Structure and Thermochemical Properties of 2-Methoxyfuran, 3

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J. Phys. Chem. A 2010, 114, 7984–7995

Structure and Thermochemical Properties of 2-Methoxyfuran, 3-Methoxyfuran, and Their Carbon-Centered Radicals Using Computational Chemistry Jason M. Hudzik and Joseph W. Bozzelli* Department of Chemistry and EnVironmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102 ReceiVed: April 2, 2010; ReVised Manuscript ReceiVed: June 15, 2010

Methoxyfurans are known components in a number of biofuel synthesis processes and their thermochemical properties are important to the stability, reaction paths, and chemical kinetics of these species. Enthalpies (∆H°f298), entropies (S°298), and heat capacities (Cp(T)) are reported for 2-methoxyfuran and 3-methoxyfuran, cyclic ethers with possible biofuel implications, and their radicals corresponding to loss of hydrogen atoms. Standard enthalpies of formation are calculated at the B3LYP/6-31G(d,p), B3LYP/6-311G(2d,2p), CBSQB3, G3MP2B3, and G3 levels of theory with isodesmic reactions to minimize calculation errors. Structures, vibrational frequencies, and internal rotor potentials are calculated at the B3LYP/6-31G(d,p) density functional level and are used to determine the entropy and heat capacities. The recommended ideal gas phase enthalpy of formation, from the average of the CBS-QB3 and G3MP2B3 levels of theory, for 2-methoxyfuran is -45.0 kcal mol-1 and for 3-methoxyfuran is -41.1 kcal mol-1. Bond dissociation energies are also calculated. The C-H bonds of the furan ring are approximately 120 kcal mol-1, which is consistent with recent data on several alkylfurans; they are significantly stronger than non-aromatic, stable heterocyclic structures. The bond energy decreases to 98 kcal mol-1 for the methoxy-methyl C-H bonds making this methyl site a favorable abstraction target and an important site for initial decomposition paths during combustion. Group additivity for furan is discussed and groups for furan and methoxyfuran carbon radicals are derived. Introduction Fuel supply and availability along with minimization of CO2 emissions is a concern for countries around the world and has created the need for development of alternative fuel supplies with minimum influx of CO2 into the environment.1,2 Several researchers indicate it is possible that within only a few decades the current petroleum industry and supply chain will not be able to meet the demands of the consumers.3,4 Biofuels, with their regeneration reusing CO2 from the atmosphere, are considered important future fuels and of high value for sustainability.5-7 Biofuels start with materials such as lignocellulosic biomass, which is high in carbohydrates; these are then converted into smaller carbon- and oxygen-containing compounds in various types of processing. This is currently used to produce bioethanol8 and is being analyzed as a possible source for other biofuel compounds.9,10 A major obstacle during the breakdown of biomass into fuels or other products is the loss of chemical energy. Minimizing this loss enhances the integrity and usefulness of the created compounds as fuels.11 One goal of an optimized conversion process is to utilize all of the carbon available in the biomass for energy efficiency. At the present time there is a wide range of research using various types of conversion processes such as pyrolysis, supercritical fluid extractions, gasification, and liquefaction in hopes to improve the chemical yields from biomass.12,13 Understanding the thermochemistry and reaction kinetics in the hydrolysis, dehydration, oxidation, and hydrogenation of biomass will be valuable to modeling and design of the processing, product selectivity, energy efficiency, and carbon recycling.14 Other considerations such as type and available amount of * To whom correspondence should be addressed.

biomass, economical limitations, and environmental concerns are also important factors controlling process selection.15 One of the more versatile compounds created from biofuel production is 5-hydroxymethylfurfural (HMF); it has been isolated through several feed-stocks including glucose, fructose, cellulose,16-18 and lignocelluloses.3,19 Although HMF cannot itself be used as a fuel,18 it can be converted into a number of promising products, such as organic acids, aldehydes, alcohols, amines, and ethers.20-22 Substituted furan molecules are a group of compounds that can also be created from HMF.17,23 Methyl and methoxyfurans are two types of high-energy furans that have been shown to have possible biofuel application.3,11,24 2- and 3-Methoxyfuran are the simplest furan ethers, and their synthesis has been studied by several research groups.25-27 These methoxyfurans have been utilized in the synthesis of a variety of compounds and reactions such as 1,2,3- trisubstituted cyclopropanes,28 (()-avenaciolide and (()-isoavenaciolide,29 maleic anhydride30,31 and N-methylmaleimide30 cycloadducts, and Friedel-Crafts catalyzed reactions with nitroalkenes32 and with selenoaldehydes to generate methyl penta-2,4-dienoates.33 2-methoxyfuran has also successfully been employed to study the reaction enthalpy and kinetic parameters in the synthesis of oxanorbornadiene.34 The structure of 2-methoxyfuran has been studied with MP2/ 6-311++G**, B3LYP/6-311++G**, and B3LYP/cc-pVTZ calculation methods by Beukes et al.;35 its lowest energy structure was determined to be when the methyl is in the syn configuration with respect to the double bonded carbons in the furan ring. Our study confirms these findings. There is little experimental information on the thermochemistry of furan ethers and radicals corresponding to loss of hydrogen atoms. Furan and methyl-substituted furans have been analyzed by Simmie and Curran36 where bond dissociation

10.1021/jp102996d  2010 American Chemical Society Published on Web 07/14/2010

Methoxyfurans energies and formation enthalpies were determined. This group of compounds poses extremely strong C-H bonds due to the aromaticity and the oxygen atom’s location in their cyclic structure. Bond dissociation energies in excess of 120 kcal mol-1 were observed for C-H bonds on the furan ring with lower energies for the bonds of substituent groups. These thermochemical properties are needed for creation of detailed, chemical kinetic models for these and related compounds. In this study we determine the thermochemistry of 2-methoxyfuran, 3-methoxyfuran, and their carbon-centered radicals for their possible biofuel applications. We calculate the standard heat of formations (∆H°f298), entropies (S°298), heat capacities (Cp(T)), bond dissociation energies, and internal rotation potentials. Data from this study are used to develop groups for use in the group additivity (GA) method. It is our intent to hopefully provide valuable thermochemical data for creating chemical kinetic models for these and similar compounds.

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Figure 1. Numbering convention for 2-methoxyfuran, 3-methoxyfuran, and radicals.

calculated at the B3LYP/6-31G(d,p), B3LYP/6-311G(2d,2p), CBS-QB3, G3MP2B3, and G3 levels of theory. The enthalpy change for each reaction (∆H°rxn,298) is calculated using Hess’s law in eq I. Combining the ∆H°rxn,298 with literature values for the enthalpies of formation (∆H°f298) of the known species results in a more accurate ∆H°f298 for the target furan moiety in eq II.

Computational Methods Ab Initio and Density Functional Theory Calculations. Structures, vibration frequencies, zero-point vibrational and thermal energies, and internal rotor potentials are initially analyzed with the hybrid density functional theory (DFT) method B3LYP. This method combines the three-parameter Becke exchange functional, B3,37 with Lee-Yang-Parr correlation functional, LYP,38 and is used here with the 6-31G(d,p) and 6-311G(2d,2p) basis sets. The G339 method initially calculates geometries and uses frequencies based on the Hartree-Fock (HF) method using the 6-31G(d) basis set. The geometry is then further optimized at the higher MP2(Full)/6-31G(d) level. A MP4 calculation is employed for higher accuracy. The G3MP2B340,41 method is a modified version of G3 with geometries and zero-point energies, scaled by 0.96, from a B3LYP/6-31G(d) calculation. A single-point energy calculation using QCISD(T)/6-31G(d) is followed by a MP2(FC) calculation using the 6-31G(d) and 6-311+G(2df,2p) basis sets on Li-Ne and 6-311+G(3d2f,2p) on Na-Ar, denoted G3MP2large. Spin-orbit and higher-level corrections are incorporated in the energy calculations. CBS-QB342 utilizes the B3LYP/6-311G(2d,d,p) level of theory to calculate geometries and frequencies followed by single-point calculations using the CCSD(T), MP4(SDQ), and MP2 levels. The final energies are determined with a CBS extrapolation. All calculations are performed using the Gaussian 03 program suite.43 The natural bond orbital (NBO) analysis is also performed as implemented in the Gaussian 03 code (NBO Version 3.1).44 We analyze the enthalpy values with density functional theory considering its lower computational costs and application to larger molecules with isodesmic work reactions. The B3LYP is thought to be one of the most reliable DFT methods available45 and has been shown by Curtiss et al.46 to have the smallest average absolute deviation, 3.11 kcal mol-1, of the seven DFT methods studied using the G2 test set of molecules. Comparison of the B3 values to the more resource demanding and higher level CBS-QB3, G3, and G3MP2B3 composite methods allow us to evaluate the DFT method’s accuracy for these furan-based compounds. The accuracy of the methods can also be increased with the use of isodesmic reactions.47 Enthalpy, ∆H°f298, Calculations. Isodesmic reactions incorporate molecules with similar bonding and atomic arrangement on both sides of a hypothetical reaction resulting in the canceling of calculation error. Total energies for the compounds are

◦ ∆Hrxn,298 )

◦ ∆Hrxn,298 )

∑ (Total Enthalpy of Products) ∑ (Total Enthalpy of Reactants)

(I)

