A Possible Mechanism for Furan Formation in the Tropospheric

México, Facultad de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México ... Publication Date (Web): October ...
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Environ. Sci. Technol. 2005, 39, 8797-8802

A Possible Mechanism for Furan Formation in the Tropospheric Oxidation of Dienes M I S A E L A F R A N C I S C O - M AÄ R Q U E Z , † J. RAU Ä L A L V A R E Z - I D A B O Y , * ,†,‡,§ ANNIA GALANO,§ AND A N N I K V I V I E R - B U N G E * ,† Departamento de Quı´mica, Universidad Auto´noma Metropolitana, Iztapalapa, 09340 Me´xico, D.F., Me´xico, Facultad de Quı´mica, Universidad Nacional Auto´noma de Me´xico, Ciudad Universitaria, 04510 Me´xico D.F., Me´xico, and Instituto Mexicano del Petro´leo, 07730 Me´xico, D. F., Me´xico

The isoprene + OH gas-phase reaction has been widely studied because of its relevance in tropospheric chemistry. However, an unsolved question remains concerning the mechanism for the formation of the observed 3-methylfuran. OH addition to dienes, such as isoprene and butadiene, is assumed to occur only at the external carbon atoms, thus restricting furan formation to a step after addition at C1 and C4. Moreover, cyclization of the carbon chain necessarily involves a cis conformation. In this work, several quantum chemistry methods have been used to model five different reaction paths for furan formation. A mechanism that is highly favored for intermediates that do not undergo collisional stabilization has been identified.

Introduction The oxidation of isoprene is a matter of great interest in tropospheric chemistry because of the abundance of isoprene and its high reactivity with OH and NO3 radicals and with ozone. In the isoprene + OH reaction, the mechanism for the formation of the main products, methyl vinyl ketone and methacrolein, is well-known (1-4), but the one for 3-methylfuran has not been established. The latter is formed with an approximate 5-7.5% yield (5, 6), depending presumably on the pressure, after reaction of isoprene with OH. It is also obtained in the reaction of isoprene with NO3 (7). Furan derivatives are dangerous toxic compounds; moreover 3-methylfuran is expected to be more reactive with OH than isoprene (6). According to Atkinson et al. (5), it may play a role, albeit minor, in the chemical cycles in the lower troposphere. It has been commonly assumed that OH may add to any of the four carbon atoms of isoprene that are attached to a double bond to form hydroxy alkyl radicals. However, addition at the terminal carbons allows the radical center to be delocalized, and the corresponding isomers are expected to be considerably more stable than the others. In ref 8, we used quantum chemistry calculations to show that addition of an OH radical to isoprene occurs significantly only at external carbon atoms. * Address correspondence to either author. E-mail: jidaboy@ servidor.unam.mx (J.R.A.-I.); [email protected] (A.V.-B.). † Universidad Auto ´ noma Metropolitana. ‡ Universidad Nacional Auto ´ noma de Me´xico. § Instituto Mexicano del Petro ´ leo. 10.1021/es0500714 CCC: $30.25 Published on Web 10/14/2005

 2005 American Chemical Society

The subsequent addition of molecular oxygen to these hydroxy alkyl radicals leads to the formation of four hydroxy peroxy radicals, HOCH2C(CH3)(OO•)CHdCH2 and HOCH2C(CH3)dCHCH2(OO•), originating from the addition of OH at C1, and CH2dC(CH3)CH(OO•)CH2OH and (•OO)CH2C(CH3)d CHCH2OH that are obtained after addition of O2 to C4. In the absence of NOx, terminating reactions occur between different RO2, including HO2. At high NOx levels, the reaction of the hydroxy peroxy radicals with NO produces NO2 and the corresponding alkoxy radicals, HOCH2C(CH3)(O•)CHd CH2 and HOCH2C(CH3)dCHCH2(O•) along with CH2d C(CH3)CH(O•)CH2OH and (•O)CH2C(CH3)dCHCH2OH. The behavior of the alkoxy radicals depends on their structure and on the position of the functional groups. Dibble (9) has studied theoretically the C-C bond fission pathways of the alkoxy radicals, using the B3LYP/6-311G(2df,2p) method. For the β-hydroxy alkoxy radicals (first and third above), he found that the C-C fission decomposition pathways have very low barriers (0.7-2.1 kcal/mol) and yield formaldehyde, methyl vinyl ketone ((CH3)C(O)CHdCH2), and methacrolein (CH2dC(CH3)CH(O)). These compounds account for over 60% of the products (9, 12). From the ratio of methyl vinyl ketone to methacrolein that is obtained, the branching ratio between addition to each of the double bonds has been inferred to be about 1.4 in favor of addition to the trisubstituted double bond (11-14). For the two δ-hydroxy alkoxy radicals (second and fourth above), Dibble finds that the C-C bond fission is endothermic (16-20 kcal/mol) and that the reaction should proceed either by isomerization through a 1,5-H-shift or by reaction with O2 to form an HO2 radical and a C5 carbonyl compound. Both pathways could be relevant in the formation of furan. The mechanism that has been proposed (5, 6, 10) to obtain furans from alkoxy radical I involves the formation of a 1,4hydroxycarbonyl, cis-OCH-CRdCH-CH2OH (structure III), which undergoes cyclization by eliminating a water molecule According to Atkinson et al. (5), the cyclization is direct, and

