Reaction Paths and Chemical Activation Reactions of 2-Methyl-5

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Reaction Paths and Chemical Activation Reactions of 2-Methyl-5-Furanyl Radical with O 3

2

Jason Hudzik, and Joseph William Bozzelli J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b06650 • Publication Date (Web): 01 Sep 2017 Downloaded from http://pubs.acs.org on September 2, 2017

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The Journal of Physical Chemistry

Reaction Paths and Chemical Activation Reactions of 2-Methyl-5-Furanyl Radical with 3O2 Jason M. Hudzik† and Joseph W. Bozzelli* Chemistry, Chemical Engineering and Environmental Science New Jersey Institute of Technology, Newark, NJ 07102

Abstract Interest in high-energy substituted furans has been increasing due to their occurrence in biofuel production and their versatility in conversion to other useful products. Methylfurans are the simplest substituted furans and understand their reaction pathways, thermochemical properties, including intermediate species stability, and chemical kinetics would aid in the study of larger furans. Furan ring C–H bonds have been shown to be extremely strong, approximately 120 kcal mol-1, due in part to the placement of the oxygen atom, and aromatic like resonance, both within the ring. The thermochemistry and kinetics of the oxidation of 2-methyfuran radical at position 5 of the furan ring, 2-methyl-5-furanyl radical (2MF5j), is analyzed. The resulting chemically activated species, 2MF5OOj radical, has a well depth of 51 kcal mol-1 below the 2MF5j + O2 reactants; this is four to five kcal mol-1 deeper than that of phenyl and vinyl radical plus O2, with both of these reactions known to undergo chain branching. Important, low energy, reaction pathways include chain branching dissociations, intramolecular abstractions, group transfers, and radical oxygen additions. Enthalpies of formation, entropies, and heat capacities for the stable molecules, radicals and transition state species are analyzed using computational methods. Calculated ∆H°f 298 values were determined using isodesmic work reaction from the CBS-QB3 composite method. Elementary rate parameters are from saddle point transition state structures and compared to variational transition state analysis for the barrier less reactions. Temperatureand pressure-dependent rate constants which are calculated using QRRK and master equation analysis is used for falloff and stabilization. †

Current Address Department of Biology and Chemistry County College of Morris Randolph, NJ 07869 *Corresponding Author Email: [email protected]

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Introduction The search for replacement of current day fossil fuels has been an ongoing process for over a decade. Compounds created during the conversion process of starting material, such as biomass, are just as important as the fuels themselves. Lignocellulosic biomass is an abundant renewable feedstock, composed of cellulose, hemicelluloses, and lignin, which serves as an excellent source for a variety of compounds. The review by Gomez et al1 highlights a variety of biomass research including technical hurdles in converting it into a liquid fuel. Demirbaş2 details four conversion categories necessary for utilizing biomass including direct combustion, thermochemical, biochemical and agrochemical processes. One such group of compounds getting significant attention are high-energy substituted furans for their possible biofuel applications.3-6 5-Hydroxymethylfurfural (HMF) is one of the furans gaining interest because it can be converted from fructose7 and cellulose biomass8-11 with the latter capable of being a large scale sustainable source. HMF can be transformed into numerous organic acids, aldehydes, alcohols, amines, and ethers12-14 making it a versatile replacement for chemicals used in industrial processes derived from petroleum sources. HMF along with furan (F), furfural, furfuryl alcohol, 2- and 3-methylfuran (2MF and 3MF), 2,5-dimethylfuran (25DMF), and nine other furan-based compounds have been shown as products created from pyrolysis of

13

C isotopically labeled D-

glucose.15 Furan has been studied both experimentally and computationally. Wei et al16 analyzed furan combustion and reports on its ignition delay times over 1320-1880 K temperatures, 1.2-10.4 atm pressures, and equivalence ratios of 0.5-2.0 test conditions. A thermal decomposition study by Vasiliou et al17 showed production of acetylene, ketene, propyne, and propargyl radicals as important products. Detailed combustion models18-20 and reaction pathways with hydroxyl radicals21 have been developed as well. One of the simplest substituted furans, 2MF, has increasingly been the focus of experimental and computation studies. It can be produced from furfural, which can be generated from pentoses,22 through vapor-phase hydrogenation23 or the simultaneous hydrogenation of furfural and dehydrogenation of cyclohexanol.24 A detailed 2MF combustion kinetic model has been developed

by Tran

et

al25

and

hydrogenation

has

been

shown

to

produce 2-

methyltetrahydrofuran, 1- and 2-pentanol, and 2-pentanone.26,27 Intramolecular hydrogen and 2 ACS Paragon Plus Environment

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methyl group shifts as initiation steps for unimolecular decomposition have been studied by Somers et al28 and Lifshitz et al.29 Several studies30-36 focused on 2MF reactions with hydrogen and methyl group intramolecular migration, hydrogen abstraction via hydrogen and hydroxyl radicals, and hydrogen, hydroxyl, and oxygen additions. Abstraction of hydrogen atoms from the 2MF and 3MF furan rings, by OH radical, have barriers of 9- 12 kcal mol-1 with abstractions from the methyl group lower at the 2- 4 kcal mol-1 range.31,37,38 These abstractions can occur under mild combustion initiation conditions. Furan ring radicals could also be formed by loss of the methyl groups on methyl substituted furans. Loss of a CH3 group would result from OH radical abstraction of a weakly bound allylic like hydrogen forming a methyl radical. This methyl radical would react with O2 to form a reactive peroxy radical group, which could dissociate to RO• + O or react with radicals, such as NO, alkyl radicals (R´•). CH3, or H forming the alkoxy radical RO• plus NO2, R´O•, OH respectively. The resulting alkoxy radical could then beta scission to form a furan ring radical and formaldehyde. Simmie and Metcalfe37 studied 25DMF thermal breakdown to 2MF. They report a hydrogen atom addition to the furan ring carbon at the methyl site followed by elimination of the methyl group is the most important reaction to form 2MF. Their rate constant calculation showed hydrogen atom addition is faster than hydrogen atom abstractions, below 1500 K. Somers et al39 conducted a comprehensive analysis using theoretical and experimental studies of the pyrolysis and oxidation of 25DMF and provided a detailed kinetic mechanism accurately depicting a range of experimental measurements. Important reactions involve hydrogen atom transfers and reactions with hydrogen atoms at high temperatures and hydroxyl radical addition to the ring at moderate temperatures. Other studies on 25DMF include a detailed reaction mechanism for thermal decomposition from 1070-1370 K40 and an analysis of combustion intermediates from premixed laminar flames concluding hydrogen abstraction and pyrolysis pathways are favored.41 Sirjean and Fournet42 computationally studied the thermal breakdown of the methyl radical of 25DMF and reported the most facile decomposition pathway was the ring opening via a C–O bond cleavage with subsequent ring expansion. Computational studies on the thermochemical properties of furan-based species including substituted derivatives,43,44 HMF,45 and tetrahydrofurans46 along with various species created during flash pyrolysis of biomass47 have been completed. The results from these studies aid in creating and predicting chemical kinetic models for other furan based species. 3 ACS Paragon Plus Environment