◦ Products) ∑ (exp ∆Hf,298

◦ ◦ Reactant + ∆Hf,298 Target Furan Moiety) ∑ (exp ∆Hf,298

(II) Entropy (S°298), Heat Capacity (Cp(T)), and Internal Rotor Analysis. The entropy and heat capacities for the compounds are calculated at the B3LYP/6-31G(d,p) level of theory using the statistical mechanics for heat capacity and entropy, SMCPS,48 program. Geometry, mass, multiplicity, symmetry, frequencies, number of optical isomers, and moments of inertia are input into the code. The contributions from the translations, vibrations, and external rotations are represented as TVR in Table 6. The Pitzer and Gwinn49-51 approximation method for entropy and heat capacity contributions from internal rotations (represented as IR in Table 6) use potential energy barriers to rotation calculated at the B3LYP/6-31G(d,p) level. Reduced moments of inertia are determined from moments of inertia for the rotational groups using their mass and radius of rotation. Total entropies and heat capacities for each compound are determined by summing the contributions from TVR and IR. Results and Discussion Configuration and Geometry. Figure 1 illustrates the numbering convention for the methoxyfurans and radicals in this study. Optimized geometries for the compounds from the B3LYP/6-31G(d,p) level of theory with corresponding nomenclature are presented in Figure 2. Optimized geometry coordinates, vibrational frequencies, and moments of inertia for all structures along with a comparison showing excellent agreement of our calculation methods to the previously determined bond lengths and angles for 2-methoxfuran by Beukes et al.35 are available in the Supporting Information. 2-Methoxyfuran and each of the radicals have two low-energy conformations for the methyl group in the methoxy substituent, shown in Figure 3. Syn signifies the methyl group is adjacent to the double bonded carbons in the furan ring and anti has it rotated toward the oxygen of the furan ring. Figure 2 shows

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Figure 2. Nomenclature and illustration of the lowest energy conformation for each species in this study at the B3LYP/6-31G(d,p) level of theory.

Figure 3. Syn and anti conformations for 2-methoxyfuran.

CHART 1

Cy(occcc), Cy() represents a cyclic structure for the compound occcc Cy(oc[och3]ccc), [och3] denotes atom attachment location for och3 substituent Cy(oc•ccc), a • (dot) represents the location for hydrogen atom dissociation C-H, a bond between two atoms cdc, a double bond that Cy(oc[och3]ccc), Cy(oc[och3]cc•c), and Cy(oc[och3]c•cc) attain their lowest energies in the syn conformation with small energy increases of 0.6, 0.6, and 1.4 kcal mol-1 for their anti conformations. See Chart 1 for key to abbreviations. The other 2-methoxyfuran radicals, Cy(oc[och3]ccc•) and Cy(oc[oc•h2]ccc), have their lowest energy in the anti configuration with only a slight 0.3 kcal mol-1 energy increase to the syn conformation. Hydrogen bonding occurs in the anti configuration radicals, where the methyl hydrogen to furan oxygen

bond distances are 2.6 and 2.4 Å for Cy(oc[och3]ccc•) and Cy(oc[oc•h2]ccc); these stabilize the anti configuration. 3-Methoxyfuran and its radicals, in contrast, consistently show preference to the conformation where the methyl group is rotated away from the base of the furan ring as shown in Figure 2. Each compound in this analysis, except Cy(oc[och3]ccc•), has its lowest energy, regardless if syn or anti, when the methyl carbon is in the same plane as the furan ring creating a planar geometry. Each of the analysis methods for Cy(oc[och3]ccc•) shows the methyl group is approximately half way between parallel and perpendicular to the furan ring at a 50° angle. The G3 method deviates slightly here with a 60° angle in the optimized geometry. Data on the geometries and the NBO charges, presented in Table 1, for the 2-methoxyfuran and the Cy(oc[och3]ccc•) radical can assist in explaining the stable out of plane conformation. The shortening of the C2-O6, 1.342 to 1.336 Å, and O1-C5, 1.379 to 1.346 Å, bonds with the elongation of the O1-C2, 1.351 to 1.388 Å, suggests a partial double bond formation is occurring for C2-O6. This is further supported by the shortened C2-C3 bond, relaxation of the C2-C3-C4 angle, increased charge on C3, decreased charges for C2 and O6, and the increase of the bond angle, closer to that of a sp2 bond, to 115.8° for C7-O6-C2. Finally, the O6-C2-O1 bond angle increase to 117.7° would position the C7 methyl group slightly further from O1. The methoxy group in the Cy(oc[och3]ccc•) radical would rotate to this out of plane methyl configuration to be able to hydrogen bond to the oxygen for increasing stability.

Methoxyfurans

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TABLE 1: Comparison of Charges, Bond Lengths, and Bond Angles for 2-Methoxyfuran and Cy(oc[och3]ccc•) at the B3LYP/6-31G(d,p) Level of Theory Cy(oc[och3]ccc)

Cy(oc[och3]ccc•)

NBO Charge -0.464 0.628 -0.440 -0.319 0.060 -0.520 -0.324

O1 C2 C3 C4 C5 O6 C7 O1-C2 C2-C3 C3-C4 C4-C5 O1-C5 C2-O6 O6-C7

Bond Length (Å) 1.351 1.367 1.440 1.356 1.379 1.342 1.424

O1-C2-C3 C2-C3-C4 C3-C4-C5 C4-C5-O1 C7-O6-C2 O6-C2-O1 C2-O1-C5

Bond Angle (deg) 111.6 105.0 106.9 110.1 114.6 113.4 106.4

-0.523 0.591 -0.366 -0.442 0.416 -0.527 -0.326 1.388 1.361 1.448 1.356 1.346 1.336 1.438 110.2 106.6 104.5 113.2 115.8 117.7 105.4

TABLE 2: Standard Enthalpies of Formation at 298.15 K for Reference Species in Isodesmic Reactions

a

species

∆H°f298 (kcal mol-1)

ref

h ch4 ch3ch3 ch3ch2ch3 ch2dch2 ch2dchch3 ch3och3 ch3ch2ch2och3 ch2dchch2och3 Cy(occcc) Cy(c6h6) Cy(c6h5)-och3 c•h 3 ch3c•h2 ch3ch2c•h2 ch2dc•h ch2dchc•h2 Cy(c•6h5)

52.10 ( 0.001 -17.89 ( 0.08 -20.04 ( 0.07 -24.82 ( 0.14 12.54 ( 0.07 4.88 ( 0.08 -43.99 ( 0.12 -56.89 ( 0.20 -25.68a -8.29a 19.81 ( 0.13 -17.27 ( 0.93 34.82 ( 0.2 28.40 ( 0.5 23.90 ( 0.5 70.90 ( 0.3 39.13 ( 0.13 81.4 ( 0.16

52 53 54 53 52 55 56 57 58 59 60 52 52 61 52 62 63 52

Error not reported.

Enthalpies of Formation. Enthalpies of formation (∆H°f298) are determined for the target furan compounds in this study using isodesmic reactions. The reaction enthalpies (∆H°rxn,298) for each reaction, eq I, are used to find ∆H°f298 of the methoxyfuran target by eq II. We note that similar values are obtained over all the calculation levels. ∆H°f298 values for the reference species and uncertainties are summarized in Table 2. The enthalpies of formation for 2-methoxyfuran and 3-methoxyfuran for each of the reactions are shown in Tables 3 and 4 for the five levels of theory. There is good consistency with the B3LYP method for the ∆H°f298 values between the 6-31G(d,p) and 6-311G(2d,2p) basis sets, with all differences less than 0.65 kcal mol-1 for the methoxyfurans and their corresponding radicals. The higher level ab initio calculations from the CBS-QB3 and G3MP2B3 methods also have consistent values within 0.04 and 0.10 kcal mol-1 for 2- and 3-methoxy-

furan but do show deviations ranging from 0.02 to 1.02 kcal mol-1 for the radical species. Values for the radicals from the G3 level of theory are presented but are excluded from the recommended averages, due to their average deviation of 1.8 kcal mol-1 (higher) than the CBS-QB3 and G3MP2B3 values. This is consistent with the discussion in the radicals section of the article by Simmie and Curran36 and references therein. Differences in the ZPVE and geometry optimization methods in G3 versus the CBS-QB3 and G3MP2B3 calculation methods could also be a reason for the consistently higher G3 energy values. We recommend the following enthalpy of formation values of -45.0, 23.7, 22.1, 23.4, and 1.7 kcal mol-1 for Cy(oc[och3]ccc), Cy(oc[och3]ccc•), Cy(oc[och3]cc•c), Cy(oc[och3]c•cc), and Cy(oc[oc•h2]ccc) and -41.1, 25.9, 27.8, 27.3, and 4.6 kcal mol-1 for Cy(occ[och3]cc), Cy(occ[och3]cc•), Cy(occ[och3]c•c), Cy(oc•c[och3]cc), and Cy(occ[oc•h2]cc). These values are the averages from the CBSQB3 and G3MP2B3 methods presented in Table 5. The DFT method is shown to provide acceptable enthalpy values by comparison to the higher level calculations. There is only a 0.5-0.8 kcal mol-1 difference for 2- and 3-methoxyfuran from the DFT method work reactions relative to the higher calculation levels. The enthalpy analysis for the radicals shows a larger deviation ranging from 0.01 to 1.23 kcal mol-1. This suggests that the B3LYP density functional calculations with work reactions provide reasonable estimates for these molecules and radicals. Enthalpies of Formation Calculated by Atomization Reaction Method. Atomization reactions can also be used to determine the enthalpy of formation for a compound by considering the compound’s decomposition to its balanced number of constituent atoms, C, H, and O. Osmont et al. developed an empirically corrected atomization method for gasphase heterocyclic oxygenated compounds using the B3LYP/ 6-31G(d,p) calculation and is presented in Tables 3 and 4. This method, while partially calibrated on furan-like molecules, has been shown to produce good agreement to experimental standard enthalpies of formation for the related compounds tetrahydrofuran, furan, and 2,5-dimethylfuran.64 Comparison of values from the Osmont method determined from our calculated DFT energies to values in this study shows good agreement. For the parent compounds, the work reaction values of -45.6 kcal mol-1 for 2-methoxyfuran and -42.1 kcal mol-1 for 3-methoxyfuran were 0.4 kcal mol-1 lower than the Osmont method values of -46.0 and -42.5 kcal mol-1, respectively. This method also shows good comparison to our recommended higher calculation level average values with deviations of 1.0 kcal mol-1 for 2-methoxyfuran and 1.4 kcal mol-1 for 3-methoxyfuran. Radical analysis from this method by incorporation of a carbon atom radical correction65,66 is also presented in Tables 3 and 4. The eight methoxyfuran radicals show acceptable results similar to the parent compounds. There is a 0.2 kcal mol-1 average difference for the work reactions to the Osmont method and only a 0.9 kcal mol-1 difference to our recommended values. This consistency validates the use and choices of our work reactions employed overall in this analysis for the B3LYP/631G(d,p) level of theory. Bond Dissociation Energies. Differences in bond dissociation energies (BDE) for a compound are important to determine the reaction pathways and kinetics for abstraction and dissociation reactions. The methoxyfurans have four different C-H bond types. Bond energies are determined by using the reaction parent f