the elimination of a water molecule involves an H atom and an OH group, both on the same carbon atom C4. Gu et al. (6) and Tuazon et al. (10) instead eliminate the same water molecule in two steps For both channels, the spontaneous

cyclization of stable cis-OCH-CRdCH-CH2OH could present a large barrier. In fact, Tuazon et al. (10) have remarked that VOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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furan formation from cis-OCH-CRdCH-CH2OH would be expected to result in a furan yield that increases with reaction time, in contradiction with experimental results. Atkinson et al. have also ruled out the possibility that furans may be formed as a result of heterogeneous surface processes on the walls of the vessel (5). Several authors have remarked (2, 6, 10) that the formation of furans requires the carbon chain of dienes to adopt a cis conformation rather than the trans conformation that is the most stable one. The energy difference between the cis and trans conformers of isoprene, however, is only 2-3 kcal/ mol, and the barrier for the trans to cis conversion is about 5.5 kcal/mol. Yet, the trans isomer represents about 95% of the total amount of diene (11), and most reactions are usually assumed to occur for the trans isomer. The trans to cis conversion of the carbon chain could conceivably occur at any of the following steps in the reaction: •before reaction, i.e., the OH attack would occur directly on the s-cis conformer, •from the trans to the cis OH-diene adduct, •from the trans to the cis peroxy radical, and •from the trans to the cis alkoxy radical. In all cases, rotation around the C2-C3 bond is involved. However, the double bond character of the C2-C3 bond is seen to increase as the reaction progresses. Dibble (9) has suggested that isomerization could occur between the E and Z hydroxyl adducts. He used quantum chemistry to determine the barriers for these processes and found them to be about 15 kcal/mol. He suggested, on the basis of Multiwell calculations (12), that isomerization of the chemically activated adducts might compete with quenching of the excess energy produced in the formation of adducts. Even though the barrier to trans-cis rotation increases as OH is added, the reaction is very exothermic, about 40 kcal/mol (8), and the adducts are formed with sufficient excess vibrational energy to overcome the rotational barrier. However, this does not occur after O2 addition, because the exothermicity of the corresponding reaction is less than the rotational barrier height. Thus, in this work, we have assumed that the alkoxy intermediate I is in the cis conformation. Quantum chemistry is especially suited to establish whether a reaction pathway is feasible or not. Thus, in this work, several mechanisms of furan formation in the OH + butadiene and OH + isoprene reactions were investigated using quantum chemistry methods. Stationary points and transition states along five possible pathways were determined, and their energies were compared to identify the most favorable channel.

Computational Methodology Electronic structure calculations for five channels of furan formation in the OH reaction with butadiene and the most probable channel for isoprene have been performed with the system of programs Gaussian 98 (13). On the basis of the experience gained in refs 1 and 8, the BHandHLYP/6-311G(d,p) and the MP2/6-311G(d,p) methods have been used. The convenience of using several methods of calculation simultaneously has often been stressed (9, 14-16), especially in the case of radical-molecule reactions. Different methods seem to be more reliable and/or efficient for different types of systems. Density functional methods (such as BHandHLYP) and perturbation theory methods (such as MP2) are based on quite different approaches to the calculation of the energy of a molecular system. Each one has its strengths and its drawbacks; density functional methods tend to underestimate long-range interactions and to yield energy barriers that are too low, while perturbation methods yield excellent geometries, but they are subject to spin contamination and often yield barriers that are too high. We have found that working simultaneously with both reinforces the validity of 8798

