Mechanisms for the Reaction of Thiophene and Methylthiophene with

Article pubs.acs.org/JPCA

Mechanisms for the Reaction of Thiophene and Methylthiophene with Singlet and Triplet Molecular Oxygen Xinli Song, Matthew G. Fanelli, Justin M. Cook, Furong Bai, and Carol A. Parish* Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond Virginia 23173, United States S Supporting Information *

ABSTRACT: Mechanisms for the reaction of thiophene and 2-methylthiophene with molecular oxygen on both the triplet and singlet potential energy surfaces (PESs) have been investigated using ab initio methods. Geometries of various stationary points involved in the complex reaction routes are optimized at the MP2/6-311++G(d, p) level. The barriers and energies of reaction for all product channels were refined using single-point calculations at the G4MP2 level of theory. For thiophene, CCSD(T) single point energies were also determined for comparison with the G4MP2 energies. Thiophene and 2methylthiophene were shown to react with O2 via two types of mechanisms, namely, direct hydrogen abstraction and addition/elimination. The barriers for reaction with triplet oxygen are all significantly large (i.e., >30 kcal mol−1), indicating that the direct oxidation of thiophene by ground state oxygen might be important only in high temperature processes. Reaction of thiophene with singlet oxygen via a 2 + 4 cycloaddition leading to endoperoxides is the most favorable channel. Moreover, it was found that alkylation of the thiophene ring (i.e., methyl-substituted thiophene) is capable of lowering the barrier height for the addition pathway. The implication of the current theoretical results may shed new light on the initiation mechanisms for combustion of asphaltenes.

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subjected to extensive study,10−21 especially for measuring kinetic rate constants. Atmospheric oxygen is in much greater concentration than these radicals and therefore we were also interested in the mechanisms whereby O2 reacts with thiophene. In 1999, Hadad et al.22 reported a computational study of the reaction of ground state oxygen with several aromatic radicals. The PES of the phenyl radical with O2 was explored in detail and several stationary points for reaction with other heteroatomic radicals were also reported. To the best of our knowledge, no mechanistic characterization of the reaction between O2 and closed shell heteroaromatics such thiophene or 2-methylthiophene has been reported. It is possible that the high temperatures available in combustion processes could make the direct oxidation of thiophene feasible. The oxidation process could be complicated by the presence of both triplet (3∑g−) and singlet (1Δg) O2, especially under the high temperature conditions available during combustion or the photoradiative environment in the atmosphere.23 The metastable O2 singlet (1Δg) is more reactive than the ground state triplet (3∑g−) and thus the reaction of thiophene with singlet oxygen might play an important role in photochemical oxidation mechanisms. In this work, we report the computational study of the reactions of thiophene and 2-methylthiophene with both triplet and singlet oxygen molecules using high-level ab initio quantum chemistry methods. Electronic structures and energies of the

INTRODUCTION Asphaltenes comprise a significant portion of the sulfurcontaining compounds in petroleum and are particularly significant in understanding the pyrolysis and combustion behavior of alternative energy fuels such as oil sand and oil shale.1 Asphaltenes are known to contain polycyclic heteroaromatic species with alkyl substitutions.2 Thiophene and methylthiophene are two basic model compounds of asphaltene. Thiophene and other heteroaromatics are also produced from the combustion of fuel oil or coal, from burning plants and other biomasses, and from the byproduct of petroleum distillation and coal gasification.3−5 For instance, various forms of thiophene, from monoalkylthiophenes to multialkylthiophenes, were detected in fluid catalytic cracking (FCC) and residue fluid catalytic cracking (RFCC) gasoline.6 Oxidation processes of thiophene and its derivatives are of great importance in the combustion and decomposition of asphaltene. In 1970, Wasserman and Strehlow, along with Skold and Schlessinger, reported the experimental characterization of the reaction product of singlet oxygen with alkyl thiophenes to form sulfines (R2CSO).7,8 In 1984, Gollnick and Griesbeck9 argued that singlet oxygen reacts with 2,5dimethylthiophene exclusively by a (2 + 4) cycloaddition to produce thiaozonide, which further decomposes to form sulfines. Other than these early reports, very little is known about the molecular mechanisms of the direct reaction of thiophene with oxygen. In the atmosphere, the main sink for thiophene and substituted thiophenes is thought to be fast reaction with OH, O3, and NO3 radicals.10 These reactions have been © XXXX American Chemical Society