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TABLE 3: Isodesmic Reactions, Calculated Enthalpies of Formations, and Bond Energies for 2-Methoxyfuran and Radicals ∆H°f298 (kcal mol-1) B3LYP isodesmic reactions

6-31G(d,p)

Cy(oc[och3]ccc) System -45.05 Cy(oc[och3]ccc) + ch4 f Cy(occcc) + ch3och3 Cy(oc[och3]ccc) + ch3ch2ch3 f Cy(occcc) + ch3ch2ch2och3 -45.89 -46.16 Cy(oc[och3]ccc) + ch2dchch3 f Cy(occcc) + ch2dchch2och3 Cy(oc[och3]ccc) + Cy(c6h6) f Cy(occcc) + Cy(c6h5)-och3 -45.15 average -45.56 Osmont et al. corrected atomization -46.0 Cy(oc[och3]ccc•) System 21.70 Cy(oc[och3]ccc ) + ch4 f Cy(oc[och3]ccc) + c h3 Cy(oc[och3]ccc•) + ch3ch3 f Cy(oc[och3]ccc) + ch3c•h2 22.19 Cy(oc[och3]ccc•) + ch3ch2ch3 f Cy(oc[och3]ccc) + ch3ch2c•h2 22.19 Cy(oc[och3]ccc•) + ch2dch2 f Cy(oc[och3]ccc) + ch2dc•h 22.98 Cy(oc[och3]ccc•) + ch2dchch3 f Cy(oc[och3]ccc) + ch2dchc•h2 23.09 Cy(oc[och3]ccc•) + Cy(c6h6) f Cy(oc[och3]ccc) + Cy(c•6h5) 24.47 average 22.77 Osmont et al. corrected atomization 22.8 bond energy Cy(oc[och3]ccc[-h]) 119.87 •

•

Cy(oc[och3]cc•c) System • • 20.77 Cy(oc[och3]cc c) + ch4 f Cy(oc[och3]ccc) + c h3 Cy(oc[och3]cc•c) + ch3ch3 f Cy(oc[och3]ccc) + ch3c•h2 21.26 Cy(oc[och3]cc•c) + ch3ch2ch3 f Cy(oc[och3]ccc) + ch3ch2c•h2 21.26 Cy(oc[och3]cc•c) + ch2dch2 f Cy(oc[och3]ccc) + ch2dc•h 22.05 Cy(oc[och3]cc•c) + ch2dchch3 f Cy(oc[och3]ccc) + ch2dchc•h2 22.16 Cy(oc[och3]cc•c) + Cy(c6h6) f Cy(oc[och3]ccc) + Cy(c•6h5) 23.54 average 21.84 Osmont et al. corrected atomization 21.9 bond energy Cy(oc[och3]cc[-h]c) 118.94 Cy(oc[och3]c•cc) System 22.01 Cy(oc[och3]c•cc) + ch4 f Cy(oc[och3]ccc) + c•h3 Cy(oc[och3]c•cc) + ch3ch3 f Cy(oc[och3]ccc) + ch3c•h2 22.51 Cy(oc[och3]c•cc) + ch3ch2ch3 f Cy(oc[och3]ccc) + ch3ch2c•h2 22.51 Cy(oc[och3]c•cc) + ch2dch2 f Cy(oc[och3]ccc) + ch2dc•h 23.30 Cy(oc[och3]c•cc) + ch2dchch3 f Cy(oc[och3]ccc) + ch2dchc•h2 23.41 Cy(oc[och3]c•cc) + Cy(c6h6) f Cy(oc[och3]ccc) + Cy(c•6h5) 24.78 average 23.09 Osmont et al. corrected atomization 23.1 bond energy Cy(oc[och3]c[-h]cc) 120.19 Cy(oc[oc•h2]ccc) System -0.57 Cy(oc[oc•h2]ccc) + ch4 f Cy(oc[och3]ccc) + c•h3 Cy(oc[oc•h2]ccc) + ch3ch3 f Cy(oc[och3]ccc) + ch3c•h2 -0.07 Cy(oc[oc•h2]ccc) + ch3ch2ch3 f Cy(oc[och3]ccc) + ch3ch2c•h2 -0.07 Cy(oc[oc•h2]ccc) + ch2dch2 f Cy(oc[och3]ccc) + ch2dc•h 0.72 Cy(oc[oc•h2]ccc) + ch2dchch3 f Cy(oc[och3]ccc) + ch2dchc•h2 0.83 Cy(oc[oc•h2]ccc) + Cy(c6h6) f Cy(oc[och3]ccc) + Cy(c•6h5) 2.21 average 0.51 Osmont et al. corrected atomization 0.5 bond energy Cy(oc[o[c-h]h2]ccc) 97.61

radical + H, where we have determined the ∆H°f298 values for both the parent and radicals combined with the established ∆H°f298 literature value52 of 52.1 kcal mol-1 for the hydrogen atom. The bond dissociation energies from the radicals are listed in Tables 3 and 4 along with a summary of the averages in Table 5. For the radicals formed from hydrogen loss on the furan ring, Cy(oc[och3]ccc•), Cy(oc[och3]cc•c), Cy(oc[och3]c•cc), Cy(occ[och3]cc•), Cy(occ[och3]c•c), and Cy(oc•c[och3]cc), we recommend the bond energy averages from the CBS-QB3 and G3MP2B3 calculations of 120.8, 119.2, 120.5, 119.1, 121.0, and 120.5 kcal mol-1. These energies create a 1.9 kcal mol-1 range for the different ring positions on these methoxysubstituted furans and are among the highest currently known

6-311G(2d,2p)

CBS-QB3

G3MP2B3

G3

-44.80 -45.37 -45.92 -45.23 -45.33

-44.86 -44.75 -45.13 -45.35 -45.02

-44.79 -44.78 -45.14 -45.21 -44.98

-45.61 -45.41 -45.70 -45.24 -45.49

22.20 22.65 22.71 23.20 23.21 24.30 23.04

23.55 22.99 22.99 23.99 23.26 22.37 23.19

25.37 24.38 24.33 24.89 23.14 23.14 24.21

25.82 24.63 24.59 25.42 24.70 26.84 25.33

120.15

120.29

121.31

122.43

21.59 22.05 22.11 22.60 22.60 23.70 22.44

22.23 21.67 21.66 22.67 21.93 21.05 21.87

23.54 22.54 22.50 23.05 21.31 21.30 22.37

23.78 22.59 22.55 23.38 22.65 24.79 23.29

119.54

118.97

119.47

120.39

22.75 23.20 23.27 23.75 23.76 24.86 23.60

23.50 22.94 22.93 23.94 23.20 22.32 23.14

24.73 23.73 23.68 24.24 22.49 22.49 23.56

25.50 24.31 24.27 25.10 24.37 26.51 25.01

120.70

120.24

120.66

122.11

-0.48 -0.03 0.04 0.52 0.53 1.63 0.37

2.04 1.48 1.48 2.48 1.75 0.86 1.68

2.82 1.82 1.78 2.34 0.59 0.59 1.66

3.59 2.40 2.36 3.19 2.47 4.61 3.10

97.47

98.78

98.76

100.20

for hydrocarbon aromatics. These furan ring bond energies are consistent with other multiposition heterocyclic compounds. For example, furan36,67 has bond energies of 118-121 kcal mol-1 depending on the C-H bond position, pyrrole67-69 with 118-120 kcal mol-1, and thiophene67 with 114-117 kcal mol-1. Unsaturated cyclic hydrocarbons such as benzene36,67,70,71 have lower bond energies of 110-113 kcal mol-1. Saturated cyclic hydrocarbons have even lower bond dissociation energies of approximately 95 kcal mol-1 for cyclopentane71-75 and 100 kcal mol-1 for cyclohexane.71,73-75 The radicals on the methyl group for 2- and 3-methoxyfuran have recommended bond energies of 98.8 and 97.7 kcal mol-1. This is 20 kcal mol-1 lower in energy than the furan ring C-H bonds but is ∼7.5 kcal mol-1 higher than the benzylic C-H bond on the methyl groups of toluene, xylenes, and other multi-

