The Atmospheric Oxidation of Furan and Methyl Substituted Furans

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The Atmospheric Oxidation of Furan and Methyl Substituted Furans Initiated by Hydroxyl Radicals Yi Yuan, Xiaocan Zhao, Sainan Wang, and Liming Wang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b09741 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017

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The Journal of Physical Chemistry

The Atmospheric Oxidation of Furan and Methyl Substituted Furans Initiated by Hydroxyl Radicals Yi Yuan,1 Xiaocan Zhao,1 Sainan Wang,1 and Liming Wang1,2* 1

School of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou

510640, China. 2

Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, South

China University of Technology, Guangzhou 510006, China.

ACS Paragon Plus Environment

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ABSTRACT Atmospheric oxidation mechanism of furan and methyl furans (MFs) initiated by OH radicals is studied using high-level quantum chemistry and kinetic calculations. The reaction starts mainly with OH addition to C2/C5-position, forming highly chemically activated adduct radical R2*/R5*, which would either be stabilized by collision or promptly isomerize to R2B*/R5B* by breaking the C2-O/C5-O bond and then isomerize to other conformers of R2B/R5B by internal rotations. Under the atmospheric conditions, the ring-retaining radical R2/R5 would recombine with O2 and be converted to a 5-hydroxy-2-furanone compound and a compound containing epoxide, ester, and carbonyl functional groups, while the ring-opening radicals R2B/R5B would react with O2 and form unsaturated 1,4-dicarbonyl compounds. RRKM-ME calculations on the fate of R2*/R5* from the addition of OH and furans predict the fractions of R2B/R5B formation, i.e., the molar yields of the corresponding dicarbonly compounds, are 0.73, 0.43, 0.26, 0.07, and 0.28 for furan, 2-MF, 3-MF, 2,3-DMF, and 2,5-DMF, respectively, at 298 K and 760 Torr when using the RHF-UCCSD(T)-F12a/cc-pVDZ-F12 reaction energies and barrier heights. The predicted yields for dicarbonyl compounds agree reasonably with recent experimental measurements. Calculations here also suggest high yields of ring-retaining 5-hydroxy-2-furanone compounds, which might deserve further study.


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1. Introduction Furan and its methyl substituents are emitted into the atmosphere from several sources such as combustion of fossil fuels, biomass, and waste,1 and natural sediment and water samples,2 etc. 2-Methylfuran and 2,5-dimethylfuran, as the promising new generation of alternative biofuels, have the potential to enter the atmosphere in a large amount.3-4 In the atmosphere, furans are also formed in situ in the degradation of conjugated dienes, e.g., formation of furan in the oxidation of 1,3-butadiene with yield of 1~4% and formation of 3-methylfuran in the oxidation of isoprene with yield of a few percent.5-8 The presence and oxidation of furans in the atmosphere may cause degradation of air quality. The atmospheric oxidation of furans could be initiated by their reactions with O3, OH, NO3, and Cl-atom etc. For furan, rate coefficients at room temperatures and air pressures have been reported as ~2.4 × 10–18, ~4 × 10–11, ~1.2 × 10–12, and ~2 × 10–10 cm3 molecule–1 s–1 for its reactions with O3,9 OH radical,9-11 NO3 radical,12-13 and Cl-atom,14 respectively, leading to its removal mainly by OH radical during daytime and by NO3 radical during nighttime. Similarly, the reported rate coefficients for 2-/3-methylfuran (2-/3-MF) and 2,3-/2,5-dimethylfurane (2,3/2,5-DMF) also suggest their fast removal by OH radical during daytime and by NO3 radical during nighttime.10,15-16 The reaction of furan with O3 in the atmosphere is rather minor; however, it might be significant and even dominant for 2,5-DMF during daytime with a reported rate coefficient of (4.2 ± 0.9) × 10–16 cm3 molecule–1 s–1 at room temperature,17 comparing to the rate coefficient of ~1.3 × 10–10 cm3 molecule–1 s–1 for its reaction with OH radical15 if assuming average concentrations of 8 × 1011 molecules cm–3 and 1 × 106 molecules cm–3 for O3 and OH, respectively. Reactions of other MFs with O3 might also be important in the atmosphere and in smog chamber studies where high levels of MFs and NOx might form a large amount of O3 in the photo-oxidation processes. The oxidation mechanism of furan initiated by OH radical has been proposed from product measurements. Bierbach et al.18 identified Z/E-butenedial as the main products with molar yield of >0.70 at ~300 K in the absence of NOx using in situ FTIR absorption spectroscopy. They proposed a possible route for butenedial formation as


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In the presence of NOx, Gómez Alvarez et al.19 obtained molar yields of 1.09 ± 0.41 and 0.90 ± 0.36 (in fraction, the same below) for butenedial in two measurements in the EUPHORE chamber using Solid Phase Microextraction sampling and GC-FID quantification, and Aschmann et al.20 obtained a molar yield of (0.75 ± 0.05) for butenedial using GC-FID detection. All these measurements found that butenedial was formed as mixtures of E- and Z-conformers while favoring the E-conformer. Aschmann et al. suggested several reaction routes based on the products observed in the reaction of furan-d4 using API-MS and API-MS/MS analysis; however, only one single pathway could likely be ruled out from the proposed mechanism. The measurements by Bierbach et al. were carried out under the conditions of reasonably high HO2 concentration because the OH radical was generated from photolysis of H2O2; while high NO conditions were present in the measurements by Gómez Alvarez et al. (~5 × 1011 molecules cm– 3 19

) and Aschmann et al. (~1.2 × 1014 molecules cm–3).20 High NO might suppress the possible

unimolecular reactions of intermediate peroxy radicals, and change the degradation pathway of furan. Furthermore, the reactions of peroxy radicals with HO2 and NO have significantly different products. However, the large uncertainties on the measured yields prohibited the analysis on the effect of NOx or HO2. Formation of unsaturated dicarbonyl compounds from the ring opening of the (D)MF-OH adducts were also reported in oxidation of (D)MFs initiated by OH radical. Aschmann et al.20 observed the markedly decreased yields of unsaturated dicarbonyl compounds from furan to MFs and to DMFs, reporting molar yields of 0.31 ± 0.05 and 0.38 ± 0.02 for 2- and 3-MF, and 0.08 ± 0.02 and 0.27 for 2,3- and 2,5-DMF. The molar yields from 2- and 3-MFs reported by Aschmann et al. were significantly lower than those of 0.60 ± 0.24 and 0.83 ± 0.33 reported by Gómez Alvarez et al.19 Without correcting the reaction loss of dicarbonyl compounds due to their secondary reaction with OH radical, Tapia et al.,21 in the oxidation of 3-MF, reported a much lower yield of 0.014 ± 0.003 for 2-methylbutenedial, but they also reported relatively higher yields of 0.039 ± 0.018 and 0.181 ± 0.046 for the ring-retaining products 3-furaldehyde and 3methyl-2,5-furanodione. The study by Tapia et al. is so far the only one reporting the ringretaining products, though Strollo and Ziemann22 also identified ring-retaining hydroxyfuranone


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in secondary organic aerosol formed in oxidation of 10 ppmv each for 3-MF, CH3ONO, and NO. No other information is available on the ring-retaining products for other (D)MFs. Also it is unclear why the yields of ring-opening products could change so much from furan to methylated furans. Theoretical studies had also been carried out on reactions of furan and methyl furans with OH radical. For the reaction of furan, Mousavipour et al.23 and Anglada24 found the dominance of OH addition to C2-position at the atmospheric temperatures, forming Furan-2-OH adduct. Both studies used UMP2 method to optimize the structures of molecules and transition states. Mousavipour et al. also estimated the effective bimolecular rate coefficients for product channels from Furan-2-OH adduct using multichannel RRKM-TST calculations, and concluded that Furan-2-OH adduct would undergo ring breakage almost completely at 760 Torr, even though the ring-opening of Furan-2-OH adduct is almost energetically neutral. The kinetic model by Mousavipour et al. is valid only when O2 is not present, and cannot be applied to the atmospheric oxidation of furan for two reasons. Firstly, Furan-2-OH adduct can be captured by oxygen and be oxidized via different routes in the atmosphere. Secondly, the ring-opening of Fural-2-OH adduct should be reversible, therefore Furan-2-OH adduct should sustain to certain extent. Zhang et al.25-26 found similar reaction pathways for the reactions of 2-MF and 3-MF with OH radical. They predicted similar relative energies and barrier heights for the ring-opening of adducts formed in 2-MF and 3-MF, and suggested similar reaction mechanism without, however, kinetic calculations. Theoretical and modeling studies on the reactions furans and OH radical were also performed in the interest of their combustion,27 where the reaction conditions and then mechanism are drastically different. In this respect, current understanding on the atmospheric oxidation mechanism of furans is considerably weak. In this work, we carried out a systematic study on the mechanism of OHinitiated atmospheric oxidation of furan and methyl furans by using quantum chemistry and kinetic calculations, attempting to rationalize the difference between furan and methyl furans using furan, 2-MF, 3-MF, 2,3-DMF, and 2,5-DMF as prototype and to propose their atmospheric oxidation mechanism. 2. Theoretical Methods