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This study reports on the reactions involving 2-methyl-5-furanyl radical (2MF5j) plus O2 in order to understand furan ring oxidation and further reactions of furans that can lead to chain branching. With the exception of O2 reaction with the methyl group radical on a furan ring, the O2 reactions with the ring carbon of the furan radicals have to the best of our knowledge, not been considered. In this study, we address O2 association with the furanyl ring radical, which can be formed under combustion conditions, forming a chemically activated furan-ring peroxy radicals. We calculate in this study that this chemically activated reaction leads to chain branching. We have previously determined48 ∆Hf °298 values for 2MF hydroxyl and hydroperoxide species, including radicals, from CBS-QB3 and CBS-APNO calculations. Notation convention and ∆Hf

°

298

values, in kcal mol-1, for several important 2MF related species that are key for

analysis are shown in Figure 1. Calculated ∆Hf °298 values for the 2MF peroxide radicals range from -0.2 to -1.0 kcal mol-1 which would produce well depths of over 50 kcal mol-1 for furan ring radical species (2MF3j, 2MF4j, and 2MF5j) plus O2 compared to 15 kcal mol-1 for the 2MF2j location. We determine the most favorable reaction pathways for the 2MF5OOj peroxy radical involves dissociation of the peroxy oxygen atom (chain branching), ring opening and subsequent CO2 loss, and ring expansion via intramolecular ipso addition of the radical peroxy oxygen. The first pathway has an initial transition state barrier of 21 kcal mol-1 and leads to the formation of a dissociation product that is 38 kcal mol-1 below the initial entrance channel. The second pathway begins with the initial transition state furan structure having a three-member, di-oxiranyl ring from ipso addition of the peroxy oxygen radical to the peroxy carbon, with a 11.5 kcal mol-1 barrier height. This creates a bicyclic intermediate product 55 kcal mol-1 below the initial entrance channel. This di-oxiranyl ring pathway is also the important low energy path in phenyl radical reactions with O2, under combustion conditions. The thermochemical properties, including enthalpies of formation (∆Hf

°

298),

entropies

(S°298), and heat capacities (Cp(T)), for the association of 2MF5j and O2 forming the chemically activated peroxy 2MF5OOj* radical along with 2MF, 2MF5j, 2MF5Oj, 2MF5OOH, and 2MF5OOj species, will be of value in evaluation of oxidation reaction pathways of furans.


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a

Ref 49. Ref 50. c Calculated from the C—H BDE of 2MF5j (Ref 51) and the ∆Hf °298 values of 2MF (Ref 50) and H atom (Ref 52). d Ref 48 b

Figure 1 Nomenclature and ∆H°f 298 (kcal mol-1) values for important 2MF species in the 2MF5j + O2 system.

Nomenclature Abbreviations are utilized as illustrated below: •

– represents a bond between two atoms,

•

= represents a double bond between two atoms,

•

# or ≡ represents a triple bond between two atoms,

•

Y represents a cyclic structure,

•

j represents a radical site on the preceding carbon atom,

•

Brackets ( ) represents a substituent on the preceding carbon atom, for example CC(=O) is acetaldehyde,

•

Ci or Oi denotes carbon or oxygen i according to numbering in Figure 1,

•

TS denotes transition state structure,

•

* denotes an activated adduct complex, 5 ACS Paragon Plus Environment


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•

F denotes furan,

•

2MF or MF denotes 2-methylfuran.

Computational Methods Optimized geometries are determined using the density functional theory (DFT) M06-2X53,54 method with the 6-31G(d,p) basis set. In some cases, M06-2X either could not determine appropriate structures or for the transition states (TS) gave negative frequencies that did not correctly represent the desired species. For these situations, MP255-59 or PM360,61 methods are used to determine appropriate structures. Sebbar et al62 used PM3 for two transition states in their benzoyl radical oxidation reactions when appropriate species could not be optimized. Stable species are verified by having all positive vibrations while TS species all had a characteristic single imaginary (negative) frequency corresponding to the mode of vibration connecting the reactant and product. High level DFT based composite calculations are performed with CBS-QB363,64 using the Gaussian 0365 and Gaussian 0966 programs. This method uses geometries and frequencies from the B3LYP/6-311G(2d,d,p) level with single point energy calculations at the CCSD(T), MP4SDQ, and MP2 levels and a final CBS extrapolation. CBS-QB3 single point energies are calculated from the optimized MP2 and PM3 geometries when necessary. For calculation of ∆H°f 298 values, isodesmic work reactions are used to cancel systematic CBS-QB3 calculation errors for all stable and TS species. The work reactions use standard reference species, which have well-established ∆H°f

298

values. For two of these standard

reference species, H2C=CHCH=O and Y(C4H4O2), enthalpies of formation from literature sources were not available so they are calculated from a set of work reactions, shown in the Supporting Information. Atomization reactions are also included as a check on our calculated values. This type of reaction relies on simply the balancing of the target species with constituent atoms, C, O, and H, in the gas phase. Although this allows for fast enthalpy estimation, there is not the same error cancelling capabilities as the work reaction method because the environment of the atoms is not the same as that of the molecule. Entropy and heat capacities are calculated using the rigid-rotor harmonic-oscillator approximation for contributions from translations, vibrations, and external rotations using the 6 ACS Paragon Plus Environment

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Statistical Mechanics for Heat Capacity and Entropy (SMCPS) program.67 The SMCPS program uses input including geometry, mass, electronic degeneracy, symmetry, frequencies, number of optical isomers, and moments of inertia for each species. Zero-point vibration energies are scaled by 0.967 for M06-2X as recommended by Alecu et al68 and 0.9608 for MP2 and 0.9761 for PM3 as recommended by Scott and Radom.69 Corrections for internal rotor torsion frequencies corresponding to single bond rotations are replaced with hindered rotor analysis from VIBIR which uses the Pitzer and Gwinn70-72 approximation method. Reduced moments of inertia are calculated from the optimized structures using the mass and radius of rotation for the rotating group and barriers to single bond rotation are determined. Rotations which would substantially change the conformation of a species were not analyzed. Summing the SMCPS and VIBIR contributions gives the total entropy and heat capacities for the species. High-pressure rate constants, k(T), are calculated for the 300–2000 K temperature range using canonical transition state theory (CTST):

∆ −∆ ‡ ∆ = ℎ ‡

‡

Degeneracy is accounted for in the symmetry of reactants and products. These rate constants are then used to determine elementary rate parameters (A, n, Ea) using a modified form of the Arrhenius equation:

− =

For the 2MF5j + O2 adduct, bond length scans are performed with calculation of optimized structure, energy and vibration frequencies at each 0.05 Å step. A complete set of thermochemical properties are calculated for the different transition state structure at each step. Rate constants are then calculated from the reactant to each structure at temperatures of 298 to 2000 K. The minimum rate constant is taken across the temperature – bond length data set, for each temperature. The minimum set of rate constants over the temperature range is fit to modified Arrhenius rate constant form. Temperature and pressure-dependent rate constants are calculated using the multichannel, multifrequency quantum Rice-Ramsperger-Kassel (QRRK) analysis for k(E) with master equation for falloff and stabilization as implemented in the CHEMASTER code.73,74 Energy 7 ACS Paragon Plus Environment


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dependence of the rate constant, k(E), must be considered to correctly account for product distribution from chemically activated reactions. The steady-state assumption is applied to the energized adduct where both forward and reverse reaction paths are calculated. In comparison, the formation of products is not reversible and only adjacent product formation is considered and subsequent dissociation needs to be handled separately. The required input for CHEMASTER includes temperature and pressure ranges of interest, the mass of the species, the Lennard-Jones transport parameters for the collider molecule, the third body bath gas, and reactants, and a reduced set of three representative vibrations and their degeneracies.

Results and Discussion Optimized geometry parameters, symmetry values, optical isomers, moments of inertia, and vibrational frequencies for all species in the 2MF5j + O2 system are presented in the Supporting Information.

Enthalpy of Formation ∆H°f 298 Error cancelling isodesmic work reactions are used to determine ∆Hf °298 values for stable and TS species from CBS-QB3 calculations. Enthalpies for each species are analyzed using several reactions with standard reference species, presented in Table 1. Some of the species have hydrogens shown, while others are not shown for clarity. Table 1 also includes the reference species used for calculation of ∆Hf °298 values for H2C=CHCH=O and Y(C4H4O2). In selecting the work reactions, care is taken to ensure that similar environments, including aromaticity where applicable, to cancel error as best as possible. The enthalpies of formation from work reactions and atomization reactions are given in the Supporting Information. There is good agreement between the ∆Hf °298 from the work reaction and atomization methods.

Table 1 Standard Enthalpies of Formation for Reference Species Used in Work Reactions Species CH4 CH3CH3 CH3CH2CH3 CH3CH2CH2CH3 CH2=CH2

∆H°f 298 (kcal mol-1) -17.6 ± 0.3 -20.0 ± 0.1 -25.2 ± 0.3 -30.0 ± 0.1 12.5 ± 0.1 8

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Reference 75 75 75 75 75

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CH2=CHCH3 4.6 ± 0.3 75 CH2=CHCH2CH3 0.00 ± 0.1 75 CH#CH 54.6 ± 0.1 75 CH#CCH3 44.3 ± 0.2 75 CH3OH -48.2 ± 0.3 75 CH3CH2OH -56.4 ± 0.4 75 CH3CH2CH2OH -61.3 ± 0.9 75 CC(=O) -39.6 ± 0.1 75 CC(=O)C -52.0 ± 0.4 75 CC(=O)CC -57.3 ± 0.2 76 n-CC(=O)CCC -62.0 ± 0.2 76 CCC(=O)CC -62.5 ± 0.2 76 CC(=O)C=C -27.4 ± 2.6 77 H2C=C=O -11.7 ± 0.1 75 CH3CH=C=O -15.1 ± 0.9 75 a H2C=CHCH=O -15.3 H2C=C=C=O 31.2 ± 0.9 75 CH3CH2CH=C=O -20.7 78 HOCH=C=O -36.2 ± 0.9 75 HOCH2CH=C=O -51.6 78 CH3CjH2 28.9 ± 0.4 75 CH3CH2CjH2 24.3 ± 0.9 75 19.3 ± 0.9 75 CH3CH2CH2CjH2 CH3CH2Oj -3.1 ± 0.4 75 CH3CH2CH2Oj -8.1 ± 0.9 75 CC(=O)Cj -8.3 ± 0.5 79 CC(=O)CCj -7.8 ± 0.4 76 n-CC(=O)CCCj -12.9 ± 0.4 76 CjCC(=O)CC -13.1 ± 0.4 76 F -8.29 ± 0.21 49 F2j 60.2 ± 1.3 51 F3j 60.3 ± 1.3 51 2MF -18.3 ± 0.3 50 b 2MF2j 15.9 ± 1.2 b 2MF3j 50.2 ± 1.3 b 2MF4j 50.0 ± 1.4 b 2MF5j 50.0 ± 1.4 2MF5OH -60.0 ± 0.7 48 2MF5OOH -39.8 ± 0.8 48 2MF5OOj -1.0 ± 1.4 48 a Y(C4H4O2) (1,4-Dioxin) -19.9 -81.4 ± 1.0 80 Y(C4H8O2) (1,3-Dioxane) a See the Supporting Information for calculation. b Calculated from the C—H BDEs of 2MF Radicals (Ref 51) and the ∆Hf °298 values of 2MF (Ref 50) and H atom (Ref 52).



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Optimized geometries for all of the stable species are depicted in the subsequent potential energy (PE) diagrams while the TS species are shown in Figure 2. For all of the species the optimized structure parameters are also included in the Supporting Information.

Figure 2 Optimized structures for transition state species studied in the 2MF5j + O2 system.

Entropy (S°(T)) and Heat Capacities (Cp(T)) S°298 and Cp(T), for 300 ≤ T ≤ 1500 K, are presented in Table 2. These values are the combination of the contributions from the translations, vibrations, and external rotations with corrections for single bond rotations with barriers below 5 kcal mol-1. Geometries, moments of inertia, vibrational frequencies, and internal rotor potentials for these species the M06-2X/631G(d,p) method is used unless otherwise noted. Other well-known small combustion species, which are not calculated in this study such as CjH3, CH#CH, CO, CO2, O, and OH, are also listed in Table 2.


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Table 2 Enthalpies of Formationa, Entropiesb, and Heat Capacitiesb Utilized in the 2MF5j + O2 System

Species CJH3 CH#CH CO CO2 O OH 2MF5J 2MF5OOJ TS 2MF5OOJ TS I 1 MF4j5OOH TS I 2 CC(=O)CH=C=C=O TS II 1 MF45Y(CjCOOj) TS II 2 MF4Oj5O TS II 3 O=CjOC(C)=CHCH(=O) TS II 4 n-CC(=O)CjHCH(=O) TS II 5 O=CHCH=C=O TS III 1 MF5Yj(COO) TS III 2 CC(=O)CH=CHCO2j TS III 3

∆H°f 298

S°298

C°p(T) (cal mol-1 K-1)

(kcal mol-1)

(cal mol-1 K-1)

300 K

400 K

500 K

600 K

800 K

1000 K

1500 K

35.2 54.6 -26.3 -94.1 59.567 8.89 50.0 -1.0 46.2 37.8 29.7 35.9 -7.7 32.9 25.2 24.6 -50.2 -43.9 -53.0 -41.8 -38.3 0.5 -40.3 10.5 -5.3 25.5 -50.8 -49.9