Received: February 27, 2012 Revised: April 16, 2012

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dx.doi.org/10.1021/jp301919g | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Scheme 1. Reaction Pathways for the Initial Reaction of O2 with Thiophenea

a

Singlet structures and pathways are distinguished with a lower case “s” at the end of the label.

states (TSs) contained only one imaginary frequency. Moreover, for each transition state intrinsic reaction coordinate (IRC)26 calculations, using a smaller basis set for computational efficiency ((U)MP2/6-31G*), were used to confirm the connections between the corresponding reactants and products. Some stationary points were also optimized at other levels of theory [B3LYP/6-311++G(d,p),27 M06−2X/6-311++G(d,p)28 and B2PLYP/6-311++G(d,p)29] so as to rule out any methodological dependence of the results. Single point energies of all stationary points were also calculated using the G4MP2//MP2 method,30 which is based upon Gaussian-4 theory31 but using reduced order perturbation theory. Like all of the Gaussian-n series of model chemistries, G4MP2 is a composite method that performs several welldefined single point calculations to arrive at a total energy for a given molecular species. The G4MP2 method has been wellcalibrated against more than 400 thermochemical energies with an average absolute deviation of 1.04 kcal mol−1.30 The default G4 and G4MP2 theories use B3LYP/6-31G(2df,p) optimized geometries and frequencies. Because some stationary points in

transition states and intermediates are presented. The significance of various product channels in the oxidation mechanisms of thiophene and substituted thiophenes are discussed in the context of barrier heights and thermochemistry.

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COMPUTATIONAL METHODS All calculations were carried out using the Gaussian09 suite of programs.24 Geometries of all species involved in the reactions were fully optimized using second-order Mö ller−Plesset perturbation theory (MP2),25 with the standard 6-311++G(d, p) basis set. All open-shell species were treated with an unrestricted approach whereas closed-shell species were described with a restricted method. Many singlet transition states suspected of containing at least some open-shell character were also examined using broken symmetry unrestricted approaches. Harmonic vibrational frequencies were calculated at the same level of theory to characterize the nature of each optimized stationary point. The local minimum was confirmed to have all real frequencies whereas transition B



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Figure 1. Energy profile (kcal mol−1) for the reaction of triplet O2 + thiophene at the G4MP2 level of theory. Note: TS4 is slightly higher in energy than IM4 before ZPE correction.

Figure 2. Structures along the triplet O2 + thiophene H-abstraction and addition/elimination pathways.

C



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this study could not be located at the B3LYP level, we used geometries optimized at MP2/6-311++G(d,p) to calculate the G4MP2 energy. Zero-point energies, E(ZPE), were determined on the basis of MP2/6-311++G(d,p) frequencies scaled by 0.95. Four single-point calculations were included, i.e., CCSD(T)/6-31G(d), MP2/G3MP2LargeXP, HF/aug-ccpVTZ, and HF/aug-cc-pVQZ. Higher-level corrections (E(HLC)) were made according to the numbers of α and β valence electrons (Nα, Nβ). The constants used to calculate E(HLC) were A = 0.009472, A′ = 0.009769, B = 0.003102, C = 0.009741, and D = 0.002115. E(HF/∞) was calculated via a linear two-point extrapolation procedure32 using the singlepoint energies obtained at the HF/aug-cc-pVTZ and HF/augcc-pVQZ levels. Therefore, the G4MP2 total energy E0 was determined using the following formula:

Table 1. G4MP2 and CCSD(T)/cc-pVTZ Relative Energies Using (U)MP2/6-311++G(d,p) Geometries for Various Species Involved in the Thiophene + O2(T) Reaction (kcal mol−1)a thiophene + O2(T) TS1 TS2 TS3 TS4 TS5 TS6 IM3 IM4 P1 + HO2 P2 + HO2 P3 + O3P

E0[G4MP2] = CCSD/6‐31G(d) + ΔEMP2 + ΔE HF + ΔE(SO) + E(HLC) + E(ZPE)

ΔE(HF) = E(HF/∞) − E(HF/G3MP2LargeXP) (closed‐shell molecules) (open‐shell molecules)

− Cnβ − D(nα − nβ )

(atoms and atomic ions)

(2.030) (2.025) (2.037) (2.087) (2.158) (2.044) (2.011) (2.000)

G4MP2

CCSD(T)

0.00 66.68 63.74 38.44 43.79 74.61 81.27 36.84 45.70 70.88 69.08 52.45

0.00 69.49 66.68 40.72 46.67 75.29 82.72 38.23 47.72 75.20 73.40 49.98

All energies are ZPE corrected using ZPEs obtained at the (U)MP2/ 6-311++G(d,p) level and a scale factor of 0.95. The S2 values were also obtained at the (U)MP2/6-311++G(d,p) level. bThe expectation values of S2 of HF reference wave functions before annihilation. The value in the brackets is after annihilation.

ΔEMP2 = [E(MP2/G3MP2LargeXP)] − [E(MP2/6‐31G(d))]

− A′nβ − B(nα − nβ )

2.261 2.228 2.292 2.427 2.641 2.407 2.212 2.068

ZPE 43.30 43.39 43.56 45.60 46.42 45.01 47.00 46.07 47.58 44.94 45.28 44.85

a

where

E(HLC) = − Anβ

⟨S2⟩HFb

species

which is within approximately 10% of the exact 2.00 value expected for triplets. Typically, strong spin contamination is expected to result in higher calculated barriers so the values presented here may be most safely thought of as upper bounds on the true values.34 The α and β direct abstraction pathways remove a H atom from C2 and C3 via transition states TS1 and TS2, respectively. As seen in Figure 1, both H-abstraction steps must overcome barriers of more than 60 kcal mol−1. TS2 is approximately 3 kcal mol−1 lower than TS1, indicating a slight preference for H abstraction from the C3 site. A similar trend was also found in a computational study of the thiophene + NO3 reaction,10 in which the barrier for H abstraction from the C3 site was approximately 4 kcal mol−1 lower than the barrier corresponding to H abstraction from the C2 site. Experimental evidence for this H-abstraction site preference is also available as the activation energy obtained via the fast-flow-discharge technique for 2-methylthiophene + NO3 radical has been reported to be approximately 1.1 kcal mol−1 higher than for 3-methylthiophene + NO3.18,20 The origins of this regioselectivity are likely due to an ability in the C2-substituted intermediate to distribute the ring radical on to additional atom centers. (Resonance structures are shown in the Supporting Information, S1) As shown in Figure 1, H-abstraction via TS1 or TS2 leads to the formation of the HO2 radical along with the 2(P1) or 3-thienyl (P2) radicals, and these reactions are endothermic by 70.88 and 69.08 kcal mol−1, respectively, depending upon the site of abstraction. Both TS1 and TS2 energies are slightly lower than their corresponding products, suggesting the formation of an additional intermediate before the formation of the final product. We identified a weakly bonded complex between P2 and HO2 at the UMP2/6-3111+ +G** level of theory. However, the high energy nature of this pathway suggests that it may be less important in the overall title reaction and therefore we did not seek a similar intermediate between P1 and HO2. As with the H-abstraction mechanism, end-on addition of triplet O2 may occur at either the α (C2) or β (C3) sites on thiophene. The corresponding transition states are denoted

In addition to G4MP2 single point energies, CCSD(T)33 calculations with the cc-pVTZ basis set were also performed for the thiophene + O2 reaction using the MP2/6-311++G(d, p) optimized geometries. All the energies discussed below correspond to the G4MP2 level unless stated otherwise.