Methoxyfurans

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TABLE 4: Isodesmic Reactions, Calculated Enthalpies of Formations, and Bond Energies for 3-Methoxyfuran and Radicals ∆H°f298 (kcal mol-1) B3LYP isodesmic reactions

6-31G(d,p)

Cy(occ[och3]cc) System -41.56 Cy(occ[och3]cc) + ch4 f Cy(occcc) + ch3och3 Cy(occ[och3]cc) + ch3ch2ch3 f Cy(occcc) + ch3ch2ch2och3 -42.40 -42.67 Cy(occ[och3]cc) + ch2dchch3 f Cy(occcc) + ch2dchch2och3 Cy(occ[och3]cc) + Cy(c6h6) f Cy(occcc) + Cy(c6h5)-och3 -41.66 average -42.07 Osmont et al. corrected atomization -42.5 Cy(occ[och3]cc•) System 24.71 Cy(occ[och3]cc ) + ch4 f Cy(occ[och3]cc) + c h3 Cy(occ[och3]cc•) + ch3ch3 f Cy(occ[och3]cc) + ch3c•h2 25.20 Cy(occ[och3]cc•) + ch3ch2ch3 f Cy(occ[och3]cc) + ch3ch2c•h2 25.20 Cy(occ[och3]cc•) + ch2dch2 f Cy(occ[och3]cc) + ch2dc•h 25.99 Cy(occ[och3]cc•) + ch2dchch3 f Cy(occ[och3]cc) + ch2dchc•h2 26.10 Cy(occ[och3]cc•) + Cy(c6h6) f Cy(occ[och3]cc) + Cy(c•6h5) 27.48 average 25.78 Osmont et al. corrected atomization 25.4 bond energy Cy(occ[och3]cc[-h]) 118.94 •

•

Cy(occ[och3]c•c) System • • 26.42 Cy(occ[och3]c c) + ch4 f Cy(occ[och3]cc) + c h3 Cy(occ[och3]c•c) + ch3ch3 f Cy(occ[och3]cc) + ch3c•h2 26.91 Cy(occ[och3]c•c) + ch3ch2ch3 f Cy(occ[och3]cc) + ch3ch2c•h2 26.91 Cy(occ[och3]c•c) + ch2dch2 f Cy(occ[och3]cc) + ch2dc•h 27.70 Cy(occ[och3]c•c) + ch2dchch3 f Cy(occ[och3]cc) + ch2dchc•h2 27.81 Cy(occ[och3]c•c) + Cy(c6h6) f Cy(occ[och3]cc) + Cy(c•6h5) 29.19 average 27.49 Osmont et al. corrected atomization 27.1 bond energy Cy(occ[och3]c[-h]c) 120.65 Cy(oc•c[och3]cc) System 25.10 Cy(oc•c[och3]cc) + ch4 f Cy(occ[och3]cc) + c•h3 Cy(oc•c[och3]cc) + ch3ch3 f Cy(occ[och3]cc) + ch3c•h2 25.59 Cy(oc•c[och3]cc) + ch3ch2ch3 f Cy(occ[och3]cc) + ch3ch2c•h2 25.59 Cy(oc•c[och3]cc) + ch2dch2 f Cy(occ[och3]cc) + ch2dc•h 26.38 Cy(oc•c[och3]cc) + ch2dchch3 f Cy(occ[och3]cc) + ch2dchc•h2 26.49 Cy(oc•c[och3]cc) + Cy(c6h6) f Cy(occ[och3]cc) + Cy(c•6h5) 27.87 average 26.17 Osmont et al. corrected atomization 25.7 bond energy Cy(oc[-h]c[och3]cc) 119.33 Cy(occ[oc•h2]cc) System 3.06 Cy(occ[oc•h2]cc) + ch4 f Cy(occ[och3]cc) + c•h2 Cy(occ[oc•h2]cc) + ch3ch3 f Cy(occ[och3]cc) + ch3c•h2 3.56 Cy(occ[oc•h2]cc) + ch3ch2ch3 f Cy(occ[och3]cc) + ch3ch2c•h2 3.55 Cy(occ[oc•h2]cc) + ch2dch2 f Cy(occ[och3]cc) + ch2dc•h 4.34 Cy(occ[oc•h2]cc) + ch2dchch3 f Cy(occ[och3]cc) + ch2dchc•h2 4.46 Cy(occ[oc•h2]cc) + Cy(c6h6) f Cy(occ[och3]cc) + Cy(c•6h5) 5.32 average 4.05 Osmont et al. corrected atomization 3.7 bond energy Cy(occ[o[c-h]h2]cc) 97.22

TABLE 5: Recommended ∆H°f298 and Bond Dissociation Energies for 2-Methoxyfuran, 3-Methoxyfuran, and Radicals DFT species

∆H°f298 (kcal mol-1)

Cy(oc[och3]ccc) Cy(oc[och3]ccc•) Cy(oc[och3]cc•c) Cy(oc[och3]c•cc) Cy(oc[oc•h2]ccc) Cy(occ[och3]cc) Cy(occ[och3]cc•) Cy(occ[och3]c•c) Cy(oc•c[och3]cc) Cy(occ[oc•h2]cc)

-45.45 22.91 22.14 23.34 0.44 -41.88 26.11 27.76 26.33 4.07

CBS-QB3/G3MP2B3

BDE (kcal mol-1) 120.01 119.24 120.44 97.54 119.27 120.92 119.49 97.23

∆H°f298 (kcal mol-1) -45.00 23.70 22.12 23.35 1.67 -41.06 25.89 27.81 27.34 4.57

BDE (kcal mol-1) 120.80 119.22 120.45 98.77 119.05 120.97 120.51 97.73

methyl-substituted benzenes. These lower bond energies are due to the increase in stability from the resonance of the electron

6-311G(2d,2p)

CBS-QB3

G3MP2B3

G3

-41.16 -41.74 -42.28 -41.60 -41.70

-40.95 -40.84 -41.22 -41.44 -41.11

-40.82 -40.81 -41.17 -41.24 -41.01

-41.61 -41.41 -41.70 -41.25 -41.49

25.58 26.04 26.10 26.59 26.59 27.69 26.43

25.95 25.39 25.39 26.39 25.66 24.78 25.59

27.35 26.35 26.31 26.87 25.12 25.12 26.19

28.05 26.86 26.82 27.65 26.93 29.07 27.56

119.59

118.75

119.35

120.72

27.18 27.64 27.70 28.19 28.19 29.29 28.03

27.89 27.33 27.33 28.33 27.60 26.72 27.54

29.25 28.25 28.20 28.76 27.02 27.01 28.08

29.86 28.67 28.63 29.46 28.74 30.88 29.37

121.19

120.70

121.24

122.53

25.63 26.09 26.15 26.64 26.65 27.74 26.48

27.91 27.35 27.34 28.35 27.61 26.73 27.55

28.31 27.31 27.26 27.82 26.07 26.07 27.14

31.15 29.96 29.92 30.75 30.03 32.17 30.66

119.64

120.71

120.30

123.82

3.21 3.67 3.73 4.22 4.22 5.45 4.08

5.26 4.70 4.70 5.70 4.97 4.09 4.91

5.39 4.39 4.35 4.91 3.16 3.16 4.23

6.35 5.16 5.12 5.95 5.23 7.37 5.86

97.23

98.07

97.39

99.02

with the adjacent oxygen atom and somewhat through to the furan ring. Benzyl radical formation in toluene71,76 and the methyl radical formation in 2-methylfuran36,71 also have similarly low C-H bond energies (resonance stabilized radicals) of 86-90 kcal mol-1. Primary C-H bond energies in dimethyl ether71,77 and secondary bond energies in ethyl methyl ether71 also have similar bond energies of 96 and 93 kcal mol-1, which are low compared to the equivalent bond energies, 98.5-101 kcal mol-1, of the primary and secondary bonds in the hydrocarbons ethane, n-propane, and n-butane.71,75,76,78 Internal Rotor Analysis. Potential energy curves for the internal rotation barriers across the methoxy and the methyl rotors in 2- and 3-methoxyfuran were calculated at the B3LYP/ 6-31G(d,p) level of theory. The lowest energy conformation was first determined, and potential energy profiles were then created

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TABLE 6: Calculated Entropy (S°298) and Heat Capacities (Cp(T)) for 2-Methoxyfuran, 3-Methoxyfuran, and Radicals from B3LYP/6-31G(d,p) Level of Theory Cp(T) (cal mol-1 K-1) species Cy(oc[och3]ccc) σ (symmetry) ) 3 Cy(oc[och3]ccc•) σ (symmetry) ) 1 Cy(oc[och3]cc•c) σ (symmetry) ) 1 Cy(oc[och3]c•cc) σ (symmetry) ) 1 Cy(oc[oc•h2]ccc)b σ (symmetry) ) 1 Cy(occ[och3]cc) σ (symmetry) ) 3 Cy(occ[och3]cc•) σ (symmetry) ) 1 Cy(occ[och3]c•c) σ (symmetry) ) 1 Cy(oc•c[och3]cc) σ (symmetry) ) 1 Cy(occ[oc•h2]cc)c σ (symmetry) ) 1

TVR IR 1 (-och3) IR 2 (oc-h3) total TVR IR 1 (-och3) IR 2 (oc-h3) total TVR IR 1 (-och3) IR 2 (oc-h3) total TVR IR 1 (-och3) IR 2 (oc-h3) total TVR IR 1 (-oc•h2) IR 2 (oc•-h2) total TVR IR 1 (-och3) IR 2 (oc-h3) total TVR IR 1 (-och3) IR 2 (oc-h3) total TVR IR 1 (-och3) IR 2 (oc-h3) total TVR IR 1 (-och3) IR 2 (oc-h3) total TVR IR 1 (-oc•h2) IR 2 (oc•-h2) total