46.38 48.00 47.21 51.07 38.47 43.88 75.13 86.36 89.45 82.57 86.73 88.50 86.26 83.18 82.86 82.45 85.15 85.91 92.91 90.64 81.39 83.33 70.86 83.20 83.17 83.87 94.19 96.22

9.26 10.53 6.96 8.90 5.23 7.16 20.79 28.50 27.84 27.03 29.10 28.95 26.23 26.33 26.94 26.44 28.61 28.77 31.45 31.30 23.70 25.10 17.48 26.85 28.28 28.09 30.49 32.04

10.05 11.97 7.02 9.85 5.14 7.08 26.48 35.17 33.77 33.91 35.86 35.33 31.16 33.38 34.29 33.40 35.35 34.86 37.16 36.66 28.70 29.65 20.45 33.60 35.37 34.62 36.78 37.47

10.81 12.97 7.13 10.65 5.08 7.05 31.46 40.84 39.03 39.75 41.51 40.70 35.31 39.26 40.44 39.26 41.00 40.02 41.98 41.30 33.11 33.45 22.94 39.32 41.26 40.09 41.97 41.79

11.54 13.73 7.27 11.31 5.04 7.05 35.55 45.39 43.41 44.48 46.01 45.02 38.71 43.94 45.33 43.94 45.58 44.28 45.98 45.19 36.80 36.61 25.00 43.93 45.95 44.50 46.10 45.29

12.90 14.93 7.61 12.30 5.01 7.15 41.62 51.96 50.00 51.37 52.46 51.30 43.82 50.71 52.35 50.72 52.34 50.72 52.11 51.22 42.46 41.51 28.14 50.66 52.73 51.00 52.16 50.65

14.09 15.92 7.94 12.97 5.01 7.33 45.87 56.42 54.62 56.01 56.80 55.61 47.42 55.31 57.09 55.34 57.01 55.26 56.50 55.55 46.50 45.13 30.37 55.25 57.33 55.48 56.34 54.55

16.26 18.00 8.41 13.93 4.98 7.87 52.16 62.89 61.42 62.50 63.09 61.94 52.71 61.93 63.85 61.97 63.77 61.91 62.99 62.01 52.55 50.75 33.61 61.89 63.94 62.01 62.47 60.48



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

CC(=O)CH=CjH 29.9 75.20 TS III 4 56.8 83.96 CCj(=O) -2.6 63.01 TS III 5 14.3 66.13 TS IV 1 20.2 84.95 2MF5Oj -47.7 78.38 TS IV 2c -39.4 78.00 c CC(=O)CHCjHC=O -30.0 80.06 TS IV 3 9.7 91.77 TS V 1d 60.1 80.41 MF2CjH25OOH -6.8 88.39 TS V 2 -1.0 83.38 MF2CH25O -45.1 75.48 TS V 3 34.5 79.66 C=C=CHCHCO2 6.5 83.07 TS VI 1 -13.2 83.93 c MF45Y(CCO)5Oj -23.7 81.85 -36.6 80.03 TS VI 2c Y(OC(C)=CCjOC(=O)) -78.9 84.12 TS VI 3 -36.9 86.23 CC(=O)CH=CHOCj=O -53.6 89.21 TS VI 4 -39.5 92.48 CC(=O)CjHCH(=O) -37.6 79.12 TS VII 1 -36.5 85.17 TS VIII 1d 56.1 84.30 MF4OH5Oj -91.3 83.64 TS IX 1 31.7 90.69 O=C=CHCH=C=O -9.8 79.73 a From CBS-QB3 calculations b From M06-2X/6-31G(d,p) calculations unless otherwise noted c ° S 298 and C°p(T) from MP2 calculations d ° S 298 and C°p(T) from PM3 calculations

20.91 23.45 12.30 13.95 27.45 24.03 22.82 24.51 27.99 27.41 30.42 29.39 23.16 24.92 25.68 27.61 26.65 26.12 28.51 27.81 30.48 30.78 23.64 28.28 28.10 28.43 29.14 21.58

25.52 26.93 14.35 15.50 34.08 30.34 28.57 30.22 32.58 34.65 37.84 36.62 29.50 30.48 31.44 34.33 33.60 32.87 35.22 34.20 36.42 36.23 28.67 34.45 34.62 35.14 34.08 24.96

29.45 29.81 16.29 16.83 39.69 35.72 33.55 35.05 36.59 40.63 43.60 42.31 34.61 34.96 36.08 39.96 39.41 38.53 40.88 39.59 41.55 40.96 33.08 39.74 40.06 40.78 38.00 27.56


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32.71 32.25 18.01 18.01 44.22 40.10 37.69 39.07 39.99 45.40 47.97 46.64 38.64 38.51 39.71 44.47 44.08 43.10 45.47 44.00 45.81 44.93 36.79 44.09 44.45 45.34 41.18 29.63

37.73 36.19 20.80 20.02 50.84 46.58 43.99 45.27 45.36 52.31 54.05 52.65 44.45 43.71 44.94 51.03 50.91 49.80 52.24 50.56 52.29 51.02 42.45 50.62 50.86 52.07 46.10 32.76

41.38 39.25 22.91 21.68 55.38 51.09 48.48 49.80 49.31 56.95 58.13 56.62 48.40 47.31 48.52 55.51 55.68 54.45 56.92 55.13 56.88 55.39 46.50 55.19 55.27 56.71 49.74 35.00

46.98 44.26 26.22 24.51 61.95 57.71 55.26 56.79 55.37 63.46 64.14 62.44 54.11 52.60 53.73 62.01 62.79 61.29 63.71 61.80 63.65 61.91 52.55 61.86 61.74 63.48 55.46 38.33

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2MF5j + O2 Reaction System Variational transition state theory (VTST) is commonly used to determine rate constants for barrier-less reactions common to association reactions such as 2MF5j + O2. A scan of the bond length for 2MF5OOj → 2MF5j + O2 is shown in Figure 3 using unrestricted M06-2X/631G(d,p). Calculation of elementary rate parameters from VTST are given in Table 3.

60 Relative Energy (kcal mol-1)

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


Dissociation of 2MF5OOj → 2MF5j + O2

50 40 uwB97X-D uM06-2X uB3LYP

30 20 10 0 1.35

1.85

2.35 2.85 Bond Length (Ǻ)

3.35

3.85

Figure 3 Scan of 2MF5OOj bond length for VTST calculation.