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RESULTS AND DISCUSSION The initial reaction of O2 with thiophene can occur via two types of pathways: direct hydrogen abstraction and the addition of O2 to the carbon atoms of the thiophene ring. Because thiophene has C2v symmetry, both H-abstraction and addition may occur at either α or β reaction sites, i.e., the C2 or C3 position of thiophene. Our results (vide infra) indicate that hydrogen abstraction occurs on both the singlet and triplet potential energy surfaces (PES) whereas 2 + 4 parallel and 2 + 2 side-on addition (simultaneously adding both atoms of O2 to thiophene), as well as O2 electrophilic attack on S, occurs exclusively on the singlet surface. Two variations of O2 end-on addition to C2 or C3 occur via distinct transitions states on the singlet and triplet surface, both resulting in an oxirane-like product and release of O3p. An overview of these mechanisms is shown in Scheme 1. Thiophene + O2(Triplet) Reaction. The PES for the reaction of triplet O2 with thiophene is summarized in Figure 1. The MP2/6-311++G(d,p) optimized geometries are shown in Figures 1 and 2. The relative energies including zero-point corrections of the stationary points from CCSD(T) and G4MP2 calculations are listed in Table 1. Because spin contamination may occur during the UMP2 calculations, we also listed the values of ⟨S2⟩. As shown in Table 1, the G4MP2 values are approximately 2−3 kcal mol−1 lower systematically than the values calculated using CCSD(T) and values of ⟨S2⟩ range from 2.068 (IM4) to 2.641 (TS5). After annihilation of the first spin contaminant, the values decreased to 2.000−2.158, D



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Figure 3. Energy profile (kcal mol−1) for the reaction of thiophene + O2(singlet) at the G4MP2 level of theory. [Upper data are relative to thiophene + O2(triplet), and lower parenthetical data are relative to thiophene + O2(singlet).]

TS3 and TS4 in Scheme 1 and Figures 1 and 2. The attack of O2(triplet) on the C2 or C3 atom leads to peroxy diradical adducts IM3 and IM4, respectively. Both additions involve endon attack, namely, one end of the O2 molecule attacks either C2 or C3 in the thiophene ring. In TS3, the distance between the C2 atom and the O atom is 1.787 Å, and this TS lies 38.44 kcal mol−1 above the initial reactants. IM3 has a relative energy of 36.84 kcal mol−1. The C···O distance in TS4 (1.746 Å) is a bit shorter than the corresponding distance in TS3, and TS4 is 5.35 kcal mol−1 higher than TS3. IM4 lies 45.70 kcal mol−1 above the initial reactants, which is slightly higher than TS4. (IM4 is slightly lower in energy than TS4 before ZPE correction.) Addition at C2 is slightly preferred over C3-site addition, which is the opposite of the structural preferences for H-abstraction. Subsequent endothermic rearrangement (52.45 kcal mol−1) of IM3 and IM4 leads to the same product: O3P atom and an oxirane-like bicyclic product P3. The transition states for this decomposition are TS5 and TS6, with relative energies of 74.61 and 81.27 kcal mol−1, respectively. In these transition states, one of the O atoms is approaching the C3 (TS5) or C2 atom (TS6) while kicking off the other O atom simultaneously to form P3. On the triplet surface, the H-abstraction channels lie almost 20 kcal mol−1 higher in energy than the addition reactions. However, the lowest barrier for addition is more than 30 kcal mol−1 above the initial reactants and both the H-abstraction and addition−elimination pathways are highly endothermic. Therefore, we conclude that the direct oxidation of thiophene by triplet O2 is likely not feasible in the lower temperature regime. Thiophene + O2(Singlet) Reaction. As shown in Scheme 1, the singlet surface also has two types of possible reaction mechanisms; namely, H-abstraction and addition/elimination. The PES for the reaction of singlet O2 with thiophene is summarized in Figure 3. The MP2/6-311++G(d,p) optimized geometries are shown in Figures 4 and 5. The total energies including zero-point corrections of the stationary points from CCSD(T) and G4MP2 calculations are listed in Table 2. For ease of comparison, Figure 3 presents all of the singlet energies relative to the energy of the thiophene + O2 triplet reactants

Figure 4. Structures along the singlet O2 + thiophene H-abstraction pathways.