S°298a

300 K

400 K

500 K

600 K

700 K

800 K

900 K

1000 K

1500 K

70.72 6.58 4.47 81.76 74.75 6.96 5.26 86.97 74.25 6.62 4.51 85.37 74.67 6.62 4.84 86.12 74.46 6.47 3.13 84.05 70.78 6.09 4.29 81.16 74.48 6.24 4.32 85.04 74.22 6.20 4.31 84.73 74.80 6.09 4.90 85.79 74.03 6.39 3.22 83.65

20.85 2.11 2.13 25.08 20.70 1.79 1.57 24.06 20.48 2.09 2.12 24.68 20.48 2.09 1.95 24.52 21.58 2.16 1.99 25.73 20.91 2.32 2.12 25.35 20.67 2.32 2.14 25.13 20.38 2.33 2.13 24.84 20.94 2.32 2.02 25.28 21.17 2.29 1.94 25.40

27.78 1.99 2.10 31.87 26.96 1.65 1.46 30.07 26.70 1.96 2.08 30.74 26.65 1.96 1.85 30.46 28.13 2.05 2.09 32.27 27.82 2.32 2.18 32.32 26.88 2.29 2.18 31.34 26.58 2.30 2.18 31.06 27.23 2.32 1.91 31.47 27.78 2.21 2.05 32.04

33.83 1.77 1.94 37.54 32.40 1.46 1.33 35.19 32.14 1.74 1.92 35.79 32.08 1.74 1.66 35.47 33.56 1.83 2.21 37.60 33.85 2.28 2.18 38.31 32.28 2.21 2.16 36.66 32.02 2.23 2.17 36.41 32.67 2.28 1.80 36.75 33.27 2.10 2.14 37.51

38.81 1.60 1.78 42.19 36.85 1.35 1.24 39.44 36.61 1.57 1.75 39.93 36.55 1.57 1.51 39.63 37.87 1.66 2.27 41.80 38.82 2.13 2.07 43.02 36.73 2.02 2.05 40.80 36.50 2.05 2.05 40.61 37.10 2.13 1.61 40.84 37.64 1.89 2.24 41.77

42.88 1.47 1.65 46.00 40.47 1.26 1.18 42.91 40.25 1.45 1.62 43.32 40.20 1.45 1.40 43.05 41.31 1.52 2.27 45.10 42.89 1.96 1.93 46.77 40.36 1.84 1.90 44.10 40.16 1.87 1.91 43.94 40.69 1.96 1.47 44.12 41.13 1.71 2.27 45.12

46.23 1.38 1.54 49.15 43.44 1.20 1.14 45.78 43.24 1.36 1.51 46.12 43.21 1.36 1.33 45.90 44.11 1.42 2.23 47.76 46.24 1.81 1.79 49.83 43.34 1.69 1.76 46.79 43.18 1.72 1.76 46.66 43.64 1.80 1.37 46.81 43.97 1.57 2.25 47.79

49.04 1.32 1.46 51.82 45.91 1.17 1.11 48.19 45.74 1.30 1.44 48.48 45.72 1.30 1.27 48.29 46.44 1.36 2.16 49.95 49.05 1.68 1.67 52.39 45.83 1.57 1.64 49.04 45.69 1.60 1.65 48.93 46.09 1.68 1.30 49.07 46.33 1.46 2.18 49.97

51.42 1.26 1.38 54.05 47.99 1.13 1.09 50.21 47.84 1.24 1.36 50.45 47.83 1.24 1.22 50.28 48.40 1.29 2.09 51.77 51.42 1.49 1.48 54.39 47.92 1.40 1.46 50.78 47.81 1.42 1.47 50.69 48.15 1.48 1.20 50.83 48.32 1.32 2.00 51.64

59.11 1.12 1.18 61.41 54.67 1.06 1.04 56.76 54.59 1.11 1.17 56.87 54.59 1.11 1.10 56.80 54.77 1.13 1.70 57.61 59.11 1.24 1.25 61.60 54.63 1.19 1.23 57.06 54.58 1.20 1.24 57.02 54.76 1.24 1.09 57.09 54.75 1.15 1.62 57.51

a Units of cal mol-1 K-1. b 152.3° H-C7-O6-H dihedral angle from B3LYP-6-31G(d,p) level of theory. c 148.2° H-C7-O6-H dihedral angle from B3LYP-6-31G(d,p) level of theory.

TABLE 7: Calculated and Literature Group Additivity and Bond Dissociation Energies for Methoxyfurans C°p(T) (cal mol-1 K-1) group

∆H°f298 (kcal mol-1)

S°298 (cal mol-1 K-1)

O/Cd2 Cd/H/Ob Cd/Cd/Hb O/C/Cd C/H3/Oa CY/FURAN

-32.80 8.60 6.74 -29.70 -10.00 -6.84

10.00 6.20 6.38 5.44 30.41 30.09

CF/O CF/OF/Od 2CF5JOC 2CF4JOC 2CF3JOC 2CFOCJ 3CF5JOC 3CF4JOC 3CF2JOC 3CFOCJ

14.34 12.26 120.80 119.22 120.45 98.77 119.05 120.97 120.51 97.73

CFJ CFJ/O/C CF/O/CJ

120.2c 120.2c 98.3

a

300 K

400 K

500 K

600 K

800 K

1000 K

1500 K

Known Groups 3.40 3.70 4.75 6.46 4.46 5.79 4.04 4.73 6.19 7.84 -6.08 -6.98

3.70 7.64 6.75 4.99 9.40 -6.71

3.80 8.35 7.42 5.04 10.79 -6.00

4.40 9.10 8.35 4.97 13.03 -4.85

4.60 9.56 9.11 4.81 14.77 -4.03

4.80 10.46 10.09 4.49 17.58 -2.95

4.90 5.02 -2.350 -1.754 -2.071 0.057 -1.652 -1.896 -1.560 -0.800

5.27 5.37 -2.752 -2.262 -2.559 -0.394 -2.220 -2.414 -2.180 -1.250

5.73 5.80 -3.370 -3.033 -3.253 -1.389 -3.039 -3.167 -3.016 -2.041

6.01 6.12 -3.835 -3.602 -3.766 -2.275 -3.605 -3.696 -3.555 -2.755

6.67 6.85 -4.647 -4.540 -4.613 -3.804 -4.543 -4.583 -4.506 -4.090

-2.65 -2.40 -0.82

-3.42 -3.15 -1.72

-3.99 -3.68 -2.51

-4.85 -4.57 -3.95

Groups Developed in -11.38 3.84 -10.96 3.86 5.207 -1.019 3.611 -0.401 4.365 -0.558 2.295 0.651 3.883 -0.220 3.570 -0.515 4.627 -0.071 2.487 0.046

This Study 4.32 4.54 -1.802 -1.132 -1.406 0.402 -0.980 -1.263 -0.854 -0.282

Recommended Bond Dissociation Groups 2.46 -0.72 -1.51 -2.15 4.21 -0.46 -1.24 -1.88 2.39 0.35 0.06 -0.37

Reference 84. b Reference 89. c Average from ref 36 and this study. d CF and OF represent carbon furan and oxygen furan.

Methoxyfurans

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TABLE 8: Calculated Entropy (S°298) for 2-Methoxyfuran, 3-Methoxyfuran, and Radicals from B3LYP/6-31G(d,p) Level of Theory Cy(oc[och3]ccc•)

Cy(oc[och3]ccc) temp (K)

TVR

100 150 200 250 298 300 400 500 600 700 800 900 1000 1500 2000 2500 3000 3500 4000 4500 5000

56.11 60.26 63.90 67.38 70.72 70.85 77.81 84.68 91.31 97.61 103.56 109.17 114.46 136.95 154.55 168.84 180.83 191.12 200.13 208.12 215.31

temp (K)

TVR

100 150 200 250 298 300 400 500 600 700 800 900 1000 1500 2000 2500 3000 3500 4000 4500 5000

59.75 64.08 67.83 71.36 74.67 74.80 81.55 88.10 94.36 100.27 105.84 111.08 116.01 136.85 153.07 166.21 177.21 186.64 194.89 202.21 208.79

IR 1 (-och3) IR 2 (oc-h3) 5.23 5.69 6.06 6.35 6.58 6.74 6.91 7.11 7.23 7.31 7.37 7.41 7.44 7.52 7.55 7.56 7.57 7.57 7.57 7.57 7.58

3.70 3.83 4.05 4.28 4.47 4.64 4.81 5.04 5.20 5.31 5.39 5.44 5.49 5.60 5.65 5.67 5.68 5.69 5.69 5.69 5.69

total

TVR

65.04 69.78 74.00 78.01 81.76 82.24 89.53 96.83 103.74 110.23 116.31 122.01 127.39 150.08 167.74 182.07 194.08 204.38 213.39 221.39 228.58

59.75 64.08 67.85 71.41 74.75 74.88 81.71 88.33 94.64 100.60 106.21 111.47 116.42 137.31 153.54 166.69 177.69 187.13 195.38 202.70 209.28

TVR

100 150 200 250 298 300 400 500 600 700 800 900 1000 1500 2000 2500

59.65 63.87 67.60 71.15 74.48 74.62 81.43 88.02 94.32 100.26 105.85 111.10 116.04 136.91 153.13 166.28

