatures at different areas in the ground level of earth’s atmosphere. The reaction in this study involves three molecules, and the detailed expression of the rate equation is given by taking the reaction HFCO···H2O + H2SO4 pathway as an example in the Supporting Information. The rates are listed in eqs –12.

In the above equations, Keq1, Keq2, Keq3, Keq4, and Keq5 refer to the equilibrium constants of the complexes HFCO···H2O, H2O···H2SO4, HFCO···H2SO4, H2O···HCOOH, and HFCO···HCOOH, respectively. k1 is the rate constant of the bimolecular reaction of HFCO + H2O. The k2′, k2″, and k2‴ represent the bimolecular rate constants of the HFCO···H2O + H2SO4, HFCO + H2O···H2SO4, and HFCO···H2SO4 + H2O reactions, respectively. k3′, k3″, and k3‴ denote the bimolecular rate constants of the HFCO···H2O + HCOOH, HFCO + H2O···HCOOH, and HFCO···HCOOH + H2O reactions, respectively. k4 is the unimolecular rate constant of FCH(OH)2 decomposition into HF and HCOOH. k5 and k6 express the bimolecular rate constants of the FCH(OH)2 + H2SO4 and FCH(OH)2 + HCOOH reactions, respectively. Here, [H2SO4] and [HCOOH] represent the concentrations of H2SO4 and HCOOH, which are 4 × 108 molecules cm–3[58] and 2 × 1011 molecules cm–3,[59] respectively. We consider the fixed concentrations of H2SO4 and HCOOH only for an understanding of the atmospheric lifetime of HFCO. We calculated the equilibrium constants, rate constants, and tunneling for the individual reaction pathway at the temperature range of 200–298 K, as listed in Tables S2 and S3. In Table S3, it can be seen that tunneling remarkably accelerate the HFCO + H2O → FCH(OH)2 and FCH(OH)2 → HF + HCOOH reactions because the tunneling coefficients are estimated to be 1011–102, 106–101 at 200–298 K, respectively.

Considering that there are the three different entrance channels in the reactions of HFCO + H2O + H2SO4 and HFCO + H2O + HCOOH, respectively, we judge the importance of the three different entrance channels by the rate ratios as provided in eqs –16.

We calculated the rate ratios of the three entrance channels for the HFCO + H2O + H2SO4 reaction and the rate ratios of the three entrance channels for the HFCO + H2O + HCOOH reaction. The results are listed in Table . The rate ratio of v2′ to v2″ is 7.75 × 10–1–9.34 × 10–1 and the rate ratio of v2‴ to v2″ is 2.43 × 10–1–2.49 × 10–1 at the temperature range of 200–298 K; this shows that the three entrance channels are important for the HFCO + H2O + H2SO4 reaction. Therefore, v2 is the sum of the rates of the three entrance channels. The similar tendency in the HFCO + H2O + HCOOH reaction is observed because the rate ratio of v3′ to v3″ is 9.70 × 10–1–9.95 × 10–1 and the rate ratio of v3‴ to v3″ is 4.95 × 10–1–4.99 × 10–1 at the temperature range of 200–298 K. Therefore, the three different entrance channels of the HFCO + H2O + HCOOH reaction should be considered and v3 is the sum of the rates of the three entrance channels.

Table 1

Rate Ratio for the Individual Reaction Pathway at the Temperature Range of 200–298 K

reaction	200 K	220 K	240 K	260 K	280 K	298 K
v2′/v2″	7.75 × 10–1	8.34 × 10–1	8.74 × 10–1	9.01 × 10–1	9.20 × 10–1	9.34 × 10–1
v2‴/v2″	2.43 × 10–1	2.46 × 10–1	2.47 × 10–1	2.48 × 10–1	2.49 × 10–1	2.49 × 10–1
v3′/v3″	9.70 × 10–1	9.81 × 10–1	9.87 × 10–1	9.91 × 10–1	9.94 × 10–1	9.95 × 10–1
v3‴/v3″	4.95 × 10–1	4.97 × 10–1	4.98 × 10–1	4.99 × 10–1	4.99 × 10–1	4.99 × 10–1
v2/v1	1.20 × 1015	6.04 × 1013	3.11 × 1012	1.62 × 1010	8.50 × 109	6.07 × 108
v3/v1	8.65 × 1011	1.28 × 1011	1.62 × 1010	1.80 × 109	1.80 × 108	2.13 × 107
v5/v4	5.50 × 1015	6.75 × 1013	9.63 × 1011	1.57 × 1010	3.03 × 108	1.07 × 107
v6/v4	1.05 × 1011	5.86 × 109	3.00 × 108	1.45 × 107	7.16 × 105	5.32 × 104
v2/vOH	2.04 × 10–5	1.40 × 10–5	1.05 × 10–5	8.42 × 10–6	7.10 × 10–6	6.29 × 10–6

We compare the HFCO + H2O reaction with the HFCO + H2O + H2SO4 and HFCO + H2O + HCOOH reactions to estimate the catalytic effects of each catalytic reactions. The rate ratios are expressed by eqs and 18.

The calculated data are listed in Table . It is worth noting that v2 is 15–8 orders of magnitude higher than v1, while v3 is 11–7 orders of magnitude higher than v1 at the temperature range of 200–298 K; this again shows that H2SO4 has a remarkably catalytic effect on the HFCO + H2O + H2SO4 reaction.

We consider the rate ratios of v5 to v4 and v6 to v4 as shown in eqs and 20.