(upper numbers) as well as relative to the energy of thiophene + O2 singlet reactants (lower numbers). For reference in the corresponding discussion, the singlet−triplet gap for molecular oxygen is calculated to be 26.86 kcal mol−1 at the G4MP2 level, which is approximately 4 kcal mol−1 higher than the experimental value of 22.66 kcal mol−1.35 As with the reaction on the triplet surface, H-abstraction on the singlet surface also occurs at the α and β sites of thiophene (Figures 3 and 4). The singlet transition states corresponding to abstraction from C2 (TS1s) and C3 (TS2s) are 74.40 and 69.52 kcal mol−1 higher in energy than thiophene + O2 (triplet), and 47.54 and 42.66 kcal mol−1 more energetic than thiophene + O2 (singlet), respectively. The same preference for C3 H-abstraction shown by triplet oxygen is also found with singlet oxygen. Both singlet and triplet H-abstraction lead to the same thienyl radicals. Singlet H-abstraction is expected to be more facile than triplet H-abstraction, judging from the difference in barrier heights. However, the overall high barriers and endothermicity of the singlet H-abstraction reactions E



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Figure 5. Structures along the singlet O2 + thiophene addition/elimination pathways.

C5 and each O atom is 1.868 Å, and the OO bond is elongated to 1.351 Å. The C3 and C4 bond gains some double bond character by shortening to 1.379 Å. The 2 + 4 parallel addition via TS3s leads to the bicyclic intermediate (IM3s). This is a Diels−Alder process. TS3s lies only 7.70 kcal mol−1 above the thiophene + O2(singlet) reactants and this pathway is exothermic by 18.98 kcal mol−1. Oxygen addition causes the thiophene moiety of IM3s to become nonplanar − with the S atom bending out of the plane of the C atoms by approximately 42°. This nonplanarity and corresponding loss of aromaticity is a result of the necessary sp3 hybridization at C2 and C5 in IM3s. This kind of concerted 2 + 4 cycloaddition has been known for many years to occur between singlet oxygen and various olefins.36−39 Although there are no direct experimental results to which we can compare our 2 + 4 cycloaddition barrier, Jursic and co-workers have previously calculated barriers of 30.1 and 24.6 kcal mol−1 for addition of the dienophiles ethylene and acrylonitrile to thiophene, respectively.40 This, combined with the low barrier determined here

suggests that these pathways are not significant in the low and mid-temperature regimes. The addition reactions on the singlet surface are distinct from the end-on O2 addition reaction on the triplet surface. Singlet oxygen can add to thiophene in five different ways; 1.) in a parallel fashion involving a 2 + 4 cycloaddition whereby both oxygen atoms add to C2 and C5 simultaneously (TS3s→ IM3s); 2.) via a 2 + 2 side-on addition whereby both oxygen atoms attack either the C2C3 (or C4C5) (TS4s→IM4s) ; 3.) via a 2 + 2 side-on addition whereby both oxygen atoms attack either the S1C1 (or S1C5) double bond (TS6s→IM6s); 4.) via an end-on attack, distinct from the end-on attack seen on the triplet surface, where one end of the O2 molecule adds to the C2C3 (or C4C5) double bond while simultaneously detaching the other O atom (TS9s→P3+O3P); and 5.) by electrophilic attack of O2 on the S atom directly (TS8s→IM8s). These options are shown in Scheme 1, as well as in Figures 3 and 5. The transition state for the parallel 2 + 4 cycloaddition is the highly symmetric TS3s. In TS3s, the distance between C2 or F