5.63 6.14 6.51 6.78 6.96 7.08 7.19 7.32 7.39 7.44 7.47 7.49 7.51 7.55 7.56 7.57 7.57 7.57 7.58 7.58 7.58

4.29 4.64 4.93 5.13 5.26 5.35 5.43 5.52 5.57 5.61 5.63 5.64 5.65 5.68 5.69 5.70 5.70 5.70 5.70 5.70 5.70

Cy(oc[och3]c•cc)

Cy(oc[oc•h2]ccc)

IR 1 (-och3) IR 2 (oc-h3)

IR 1 IR 2 (-oc•h2) (oc•-h2)

5.27 5.72 6.10 6.39 6.62 6.78 6.94 7.13 7.25 7.33 7.38 7.41 7.45 7.52 7.55 7.56 7.56 7.57 7.57 7.57 7.57

3.89 4.13 4.40 4.65 4.84 4.98 5.12 5.30 5.40 5.47 5.52 5.55 5.59 5.65 5.67 5.69 5.69 5.69 5.70 5.70 5.70

total

TVR

68.91 73.93 78.33 82.41 86.12 86.56 93.61 100.52 107.00 113.08 118.74 124.05 129.05 150.02 166.28 179.45 190.46 199.90 208.15 215.48 222.06

59.47 63.64 67.37 71.00 74.46 74.61 81.73 88.62 95.13 101.24 106.94 112.27 117.27 138.25 154.50 167.65 178.65 188.09 196.34 203.66 210.24

Cy(occ[och3]cc•) temp (K)

IR 1 IR 2 (-och3) (oc-h3)

IR 1 (-och3) IR 2 (oc-h3) 4.96 5.38 5.72 6.02 6.24 6.24 6.45 6.62 6.88 7.05 7.17 7.25 7.36 7.48 7.52 7.54

3.63 3.73 3.92 4.12 4.32 4.32 4.50 4.66 4.91 5.09 5.22 5.31 5.44 5.58 5.63 5.66

5.13 5.58 5.94 6.24 6.47 6.64 6.81 7.02 7.16 7.25 7.31 7.35 7.39 7.48 7.51 7.52 7.53 7.54 7.54 7.54 7.54

5.33 5.33 2.83 2.98 3.13 3.27 3.42 3.68 3.91 4.11 4.28 4.41 4.54 4.90 5.07 5.15 5.21 5.24 5.26 5.27 5.29

Cy(oc[och3]cc•c) total

TVR

69.66 74.86 79.29 83.32 86.97 87.31 94.33 101.16 107.60 113.65 119.31 124.60 129.58 150.54 166.79 179.95 190.96 200.40 208.65 215.98 222.56

59.55 63.76 67.45 70.95 74.25 74.39 81.14 87.70 93.97 99.90 105.47 110.71 115.65 136.49 152.70 165.84 176.84 186.27 194.52 201.84 208.42

TVR

68.24 72.98 77.24 81.29 85.04 85.18 92.38 99.31 106.10 112.39 118.23 123.66 128.83 149.96 166.28 179.48

59.62 63.79 67.45 70.94 74.22 74.35 81.08 87.61 93.86 99.77 105.33 110.57 115.50 136.33 152.54 165.68

IR 1 IR 2 (-och3) (oc-h3) 4.92 5.35 5.68 5.97 6.20 6.20 6.41 6.58 6.84 7.02 7.14 7.23 7.34 7.47 7.52 7.54

3.63 3.72 3.91 4.12 4.31 4.31 4.49 4.65 4.90 5.08 5.21 5.31 5.43 5.58 5.63 5.66

5.27 5.72 6.10 6.39 6.62 6.78 6.94 7.13 7.25 7.33 7.38 7.41 7.45 7.52 7.55 7.56 7.56 7.57 7.57 7.57 7.57

3.72 3.85 4.08 4.31 4.51 4.68 4.84 5.07 5.22 5.33 5.40 5.45 5.50 5.61 5.65 5.67 5.68 5.69 5.69 5.69 5.69

total 68.53 73.34 77.63 81.66 85.37 85.84 92.92 99.90 106.44 112.55 118.25 123.58 128.59 149.62 165.89 179.07 190.08 199.53 207.78 215.11 221.69

Cy(occ[och3]cc) total

TVR

69.94 74.55 76.14 80.22 84.05 84.52 91.97 99.32 106.20 112.59 118.53 124.03 129.20 150.64 167.08 180.33 191.39 200.86 209.14 216.47 223.06

56.17 60.30 63.94 67.44 70.78 70.92 77.90 84.77 91.40 97.70 103.65 109.27 114.56 137.05 154.65 168.95 180.93 191.22 200.23 208.22 215.41

Cy(occ[och3]c•c) total

IR 1 (-och3) IR 2 (oc-h3)

IR 1 (-och3) IR 2 (oc-h3) 4.83 5.25 5.57 5.86 6.09 6.10 6.30 6.47 6.75 6.94 7.07 7.18 7.31 7.45 7.51 7.53 7.55 7.56 7.56 7.57 7.57

3.62 3.70 3.88 4.09 4.29 4.30 4.47 4.63 4.88 5.06 5.20 5.30 5.42 5.57 5.63 5.65 5.67 5.68 5.69 5.69 5.69

total 64.62 69.25 73.40 77.39 81.16 81.32 88.66 95.87 103.03 109.70 115.92 121.74 127.29 150.07 167.78 182.13 194.15 204.46 213.48 221.48 228.67

Cy(oc•c[och3]cc) total

TVR

68.17 72.85 77.04 81.02 84.73 84.86 91.98 98.85 105.60 111.87 117.69 123.11 128.27 149.38 165.69 178.88

59.74 64.05 67.83 71.42 74.80 74.94 81.84 88.52 94.88 100.88 106.51 111.79 116.76 137.70 153.95 167.12

IR 1 (-och3) IR 2 (oc-h3) 4.84 5.25 5.58 5.86 6.09 6.10 6.30 6.47 6.75 6.94 7.07 7.17 7.30 7.45 7.51 7.53

3.93 4.19 4.47 4.71 4.90 4.91 5.05 5.17 5.33 5.43 5.49 5.54 5.60 5.65 5.68 5.69

total 68.51 73.48 77.88 81.99 85.79 85.95 93.19 100.16 106.95 113.24 119.08 124.51 129.66 150.80 167.14 180.33

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TABLE 8: Continued Cy(occ[och3]cc•) temp (K)

TVR

3000 3500 4000 4500 5000

177.28 186.72 194.97 202.29 208.87

Cy(occ[och3]c•c)

IR 1 (-och3) IR 2 (oc-h3) 7.55 7.56 7.56 7.57 7.57

5.67 5.68 5.69 5.69 5.69

total

TVR

190.50 199.96 208.21 215.54 222.13

176.68 186.12 194.36 201.68 208.26

IR 1 IR 2 (-och3) (oc-h3) 7.55 7.56 7.56 7.57 7.57

5.67 5.68 5.69 5.69 5.69

Cy(oc•c[och3]cc) total

TVR

189.90 199.35 207.61 214.94 221.52

178.13 187.57 195.82 203.14 209.72

Cy(occ[oc•h2]cc)

IR 1 (-och3) IR 2 (oc-h3) 7.55 7.55 7.56 7.56 7.57

5.69 5.69 5.70 5.70 5.70

total 191.36 200.82 209.08 216.41 222.99

Cy(occ[oc•h2]cc)

temp (K)

TVR

IR 1 (-oc h2)

IR 2 (oc -h2)

total

temp (K)

TVR

IR 1 (-oc•h2)

IR 2 (oc•-h2)

total

100 150 200 250 298 300 400 500 600 700 800

59.48 63.50 67.11 70.64 74.03 74.17 81.19 88.00 94.47 100.54 106.23

5.07 5.51 5.87 6.16 6.39 6.39 6.60 6.75 6.98 7.12 7.22

5.33 2.77 2.90 3.07 3.22 3.22 3.38 3.53 3.80 4.03 4.23

69.88 71.78 75.89 79.88 83.65 83.78 91.17 98.28 105.25 111.70 117.67

900 1000 1500 2000 2500 3000 3500 4000 4500 5000

111.55 116.53 137.50 153.74 166.89 177.89 187.33 195.57 202.90 209.47

7.29 7.37 7.47 7.50 7.52 7.53 7.53 7.54 7.54 7.54

4.39 4.64 4.96 5.11 5.18 5.23 5.26 5.27 5.28 5.29

123.23 128.54 149.93 166.35 179.59 190.65 200.11 208.38 215.72 222.31

•

•

using a relaxed scan at 10° intervals. Graphs of the potential energies of the two hindered rotors versus the dihedral angle for the compounds are available in Figures 4.1 to 4.10 in the Supporting Information. The methyl rotors Cy([o-ch3]) in the 2- and 3-methoxyfuran parent molecules and on the radicals of the furan ring all have 3-fold symmetric barriers at near 3 kcal mol-1 with two exceptions, Cy(oc[och3]ccc•) and Cy(oc•c[och3]cc) which have barriers of only 1.5 and 2 kcal mol-1. Overall the methyl rotor potentials are similar to the 3 kcal mol-1 typically found for methyl groups in hydrocarbons.79-82 The methoxy rotors Cy([-och3]) in the 2- and 3-methoxyfuran parent molecules and on the radicals of the furan ring all have a single minimum point with a broad potential (∼300°) at 1.5-2.8 kcal mol-1. This barrier has a central minimum that decreases from several tenths to 1.5 kcal mol-1 from the maximum with one exception. The Cy(oc[och3]ccc•) barrier is 3-fold and is nonsymmetrical. Methyl vinyl ether was calculated to have a similar (between 2 and 4 kcal mol-1) rotation barrier by da Silva, Kim, and Bozzelli.83 The methylene and methoxy rotors in the methylene radicals Cy(oc[oc•h2]ccc) and Cy(occ[oc•h2]cc) have similar 2-fold symmetric barriers; both are ca. 2.3 kcal mol-1 at approximately equivalent dihedral angles. The Cy([o-c•h2]) rotors both show an expected decrease from 3-fold to 2-fold symmetry with higher barriers of near 6.0 kcal mol-1, which results from partial π bonding between the methyl radical and the oxygen of the ether. These energies are similar to the 5 kcal mol-1 barriers found for methyl vinyl ether radicals.83 Entropy and Heat Capacity. Contributions to S(T) and Cp(T) from the B3LYP/6-31G(d,p) structure, translations, vibration frequencies, external rotations, multiplicity, and number of optical isomers, are represented as TVR in Table 6 and are calculated using the rigid-rotor-harmonic-oscillator approximation SMCPS48 code. Here the two torsion frequencies have been omitted. IR represents the contributions from the two internal rotors of the Cy([-och3]) and Cy([o-ch3]) bond rotations where the barrier energies are determined from the potential energy graphs of the internal hindered rotations. Summing the TVR and IR determines the overall entropy and heat capacities at the temperatures. Entropy, S°298, and heat capacities, Cp(T) for