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The molecular structures were optimized at the M06-2X/6-311++G(2df,2p) level, which was found to be suitable for kinetics study.28 The transition states were confirmed by the presence of one imaginary frequency associated with the reaction coordinate, by viewing the structure and vibration vector, and for some of them by integrating the intrinsic reaction coordinate (IRC).29 The optimized M06-2X structures were submitted to single-point energy calculation at theoretical levels of ROCBS-QB3 model chemistry (CBS in short)30 and explicitly correlated coupled-cluster theory at the RHF-UCCSD(T)/F-12a level with cc-pVDZ-F12 and cc-pVTZ-F12 basis sets (F12/VDZ ond F12/VTZ in short).31-32 The quality of electron correlation in CCSD(T)F12 with VDZ-F12/VTZ-F12 basis set is better than that in standard CCSD(T) with ccpVQZ/cc-pV5Z basis set, but CCSD(T)-F12 is two orders of magnitude faster. The M06-2X and CBS calculations were carried out using the Gaussian 09 package,33 and F12 using Molpro 2015 package.34-35 The reaction rate constants at high-pressure limit were estimated by using the traditional transition states,36-37 () = ∙ ∙

10 ∆ G ∙ exp − ∙ ° ∙ ℎ

where is the Boltzmann constant, ∆ G is the Gibbs energy barrier, is the reaction path

degeneracy, ! is 1 or 2 for uni- or bi-molecular reaction, and is the tunneling correction factor

which is calculated using the asymmetric Eckart model.38 The kinetics of the chemically

activated Furan-OH and (D)MF-OH adducts should be pressure dependent; therefore the kinetics were also modeled by the unimolecular rate theory coupled with master equation for collisional energy transfer (RRKM-ME)39 by using the MESMER code.40 Single exponential down model

was used to approximate the collisional energy transfer with 〈∆#〉%&'( of 250 cm–1. The

collision frequencies were estimated by the method of Gilbert and Smith41 and the tunneling correction factors were included to the microcanonical rates by the method of Miller.42 3. Results and Discussion: The Oxidation of Furan 3.1 The Initial OH Addition to Furan

As being proposed by Bierbach et al.18 and confirmed theoretically by Mousavipour et al.,23 the oxidation of furan initiated by OH radical proceeds as OH addition to C2 and C3 positions, forming activated R2* and R3* adducts. Because of the electron delocalization in R2, the


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addition to C2 is much more exothermic than that to C3 (–139.3 versus –66.5 kJ/mol at F12/VTZ level), and the barrier height for the addition to C2 is ~12 kJ/mol lower than that to C3, making the addition to C3 negligible. Therefore, only the addition to C2 is considered here. The addition is mediated by formation of two types of pre-reaction complexes (PRCs, see Figure S1 for structures), which would isomerize to adduct R2. The chemically activated R2* could be deactivated by collision or break the ring promptly, forming R2B-Z1 and then other R2B conformers. Figure 1 shows the potential energy surface for the OH addition and the ensuing ring-opening and isomerization processes, and Tables 1 and S1 list the detailed energy values of the PRCs, transition states, radical intermediates, and product channels at different levels of theory. The relative energies of intermediate radicals and transition states agree within 5 kJ/mol between correlation levels of CBS, F12/VDZ, and F12/VTZ. The differences between F12/VDZ and F12/VTZ are all within 1 kJ/mol except for transition states for OH additions and ring breakages of R2 radical. By using the traditional transition state theory, a rate coefficient at 298 K could be obtained as 9.63 × 10–11 cm3 molecule–1 s–1 if using CBS barrier heights, 7.30 × 10–11 cm3 molecule–1 s–1 if using F12/VDZ barrier heights, or 4.64 × 10–11 cm3 molecule–1 s–1 if using F12/VTZ barrier heights, all agreeing reasonably with the experimental value of ~4 × 10–11 cm3 molecule–1 s–1.9-11 Two transition states were identified for the ring opening of R2 radical. One is the transition state to R2B–Z1 conformer with a lower barrier (–60.9 kJ/mol, relative to furan + OH, F12/VTZ) and the other one is to R2B–Z3 conformer with a higher barrier (–38.7 kJ/mol). R2B radical can be viewed as a mixing of three resonance structures,

The bonds C2-C3, C3-C4, and C4-C5 are all rotatable, resulting in eight possible conformers for R2B (Scheme 1). Tables 1 and S1 list the relative energies of the eight conformers and transition states for these internal rotations. At M06-2X level, the bond lengths in R2B-Z1 are 1.369, 1.393, and 1.441 Å for the C2–C3, C3–C4, and C4–C5 bonds, respectively. Consequently, transition states for internal rotations around the C4–C5 bond (rotation of –CHO group) are the lowest in energy, all being less than –92 kJ/mol (relative to Furan + OH, F12/VTZ), while the transition states for rotations around the C2–C3 bonds (rotation of –CHOH group) are the highest in energy, all being higher than –57 kJ/mol. From the initially formed R2B–Z1 conformer in ring-opening


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of R2, conformers R2B–Z2, –E1, and –E2 might be reached via transition states with their relative energies less than –75 kJ/mol. In the atmosphere, all R2B conformers would also react with O2 rapidly at effective rates of >105 s–1 at 298 K (see below for kinetics between R2B and O2). We carried out RRKM-ME calculations on the reaction shown in Figure 1 (with all eight R2B conformers) in the absence of O2. Figure 2A shows the time profiles for the yields of R2 and R2B isomers in the course of reaction. Formation of R2* is instantaneous, but a fraction of the energized R2* radical would isomerize promptly to the R2B-Z conformers and then to R2B-E conformers within 10–7 s–1 of reaction time. As the reaction proceeds, the promptly formed R2B isomers become thermalized, and fast isomerizations within Z-conformers and within Econformers lead to equilibria within them in 10–7 s–1 at 298 K and even down to 243 K. After 10– 7

s, the yields for R2, ΣR2B-Zi, and ΣR2B-Ei remain almost constant. The E-conformers account

for 55~60% of R2B formed in temperature range of 243–343 K. Figure S2A shows the distributions of internal energy possessed by the chemically activated R2* radical at reaction time of 10–9, 10–8, and 10–7 s, showing Boltzmann distribution in R2 at 10-7 s while substantial activation at 10–8 s. In the presence of O2, the radicals R2, R2B-Z, and R2B-E would react with O2. Recombination between R2 and O2 under the atmospheric pressure and temperatures are fast, usually at effective rates of ~5 × 107 s–1 with the estimated high-pressure-limit rate coefficients of ~10–11 cm3 molecule–1 s–1 and [O2] of ~5 × 1018 molecules cm–3; however, within the first 5 × 10–8 s of reaction in furan + OH, the recombination of the chemically activated R2* with O2 would be very slower in capturing R2. Only after the initial ~10–7 s when the fractions of R2, ΣR2B-Zi, and ΣR2B-Ei are almost constants, the radicals become thermalized and their reactions with O2 become efficient. Therefore the branching ratios for R2, ΣR2B-Zi, and ΣR2B-Ei can represent the product yields from these radicals. It is shown below that the reaction of R2 with O2 forms peroxy radical (R2-OO) and the reaction of R2B-Zi/-Ei forms E-/Z-butenedial. The branching ratios of R2B-Z and R2B-E radicals could then compare directly with the experimental yield of butenedial and the ratio of E- to Z-butenedial. Table 2 lists the yield of R2, ΣR2B-Zi, and ΣR2B-Ei at 760 Torr and 243–343 K. At 298 K, the predicted butenedial yield of 0.76 if using F12/VTZ energies (0.73 or 0.77 if using F12/VDZ or CBS energies) agrees with the experimental value of >0.70 by Bierbach et al.,18 of 1.09 ± 0.41 and 0.90 ± 0.36 by Gómez