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the results and conclusions, especially when both give similar results since, in such cases, the intrinsic error of each method seems to be unimportant. Other methods such as fourthorder perturbation theory (MP4) or the coupled cluster method (CCSD(T)) are considered to be more reliable than either MP2 or BHandHLYP. However, they are also considerably more costly, and it is common practice to optimize geometries at a lower level and then use these methods to improve the energy values by recalculating them at the lower level geometries. Thus, originally, all geometries were fully optimized at the BHandHLYP/6-311G(d,p) level for the butadiene + OH reaction along the five proposed channels. The character of the transition states was confirmed by frequency calculations performed at the same level as the structure optimization and presenting only one imaginary frequency corresponding to the expected transition vector. Frequency calculations provided zero-point vibrational energy corrections (ZPEs). When necessary, intrinsic reaction coordinate (IRC) calculations were performed to ensure that transition states properly connect reactants and products. These will be discussed in the text. Once the best mechanism was identified for the butadiene + OH reaction, geometries of all stationary points were reoptimized at the MP2/6-311G(d,p) level, for this mechanism only. The MP2 energy values were projected to remove spin contamination, and projected MP2 values (PMP2) are used. In all cases, all reported energy differences include the ZPE corrections calculated at the MP2/6-311G(d,p) level, while free energy values were corrected for the zero-point vibrational energy and for the thermal energy at the same level. In addition, to improve the energy values, the stationary points along the best channel for the butadiene + OH reaction were recalculated with other methods and basis sets. In this context, the following methods were used: MP4/6-311G(d,p)//MP2/6-311G(d,p), CCSD(T)/6-311G(d,p)//MP2/6311G(d,p), and MP2/AUG-cc-pVTZ//MP2/6-311G(d,p). The possible improvement gained by using a very large basis set, the AUG-cc-pVTZ basis set, that includes diffuse functions to better represent long-range interactions, was considered, and energies were recalculated at the MP2/AUG-cc-pVTZ// MP2/6-311G(d,p) level. For the isoprene + OH reaction, only the best mechanism of furan formation was considered. Calculations were performed at the MP2/6-311G(d,p) level and with the BHandHLYP method with two different basis sets. Total energies are available in the Supporting Information. Relative energies are given in tables for the different methods employed. Barriers are indicated in the text and on the transition state figures. Energy profiles for the reaction paths are reported. Reaction mechanisms have elementary steps that involve different numbers of atoms; thus, the energy values in the profiles are obtained in the usual way, i.e., relative to the sum of reactants participating in the whole process.

Results and Discussion Butadiene + OH. Calculations were first performed on the butadiene system because its smaller size is more convenient for quantum chemistry calculations. The subsequent extension to isoprene is straightforward. The last steps in the mechanisms proposed by Atkinson et al. (5) (eq 1) and Gu et al. (6) and Tuazon et al. (10) (eq 2) could not be modeled as such. It was not possible to obtain transition states for the water elimination process. The apparent reason for this behavior is that the transition state would imply the breaking of two bonds without the formation of a new H-O bond, since the OH and H are too far apart. On the contrary, when the OH and H that form a water molecule are on different carbons, the new bond can begin

SCHEME 1. Potential Mechanisms for Furan Formation

to form at the same time as the homolytic cleavage of the original bonds, with the corresponding lowering of the reaction barrier. There are several reactions in solution in which water elimination occurs from OH and H on the same carbon atom, but it is probably a two-step ionic process assisted by acidic or basic catalysis. Thus, we propose mechanism 1, which is analogous to those suggested by Atkinson et al. (5) (eq 1) and Gu et al. (6) and Tuazon et al. (10) except that it involves a hydrogen migration before eliminating the water molecule in the last step. In this way, the H atom and the OH group involved do not belong to the same carbon atom. Four other different channels for furan formation were also investigated, all of them starting from the cis form of the hydroxy alkoxy radical, structure I (which, in butadiene, is identical to II). Mechanisms 1-5 are given in Scheme 1.Mechanisms 1-3 start with an H migration or 1,5-H-shift. Mechanism 2 differs from mechanism 1 in the position of the hydrogen atom that is abstracted in the second step, but the cyclization step is the same. In mechanism 3, water elimination occurs before hydrogen abstraction by an oxygen molecule. In mechanisms 4 and 5, the hydroxy alkoxy radical does not undergo a 1,5-H-shift. In mechanism 4, the alkoxy reacts via a hydrogen abstraction from C4, while in mechanism 5 a water molecule is eliminated immediately by the alkoxy radical. The transition state TS1 (M123) corresponding to the hydrogen migration in the first step of mechanisms 1-3 is represented in Figure S1 (Supporting Information). In this structure, whose energy is only 8.8 kcal/mol above the energy of hydroxy alkoxy radical I, a six-member cycle is formed that involves atoms C1, C2, C3, C4, O13, and H6. The transition vector clearly shows the motion of H6 between C1 and O13,