300-1500 K, are listed in Table 6. Data similar to JANNAF format tables over the larger temperature range of 100-5000 K for both the entropy and heat capacities are listed in Tables 8 and 9 in the Appendix. Group Additivity Values. The group additivity method, as developed by Benson,84 is a practical method for rapid estimation of thermochemical properties, particularly for larger compounds. The method relies on the knowledge of the contributions of representative groups in the molecule(s), usually obtained from smaller molecules, and their established linear consistency in thermochemical property contribution. A significant number of molecular properties such as molar volume, molar refraction, refractive index, and magnetic susceptibility along with thermodynamic properties such as entropy, molar heat capacity, and heat of formation can be accurately approximated as the sum of the individual groups.84-87 Group additivity for the thermochemical properties of biofuel molecules and intermediates will be needed for use in engineering models, with the increasing importance of biofuels in our energy supply. The 120 kcal mol-1 C-H bond energies determined in this study and in previous studies on furan ring carbons are approximately 7-9 kcal mol-1 stronger than those on benzenes36,67,70,71 and 9-13 kcal mol-1 stronger than those on primary and secondary vinyl groups.83,88 We feel that the furan-based molecules are unique and they should have special groups for the ring oxygen and carbons similar to group names for benzene. Presently the method of group additivity for furans involves using carbon double bond groups; one O/Cd2, two Cd/H/O, two Cd/Cd/H, and one furan ring group to correct for the inadequate components of the carbon double bond groups. The carbon double bond notation becomes untenable when there is a group on a furan ring carbon that can interact uniquely with the furan ring. The methoxy or ether group on the furan ring carbon in this study is one example where resonance can occur with the furan ring, that is different from that of a vinyl system. Here one would implement a Cd/O2 group for 2-methoxyfuran, but this group in furan is very different from a Cd/O2 group in a dialcohol, diether, or alcohol plus an ether, that would be bonded to a carbon double bond. One Cd/O2 group cannot represent these very different systems.

Methoxyfurans

J. Phys. Chem. A, Vol. 114, No. 30, 2010 7993

TABLE 9: Calculated Heat Capacities (Cp(T)) for 2-Methoxyfuran, 3-Methoxyfuran, and Radicals from B3LYP/6-31G(d,p) Level of Theory Cy(oc[och3]ccc•)

Cy(oc[och3]ccc) temp (K) TVR IR 1 (-och3) IR 2 (oc-h3) 100 150 200 250 298 300 400 500 600 700 800 900 1000 1500 2000 2500 3000 3500 4000 4500 5000

9.34 11.38 14.11 17.35 20.71 20.85 27.78 33.83 38.81 42.88 46.23 49.04 51.42 59.11 62.96 65.08 66.34 67.14 67.68 68.06 68.34

2.04 2.21 2.31 2.31 2.23 2.11 1.99 1.77 1.60 1.47 1.38 1.32 1.26 1.12 1.07 1.04 1.03 1.02 1.01 1.01 1.01

1.03 1.58 1.91 2.08 2.15 2.13 2.10 1.94 1.78 1.65 1.54 1.46 1.38 1.18 1.11 1.07 1.05 1.03 1.02 1.02 1.01

total

TVR IR 1 (-och3) IR 2 (oc-h3)

total

TVR IR 1 (-och3) IR 2 (oc-h3)

total

12.41 15.18 18.32 21.73 25.09 25.08 31.87 37.54 42.19 46.00 49.15 51.82 54.05 61.41 65.14 67.19 68.41 69.19 69.72 70.09 70.36

9.71 11.89 14.52 17.51 20.57 20.70 26.96 32.40 36.85 40.47 43.44 45.91 47.99 54.67 57.98 59.79 60.86 61.54 62.00 62.32 62.56

13.56 16.20 18.73 21.43 24.19 24.06 30.07 35.19 39.44 42.91 45.78 48.19 50.21 56.76 60.03 61.82 62.88 63.55 64.01 64.33 64.56

9.41 11.58 14.27 17.30 20.35 20.48 26.70 32.14 36.61 40.25 43.24 45.74 47.84 54.59 57.93 59.75 60.84 61.53 61.99 62.31 62.55

12.52 15.41 18.51 21.68 24.72 24.68 30.74 35.79 39.93 43.32 46.12 48.48 50.45 56.87 60.09 61.85 62.91 63.57 64.02 64.34 64.57

Cy(oc[och3]c•cc) temp (K) TVR IR 1 (-och3) IR 2 (oc-h3) 100 150 200 250 298 300 400 500 600 700 800 900 1000 1500 2000 2500 3000 3500 4000 4500 5000

9.73 11.85 14.44 17.37 20.36 20.48 26.65 32.08 36.55 40.20 43.21 45.72 47.83 54.59 57.93 59.76 60.84 61.53 61.99 62.32 62.55

2.05 2.23 2.31 2.30 2.21 2.09 1.96 1.74 1.57 1.45 1.36 1.30 1.24 1.11 1.06 1.04 1.03 1.02 1.01 1.01 1.00

1.29 1.82 2.06 2.10 2.05 1.95 1.85 1.66 1.51 1.40 1.33 1.27 1.22 1.10 1.06 1.03 1.02 1.02 1.01 1.01 1.00

100 150 200 250 298 300 400 500 600 700 800 900 1000 1500 2000 2500 3000 3500 4000

9.41 11.67 14.44 17.49 20.54 20.67 26.88 32.28 36.73 40.36 43.34 45.83 47.92 54.63 57.96 59.77 60.85 61.54 62.00

1.97 2.12 2.24 2.31 2.32 2.32 2.29 2.21 2.02 1.84 1.69 1.57 1.40 1.19 1.11 1.07 1.05 1.03 1.03

0.94 1.49 1.82 2.03 2.13 2.14 2.18 2.16 2.05 1.90 1.76 1.64 1.46 1.23 1.13 1.09 1.06 1.04 1.03

2.17 2.31 2.25 2.10 1.94 1.79 1.65 1.46 1.35 1.26 1.20 1.17 1.13 1.06 1.03 1.02 1.01 1.01 1.00 1.00 1.00

1.69 1.99 1.95 1.82 1.68 1.57 1.46 1.33 1.24 1.18 1.14 1.11 1.09 1.04 1.02 1.01 1.01 1.00 1.00 1.00 1.00

Cy(oc[oc•h2]ccc)

2.05 2.23 2.31 2.30 2.21 2.09 1.96 1.74 1.57 1.45 1.36 1.30 1.24 1.11 1.06 1.04 1.03 1.02 1.01 1.01 1.00

1.06 1.61 1.93 2.09 2.15 2.12 2.08 1.92 1.75 1.62 1.51 1.44 1.36 1.17 1.10 1.06 1.04 1.03 1.02 1.02 1.01

Cy(occ[och3]cc)

total

TVR IR 1 (-oc h2) IR 2 (oc -h2)

total

TVR IR 1 (-och3) IR 2 (oc-h3)

total

13.07 15.90 18.81 21.77 24.62 24.52 30.46 35.47 39.63 43.05 45.90 48.29 50.28 56.80 60.05 61.83 62.89 63.56 64.01 64.33 64.56

9.27 11.57 14.63 18.06 21.44 21.58 28.13 33.56 37.87 41.31 44.11 46.44 48.40 54.77 58.00 59.78 60.85 61.53 61.99 62.31 62.55

11.29 13.76 18.49 22.15 25.60 25.73 32.27 37.60 41.80 45.10 47.76 49.95 51.77 57.61 60.53 62.15 63.11 63.72 64.14 64.43 64.64

9.28 11.37 14.15 17.41 20.77 20.91 27.82 33.85 38.82 42.89 46.24 49.05 51.42 59.11 62.97 65.08 66.34 67.15 67.68 68.06 68.34

12.14 14.92 18.14 21.70 25.21 25.35 32.32 38.31 43.02 46.77 49.83 52.39 54.39 61.60 65.25 67.27 68.47 69.24 69.75 70.11 70.38

•

Cy(occ[och3]cc•) temp (K) TVR IR 1 (-och3) IR 2 (oc-h3)

Cy(oc[och3]cc•c)