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Alvarez et al.,19 and of 0.75 ± 0.05 by Aschmann et al.,20 and the E/Z ratio of 0.43 : 0.33 (0.41 : 0.32 or 0.47 : 0.33) also agrees with the favoring production of E-butenedial observed in the experimental studies.19-20 Expectedly, yield of R2B radicals increases with the increase of reaction temperature (Figure 2B, Table S2). Moreover, the presence of HO2 or NO would have negligible effect on the yield of butenedial from the reaction of R2B radicals and O2 because the intermediate peroxy radicals R2B-O2 decompose rapidly to butenedial and HO2 at rates of >105 s–1 (see Section 3.5). 3.2 The Initial OH Additions to Methyl Furans Addition of OH radical to methyl furans is similar to that to furan. Tables 1 and S1 list the relative energies of transition states and intermediate radicals formed in the reactions of furans and OH radical at levels of ROCBS-QB3, F12/VDZ, and F12/VTZ. The overall rate coefficients at 298 K, in 10–11 cm3 molecule–1 s–1, were obtained as 11.6, 18.8, 125, and 63.3 if using F12/VTZ barrier heights (18.9, 25.5, 203, and 105 if using F12/VDZ barrier heights, or 34.1, 54.1, 583, and 231 if using CBS barrier heights), comparing to the experimental values of (7.31 ± 0.85), (8.73 ± 0.18), (12.6 ± 0.4), and (12.5 ± 0.4) for 2-MF, 3-MF, 2,3-DMF, and 2,5-DMF, respectively.15 The increase of rate coefficients from furan to MFs and to DMFs arises from the electron-donating properties of methyl group. The values obtained using F12/VTZ barrier heights agree best with the experiments, though all being higher, particularly for DMFs, than the experimental ones. However, F12/VTZ is computationally too demanding for other processes. Given the small difference between F12/VTZ and F12/VDZ (within 2 kJ/mol for all relative energies in Table S1), energies at F12/VDZ level are used for calculation of rate coefficients and analysis of reaction kinetics. The reactions of methyl furans are also mediated by the formation of PRCs as PRC-HO and PRC-R2/R5, which then isomerize to chemically activated adducts R2* and R5*. The barriers for additions to C2 and C5 are much lower than those to C3 and C4. A rough estimation based on traditional transition state theory found the dominant adduct channels are R2 (>99%), R2 (56%) and R5 (43%), R2 (87%) and R5 (13%), R2 (92%) and R5 (7%), and R2 (>97%) for OH additions to furan, 2-MF, 3-MF, 2,3-DMF, and 2,5-DMF, respectively, at 298 K amongst additions. Due to the electron donation from –CH3 group at C2/C3 position, addition to C2 prevails in 2-MF, 3-MF, and 2,3-DMF for the electrophilic additions. The H-abstractions from methyl group(s) might account for a small fraction of the overall reaction for (D)MFs under the


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atmospheric conditions as in the oxidation of methyl benzenes, e.g., ~7% in toluene,43 although H-abstraction dominates at high temperatures.27,44 High barriers have been found for Habstractions, e.g., 23–28 kJ/mol for H-abstraction from C3-C5 of 2-MF at CBS-QB3 level27 and 3.1 kJ/mol for H-abstraction from -CH3 from our ROCBS-QB3 calculations (2.8 kJ/mol at F12a/VDZ level). The H-abstraction barriers are all much higher than those for additions, rendering their small fractions at the atmospheric temperatures. In this study, we focused only on the addition channels to C2 and C5. The chemically activated radicals R2* and R5* from OH addition would again be deactivated by collision, or open the ring promptly to the corresponding R2B-Z1/Z3 and R5B-Z1/Z3 radicals and subsequently convert to other conformers via internal rotations. We have also carried out RRKM-ME calculations with the inclusion of all R2B and R5B conformers. Table 2 lists the obtained yields of R2, R5, ΣR2B-Ei/-Zi, and ΣR5B-Ei/-Zi, and Table S2 lists their temperature dependence. The predicted total yields of R2B + R5B at 298 K and 760 Torr are 0.73, 0.43, 0.26, 0.07, and 0.28 if using the F12/VDZ energies (0.20, 0.35, 0.24, 0.05, and 0.19 if using ROCBSQB3 energies) for furan, 2-MF, 3-MF, 2,3-DMF, and 2,5-DMF, respectively. The fractions of ring-opening in R2*/R5* are in line with the reduced microcanonical rates k(E) for ring-opening of R2 and R5 (Figure S2B). Barrier heights of 76.6, 77.4, and 77.0 kJ/mol (F12/VDZ, relative to R2) for ring-opening of R2 radical in furan, 2-MF, and 2,5-DMF, are about the same; however, the microcanonical rates k(E) are greatly reduced due to the increased size of R2 radical from furan to 2-MF and to 2,5-DMF. In 2,3-DMF, high barrier of 85.6 kJ/mol for ring-opening of R2 radical and the large size of R2 result in the smallest fraction of ring-opening amongst the furans studied here. As for furan, the reactions of all R2B and R5B radicals with O2 form the corresponding unsaturated dicarbonyl compounds and HO2 radical under the atmospheric conditions. Note that R2B and R5B form the same dicarbonyl compounds. The presence of NO would unlikely affect the yield of dicarbonyl compounds in the reactions of R2B/R5B with O2 because HO2eliminations in R2B/R5B-OO are very fast, e.g., at a rate of >105 s–1 at 298 K (see Section 3.5). Therefore, the yields for the carbonyl compounds would be the same as those of R2B + R5B radicals, resulting in yields of 0.73, 0.43, 0.26, 0.07, and 0.28 (with F12-VDZ energies) for dicarbonyl compounds for furan, 2-MF, 3-MF, 2,3-DMF, and 2,5-DMF, respectively. The values agree closely with the measured yields of 0.75 ± 0.05, 0.31 ± 0.05, 0.38 ± 0.02, 0.08 ± 0.02, and


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0.27 in the study by Aschmann et al. with GC-FID detection method at NO of ~1.2 × 1014 molecules

cm–3.20

Aschmann

CH3C(O)CH=CHCHO

in

et

al.

also

found

2-MF

is

primarily

that formed

the as

dicarbonyl

compound

E-conformer,

while

HC(O)C(CH3)=CHCHO in 3-MF is a mixture of E- and Z-conformers. These are supported by our predicted E- to Z-ratio of 0.30 to 0.13 in 2-MF and of 0.12 to 0.14 in 3-MF. For 2-MF, Bierbach et al.,18 using FT-IR detection and assuming the same absorption coefficients for CH3C(O)CH=CHCHO (unknown) and CH3C(O)CH=CHC(O)CH3 (known), obtained a high yield of ~0.70 for CH3C(O)CH=CHCHO in the absence of NO. Gómez Alvarez et al.,19 using SPME sampling and GC-FID quantification in EUPHORE chamber, also obtained higher yields of 0.60 ± 0.24 and 0.83 ± 0.33 for dicarbonyl compounds in 2-MF and 3-MF, respectively, in the presence of NO. For 3-MF, Tapia et al.21 obtained a very low yield of 0.014 ± 0.003) (in C) for HC(O)C(CH3)=CHCHO, and they suspected that the low yield was due to its secondary reaction to product like 3-methyl-2(3H)-furanone. 3.3 Reaction of Furan-2-OH Radical (R2) Reaction between Furan-R2 and O2 could form four peroxy radicals as R2-3OO-a/s and R25OO-a/s (a/s = anti/syn represents the opposite or same side of the ring for –OH and –OO groups), which would react with NO, HO2, and other RO2 radicals, or undergo unimolecular isomerizations as intramolecular H-migration or ring-closure as for the peroxy radicals in the oxidation of benzenes45-50 and furfural.51 The reaction between R2 and O2 can be modeled as,

where kF[O2] is the bimolecular reaction rate (in s–1) for the re-combinations between R2 and O2, kR and kUni are the unimolecular rate coefficients (in s–1) for back-decomposition and forwardisomerization processes of the RO2 radicals, kBi[X] is the bimolecular rate (in s–1) for reaction of RO2 radicals with X such as NO, HO2, and other peroxy radicals, kIso is the unimolecular rate for isomerization of R2 to R2B, and kXO[XO] is the bimolecular rate for reaction of R2 with XO (mainly NO2 and O3, and also halogen oxides in coastal regions). Typical value for kBi is ~10–11 cm3 molecule–1 s–1 as for other RO2 radicals with NO/HO2,52 and value for kXO is ~2 × 10–12 cm3 molecule–1 s–1 for XO = O3 and ~2 × 10–11 cm3 molecule–1 s–1 for XO = NO2 if assuming the same values as for the reactions of CH3 radical.53 The atmospheric concentrations of HO2 and