with an imaginary frequency of 1753i cm-1, indicating a tight transition state. A rather large tunneling effect is expected for this thin barrier, thus increasing the possibility of the hydrogen atom migration. Transition states for the first step of mechanisms 4 and 5 are represented in Figure S2 (Supporting Information). In TS1 (M4), a hydrogen atom (H9) attached to C4 is abstracted by an oxygen molecule to form a hydrogen peroxy radical. The imaginary frequency νi for the corresponding motion is 2409 cm-1, and tunneling could be important. The position of H9 indicates a very early transition state. For TS1 (M5), the structure presents a 570 cm-1 negative frequency corresponding to the expected motion. The activation energies for TS1 (M4) and TS1 (M5) are 15.9 and 36.2 kcal/mol respectively, above radical III. The latter value is so much larger than Ea for the 1,5-H-shift (Figure S1) that mechanism 5 may be discarded without refining the calculations or including the entropy contribution. Results for all intermediates in mechanism 5, obtained at the BHandHLYP level, are available from the authors. For mechanisms 1-4 of butadiene + OH, all structures along the reaction paths were optimized at the BHandHLYP/ 6-311G(d,p) level. The last two steps in mechanism 4 are identical to those in mechanism 1. Total energies and zero-point vibrational, thermal energy, and thermal free energy corrections (ZPEs, TCEs, and TCGs) of the alkoxy radical, the transition states, and the adducts of the OH + butadiene reaction along pathways 1 and 2 are given in Tables 1 and 2. Next, we consider the reaction with an oxygen molecule in step 2 of mechanisms 1 and 2. For the latter, a direct hydrogen abstraction by the O2 molecule occurs, and a transition state (TS2 (M2)) is found that presents a barrier VOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Energy of Intermediates and Transition States (in kcal/mol) Relative to Reactants and Unscaled Imaginary Frequencies at the Transition States for the Formation of Furan by Mechanism 3 in the Butadiene + OH Reaction BHandHLYP/6-311G(d,p)

TS 1 (M123) III TS 2 (M3) VIII TS 3 (M3) IX TS 4 (M3) furan

PUMP2/6-311G(d,p)

PUMP4(SDTQ)/6-311G(d,p)// PUMP2/6-311G(d,p)

CCSD(T)/6-311G(d,p)// PUMP2/6-311G(d,p)

PUMP2/AUG-cc-pVTZ// PUMP2/6-311G(d,p)

Ea

νi (cm-1)

Ea

νi (cm-1)

Ea

Ea

Ea

8.8 -22.8 -5.5 -25.8 5.6 -21.1 -9.6 -31.5

-1753.7

5.2 -37.5 -18.5 -34.9 -7.6 -32.3 -23.1 -39.6

-1785.7

6.8 -28.4 -11.9 -23.1 -0.5 -22.1 -19.5 -30.9

5.5 -30.2 -10.0 -26.9 -0.6 -24.4 -17.2 -38.5

3.5 -38.9 -21.5 -36.0 -13.4 -37.1 -34.7 -48.8

-779.5 -774.2 -1299.6

-955.1 -1394.0 -1097.7

TABLE 2. Relative Energies (in kcal/mol) and Unscaled Imaginary Frequencies at the Transition States for the Formation of 3-Methylfuran Following Mechanism 3 for the Isoprene + OH Reaction BHandHLYP/6-311G(d,p) C1

TS1 (M123) III TS2 (M3) VIII TS3 (M3) IX TS4 (M3) 3-methylfuran

νi (cm-1)

Ea

νi (cm-1)

Ea

νi (cm-1)