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mol−1. IM4s can rearrange into another bicyclic intermediate IM5s by shifting the O atom from the C5 to the C3 site. This process needs to overcome a barrier of 54.26 kcal mol−1 via transition state TS5s. IM5s is 35.40 kcal mol−1 higher in energy than IM4s. The second 2 + 2 side-on addition pathway has O2 attaching to the C−S bond via TS6s. In this transition state, the forming S···O and C···O distances are 2.295 and 1.453 Å in TS6s, whereas the OO bond has elongated to 1.313 Å. The barrier for TS6s is 20.66 kcal mol−1 with respect to thiophene + O2 (singlet), which is approximately 18 kcal mol−1 lower than TS4s. This indicates that the addition of O2 to the CS bond is preferred over CC double bond addition in thiophene. TS6s leads to the bicyclic intermediate IM6s, which is 11.46 kcal mol−1 above the reactants. In IM6s, the SO and CO distances have shortened to 1.960 and 1.392 Å, respectively, whereas the OO bond has stretched to 1.466 Å. IM6s can further rearrange via CS and OO bond rupture via TS7s to produce the linear sulfine product IM7s. The formation of IM7s is highly exothermic by 66.44 kcal mol−1 with respect to the reactants. The fourth type of addition occurs via an O2 end-on attack on one of the CC double bonds of thiophene while simultaneously detaching an O atom. The corresponding transition state is TS9s, which is 24.70 kcal mol−1 above the thiophene + O2 (singlet) reactants. This process leads to the same bicyclic oxirane-like product P3 + O3P, as was formed on the triplet surface but via a different, unique transition state. The fifth type of addition occurs via O2 attacking the S atom directly. The corresponding transition state is TS8s in which the S···O bond is 1.639 Å. TS8s lies 29.65 kcal mol−1 above thiophene + O2(singlet) and leads to the formation of IM8s. The geometry of IM8s is very similar to TS8s, except that the SO bond length is shorter (1.582 Å) whereas the OO distance is longer (1.449 Å). IM8s is calculated to be approximately 1 kcal mol−1 higher than TS8s at the G4MP2 level, but slightly lower than TS8s at the MP2/6-311++g(d, p) level of theory. Finally, it should be noted that attempts to identify a transition state for singlet O2 adding to the C3C4 double bond were unfruitful at all levels of theory.

Table 2. G4MP2 and CCSD(T)/cc-pVTZ Relative Energies (kcal mol−1) Using (U)MP2/6-311++G(d,p) Geometries for Various Species Involved in the Thiophene + O2(Singlet) Reactiona species

ZPE

G4MP2

CCSD(T)

thiophene + O2(S) TS1s TS2s TS3s TS4s TS5s TS6s TS7s TS8s TS9s IM3s IM4s IM5s IM6s IM7s IM8s P1 + HO2 P2 + HO2 P3 + O3P

43.02 43.46 43.63 45.10 44.91 45.82 45.19 45.59 44.24 45.24 47.34 47.55 46.22 46.22 46.22 46.22 44.94 45.28 44.85

0.00 47.54 42.76 7.70 38.46 35.06 20.66 12.60 29.65 24.70 −18.98 −19.20 16.20 11.46 −66.44 30.74 44.02 42.22 25.59

0.00 50.39 44.69 7.82 37.96 37.28 21.15 13.24 35.47 25.68 −21.34 −21.56 16.14 12.54 −64.02 36.32 45.30 43.49 20.08

a

All energies are ZPE-corrected using ZPEs obtained at the (U)MP2/ 6-311++G(d,p) level and a scale factor of 0.95. (S2 values obtained at the UMP2/6-311++G(d,p) level are not shown as all values for transition states and intermediates were equal to 0.0000.)

for singlet O2 addition, suggests that thiophene is relatively unreactive toward conventional dieneophiles but more reactive with electron-deficient molecules such as O2. There are two types of 2 + 2 side-on addition. One pathway has O2 attacking the CC double bond via TS4s. In TS4s, the C···O distances are 1.908 and 1.923 Å. The OO bond is 1.318 Å, and the C4C5 bond is elongated to 1.494 Å. TS4s is 30.76 kcal mol−1 higher than TS3s. TS4s also leads to a bicyclic intermediate IM4s, which is quite exothermic by 19.20 kcal

Scheme 2. Reaction Pathways for the Initial Reaction of O2(Triplet) with 2-Methylthiophene

G



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Scheme 3. Rearrangement Pathways of the mIM1 Adducta

a

Energies shown are relative to 2-methylthiophene + O2(T).