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RO2 are usually less than 100 pptv, hence kBi[X] is often determined by NO concentration in the atmosphere. Typical values of kBi[X] are in the range of 0.1–10 s–1 with NO of 0.4–40 ppbv. Typical values of kXO[XO] are in the range of 0.25–2.5 s–1 due to O3 of 5–50 ppbv or of 0.2–20 s–1 due to NO2 of 0.4–40 ppbv, comparing to slow isomerization from R2 to R2B-Z1 with kIso of ~0.1 s–1 at 298 K and ~0.6 s–1 at 313 K. Table S3 lists the high-pressure-limit rate coefficients for kF, kR, and kIso at 298 K, and values at other temperatures are given in Table S4. Values of kF and kR might have high uncertainty because of the multireference nature of the wavefunctions for the transition states for O2 additions as being indicated by the high T1 diagnostic of >0.035 in RHFUCCSD calculations.54 Figure 3 shows the potential energy surface for the reaction of R2 and O2. Detailed reaction pathways and the relative energies of intermediate radicals and transition states are available in Table S5. Barriers for all initial O2 additions are below the separate R2 + O2, due to the existence of pre-reactant complexes. For the four peroxy radicals, all their forward unimolecular isomerizations are hindered by high barriers of > 90 kJ/mol (relative to RO2 radicals, at CBS level) except for the following process from R2-3OOs radical as

Barrier height for this H-migration is 85.7 kJ/mol at F12/VTZ level (83.2 kJ/mol at F12/VDZ level and 81.9 kJ/mol at CBS level). This process, with kUni,298K of ~10–2 s–1, is the only sensible forward isomerization for the four RO2 radicals under the atmospheric conditions, and is still much slower than kBi[X] of 0.1–10 s–1. We have obtained an analytical solution to the kinetic equations staring from R2 radicals (see ESI for details). For all the four RO2 radicals in the atmosphere (T < 323 K, [NO2] < 100 ppbv), their values of kF(kUni + kBi[X])[O2] are orders of magnitude higher than those of (kIso + kXO[XO])⸳(kR + kUni + kBi[X]). Namely, the ratio between product channel (P) via R2-3OO/-

5OO and product channels (R2B + R2O) via R2 is ~kF[O2]/kR∙(kUni + kBi[X])/(kIso + kXO[XO]) = KEq[O2] ∙(kUni + kBi[X])/(kIso + kXO[XO]) with an effective first-order rate as (kUni + kBi[X]).

Numerical simulations show that R2 recombines rapidly with O2 and establishes an equilibrium between R2 + O2 and R2-OO within 0.1 µs, and then R2-OO decays at a rate of ~(kUni + kBi[X]). Figure S3 shows the profiles for R2 and R2-5OO-s with typical parameters of k’s. For the four


12

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peroxy radicals, values for (kUni + kBi[X])/(kIso + kXO[XO]) change slightly, but equilibrium clearly favors the most stable R2-5OO-s radical, for which KEq is orders of magnitude higher than other RO2 radicals (Table 3). Therefore the removal of R2 is dominated by the route R2 → R2-5OO-s → P, and the second most stable radical R2-5OO-a accounts for only ~2% of R2 removal at 298 K. Noticeably, even in pristine atmosphere with (kUni + kBi[X]) of 0.025 s–1 (equivalent to NO and/or HO2 of ~100 pptv), the route R2 → R2-OO → P still has dominance of >99%. Both the fractions of different R2-OO routes and the ratios of P and (R2B + R2O) are proportional to KEq between R2 + O2 and R2-OO; therefore, the high uncertainty in the energies of transition states for O2 additions and subsequently the uncertainty in kF and kR, as being mentioned above, would be cancelled out. Calculation using reaction energies at CBS level leads to virtually the same dominance of R2-5OO-s route and the same dominance of product P. The dominance of reaction route via R2-5OO-s in Furan-R2 radical is different to that in FurfuralR2/R5 radicals, for which we found the reaction Furfural-R2/R5 + XO should be important because the recombination of Furfural-R2/R5 and O2 are endothermic and therefore are highly reversible and extremely slow.51 Product P is formed mainly from the reaction of R2-5OO with HO2 and NO. Reaction of R25OO with HO2 might occur in the pristine atmosphere, and might even dominate in smog chamber studies when OH is generated from photolysis of H2O2. Products from reaction of R25OO and HO2 include 2-hydroxy-5-hydroperoxyl furan, 2,5-dihydroxyfuran, and alkoxy radical R2-5O (+ O2 + OH), although the branching ratios are not available.52 In most of cases, the atmospheric HO2 level is extremely low with typical concentrations less than 10 pptv,55-56 and R2 radical will be converted to alkoxy radicals R2-5O via the reaction of R2-5OO with NO,

Organic nitrates could be formed with a yield of ~10% if assuming the similar yield as other RO2 radicals of similar size.52 Radical R2-5O-s/a can undergo the following reactions as

The cyclization to R2-45O radical was also found in similar radicals formed in the oxidation of benzenes and naphthalenes45-48,50,57-59 and in alkoxy radicals with unsaturated >C=C< bond


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adjacent to the radical center, e.g., the alkoxy radical formed in oxidation of isoprene.60 For R25O radicals, we have identified the transition states for the H-elimination and cyclization processes. Table 4 lists the reaction energies and barrier heights for the cyclization and Helimination. The barrier for cyclization is as low as 22.5 kJ/mol at F12/VTZ level (21.6 kJ/mol at F12/VDZ level), being close to those of 10–25 kJ/mol for the similar radicals in oxidation of benzenes and naphthalenes. However, the cyclization is also endothermic and therefore highly reversible. Equilibrium between R2-5O-s and R2-45O-s is expected with 10–9 s at 298 K with kF + kR > 109 s–1 (from RRKM-ME calculations). The fate of R2-5O radicals can also be modelled in the same way of R2. Figure S4 shows the modelled time profiles of species for the reaction starting with R2-5O-s. The ratio between products R2-45O-s-3OO and 5-hydroxy-2(5H)-furanone can again be approximated as kFkBi2[O2]/kR(kUni

+ kBi-1[O2]) = KEqkBi-2[O2]/(kUni + kBi-1[O2]), of which KEq is the equilibrium constant

between R2-5O-s and R2-45O-s, and kUni can be obtained by RRKM-ME calculations. For the other two parameters, kBi-2 might be assumed as ~10–11 cm3 molecule–1 s–1 with a weak negative T-dependence,52 while kBi-1 is difficult to determine both experimentally and theoretically. Typical kBi-1 is in the range of 10–15 – 10–14 cm3 molecule–1 s–1 with a weak positive Tdependence.61-62 Therefore, a range, instead of a single value, is given for the yield of furanone, varying from 0.78 with kBi-1 of 10–15 cm3 molecule–1 s–1 to 0.86 with kBi-1 of 10–14 cm3 molecule–1 s–1 for R2-5O-s if using F12/VTZ energies (0.65 to 0.79 if using F12/VDZ energies, or 0.33 to 0.53 if using CBS energies, Table 4). Calculations here clearly suggest the existence of the two reaction channels for R2-5O-s; however, the exact branching ratio await further experimental measurement. When temperature increases, (kUni + kBi-1[O2]) increases more rapidly than KEq would and kBi-2 decreases slowly with increasing temperature, leading to smaller ratio and higher yield of furanone. The formation of 5-hydroxy-2(5H)-furanone is consistent with the observed C4H4O3-d3 composition in the oxidation of furan-d4 by Aschmann et al.,20 though those authors assigned the C4H4O3-d3 composition to a dicarbonyl compound in equilibrium with 5-hydroxy2(5H)-furanone. The peroxy radical from ring-cyclization would react also with NO, forming R2-45O-s-3O,


14

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The radical R2-45O-s-3O-a/-s is ready to break the C2-C3 bond with bond lengths of 1.614 and 1.575 Å, comparing to C3-C4 lengths of 1.517 and 1.539 Å. The barrier heights for breaking C2C3 bonds are only 13.7 and 17.8 kJ/mol (ΔE0K by CBS), leading to extremely fast ring-opening and eventual formation of HO2 radical and a multiple functional product (MFP, C4H4O4) with epoxide, ester, and carbonyl groups. 3.4 Reactions of (D)MF-2OH (R2) and (D)MF-5-OH (R5) Radicals with O2 The radicals R2 and R5 formed in 2-/3-MFs and 2,3-/2,5-DMFs would react with O2 similarly as the Furan-R2 radical.