8.0 -20.5 -4.0 -25.7 5.8 -21.0 -10.0 -31.4

-1782.4

7.5 -22.3 -3.6 -24.5 6.0 -21.5 -11.2 -31.2

-1695.2

4.5 -35.1 -16.9 -33.9 -6.1 -30.7 -22.4 -38.0

-1820.6

-807.6 -782.5 -1271.8

HOCH2CCH3dCHCH2(OO•) + NO f

HOCH2CCH3dCHCH2(O•) + NO2

is known to be about 10-11 kcal/mol (17). Thus, we assume that the original hydroxyalkoxy radical excess of vibrational energy is at least 10 kcal/mol. Transition state TS4 (M1), which is the same as TS2 (M4), is shown in Figure S5 (Supporting Information). It has a large barrier of 30.8 kcal/mol. Even though tunneling could be important, we shall see that mechanisms 1 and 4 are definitely discarded at a later step. The final step in mechanisms 1, 2, and 4 is a water elimination from the 1,3-butadiene-1,4-diol resulting in a cyclization of the carbon chain to form furan. The barrier for this step is found to be prohibitively large, at 66.2 kcal/mol, which is much larger than any possible vibrational excess. 9

MP2/6-311G(d,p) C1

Ea

of 18.7 kcal/mol. It is represented in Figure S3 (Supporting Information). The very symmetrical position of H9 with respect to O14 and C4 is unusual. In this case, the hydrogen atom that is to be abstracted from the intermediate radical III is not attached to the radical carbon atom but rather to a saturated carbon. This may explain why TS2 (M2) is a late transition state. For mechanism 1, however, hydrogen abstraction by an oxygen molecule occurs in two steps. First, oxygen is attached to C1, and then an HO2 radical is lost as O2 abstracts a hydrogen atom from the OH attached to C1. Barriers for these two steps are 5.0 and 14.2 kcal/mol, respectively. The corresponding transition states are shown in Figure S4 (Supporting Information). The transition state for the second step is less symmetrical than TS2 (M2), because the hydrogen atom that is being abstracted is originally attached to an oxygen atom. Even though its barrier is somewhat high, it is clear that the alkoxy intermediate I is formed in a vibrationally excited state and that its excess energy could be larger than this value. The experimental exothermicity of the reaction

8800

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-924.8 -789.3 -1124.9

-1016.7 -1462.5 -1055.7

The corresponding transition vector shows the motion of the water molecule receding from C4, at a distance of 2.57 Å. That this transition state is correct was further verified by performing an IRC calculation (18), at the same level, BHandHLYP/6-311(d,p), to show that indeed the reaction path connects compound VII with furan (Figure S6, Supporting Information). It is possible that 1,3-butadiene-1,4diol, if it is formed, reacts in some other way to yield different products. In mechanism 3, water elimination is assumed to occur immediately after the 1,5-H-shift. The barrier for the elimination of a water molecule is 17.3 kcal/mol (TS2 (M3)). However, this water elimination does not lead directly to the formation of the cycle. Rather, it is observed that a stable carbonyl radical (structure VIII) is formed first and that cyclization of this radical then presents an energy barrier (TS3 (M3)) of 31.4 kcal/mol measured with respect to VIII. Both transition states are shown in Figure 1. The TS3 barrier is, of course, very large, and it is the largest one in the whole mechanism. However, it is within the vibrational excess of compound I, as can be seen in the energy profile shown in Figure 2. Here, the largest relative energy in the whole reaction profile for mechanism 3 is the one corresponding to transition state 1. It is necessary to perform a refinement of this energy barrier using high-level calculations and including entropy contributions to determine if this process is feasible. The last step in mechanism 3 involves the abstraction of a hydrogen atom (Ea ) 11.5 kcal/mol). The corresponding transition state (TS4 (M3)) is represented in Figure S7 (Supporting Information). In Figure 2, the energy profiles for mechanisms 1-4 for the butadiene + OH reaction have been represented, relative to the reactants. Mechanism 3 is clearly the most favorable one. Profiles according to free energy values are represented in Figure 3. It can be seen that including the entropy contribution does not change qualitatively the reaction profiles, although there is a clear quantitative change. Free energy profiles show that the proposed mechanism is highly

FIGURE 3. Free energy profiles at different levels for mechanism 3 of butadiene.

FIGURE 1. BHandHLYP/6-311(d,p) transition state TS2 (M3) linking structures III and VIII and TS3 (M3) linking structures VIII and IX in mechanism 3. The transition vectors as well as a few relevant bond distances (in Å) and the activation energies (in kcal/mol) are indicated on the figure.