Table 3 Elementary Rate Parameters for 2MF5OOj Association 2MF5j + O2→TS 2MF5OOj Method A n Ea 3 uB3LYP (VTST) 5.79x10 2.23 -1.1 3 uwB97X-D (VTST) 1.40x10 2.30 -2.4 uM06-2X (VTST) 3.13x102 2.45 0.7 3 uM06-2X (TS) 1.67x10 2.46 -4.2 Units: A (mol cm-3 s-1), Ea (kcal mol-1)

Dissociation 2MF5OOj→TS 2MF5OOj A n Ea 14 1.36x10 -0.11 50.9 13 3.26x10 -0.04 49.6 7.35x1012 0.11 52.7 3.93x1013 0.11 47.8

To further validate these rate parameters, calculations using unrestricted wB97X-D81,82 with the 6-311+G(d,p) and B3LYP83,84 with the 6-31G(d,p) basis sets are shown. The bond dissociation scans for B3YLP and wB97X-D are similar while M06-2X shows a saddle point at approximately 2.20 Å. M06-2X was able to converge on an optimized TS geometry with a bond length of 2.18 Å. Using M06-2X similar elementary rate parameters are determined using CTST, uM06-2X (TS), to those from VTST, uM06-2X (VTST) in Table 3. Rate parameters from uM06-



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2X (TS), further denoted TS 2MF5OOj, are used in this study with the modification of 0.0 kcal mol-1 for the activation energy instead of -4.2 kcal mol-1. The focus of this analysis is on the initial reaction pathways of the 2MF5j + O2 system given in Figure 4. Additional ring opening products of some furan-based species are also determined. Values in the figure are calculated enthalpies of formation, kcal mol-1, where values in parenthesis are the total enthalpies when more than one species is formed. Note that species in blue are optimized using either MP2 or PM3 methods. Pathways are denoted with roman numerals followed by sequential numbering.


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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


Figure 4 Chemical activation, isomerization, and dissociation pathways studied for the 2MF5j + O2 → 2MF5OOj reaction system. Units are in kcal mol-1. 15 ACS Paragon Plus Environment


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Reaction Pathways The initial reaction pathways of the 2MF5j + O2 system, given in Figure 4, are separated based upon their type of reaction and are presented in more detail in the potential energy diagrams in Figures 5 and 6. The furan ring nomenclature and atom numbering in the subsequent analysis, continues to follow that in Figure 1. The high electronegativity of oxygen relative to carbon atoms in and on the ring is important in understanding and evaluation for a number of the reaction paths described below. The pi bond of a carbon to oxygen in a carbonyl C=O group is on the order of 20 kcal mol-1 stronger than the C=C pi bond, and the aromatic system adds an additional stabilization to the ring. When one adds an oxygen on the ring it can draw electrons from the ring – trying to form a carbonyl C=O bond. In this system, there is an added complexity of the oxygen in the ring; it can also form a carbonyl bond while cleaving a carbon – oxygen bond and opening the ring. The energies and descriptions of these reactions are described below. The initial association of 2MF5j + O2 creates a chemically activated 2MF5OOj* radical with a well depth of 51 kcal mol-1. This is similar to the 50 kcal mol-1 findings of Sebbar et al85 and 46 kcal mol-1 of Tokmakov et al86 for phenyl radical plus oxygen as well as 46 kcal mol-1 for vinyl radical plus oxygen from Mebel et al87 which are known to undergo chain branching. Lower well depths of 35-42 kcal mol-1 for C3-C5 cyclic alkane radicals plus oxygen are reported by Auzmendi-Murua and Bozzelli.88 The chemically activated 2MF5OOj* can dissociate back to the original reactants or undergo a number of different reaction as illustrated in Figure 4. Figure 5 illustrates the reactions of 2MF5OOj* that occur via formation of new isomers through intramolecular abstraction, group transfer, or by dissociations to new products. Figure 6 illustrates the reaction where the peroxy oxygen radical undergoes intramolecular addition to the double bonds on the furan ring at the ipso and beta carbons relative to the peroxy carbon. These initiate ring opening or expansion.


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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


Figure 5 Potential energy diagram for abstraction, dissociation, and group transfers studied in the 2MF5j + O2 → 2MF5OOj reaction system. Units are in kcal mol-1; overall these are higher barrier reaction paths.



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

Figure 6 Potential energy diagram for radical peroxy oxygen intramolecular addition initiating ring opening and expansion in the 2MF5j + O2 → 2MF5OOj reaction system. Units are in kcal mol-1. 18 ACS Paragon Plus Environment

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Intramolecular Hydrogen Transfer (Abstraction) from C4 by 2MF5OOj Radical Peroxy Oxygen (Pathway I) This pathway begins with an intramolecular hydrogen transfer from carbon C4 to the radical peroxy oxygen on 2MF5OOj with a 39 kcal mol-1 barrier (TS I 1) and an endothermicity of 31 kcal mol-1 to form the hydroperoxide furan ring radical (MF4j5OOH). A radical initially on the C4 carbon is now in the ring and carbonyl. Further reaction of this MF4j5OOH isomer involves one of the ring electrons starting to undergo carbonyl formation - beta scission to try and form a carbonyl bond (C=O) through a 6 kcal mol-1 barrier (TS I 2) with elimination of the OH group. Simultaneously this C5 carbon bond to the ring oxygen now cleaves opening the ring and forms a carbonyl (C=O) group generating a linear CC(=O)CH=C=C=O with no further barrier other than ∆Hrxn. A stable cyclic intermediate species could not be identified. The ring opened CC(=O)CH=C=C=O structure can rotate 180° around the CC(=O)–CH bond, over a 5 kcal mol-1 barrier, to a more stable conformation which is 0.4 kcal mol-1 lower in energy.

Intramolecular Hydrogen Transfer (Abstraction) from the Methyl group by 2MF5OOj Radical Peroxy Oxygen (Pathway V) A high, 61 kcal mol-1, barrier (TS V 1) is calculated for intramolecular hydrogen transfer from the methyl group to the radical peroxy oxygen on 2MF5OOj, due to the strain created in the TS structure. This puts an added electron in the methyl-furan ring system MF2CjH25OOH (methyl radical) which allows formation of a carbonyl on the peroxy carbon with elimination of OH radical, TS V 2. This reaction has a 6 kcal mol-1 energy barrier and forms a substituted furan MF2CH25O and hydroxyl radical. The overall reaction is 86 kcal mol-1 exothermic from the initial entrance channel. Ring opening of MF2CH25O requires surpassing a high 80 kcal mol-1 barrier (TS V 3) to form C=C=CHCHCO2. The initial ring opened species rotates 180° over a 2 kcal mol-1 barrier across the C–C bond to move the CO2 group further away from the double bond.