Reaction energies, barrier heights, kF, kR, kUni, and KEq are listed in Tables S4 and S6. The reaction energies and barrier heights for 2-MF agree closely with the recent study by Davis and Sarathy.27 Again, the products (P) via RO2 and products (R2B/R5B + R2O/R5O) via R2/R5

would be formed at ratio of ~kF[O2]/kR∙(kUni + kBi[X])/(kIso + kXO[XO]) = KEq[O2] ∙(kUni +

kBi[X])/(kIso + kXO[XO]) with an effective first-order rate (kUni + kBi[X]). The fractions of different reaction routes are obtained from these parameters and the results are listed in Table 3. For furan, 2-MF, and 2,5-DMF, formation of products via R2-5OO-s and R5-2OO-s dominates; while for 3-MF and 2,3-DMF, the routes via R2-5OO-a and R5-2OO-a are also important, though the branching ratios predicted using F12/VDZ and CBS energies are different. Under typical conditions with a few ppbv of NO, fate of R2 and R5 from 2-MF and 2,5-DMF would be R2-5O-s and R5-2O-s, while that from 3-MF and 2,3-DMF would be R2-5O-s/-a and R5-2O-s/a. Note that the predicted fractions depends strongly on the level of theory for reaction energies and barrier heights for 3-MF and 2,3-DMF. The R2-5O and R5-2O radicals would also undergo H- or CH3-elimination, cyclize, or react with O2, in example R2-5O-s and R5-2O-s from 2,3-DMF, as


15


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All the cyclizations are endothermic with low barrier, leading to rapid establishment of equilibrium between R2-5O/R5-2O and R2-45O/R5-23O. The barrier heights for CH3elimination are 10–15 kJ/mol lower than those for H-elimination. Again, the yields of furanone products are given as a range of value due to the uncertainties in kBi-1, kBi-2, kEq (= kF/kR), and kUni. Table 4 lists the reaction energies and barrier heights for the unimolecular processes and the branching ratios of furanone products for all possible R2-5O/R5-2O radicals. Tapia et al.21 in the reaction of 3-MF identified the production of 5-hydroxy-4-methyl-2(5H)-furanone and 3-methyl2,5-furandione, of which the latter is likely the product from the secondary oxidation of the former. 3.5 Reaction of R2B and R5B Radicals with O2 Reactions of R2B and R5B with O2 also start with the O2 additions. Scheme 2 shows the O2 additions to the four conformers R2B–Z1, –Z2, –E1, and –E2 from furan, along with the rates or rate coefficients at 298 K. Figure 4 shows the potential energy and free energy diagrams for the reaction of Furan-R2B-Z1 conformer and O2, and the reaction energies and barrier heights are available from Table S3. Reactions between other R2B/R5B conformers and O2 have similar potential energy and free energy surfaces. For additions to C2-positions of R2B conformers and C5-position of R5B conformers, the fate of the peroxy radicals formed is to eliminate the HO2, forming the corresponding unsaturated dicarbonyl compounds. At 298 K, the rates for HO2eliminations are all >105 s–1, which is much faster than the possible bimolecular reaction of peroxy radicals with the atmospheric NO or other trace species. Namely, the formation yield of dicarbonyl compounds through this route is not affected by the presence of NO. On the other hand, O2 additions to C4–positions of R2B conformers (C3 of R5B) are endergonic with positive ∆G at ambient temperatures and are hence reversible, although these additions are exothermic with negative ∆E; besides, the possible forward unimolecular processes in these peroxy radicals are extremely slow due to high barriers such as the H-migration in Furan-R2B-4OO,


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for which barrier height was obtained as 71.7 kJ/mol at ROCBS-QB3 level (46.8 kJ/mol relative to R2B-Z1 + O2). The R2B-4OO/R5B-3OO radicals would decompose rapidly back to R2B/R5B + O2. Consequently, O2 additions to C4 of R2B and C3 of R5B contribute negligibly to the removal of R2B and R5B. Therefore, the reactions of R2B and R5B conformers with O2 would form the corresponding unsaturated dicarbonyl compounds, along with HO2 radical. The yields of dicarbonyl compounds are listed in Table 2. The values agree at least semi-quantitatively with the reported experimental measurements by Aschmann et al.20 The reactions of R2B and R5B radicals in furans are different to that of Furfural-R2B/-R5B radicals, in which all the O2 additions are endothermic/endergonic and highly reversible, resulting in extremely slow removal of FurfuralR2B/-R5B via peroxy radical. Instead, direct reaction of Furfural-R2B/-R5B with NO2 and/or O3 is important.51 In 2,5-DMF, Aschmann et al.15 reported NO-dependent yields for 3-hexene-2,5-dione with molar yields of 0.24 ± 0.3 in the presence of NO (~2.4 × 1014 molecules cm–3) and 0.34 ± 0.03 in the absence of NO. Our prediction here is 0.28 with F12/VDZ energies (0.19 with ROCBS-QB3 energies) (Table 2). RRKM-ME modeling shows that the 2,5-DMF-R2B radical is formed in E1conformer, which reacts with O2 as

The estimated rate kD for HO2 elimination in the peroxy radical is ~1 × 106 s–1. Reaction of RO2 with NO is unlikely to compete with the unimolecular decomposition here. It is unclear to us the origin of the NO-dependent yields for 3-hexene-2,5-dione in 2,5-DMF by Aschmann et al. 4. Conclusions The atmospheric oxidation mechanism of furan and methyl furans initiated by the OH radicals is studied by using high-level quantum chemistry and kinetic calculations. The main oxidation routes for furan and 2,5-DMF are shown in Schemes 3 and 4 for reactions at 298 K and 760 Torr. Similar oxidation routes for 2-MF, 3-MF, and 2,3-DMF are shown in Scheme S1-S3. Table 5 lists the yields of products under the typical atmospheric conditions of 760 Torr and 298 K. The yields are unlikely affected by NOx concentration.


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Generally, the reaction of furans and OH radical starts with OH addition to C2/C5-positions, forming chemically activated adduct R2*/R5*, which may be deactivated by collision or promptly break the ring and then isomerizes to various R2B/R5B conformers. Subsequent reaction of R2B/R5B radicals with O2 lead to the formation of their corresponding unsaturated dicarbonyl compounds, and the reaction of R2/R5 radical lead to the formation of furanone compounds and a substituted epoxide compounds. We have modelled, in R2*/R5*, the competition between collisional deactivation and prompt ring-opening using RRKM-ME calculations. The predicted formation yields of dicarbonyl compounds agree well with the experimental values reported by Aschmann et al.,20 while the predict yields in 2-MF and 3-MF are lower than the values reported by other studies.18-19,21 The predicted ratios of E- and Zdicarbonly conformers agree also with the experimental measurements for furan, and 2-/3-MF. In the atmosphere, radical R2/R5 is removed dominantly via peroxy radical R2-5OO/R52OO, which reacts with NO to form the corresponding alkoxy radical R2-5O/R5-2O. The alkoxy radicals can cyclize to epoxide structures, resulting in the formation of epoxide compounds, or decompose unimolecularly by eliminating H-atom or CH3-group or react with O2 bimolecularly, resulting in the formation of furanone compounds, though it is difficult to determine the ratios between the unimolecular and bimolecular processes at this moment. While the atmospheric fate of the unsaturated 1,4-dicarbonyl compounds has been the subject of a few studies due mainly to their high formation yields in the atmospheric oxidation of aromatic benzenes and 1,3butenediene etc,63-66 the high formation yield of 5-hydroxyl-2-furanone compounds in the atmospheric oxidation of furan and (D)MFs calls for further study on the fate of these furanone compounds. ASSOCIATED CONTENT Supporting Information. Analytical solution to kinetic equation, Table S1-S6 (detailed reaction energies, barrier heights, and rate coefficients), Figures S1-S4 (molecular structures of prereactant complexes and species time profiles from modeling), and Scheme S1-S3 (Mechanism for 2-MF, 3-MF, and 2,3-DMF. These files are available free of charge from ACS Web. AUTHOR INFORMATION Corresponding Author


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* Liming Wang, [email protected]. Notes The authors declare no competing financial interest.

Funding Sources National Natural Science Foundation of China and Natural Science Foundation of Guangdong Province.

ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (No. 21477038 and 21677051) and Natural Science Foundation of Guangdong Province (No. 2016A030311005).