2.02 2.19 2.30 2.32 2.27 2.16 2.05 1.83 1.66 1.52 1.42 1.36 1.29 1.13 1.07 1.05 1.03 1.02 1.02 1.01 1.01

•

0.00 0.00 1.57 1.76 1.89 1.99 2.09 2.21 2.27 2.27 2.23 2.16 2.09 1.70 1.46 1.32 1.23 1.17 1.13 1.10 1.08

Cy(occ[och3]c•c)

1.94 2.08 2.19 2.28 2.32 2.32 2.32 2.28 2.13 1.96 1.81 1.68 1.49 1.24 1.14 1.09 1.06 1.04 1.03 1.03 1.02

0.91 1.47 1.80 2.01 2.12 2.12 2.18 2.18 2.07 1.93 1.79 1.67 1.48 1.25 1.14 1.09 1.06 1.05 1.03 1.03 1.02

Cy(oc•c[och3]cc)

total

TVR IR 1 (-och3) IR 2 (oc-h3)

total

TVR IR 1 (-och3) IR 2 (oc-h3)

total

12.31 15.28 18.50 21.82 25.00 25.13 31.34 36.66 40.80 44.10 46.79 49.04 50.78 57.06 60.20 61.93 62.96 63.61 64.05

9.34 11.49 14.19 17.21 20.25 20.38 26.58 32.02 36.50 40.16 43.18 45.69 47.81 54.58 57.93 59.75 60.84 61.53 61.99

12.23 15.08 18.23 21.53 24.70 24.84 31.06 36.41 40.61 43.94 46.66 48.93 50.69 57.02 60.18 61.92 62.95 63.61 64.05

9.60 11.89 14.64 17.71 20.81 20.94 27.23 32.67 37.10 40.69 43.64 46.09 48.15 54.76 58.04 59.83 60.89 61.57 62.02

12.88 15.83 18.89 22.07 25.15 25.28 31.47 36.75 40.84 44.12 46.81 49.07 50.83 57.09 60.23 61.95 62.97 63.63 64.06

1.96 2.11 2.22 2.30 2.33 2.33 2.30 2.23 2.05 1.87 1.72 1.60 1.42 1.20 1.12 1.07 1.05 1.04 1.03

0.93 1.49 1.82 2.03 2.13 2.13 2.18 2.17 2.05 1.91 1.76 1.65 1.47 1.24 1.14 1.09 1.06 1.04 1.03

1.94 2.08 2.19 2.28 2.32 2.32 2.32 2.28 2.13 1.96 1.80 1.68 1.48 1.24 1.14 1.09 1.06 1.04 1.03

1.34 1.86 2.07 2.09 2.02 2.02 1.91 1.80 1.61 1.47 1.37 1.30 1.20 1.09 1.05 1.03 1.02 1.01 1.01

7994

J. Phys. Chem. A, Vol. 114, No. 30, 2010

Hudzik and Bozzelli

TABLE 9: Continued Cy(occ[och3]cc•)

Cy(occ[och3]c•c)

temp (K) TVR IR 1 (-och3) IR 2 (oc-h3) 4500 5000

62.32 62.56

1.02 1.02

1.02 1.02

total

Cy(oc•c[och3]cc)

TVR IR 1 (-och3) IR 2 (oc-h3)

64.36 62.31 64.59 62.55

1.02 1.02

1.03 1.02

total

TVR IR 1 (-och3) IR 2 (oc-h3)

64.36 62.34 64.59 62.57

Cy(occ[oc•h2]cc)

1.03 1.02

1.01 1.00

total 64.37 64.59

Cy(occ[oc•h2]cc)

temp (K)

TVR

IR 1 (-oc•h2)

IR 2 (oc•-h2)

total

temp (K)

TVR

IR 1 (-oc•h2)

IR 2 (oc•-h2)

total

100 150 200 250 298 300 400 500 600 700 800

9.00 11.17 14.19 17.63 21.03 21.17 27.78 33.27 37.64 41.13 43.97

2.00 2.17 2.28 2.32 2.29 2.29 2.21 2.10 1.89 1.71 1.57

0.00 1.32 1.62 1.81 1.94 1.94 2.05 2.14 2.24 2.27 2.25

11.00 14.66 18.09 21.76 25.25 25.40 32.04 37.51 41.77 45.12 47.79

900 1000 1500 2000 2500 3000 3500 4000 4500 5000

46.33 48.32 54.75 57.99 59.78 60.85 61.53 61.99 62.32 62.55

1.46 1.32 1.15 1.08 1.05 1.03 1.03 1.02 1.01 1.01

2.18 2.00 1.62 1.40 1.27 1.19 1.14 1.11 1.09 1.07

49.97 51.64 57.51 60.47 62.10 63.07 63.70 64.12 64.41 64.63

We feel that furan should have its own groups, for example OF/CF2, CF/OF/H, and CF/H groups could be developed, where a separate ring correction group would not be needed. We will work on this in a future study and in this paper we present two groups for the bonding of the methoxy group to a furan carbon; CF/OF/O (replaces Cd/O2) for 2-methoxyfuran and CF/O for 3-methoxyfuran. The previously used84,89 and our developed groups from this study are summarized in Table 7; future studies will likely supplement these. We also list the separate bond groups in Table 7, which result in thermochemical properties of the radicals when the entropy and heat capacity properties are added to the corresponding methoxyfuran parent molecules. The bond group enthalpy value is different however; it corresponds to the C-H bond energy at the indicated site.63 The enthalpy of the radical is determined by a C-H bond dissociation reaction, knowing the enthalpy of the parent, the H atom, and the bond energy. Upon inspection of these bond groups, we find that there is only a 2 kcal mol-1 range in the bond energies in this study as well in the study of Simmie and Curran.36 The average bond energy over the species in both studies is 120.2 kcal mol-1. Separately we determine the entropy and heat capacity data for the parent furan and its two radical sites and observe that the entropy and heat capacity values in these bond groups from the two methoxyfurans and in the two sites of furan are also similar. The average bond energy for CFJ from our study and that of Simmie (120.2 kcal mol-1) is used. We therefore suggest the use of the CFJ, CFJ/O/C, and CF/ O/CJ bond groups (J represents radical site) and report recommended values from average values of the sites in the lower section of Table 7. These groups are intrinsic and that one must also consider contributions from symmetry, electron degeneracy, optical isomers, gauche, and other interactions as clearly illustrated by Benson.84 Summary Density functional theory and higher level composite calculation methods are used to determine thermochemical properties for 2-methoxyfuran, 3-methoxyfuran, and radicals corresponding to loss of hydrogen atoms. There is a close agreement between the density functional theory B3LYP and the composite CBSQB3 and G3MP2B3 calculations for both the parent and the radical compounds when work reactions are used. The recommended ∆H°f298 values for Cy(oc[och3]ccc) and Cy(occ[och3]cc) are -45.0 and -41.1 kcal mol-1. Bond energies for the C-H

bonds on the furan ring are approximately 120 kcal mol-1 and decrease to 98 kcal mol-1 for the methoxy-methyl C-H bonds. The furan ring bond energies are similar to those reported for furan and hydrocarbon-substituted furans. The lower energy methyl C-H bonds are a more favored abstraction site. Thermodynamic properties for group additivity were determined for CF/O, CF/OF/O, furan ring carbon-hydrogen bonds, and methyl radicals of the methoxy groups. Acknowledgment. The authors acknowledge helpful discussions on furan group additivity with Wayne Metcalf of National University of Ireland, Galway. Appendix Table 8 has the entropies and Table 9 has the heat capacities for each species in the 100-5000 K temperature range. Supporting Information Available: Internal rotor potential curves, geometries, moments of inertia, and vibrational frequencies for target molecules and bond length and angle comparisons for 2-methoxyfuran. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gomez, L. D.; Steele-King, C. G.; McQueen-Mason, S. J. New Phytol. 2008, 178, 473. (2) Hill, J.; Nelson, E.; Tilman, D.; Polasky, S.; Tiffany, D. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 11206. (3) Binder, J. B.; Raines, R. T. J. Am. Chem. Soc. 2009, 131, 1979. (4) Greene, D. L.; Hopson, J. L.; Li, J. Energy Policy 2006, 34, 515. (5) Huber, G. W.; Iborra, S.; Corma, A. Chem. ReV. 2006, 106, 4044. (6) Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.; Cairney, J.; Eckert, C. A.; Frederick, W. J., Jr.; Hallett, J. P.; Leak, D. J.; Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T. Science 2006, 311, 484. (7) Zidansˇek, A.; Blinc, R.; Jeglicˇ, A.; Kabashi, S.; Bekteshi, S.; Sˇlaus, I. Int. J. Hydrogen Energy 2009, 34, 6980. (8) Sukumaran, R. K.; Singhania, R. R.; Mathew, G. M.; Pandey, A. Renew. Energy 2009, 34, 421. (9) Catoire, L.; Yahyaoui, M.; Osmont, A.; Go¨kalp, I.; Brothier, M.; Lorcet, H.; Gue´nadou, D. Energy Fuels 2008, 22, 4265. (10) Kotchoni, S. O.; Gachomo, E. W. J. Biol. Sci. 2008, 8, 693. (11) Schmidt, L. D.; Dauenhauer, P. J. Nature 2007, 447, 914. (12) Demirba, A. Energy ConVers. Manage. 1998, 39, 685. (13) Demirba, A. Energy ConVers. Manage. 2001, 42, 1357. (14) Chheda, J. N.; Huber, G. W.; Dumesic, J. A. Angew. Chem., Int. Ed. 2007, 46, 7164. (15) McKendry, P. Bioresour. Technol. 2002, 83, 47.

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