The calculated results show that H2SO4 and HCOOH can effectively catalyze the decomposition of FCH(OH)2 because the rate ratios of v5/v4 and v6/v4 are calculated to be 1015–107 and 1011–104 at the temperature range of 200–298 K, respectively. Therefore, the calculated kinetics again shows that H2SO4 can accelerate the decomposition of FCH(OH)2 responsible for the formation of formic acid.

Atmospheric Implications

Previous studies have shown that the dominant sink of HFCO is its reaction with OH radical.[28,29] Therefore, it is necessary to compare the hydrolysis of HFCO catalyzed by H2SO4 in the gas phase with the HFCO + OH reaction. The rate ratio is written in eq

kOH is the rate constant of the HFCO + OH reaction in the atmosphere, which is an upper limit of 4 × 10–15 cm3 molecule–1 s–1 at 298 K obtained from experimental results.[31] We use the fixed concentrations of OH and H2O just to understand the atmospheric lifetime of FCHO. When [OH] is 1 × 106 molecules cm–3 [32] and [H2O] is 3.80 × 1017 molecules cm–3,[60] the ratio of v2/vOH is about 10–5–10–6 at the temperature range of 200–298 K; this shows that the reaction between HFCO and the OH radical is still the most dominant pathway when tropospheric degradation of HFCO is considered. Furthermore, the calculated atmospheric lifetime of HFCO with respect to the HFCO hydrolysis reaction in the presence of H2SO4 is ∼1.26 × 106 years at 298 K (see Table S5) when the concentrations of H2O and H2SO4 are 3.80 × 1017 molecule cm–3 [60] and 4 × 108 molecules cm–3,[58] respectively, which is about 6 orders of magnitude higher than the above-said ∼8 years in the HFCO + OH reaction. Hence, it is negligible for the atmospheric impact of the gas-phase hydrolysis of HFCO in the presence of H2SO4 in comparison to its reaction with the OH radical. In general, the concentration of the OH radical is quite high during both daytime and nighttime.[61,62] However, when there is the absence of sunlight, the concentration of the OH radical in the troposphere is decreased significantly.[63−66] Previous investigations have shown that the concentration of OH at night is between 0 and 2 × 105 molecule cm–3.[67,68] Thus, it is possible that the concentration of OH is decreased to be 101 molecule cm–3 at some special areas. When the concentration is 101 molecule cm–3, the rate ratio of v2/vOH is 2.04 × 100–6.29 × 10–1 at the temperature range of 200–298 K. This shows that the gas-phase hydrolysis of HFCO can only occur in the presence of H2SO4 when the atmospheric concentration of OH is about 101 molecules cm–3 or less.

The HFCO + H2O + H2SO4 reaction is also expected to occur on the surface of secondary organic aerosols (SOAs) in the troposphere to form FCH(OH)2 firstly and then the hydrogen fluoride (HF) and HCOOH from the decomposition of FCH(OH)2 in the presence of H2SO4 and HCOOH. This is because the gas-phase HFCO + H2O + H2SO4 reaction may be more facile on the surface of secondary organic aerosols (SOAs), as secondary organic aerosols contain H2SO4, HCOOH, and H2O.[69,70] Moreover, some studies have shown that some atmospheric reactions occur more facilely on the surface of the secondary organic aerosol (SOA) by the assistance of atmospheric acids,[71] where the H2SO4···H2O complexes exist.[69,72] The formed HCOOH can strongly interact with H2SO4 to lead to the formation of the nucleation precursor.[70,73,74] Therefore, the HFCO + H2O + H2SO4 reaction may contribute a part to the formation of secondary organic aerosols.

Conclusions

In this article, we have studied the gas-phase hydrolysis of HFCO catalyzed by H2SO4 and HCOOH via using quantum chemical methods and conventional transition state theory with Eckart tunneling. We have found that H2SO4 has a more significantly catalytic ability to promote the gas-phase hydrolysis of HFCO responsible for the formation of FCH(OH)2 and the decomposition of FCH(OH)2 into HCOOH and HF than HCOOH. Our results from the comparative rate analysis indicate that the reaction between HFCO and the OH radical is still the most dominant pathway for the tropospheric degradations of HFCO and that the gas-phase hydrolysis of HFCO catalyzed by H2SO4 has a very limited role possibly in the absence of sunlight. In addition, the hydrolysis of HFCO catalyzed by H2SO4 may occur on the surface of secondary organic aerosols (SOAs).

Computational Methods

All of the electronic calculations in this article were carried out using Gaussian 09[75] and Molpro 2018.[76] The geometrical structures of all reactants, transition states, complexes, and products were optimized by the functional of M06-2X[77] with the 6-311++G(3df,3pd) basis set.[78,79] Previous investigations have shown that M06-2X/6-311++G(3df,3pd) theoretical method can obtain more accurate structures and free energies for the atmospheric clusters with sulfuric acid involved than other methods.[80] The frequencies of the optimized structures were computed at the same theoretical level to show the optimized transition state with only one imaginary frequency and other stationary points without imaginary frequencies. Additionally, an intrinsic reaction coordinate[81] was used to obtain the reaction path, where the transition state is connected with the corresponding reactants and products. To obtain the relative energies more reliably, coupled cluster theory with single and double excitations and noniterative triple excitations with the simplified explicit correlation (CCSD(T)-F12a)[82,83] theoretical method was used to calculate the single-point energies of optimized geometries at the cc-pVTZ-F12 basis set based on M06-2X/6-311++G(3df,3pd) optimized geometries. Finally, the rate constants for each reaction path were calculated by conventional transition state theory[84−86] with Eckart tunneling,[87] which executes using TheRate code.[88,89]