Dissociation of O Atom from 2MF5OOj (Pathways IV and IX) 2MF5OOj dissociation of the terminal oxygen to form 2MF5Oj + O occurs over a 21 kcal mol-1 energy barrier, TS IV 1. A transition state (TS IV 2) and subsequent stable ring opened product (CC(=O)CHCjHC=O) could not be determined with the M06-2X method. Optimization of these structures reformed the C5–O1 bond back to the five-member furan ring. MP2 converged on appropriate structures, but the resulting energy for TS IV 2 is lower than that of the 19 ACS Paragon Plus Environment


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CC(=O)CHCjHC=O product. Simmie and Metcalfe37 determined a 19.6 kcal mol-1 barrier for ring opening of 2MF5Oj with G3 calculations for this same system. The energy difference from 2MF5Oj and CC(=O)CHCjHC=O is calculated to be 18 kcal mol-1, consistent with their findings. As the terminal CO group in CC(=O)CHCjHC=O moved further from O1, the rotational barrier for the CC(=O)–CH bond lowered to 4.9 kcal mol-1 allowing the 180° rotation depicted in TS IV 3. Loss of the terminal CO group generates CC(=O)CH=CjH whose subsequent breakdown is further described in Pathway III. The reactions of this CC(=O)CH=CjH species are shown in Figure 6. From CC(=O)CHCjHC=O, Simmie and Metcalfe37 suggested that the methyl group could also be dissociated. Pathway IX, Figure 5, depicts this with an energy barrier of 62 kcal mol-1 (TS IX 1). There is bond rotation over the elongated C3–C4 bond in the center of the species created upon transition state formation leading to the stable O=C=CHCH=C=O species.

Intramolecular Hydroxyl Group Transfer to C4 in 2MF5OOj (Pathway VIII) A hydroxyl group dissociation from the peroxide with simultaneous ring opening was previously described in Pathways I and V. It is also possible to have the OH group transfer to the original hydrogen abstraction site on C4 from MF4j5OOH with a 26 kcal mol-1 barrier of TS VIII 1. This cleaves the weak RO-OH bond and allows formation of a new carbon – oxygen double bond. The resulting MF4OH5Oj species is calculated to be the most stable substituted furan, with over 120 kcal mol-1 of chemical activation energy relative to the reacting MF4j5OOH*.

Intramolecular Addition of Peroxy Oxygen Radical to C4 in 2MF5OOj and Subsequent Weak RO—OR' Bond Cleavage (Pathway II) The peroxy oxygen radical of 2MF5OOj attacks the unsaturated bond on C4 forming a fourmember peroxide ring over a 34 kcal mol-1 barrier of TS II 1. Formation of the strained bicyclic MF45Y(CjCOOj) species begins to break the O–O bond in the peroxide ring through TS II 2 which is calculated to be slightly lower in energy than MF45Y(CjCOOj). The resulting MF4Oj5O completes the oxygen transfer by forming a carbonyl bond and ring opening with a substantial 75 kcal mol-1 exothermicity. Ring opening of MF4Oj5O through TS II 3 has a low 6 kcal mol-1 barrier forming the resulting O=CjOC(C)=CHCH(=O) structure. Initial ring opening has the terminal CO group at 20 ACS Paragon Plus Environment

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the peak of its calculated rotational barrier where rotation by 180°, through a small 0.5 kcal mol-1 barrier, over the O=CjO–C bond to a 2 kcal mol-1 lower energy conformation. Elimination of CO can occur through a low 11 kcal mol-1 barrier, TS II 4, generating a straight chain nCC(=O)CjHCH(=O) followed by beta scission (elimination) of the terminal methyl group requiring approximately 39 kcal mol-1 over TS II 5 yielding O=CHCH=C=O and a methyl radical.

Ipso Addition of Radical Peroxy Oxygen in 2MF5OOj and Subsequent Ring Opening from Three-Member (Di-Oxiranyl) Ring (Pathway III) The terminal peroxy radical oxygen can also undergo addition to the ipso carbon, C5, forming a three-member di-oxiranyl ring over a low 11 kcal mol-1 energy barrier, TS III 1, placing a radical initially on C5 in the ring. The stable bicyclic formed, MF5Yj(COO), has a high energy reaction Pathway III, and a low energy reaction Pathway VI. Cleavage of the C5–O1 bond in MF5Yj(COO) has a 30 kcal mol-1 energy barrier (TS III 2) for CC(=O)CH=CHCO2j formation which is 45 kcal mol-1 lower in energy than MF5Yj(COO). The overall reaction is 101 kcal mol-1 exothermic from the initial reactants with all barriers below the entrance channel. Loss of CO2 from CC(=O)CH=CHCO2j occurs over a 1 kcal mol-1 barrier for TS III 3. In this reaction, the bond lengthening for the CO2 group lowers the CC(=O)–CH rotational barrier to 2.5 kcal mol-1, allowing for a 180° rotation to a lower energy conformation. Complete removal of CO2 results in CC(=O)CH=CjH formation, the same product created by the CO removal from CC(=O)CHCjHC=O in Pathway IV. A barrier of 27 kcal mol-1 is required for further reaction, beta scission forming acetylene plus CCj(=O) from CC(=O)CH=CjH via TS III 4; then further beta scission by CCj(=O) forms a methyl radical and CO over TS III 5. The overall reaction is 81 kcal mol-1 exothermic from the entrance channel, with all barriers below the entrance channel.

Ring Expansion from Ipso Addition of Radical Peroxy Oxygen in 2MF5OOj (Pathways VI and VII) A second reaction path for the di-oxiranyl ipso addition adduct, MF5Yj(COO), has one oxygen of the three-member ring move towards C4 radical site, and the O–O bond of the COO ring cleaving with the new C4–O bond forming MF45Y(CCO)5Oj. The CBS-QB3 calculations show that there is no barrier to this reaction. The oxygen on C5 is a radical and forms a new double bond with C5, this cleaves the ring C4–C5 carbon bond and forms a new six-member ring, which



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is calculated to be 129 kcal mol-1 below the entrance channel and 55 kcal mol-1 lower than MF45Y(CCO)5Oj. The structure of this reaction path has the oxygen inserting between the C4 and C5 bond slightly elevated above the plane of the furan ring in MF45Y(CCO)5Oj. The bicyclic TS VI 2 has a negative vibrational frequency of -771 cm-1, corresponding to the bond breaking between the C4–C5 furan carbons leading to the oxygen insertion into the furan ring and creating the sixmember ring Y(OC(C)=CCjOC(=O)) some 78 kcal mol-1 below the 2MF5OOj radical. Formation and reaction through the equivalent of this Y(OC(C)=CCjOC(=O)) intermediate is the lowest energy channel in oxidation of benzene89,90 and toluene rings91 and is an important intermediate in this furan ring carbon oxidation path. This ring expansion to a stable six-member ring was also shown to be a favorable pathway for the thermal decomposition of 5-methyl-2-furanylmethyl radical by Sirjean and Fournet.42 Two locations for ring opening of this six-member ring Y(OC(C)=CCjOC(=O)) involve breaking the C(=O)–O bonds. Beta scission cleavage on the C5 carbon and the newly inserted oxygen (further from the methyl substituent) generates the ring opened O=CjOC(C)=CHCH(=O) through TS VII 1 with a barrier of 42 kcal mol-1. The ring opening allows for rotation of both the terminal CO and aldehyde groups by 180°. Upon initial ring opening, the terminal CO group is at the peak of the rotational barrier while the aldehyde group overcomes a 5 kcal mol-1 energy barrier which could be present from the large excess energy of the chemical activation process. The resulting structure, O=CjOC(C)=CHCH(=O), is equivalent to the product from the ring opening of MF4Oj5O. The second ring opening location has an almost equivalent 42 kcal mol-1 barrier for breaking the C(=O)–O bond to O1 over TS VI 3. In the stable ring opened product, CC(=O)CH=CHOCj=O, there are two internal bond rotations, terminal CO group rotation over a 3 kcal mol-1 barrier and a 180° rotation across the CC(=O)–CH bond over the 4 kcal mol-1 barrier, to form the more stable species. Elimination of CO occurs over a 14 kcal mol-1 TS VI 4 barrier to form CC(=O)CjHCH(=O). CC(=O)CjHCH(=O) and n-CC(=O)CjHCH(=O) are within 1 kcal mol-1 of each other, where the internal rotational of two bonds can convert one structure to the other. The rotational CC(=O)–Cj bond in CC(=O)CjHCH(=O) has an approximate 6.7 kcal mol-1 barrier and a larger 10 kcal mol-1 barrier for rotation of the terminal CH(=O). 22 ACS Paragon Plus Environment