FIGURE 2. BHandHLYP/6-311(d,p) energy profiles for mechanisms 1-4. favored for those intermediates that do not undergo collisional stabilization. To refine the energy results, the structures along mechanism 3 were recalculated using other methods and basis sets. Total electronic energies and zero-point vibrational, thermal energy, and thermal free energy corrections (ZPEs, TCEs, and TCGs) of the alkoxy radical, the transition states, and the adducts of the OH + butadiene reaction along pathway 3, calculated with different methods and basis sets, are available as Supporting Information. The highest elementary energy barrier (corresponding to TS3 (M3)) is 31.4 kcal/mol at the BHandHLYP level. This

value slightly decreases to 27.3, 26.3, and 22.6 kcal/mol when the calculations are performed at MP2/6-311(d,p), CCSDT/ 6-311G(d,p), and MP4/6-311G(d,p), respectively. When the level of theory is MP2/aug-cc-pVTZ, the value is identical to that calculated at the MP4/6-311G(d,p) level (22.6 kcal/mol); i.e., the barrier is quite high regardless of the method of choice. Energies of intermediates and transition states for the butadiene + OH reaction are given in Table 1, with respect to the energy of the reactants. All energy values include the zero-point energy correction. The negative frequency corresponding to the motion along the reaction path has been indicated for the transition states. In Figure 3, free energy profiles obtained using different methods are compared for mechanism 3 of the butadiene + OH reaction. It can be seen that tendencies are very similar for all methods and that the lowest values are obtained with the MP2 method using the largest basis set. At this point it becomes clear that the formation of furan derivatives in the butadiene reaction with OH radicals is definitely pressure-dependent and that the furan yield should decrease as pressure increases due to collisional stabilization of intermediates. This also explains the difference between the 5% 3-methylfuran yield obtained by Atkinson et al. at (5) 740 Torr and the 7.5% value obtained by Gu et al. (6) at 10 Torr. Isoprene + OH Reaction. For isoprene, mechanism 3 was first investigated using the BHandHLYP method with the 6-311G(d,p) basis set. In this case, the terminal carbons C1 and C4 are different, and both channels have to be considered. Energies of all intermediates and transition states, relative to reactants, and imaginary frequencies at the transition states are given in Table 2. It can be seen that energy barriers and reaction energies for 3-methylfuran formation are very similar for OH addition to C1 and C4, and both channels probably contribute about equally to the total rate constant. Energies are also very similar to those occurring in furan formation from butadiene + OH, implying that the mechanism is the same. The free energy profiles for OH addition to C1 and C4 in the isoprene + OH reaction are compared in Figure S8 (Supporting Information). The BHandHLYP free energy profile for this mechanism involves only one rather large barrier, corresponding to TS3 (M3), whose free energy, however, is lower than that for the 1,5-H-migration in the original alkoxy radical. The PMP2 free energy profile for mechanism 3 of the isoprene + OH reaction is shown in Figure 4. As in the case of butadiene, the proposed mechanism is highly favored for those intermediates that do not undergo collisional stabilization. VOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. PMP2/6-311(d,p) free energy profiles for mechanism 3 for channel C1 in the isoprene + OH reaction.

Acknowledgments The authors gratefully acknowledge the financial support from Conacyt through Project No. 400200-5-34043-E and from the Instituto Mexicano del Petro´leo through Project No. D.00179.

Supporting Information Available Total energies, transition states, total electronic energies, zero-point vibrational, thermal energy, and thermal free energy corrections, and free energy profiles for OH addition to C1 and C4 in the isoprene + OH reaction. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Paulson, S. E.; Seinfeld, J. E. Development and Evaluation of Photooxidation Mechanism for Isoprene. J. Geophys. Res. 1992, 97, 20703. (2) Paulson, S. E.; Flagan, R. C.; Seinfeld, J. H. Atmospheric Photooxidation of Isoprene Part I: The Hydroxyl Radical and Ground-State Atomic Oxigen Reactions. Int. J. Chem. Kinet. 1992, 24, 79. (3) Carter, W. P.; Atkinson, R. Development and Evaluation of a Detailed Mechanism for the Atmospheric Reactions of Isoprene and NOx. Int. J. Chem. Kinet. 1996, 28, 497. (4) Tuazon, E. C.; Atkinson, R. J. A Product Study of the Gas-Phase Reaction of Isoprene with the OH Radical in the Presence of NOx. Int. J. Chem. Kinet. 1990, 22, 1221. (5) Atkinson, R.; Aschmann, S. A.; Tuazon, E. C.; Arey, J.; Zielinska, B. Formation of 3-Methylfuran from the Gas-Phase Reaction of OH Radicals with Isoprene and the Rate Constant for its Reaction with the OH Radical. Int. J. Chem. Kinet. 1989, 21, 593.

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Received for review January 13, 2005. Revised manuscript received August 8, 2005. Accepted September 13, 2005. ES0500714