19


O

O HO H O

OH

H H

H

O

OH

H H O

O

OH

OH

O H OH H

H

H

H

H

H

OH H

H

H

O

H

OH

H

H O

H

Figure 1. Potential energy diagram for reaction between furan and OH at levels of ROCBS-QB3, (RHF-UCCSD(T)-F12a/cc-pVDZ-F12), and [RHF-UCCSD(T)-F12a/cc-pVTZ-F12]. (A)

(B)

R2 (Calc) R2-E (Calc)

0.6

1.0

R2-Z (Calc) R2B-E + R2B-Z

R2

0.5

Butenedian (Expt.1)

R2B-E

Butenedian (Expt.2-1)

0.8

R2B-Z

0.4

Butenedian (Expt.2-2)

Yields

Branching Ratios

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6

0.3

0.4 0.2

0.2 0.1 1E-9

1E-8

1E-7

1E-6

Reaction Time / s

1E-5

1E-4

240

260

280

300

320

340

Reaction Temperature / K

Figure 2. Results from RRKM-ME calculations for reaction of furan and OH radical in the absence of O2 at 760 Torr based on RHF-UCCSD(T)-F12a/cc-pVDZ-F12 energies. (a) Time profiles of branching ratios of intermediate radicals at 298 K (R2B-E = ΣR2B-Ei and R2B-Z = ΣR2B-Zi); and (b) Changes of branching ratios with temperature and experimental measurements (Expt. 1 from Aschmann et al.,20 Expt.2-1/2-2 from Gómez Alvarez et al.19)


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O

O

OH

O OOH

O

O

O

O

OH OO OH

OO

O

OO

OO

O

OH

O

OH

Figure 3. Potential energy diagram for reactions between Furan-2-OH and O2 at levels of ROCBS-QB3, (RHF-UCCSD(T)-F12a/cc-pVDZ-F12), and [RHF-UCCSD(T)-F12a/cc-pVTZF12].

O H 5

2 4

H

OH

3

O H

H

O O H O H OH OO

OH OO H

H

H

H H

H

Figure 4. Potential energy and free energy diagrams for reactions between Furan-R2B-Z1 and O2 at RHF-UCCSD(T)-F12a/cc-pVDZ-F12 level


21


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Scheme 1. Isomers of R2B and interconversions in furan. Energies (E0K, in kJ/mol) are relative to furan and OH at levels of ROCBS-QB3, (RHF-UCCSD(T)-F12a/cc-pVDZ-F12), and [RHFUCCSD(T)-F12a/cc-pVTZ-F12]. H H O O H OH H OH H OO 3.1E-14 H 5 2 OH 5.8E-14 7.1 4 3 H H 26.2 8.5E+3 H H 2.5E+2 H H O R2B-E1 6.9E-15 R2B-Z1 4.4E-15 R2B-Z1-2OO 3.1E+5 4.7E+6 H 1.1E+7 OHC CHO H OH 3.0E+4 1.2E+4 6.4E+4 + HO2 2.3E+6 OHC 2.3E-14

6.6E+5

H

H H OH OO 2.2E-13 O

O

3.4E+2

H

H R2B-Z2-2OO

H

OO H R2B-4OO

H

R2B-Z2

20.6

CHO

OH 6.7E-14

O

+ HO2

OHC 4.4E+5

2.2E+3

H

H O R2B-E1-2OO

2.0E+5

H

OH H

H

5.3E-15

1.9E+7

7.8E+5

H OH OO

H

H

H OH OO

H 1.3E+2 O

H R2B-E2

H H R2B-E2-2OO

Scheme 2. Interconversion between R2B isomers and their reaction with O2 with their highpressure-limit rate coefficients (in cm3 molecule–1 s–1 for O2 additions and in s–1 for unimolecular processes) at 298 K based on RHF-UCCSD(T)-F12a/cc-pVTZ-F12 energies for furan.


22

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Scheme 3. Oxidation pathways of furan proposed based on calculation at 298 K and 760 Torr

Scheme 4. Oxidation pathways of 2,5-DMF proposed based on calculation at 298 K and 760 Torr


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Table 1. Calculated relative energies (∆E0K, in kJ/mol) of R2B/R5B isomers and their interconversion barriers at levels of ROCBS-QB3 and [RHF-UCCSD(T)-F12a/cc-pVDZ-F12] (a) Furan + OH R2 TS(R2 → R2B-Z1) TS(R2 → R2B-Z3) R2B-Z1 R2B-Z2 R2B-E1 R2E-E2 TS(R2B-Z1 → Z2) TS(R2B-Z1 → E1) TS(R2B-E1 → E2) TS(R2B-Z2 → E2)

Furan 0.0 –142.9 [–139.3] –64.2 [–62.6] –41.3 [–40.7] –144.7 [–142.3] –133.5 [–130.0] –141.1 [–138.4] –146.3 [–142.5] –99.6 [–95.0] –74.9 [–73.7] –97.8 [–93.3] –78.3 [–77.2]

R5 TS(R5-R5B → Z1) TS(R5-R5B → Z3) R5B-Z1 R5B-Z2 R5B-E1 R5B-E2 TS(R5B-Z1 → Z2) TS(R5B-Z1 → E1) TS R5B-E1 → E2) TS(R5B-Z2 → E2) R2B-Z3 R2B-Z4 R2B-E3 R2B-E4 TS(R2B-Z1 → Z3) TS(R2B-Z2 → Z4) TS(R2B-Z3 → Z4) TS(R2B-E1 → E3) TS(R2B-E2 → E4) TS(R2B-E3 → E4) TS(R2B-Z3 → E3) TS(R2B-Z4 → E4) R5B-Z3 R5B-Z4 R5B-E3 R5B-E4 TS(R5B-Z1 → Z3) TS(R5B-Z2 → Z4) TS(R5B-Z3 → Z4) TS(R5B-E1 → E3) TS(R5B-E2 → E4) TS(R5B-E3 → E4) TS(R5B-Z3 → E3) TS(R5B-Z4 → E4)

–184.3 [–180.6] –135.0 [–130.9] –145.1 [–141.9] –147.3 [–143.0] –41.8 [–41.1] –54.3 [–53.9] –102.8 [–98.2] –49.1 [–49.1] –57.3 [–57.6] –101.9 [–98.7] –82.4 [–81.1] –80.2 [–78.8]

2-MF 0.0 –147.4 [–142.9] –67.3 [–65.5] –52.6 [–51.8] –126.7 [–124.0] –123.5 [–120.5] –141.3 [–139.6] –146.8 [–143.8] –93.9 [–89.6] –76.4 [–76.2] –97.4 [–93.3] –79.5 [–79.5]

3-MF 0.0 –148.0 [–143.3] –62.6 [–60.3] –42.4 [–41.1] –143.0 [–140.7] –135.2 [–131.4] –138.7 [–135.9] –140.0 [–136.9] –100.2 [–96.9] –77.8 [–76.8] –96.4 [–93.0] –80.0 [–79.0]

2,3-DMF 0.0 –155.9 [–150.1] –67.1 [–64.5] –55.0 [–53.8] –126.2 [–123.1] –127.2 [–123.8] –140.4 [–138.2] –139.3 [–136.2] –89.6 [–86.6 –78.1 [–77.5] –94.1 [–91.1] –81.4 [–81.1]

–148.8 [–143.8] –69.1 [–67.8] –46.0 [–45.7] –147.3 [–146.2] –128.6 [–126.6] –145.0 [–143.8] –145.0 [–142.9] –109.1 [–105.7] –77.3 [–77.7] –107.3 [–103.9] –80.6 [–81.0]

–143.9 [–140.7] –66.7 [–64.6] –43.1 [–42.0] –143.9 [–142.0] –139.3 [–135.8] –145.4 [–143.2] –153.5 [–150.6] –102.5 [–97.7] –87.4 [–85.9] –96.4 [–94.2] –92.9 [–91.8]

–149.8 [–145.5] –68.7 [–66.7] –46.1 [–45.1] –145.8 [–145.0] –136.2 [–134.4] –148.8 [–148.1] –151.3 [–150.2] –114.0 [–110.3] –85.4 [–85.3] –109.3 [–107.5] –95.2 [–95.1]

–186.2 [–185.1] –135.8 [–132.6] –148.1 [–145.9] –148.9 [–145.5] –41.8 [–41.0] –53.4 [–53.3] –103.5 [–100.2] –48.3 [–48.7] –56.7 [–57.0] –101.9 [–99.2] –79.4 [–79.0] –80.7 [–80.5]

–177.8 [–176.3] –126.8 [–122.8] –139.3 [–135.8] –138.5 [–134.9] –50.2 [–50.0] –60.4 [–60.5] –98.3 [–95.8] –48.4 [–48.8] –52.7 [–53.1] –99.3 [–96.7] –77.5 [–76.0] –79.5 [–78.1]

–177.6 [–176.4] –127.4 [–123.2] –135.9 [–135.2] –137.0 [–134.1] –52.2 [–51.1] –62.3 [–62.2] –96.6 [–94.1] –49.8 [–50.2] –53.5 [–54.0] –93.8 [–91.7] –74.9 [–74.1] –75.2 [–74.8]

–185.8 [–185.0] –119.4 [–116.7] –149.6 [–147.9] –145.2 [–142.9] –49.1 [–49.8] –51.7 [–52.3] –116.4 [–114.0] –57.5 [–59.0] –58.4 [–59.8] –111.9 [–109.8] –81.0 [–81.1] –83.5 [–83.3]