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Kinetic Analysis A summary of the elementary high pressure rate parameters for forward and reverse reactions studied in the 2MF5j + O2 system are listed in Table 4. Elementary high pressure rate parameters for all reactions are given in the Supporting Information.

Table 4 Summary of Elementary High Pressure Rate Parameters for Reactions Studied in the 2MF5j + O2 System Reactions A n Ea 3 2MF5j + O2 → TS 2MF5OOj 1.67x10 2.46 0.0 2MF5OOj → TS 2MF5OOj 3.93x1013 0.11 47.8 2MF5OOj → TS I 1 3.46x1010 0.62 38.9 MF4j5OOH → TS I 1 1.78x1010 0.67 8.1 MF4j5OOH → TS I 2 2.31x1011 0.77 6.3 MF4j5OOH → TS VIII 1 3.00x1010 0.74 26.5 2MF5OOj → TS II 1 4.71x1011 0.28 34.3 MF45Y(CjCOOj) → TS II 1 9.77x1011 0.46 8.1 MF45Y(CjCOOj) → TS II 2 7.78x1011 0.44 1.0 2MF5OOj → TS III 1 2.74x1012 0.04 12.1 MF5Yj(COO) → TS III 1 2.08x1013 -0.02 16.5 MF5Yj(COO) → TS III 2 2.21x1012 0.38 31.3 MF5Yj(COO) → TS VI 1 1.25x1012 0.45 1.0 2MF5OOj → TS IV 1 5.46x1011 0.41 21.6 2MF5OOj → TS V 1 7.73x108 1.02 61.0 MF2CjH25OOH → TS V 1 3.56x1010 0.26 67.1 MF2CjH25OOH → TS V 2 1.12x109 1.04 5.7 -3 -1 Units: A (mol cm s ), Ea (kcal mol-1) Reactions of the chemically activated 2MF5OOj*, formed from 2MF5j + O2 association, have at its initial formation, just over 50 kcal mol-1 activation energy from the new bond formed. This 2MF5OOj* adduct can undergo unimolecular, the above noted isomerization and dissociation reactions, or become deactivated through collisional stabilization. The temperature- and pressure-dependent rate constants are calculated using the multichannel, multifrequency quantum Rice-Ramsperger-Kassel (QRRK) analysis for k(E) with master equation for falloff and stabilization as implemented in the CHEMASTER code.73 A reduced set of three representative vibrations, from the full set of 3n-6 vibrations, that reproduces heat capacity including one external rotation have been shown to compare well to direct count



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methods92 and are determined for each well species. These calculated vibrations are listed in Table 5 while the rest of the input data for CHEMASTER is tabulated in Table 6.

Table 5 Reduced Frequencies for Well Species in 2MF5j + O2 System Species

Frequency (cm-1)

2MF5OOj

426.8 1393.9 3999.7 455.5 1358.4 3797.3 250.3 865.6 2090.5 431.4 1194.1 3017.0 406.8 1115.7 3216.8

MF4j5OOH

MF5Yj(COO)

MF45Y(CjCOOj)

MF2CjH25OOH

Number of Vibration Modes 13.351 15.907 3.742 14.338 13.888 4.274 6.394 16.045 9.061 11.049 16.138 5.313 12.618 14.666 5.216

Table 6 CHEMASTER Input Parameters for 2MF5j + O2 System CHEMASTER Input Parameters Temperatures (K) 300-2100 Pressures (ATM) 0.001-100 N2 (Bath Gas): σ (A) 3.5 e/k (K) 98.3 2MF5OOj: Mass 113.02 σ (A) 6.3 e/k (K) 692.0 ∆Edown (cal mol-1) 900 Integration Interval (kcal) 1.0 -1 Ehead (kcal mol ) 75

Chemically Activated Analysis: 2MF5OOj* Figures 7 and 8 provide the rate constant versus pressure (1 and 100 atm) and temperature (300 and 1000 K) for the chemically activated reactions of the 2MF5j + O2 system forming the 24 ACS Paragon Plus Environment

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chemically activated 2MF5OOj* radical in Figure 4. Several modifications are made to these reaction systems. Species CC(=O)CH=CHCO2j and MF5Yj(CCO)5Oj are removed from consideration as wells, because further reaction barriers were less than 1 kcal mol-1 for the former, with no barrier for the latter. Comparison to calculations that included the wells showed these exclusions did not significantly affect the calculate rate constants for chemical activation. Activation energies for TS II 2 and TS VI 1 are also set to 1.0 kcal mol-1 to in order to include some type of barrier for the calculation to show products. The species MF4j5OOH, MF45Y(CjCOOj), MF5Yj(COO), and MF2CjH25OOH are included in these calculations, but their rates are extremely low compared to the other species so they are not included in Figures 7 and 8. Results of the chemically activated kinetic calculations show two favored product distributions: loss of oxygen (2MF5Oj + O), which is chain branching, and ring expansion (Y(OC(C)=CCjOC(=O))) across the temperatures and pressures considered in Figures 7 and 8. This is consistent with the two lowest energy barriers shown in the PE diagram in Figure 4. 2MF5Oj + O and Y(OC(C)=CCjOC(=O)) are calculated to be 38 and 129 kcal mol-1, respectively, lower than the initial entrance channel.