–179.6 [–178.5] –149.5 [–145.9] –144.4 [–141.3] –149.4 [–145.8] –46.1 [–46.1] –62.1 [–62.2] –105.5 [–100.6] –52.3 [–52.6] –64.8 [–65.0] –96.0 [–92.2] –91.2 [–89.3] –96.1 [–94.7]

–181.6 [–181.3] –127.4 [–125.1] –140.2 [–139.3] –149.1 [–147.7] –55.0 [–56.0] –57.6 [–58.5] –121.0 [–116.4] –57.7 [–59.7] –64.3 [–65.8] –108.3 [–105.4] –90.2 [–89.6] –100.1 [–99.4]

2,5-DMF 0.0 –155.0 [–149.0] –73.7 [–72.0] –59.3 [–58.7] –131.7 [–130.1] –119.1 [–117.5] –154.3 [–153.4] –149.3 [–148.0] –107.4 [–105.1] –85.6 [–86.5] –109.6 [–107.8] –89.1 [–89.7]

–191.6 [–191.4] –126.9 [–125.0] –156.6 [–155.4] –156.1 [–154.3] –52.7 [–53.0] –58.3 [–59.1] –119.1 [–116.2] –62.3 [–64.1] –64.0 [–65.6] –117.2 [–115.0] –88.4 [–89.4] –95.2 [–95.9]

(a) M06-2X = M06-2X/6-311++G(2df,2p), all at M06-2X geometries and ZPEs; see Table S1 for Furan + OH at levels of M06-2X and RHF-UCCSD(T)-F12a/cc-pVTZ-F12


24

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Table 2. Molar yields of radicals in the addition of OH to furans at 298 K and 760 Torr along with the experimental yields of dicarbonly compounds from previous studies from RRKM-ME calculations Furans or Products Furan (a) Furan (b) Furan (c) HC(O)CH=CHCHO

R2 0.267 0.199 0.244

R2B-E R2B-Z R5 R5B-E 0.414 0.319 0.473 0.329 0.429 0.328 0.75 ± 0.05;(d) >0.70;(e) 1.09 ± 0.41;(f) 0.90 ± 0.36 (f)

R5B-Z

2-MF (a) 2-MF (b) CH3C(O)CH=CHCHO

0.309 0.385

0.143 0.144

0.036 0.255 0.160 0.031 0.259 0.115 0.31 ± 0.05;(d) ~0.70;(e) 0.60 ± 0.24 (f)

0.097 0.066

3-MF (a) 3-MF (b) HC(O)C(CH3)=CHCHO

0.644 0.682

0.067 0.064

0.113 0.095 0.056 0.108 0.081 0.045 0.38 ± 0.02;(d) 0.83 ± 0.33;(f) 0.014 ± 0.003 (g)

0.027 0.022

2,3-DMF (a) 2,3-DMF (b) CH3C(O)C(CH3)=CHCHO

0.819 0.869

0.027 0.024

2,5-DMF (a) 2,5-DMF (b) CH3C(O)CH=CHC(O)CH3

0.724 0.808

0.190 0.088 0.140 0.054 0.27;(d) 0.24 ± 0.03 (with NO) or 0.34 ± 0.03 (without NO) (h)

0.009 0.007

0.114 0.081 0.08 ± 0.02 (d)

0.016 0.010

0.016 0.010

(a) Based on F12/VDZ energies; (b) Based on ROCBS-QB3 energies; (c) Based on F12/VTZ energies; (d) From Aschmann et al.;20 (e) From Bierbach et al.;18 (f) From Gómez Alvarez et al.;19 (g) From Tapia et al.;21 (h) From Aschmann et al.15

Table 3. Equilibrium constants (in molecules cm–3) for recombination of O2 with R2 and R5 radicals and the branching ratios (Γ) of the reaction routes via the corresponding radicals at 298 K and 760 Torr (a)(b) KEq(R2/R5 + O2 → RO2) R2-5OO-s Γ R2-5OO-a Γ R2-3OO-s Γ R2-3OO-a Γ

Furan 1.08 ×107 98% [98%] 2.15 ×105 2% [2%] 2.23 ×103 0 1.72 ×103 0

2-MF 2.51 ×107 99% [99%] 2.59 ×105 1% [1%] 1.03 ×104 0 8.49 ×102 0

3-MF 9.08 ×107 76% [16%] 3.43 ×106 24% [84%] 2.25 ×104 0 4.22 ×103 0

2,3-DMF 2.53 ×108 88% [65%] 5.58 ×106 12% [35%] 2.74 ×103 0 5.24 ×102 0

2,5-DMF 6.65 ×107 99% [98%] 8.76 ×105 1% [2%] 7.64 ×104 0 1.03 ×104 0

R5-2OOs 2.04 ×107 6.08 ×108 1.84 ×109 Γ 98% [100%] 71% [12%] 98% [91%] R5-2OOa 4.44 ×105 2.13 ×107 3.15 ×107 Γ 2% [0%] 29% [88%] 2% [9%] R5-4OOs 1.39 ×104 6.53 ×103 3.29 ×104 Γ 0 0 0 3 3 R5-4OOa 7.38 ×10 5.36 ×10 1.48 ×104 Γ 0 0 0 (a) All based on geometries and ZPE corrections at M06-2X/6-311++G(2df,2p) level, and energies at F12/VDZ level; (b) Branching ratios are amongst R2-OOs or R5-OOs; (c) Branching ratios are obtained with the forward removal rate, kUni + kBi[X], of 1 s–1 for RO2 radicals; values in square brackets are obtained using energies at ROCBS-QB3 level.


25


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Table 4. Reaction energies and barrier heights (all in kJ/mol), the high-pressure-limit rate coefficients (kF, in s–1), equilibrium constants (KEq) at 298 K and 760 Torr, and the estimated branching ratios for furanone products (Γ) in the reaction of R2-5O and R5-2O radicals (a)(b) Reactions Furan R2-5O-s → R2-45O-s R2-5O-s → Furanone + H R2-5O-a → R2-45O-a R2-5O-a → Furanone + H 2-MF R2-5O-s → R2-45O-s R2-5O-s → Furanone + H R2-5O-a → R2-45O-a R2-5O-a → Furanone + H R5-2O-s → R2-23O-s R5-2O-s → Furanone + CH3 R5-2O-a → R2-23O-a R5-2O-a → Furanone + CH3 3-MF R2-5O-s → R2-45O-s R2-5O-s → Furanone + H R2-5O-a → R2-45O-a R2-5O-a → Furanone + H R5-2O-s → R2-23O-s R5-2O-s → Furanone + H R5-2O-a → R2-23O-a R5-2O-a → Furanone + H 2,3-DMF R2-5O-s → R2-45O-s R2-5O-s → Furanone + H R2-5O-a → R2-45O-a R2-5O-a → Furanone + H R5-2O-s → R2-23O-s R5-2O-s → Furanone + CH3 R5-2O-a → R2-23O-a R5-2O-a → Furanone + CH3 2,5-DMF R2-5O-s → R2-45O-s R2-5O-s → Furanone + CH3 R2-5O-a → R2-45O-a R2-5O-a → Furanone + CH3

*+,-./0

123.

∆E0K

∆G298K

∆E0K‡

∆G298K‡

17.4 –1.5 22.3 –6.3

19.7 –25.9 23.9 –30.7

21.6 48.3 27.3 46.6

24.6 49.2 29.4 47.8

3.03 × 108 5.75 × 104 4.38 × 107 1.11 × 105

3.51 × 10–4 0.64~0.79 [0.33~0.53] 6.45 × 10–5 0.94~0.96 [0.86~0.90]

18.4 –3.2 14.1 –6.1 9.6 –27.3 16.2 –32.0

21.4 –28.2 16.1 –30.9 6.9 –67.4 18.2 –71.1

20.7 47.4 21.7 46.1 14.3 38.7 22.9 27.9

23.6 48.3 24.2 46.7 13.1 34.6 25.4 37.4

4.49 × 108 8.65 × 104 3.55 × 108 1.77 × 105 3.21 × 1010 7.89 × 106 2.19 × 108 2.58 × 106

2.06 × 10–4 0.90~0.93 [0.56~0.71] 1.52 × 10–3 0.70~0.75 [0.95~0.96] 6.25 × 10–2 0.71 [0.48] 6.57 × 10–4 0.99[0.97]

21.4 –3.2 23.7 –9.0 14.8 –3.9 22.3 –7.4

23.3 –26.9 24.1 –34.2 17.1 –28.5 25.0 –31.8

22.1 48.1 25.6 44.2 19.8 44.8 28.3 42.8

25.6 49.6 26.6 44.6 21.2 45.7 30.8 43.8

2.05 × 108 5.17 × 105 1.38 × 108 4.28 × 105 1.21 × 109 2.32 × 105 2.52 × 107 5.49 × 105

8.36 × 10–5 0.93~0.96 [0.63~0.80] 6.02 × 10–5 0.99~0.99 [0.95~0.96] 9.99 × 10–4 0.82~0.85 [0.40~0.47] 4.17 × 10–5 1.00~1.00 [0.98~0.98]

20.8 –4.9 25.9 –7.4 7.5 –29.1 14.0 –33.5

22.1 –29.8 25.6 –33.4 9.5 –69.0 15.5 –73.2

20.9 47.1 26.8 45.3 13.1 35.9 20.3 35.1

23.8 48.1 27.7 44.4 15.7 35.9 21.9 33.9

4.17 × 108 9.60 × 104 8.89 × 107 4.51 × 105 1.10 × 1010 4.59 × 106 8.98 × 108 1.02 × 107

1.32 × 10–4 0.94~0.96 [0.67~0.75] 3.25 × 10–5 1.00~1.00 [0.97~0.98] 2.14 × 10–2 0.81 [0.65] 1.95 × 10–3 0.99 [0.98]

11.7 –27.4 25.9 –32.5

14.4 –67.1 25.6 –72.7

15.4 40.4 21.2 38.4

18.9 41.0 22.9 38.1

3.02 × 109 5.79 × 105 6.05 × 108 1.96 × 106

3.05 × 10–3

Γ

0.79 [0.55] 3.24 × 10–5 1.00 [1.00]

(a) Energies and barrier heights are at F12/VDZ // M06-2X/6-311++G(2df,2p) level; (b) The two values for branching ratios correspond to kBi-1 of 10–15 and 10–14 cm3 molecule–1 s–1.


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Table 5. Molar yields of products in the OH-initiated oxidation of furans at 298 K and 760 Torr (a)

Products Dicarbonyl (E + Z) (E) (Z) (E+Z)-Expt-1 (b) (E+Z)-Expt-2 (c) (E+Z)-Expt-3 (d)

Furan 0.73 (0.41) (0.32) 0.75 ± 0.05 >0.70 1.09 ± 0.41 0.90 ± 0.36

2-MF 0.44 (0.30) (0.13) 0.31 ± 0.05 ~0.70 0.60 ± 0.24

(E+Z)-Expt-4 (e) (E+Z)-Expt-5 (f)

3-MF 0.26 (0.12) (0.14) 0.38 ± 0.02

2,3-DMF 0.07 (0.04) (0.03) 0.08 ± 0.02

2,5-DMF 0.28 (0.19) (0.09) 0.27

0.83 ± 0.33 0.014 ± 0.003 0.24 ± 0.03 or 0.34 ± 0.03

Nitrates (g)

0.024

Furanones

HO

0.06 O

0.14~0.17

0.07

0.09

0.007

0.57

0.74

0.51

0.17

0.08

0.08

0.09

0.01

0.02

O

0.26 HO

O

O

Epoxides

0.08~0.05

0.14

(a) Based on F12/VDZ energies; (b) From Aschmann et al.;20 (c) From Bierbach et al.;18 (d) From Gómez Alvarez et al.;19 (e) Yield in Carbon, from Tapia et al.;21 (f) From Aschmann et al.,15 with or without NO; (g) Assuming 10% yield in ROO + NO reactions;


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References: 1. Yokelson, R. J.; Karl, T.; Artaxo, P.; Blake, D. R.; Christian, T. J.; Griffith, D. W. T.; Guenther, A.; Hao, W. M. The Tropical Forest and Fire Emissions Experiment: Overview and Air Borne Fire Emission Factor Measurements. Atmos. Chem. Phys. 2007, 7, 5175-5196. 2. Krause, T.; Tubbesing, C.; Benzing, K.; Scholer, H. F. Model Reactions and Natural Occurence of Furans from Hypersaline Environments. Biogeosci. 2014, 11, 2871-2882. 3. Qian, Y.; Zhu, L.; Wang, Y.; Lu, X. Recent Progress in the Development of Biofuel 2,5Dimethylfuran. Renew. Sust. Energ. Rev. 2015, 41, 633-646. 4. Thewes, M.; Muether, M.; Pischinger, S.; Budde, M.; Brunn, A.; Sehr, A.; Adomeit, P.; Klankermayer, J. Analysis of the Impact of 2-Methylfuran on Mixture Formation and Combustion in a Direct-Injection Spark-Ignition Engine. Energy Fuels 2011, 25, 5549-5561. 5. Francisco-Marquez, M.; Alvarez-Idaboy, J. R.; Galano, A.; Vivier-Bunge, A. A Possible Mechanism for Furan Formation in the Tropospheric Oxidation of Dienes. Environ. Sci. Technol. 2005, 39, 87978802. 6. Sprengnether, M. M.; Demerjian, K. L. Product Analysis of the OH Oxidation of Isoprene and 1,3Butadiene in the Presence of NO. J. Geophys. Res. 2002, 107, ACH 8-1-ACH 8-13. 7. Berndt, T.; Boge, O. Atmospheric Reaction of OH Radicals with 1,3-Butadiene and 4-Hydroxy-2butenal. J. Phys. Chem. A 2007, 111, 12099-12105. 8. Lee, W.; Baasandorj, M.; Stevens, P. S.; Hites, R. A. Monitoring OH-Initiated Oxidation Kinetics of Isoprene and Its Products Using Online Mass Spectrometry. Environ. Sci. Technol. 2005, 39, 1030-1036. 9. Atkinson, R.; Aschmann, S. M.; Carter, W. P. L. Kinetics of the Reactions of O3 and OH Radicals with Furan and Thiophene at 298 ± 2 K. Int. J. Chem. Kinet. 1983, 15, 51-61. 10. Bierbach, A.; Barnes, I.; Becker, K. H. Rate Coefficients for the Gas-Phase Reactions of Hydroxyl Radicals with Furan, 2-Methylfuran, 2-Ethylfuran and 2,5-Dimethylfuran at 300 ± 2 K. Atmos. Environ. A 1992, 26, 813-817. 11. Wine, P. H.; Thompson, R. J. Kinetics of OH Reactions with Furan, Thiophene, and Tetrahydrothiophene. Int. J. Chem. Kinet. 1984, 16, 867-878. 12. Atkinson, R.; Aschmann, S. M.; Winer, A. M.; Carter, W. P. L. Rate Constants for the Gas-Phase Reactions of NO3 Radicals with Furan, Thiophene, and Pyrrole at 295 ± 1 K and Atmospheric Pressure. Environ. Sci. Technol. 1985, 19, 87-90. 13. Cabanas, B.; Baeza, M. T.; Salgado, S.; Martin, P.; Taccone, R.; Martinez, E. Oxidation of Heterocycles in the Atmosphere: Kinetic Study of Their Reactions with NO3 Radical. J. Phys. Chem. A 2008, 108, 10818-10823. 14. Cabanas, B.; Villanueva, F.; Martin, P.; Baeza, M. T.; Salgado, S.; Jimenez, E. Study of Reaction Processes of Furan and Some Furan Derivatives Initiated by Cl Atoms. Atmos. Environ. 2005, 39, 19351944. 15. Aschmann, S. M.; Nishino, N.; Arey, J.; Atkinson, R. Kinetics of the Reactions of OH Radicals with 2- and 3-Methylfuran, 2,3- and 2,5-Dimethylfuran, and E- and Z-3-Hexene-2,5-dione, and Products of OH + 2,5-Dimethylfuran. Environ. Sci. Technol. 2011, 45, 1859-1865. 16. Colmenar, I.; Cabanas, B.; Martinez, E.; Salgado, M. S.; Martin, P. Atmospheric Fate of a Series of Furanaldehydes by Their NO3 Reactions. Atmos. Environ. 2012, 54, 177-184. 17. Matsumoto, J. Kinetics of the Reactions of Ozone with 2,5-Dimethylfuran and Its Atmospheric Implications. Chem. Lett. 2011, 40, 582-583. 18. Bierbach, A.; Barnes, I.; Becker, K. H. Products and Kinetic Study of the OH-Initiated Gas-Phase Oxidation of Furan, 2-Methylfuran, and Furanaldehydes at ≈ 300 K. Atmos. Environ. 1995, 29, 26512660. 19. Gómez Alvarez, E.; Borrás, E.; Viidanoja, J.; Hjorth, J. Unsaturated Dicarbonyl Products from the OH-Initiated Photo-Oxidation of Furan, 2-Methylfuran and 3-Methylfuran. Atmos. Environ. 2009, 43, 1603-1612.


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TOC Graphic


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The Atmospheric Oxidation of Furan and Methyl Substituted Furans

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