Isomerization and Dissociation Analysis: 2MF5OOj Rate constants for isomerization and dissociation of the stabilized 2MF5OOj are illustrated in Figures 9 and 10. Under the studied conditions of constant pressure (1 and 100 atm) and temperature (300 and 1000 K), the important channels for 2MF5OOj reaction are MF5Yj(COO) and 2MF5Oj + O. Formation of these two products corresponds to the two lowest energy barriers in Figure 4. Preference of 2MF5OOj isomerization to MF5Yj(COO) generates the favored sixmember ring Y(OC(C)=CCjOC(=O)) depicted in Figure 11 and is consistent with the product formation channels described in the chemical activation analysis. Note in Figure 11 that the rate for Y(OC(C)=CCjOC(=O)) at 300 K and 1000 K are almost identical, creating overlap of the solid and dashed lines. Reaction over the next highest energy barrier is for the isomerization to MF45Y(CjCOOj) as depicted in Figure 12 where ring opening to MF4Oj5O is more important than ring opening back to 2MF5OOj. Dissociation of 2MF5OOj back to the reactants 2MF5j + O2 and the isomerization to MF4j5OOH have the lowest rates at constant temperature and pressure.



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Formation of both products results in going through transition states with high barriers of 51 and 39 kcal mol-1, which accounts for the relative decreased rates. MF2CjH25OOH is included in the analysis calculation, it is not shown since the rate is very small in comparison and has an energy barrier over 61 kcal mol-1. Overall, both the chemical activation and isomerization and dissociation analysis conclude that Pathways III to VI and IV are the major product channels for 2MF5OOj across the temperature and pressures considered in this analysis.


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2MF5OOj* (P = 1 atm)

12

2MF5OOj* (P = 100 atm)

8

8

log (k) (cm3 mol-1 s-1)


12

4 0 -4

4 0 -4 -8

-8

-12 0

1

2 1000/T

3

0

1

(K-1)

2 1000/T

3

(K-1)

Figure 7 Chemical activation rate constants vs. temperature for the 2MF5OOj* radical at pressures of 1 and 100 atm.

12

2MF5OOj* (T = 300 K)

12

2MF5OOj* (T = 1000 K)

8 log (k) (cm3 mol-1 s-1)

8 log (k) (cm3 mol-1 s-1)

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


4 0 -4 -8

4 0 -4 -8

-12

-12 -3

-2

-1 log (P) (atm)

0

1

2

-3

-2

-1 log (P) (atm)

0

1

2

Figure 8 Chemical activation rate constants vs. pressure for the 2MF5OOj* radical at temperatures of 300 and 1000 K. 27 ACS Paragon Plus Environment


10

2MF5OOj (P = 1 atm)

15



2MF5OOj (P = 100 atm)

10

5 0 -5 -10 -15 -20 -25

5 0 -5 -10 -15 -20 -25 -30 -35

-30 0

1

2 1000/T

0

3

1

(K-1)

2 1000/T

3

(K-1)

Figure 9 Rate constants vs. temperature for 2MF5OOj isomerization and dissociation at pressures of 1 and 100 atm.

5

2MF5OOj (T = 300 K)

10

0

2MF5OOj (T = 1000 K)

5

-5



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-10 -15 -20 -25 -30 -35

0 -5 -10 -15 -20

-40

-25

-45 -3

-2

-1 0 log (P) (atm)

1

2

-3

-2

-1 0 log (P) (atm)

1

2

Figure 10 Rate constants vs. pressure for 2MF5OOj isomerization and dissociation at temperatures of 300 and 1000 K. 28 ACS Paragon Plus Environment

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MF5Yj(COO) (P = 1 and 100 atm) 15


10 5 0 -5 -10 -15 -20 -25 -30 0

1

2 1000/T

3

(K-1)

MF5Yj(COO) (T = 300 and 1000 K) 15 10 log (k) (cm3 mol-1 s-1)

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


5 0 -5 -10 -15 -20 -25 -30 -35 -3

-2

-1 0 log (P) (atm)

1

2

Figure 11 Isomerization and dissociation rate constants of MF5Yj(COO) at constant pressure and temperature. Note that rate for Y(OC(C)=CCjOC(=O)) at 300 and 1000 K are almost exactly the same leading to their lines overlapping. 29 ACS Paragon Plus Environment


MF45Y(CjCOOj) (P = 1 and 100 atm) 12


10 8 6 4 2 0 -2 -4 -6 0

1

2 1000/T

3

(K-1)

MF45Y(CjCOOj) (T = 300 and 1000 K) 12 8 log (k) (cm3 mol-1 s-1)

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

4 0 -4 -8 -12 -3

-2

-1 0 log (P) (atm)

1

2

Figure 12 Isomerization and dissociation rate constants of MF45Y(CjCOOj) at constant pressure and temperature.


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Conclusion A detailed study of the oxidation of 2-methyl-5-furanyl radical (2MF5j) including the thermochemistry and kinetics of stable and transition state species is presented using theoretical methods. Enthalpies of formation, entropies, and heat capacities are determined for all species. Elementary rate parameters are fit to a three-parameter form of the Arrhenius equation and then a QRRK analysis, along with master equation for falloff and stabilization, is used to calculate temperature- and pressure-dependent rate constants. The resulting 2MF5OOj radical has a well depth of 51 kcal mol-1 below the 2MF5j + O2 entrance

channel.

Reaction

pathways

analyzed

include

unimolecular

dissociations,

intramolecular: hydrogen transfers, group transfers, and radical peroxy oxygen additions leading to ring opening or expansion. Important species formed from the chemically activated reaction to the 2MF5OOj* peroxy radical are dissociation of a peroxy oxygen atom forming (2MF5Oj plus oxygen atom) and ring expansion to Y(OC(C)=CCjOC(=O)) from the three-member, di-oxirane ring, formed via ipso addition of radical peroxy oxygen to the unsaturated peroxy carbon site. The 2MF5Oj plus oxygen atom and Y(OC(C)=CCjOC(=O)) products are calculated to be 38 and 129 kcal mol-1, respectively, lower than the initial entrance channel. Isomerization and dissociation analysis of the stabilized 2MF5OOj radical confirm the chemical activation findings with preferred formation of 2MF5Oj + O (Pathway IV) and MF5Yj(COO) (Pathway III). The latter species is shown to further isomerize to Y(OC(C)=CCjOC(=O)).

Overall, Pathways III (ipso addition MF5Yj(COO)), IV, and VI are the major product channels for 2MF5OOj across the temperatures and pressures considered. This initial set of thermochemical and kinetic parameters can serve as a basis for other furan-based oxidation systems.

Supporting Information Available Optimized structures, vibration frequencies, moments of inertia, calculated enthalpy of formation for TS 2MF5OOj, 2MF5j + O2 reaction species, and work reaction species, elementary rate parameters and complete references. This information is available in the supplemental electronic material, free of charge via the internet at http://pubs.acs.org.



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Author Information: Corresponding Author *E-mail: [email protected]. ORCID Joseph W. Bozzelli: 0000-0003-4822-150X Jason M. Hudzik: 0000-0002-6550-3675 Present Address † J.M.H.: Department of Biology and Chemistry, County College of Morris, Randolph, NJ 07869. Notes The authors declare no competing financial interest. Acknowledgements The authors would like to thank the NJIT Advanced Research Computing (ARC) group for help and facility maintenance.

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Reaction Paths and Chemical Activation Reactions of 2-Methyl-5

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