Scheme 4. Rearrangement Pathways of the mIM2 Adducta

a


each label to distinguish the methylthiophene results from the thiophene reactions described above. Similarly, a lower case “s” at the end of the label signifies reaction with singlet oxygen. (For example, mTS1 indicates the transition state for methylthiophene reacting with triplet oxygen whereas mTS1s indicates the transition state for methylthiophene reacting with singlet oxygen.) 2-Methylthiophene + O2(Triplet) Reaction. The initial pathways for 2-methylthiophene reacting with O2(triplet) are summarized in Scheme 2. The optimized geometries and G4MP2 relative energies are shown in Figure S3 and Table S1, respectively, in the Supporting Information. The reactant, 2methylthiophene, has Cs symmetry due to the addition of the methyl group and there are two distinct types of C−H bonds; namely, the three inequivalent C−H bonds on the thiophene ring and the C−H bond in the methyl group. (The optimized geometry of 2-methylthiophene can be found in Figure S3, Supporting Information.) Therefore, there are four possible pathways for H-abstraction as shown in Scheme 2. The barrier heights for these H-abstraction reactions are in the range 41− 65 kcal mol−1, and our results show that it is far easier to abstract a H atom from the methyl group than from the thiophene ring. All H-abstraction reactions from methylthiophene are endothermic, in agreement with results shown above for thiophene. Endothermicities range from 38−70 kcal mol−1,

A comparison of the barriers associated with all of the Habstraction and addition/elimination channels on the singlet surface indicates that the (2 + 4) cycloaddition via TS3s has the lowest barrier (7.70 kcal mol−1). This barrier is likely lower than the other barriers because of higher symmetry and better orbital overlap. For instance, as seen in the HOMO images of TS3s (2 + 4 cycloaddition) and TS4s (2 + 2 cycloaddition) in the Supporting Information (S2), the (2 + 2) addition of O2 to one double bond in thiophene reduces the symmetry of the ring whereas the more symmetric (2 + 4) addition to both double bonds preserves some of the ring symmetry. This concerted (2 + 4) cycloaddition process is also exothermic, suggesting that the channel via TS3s is the dominant pathway on the singlet surface. Substituent Effect. Asphaltenes are known to contain heteroaromatic species with long alkyl chains. To better understand the effect of such alkylation on the oxidation mechanism of thiophene, our study was extended to 2methylthiophene using the same theoretical method as described above. From our previous work,41 we know that the introduction of a weakly electron-donating methyl group does not change the ring structure of thiophene significantly. Therefore, here we focus on how a methyl substituent affects the reaction mechanism with singlet and triplet oxygen. For clarity, a lower case “m” has been added at the beginning of H



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Scheme 5. Rearrangement Pathways of mIM3 and mIM4 Adductsa

a


Figure 6. Energy profile (kcal mol−1) for 2-methylthiophene + O2(singlet) reaction at the G4MP2 level of theory. Additional structures are shown in Figures 7 and 8.

with H abstraction from the methyl group to form the 2thienylmethyl radical (mP1) corresponding to the least endothermic pathway. Accordingly, there are also four possibilities for O2 addition with reactions occurring at C2, C3, C4, and C5. Each of these reactions may be characterized as electrophilic addition to the π system. The barrier heights range from 36 to 41 kcal mol−1, barriers that are in each case lower than the corresponding Habstractions. The addition reactions lead to four diradical adducts labeled mIM1, mIM2, mIM3, and mIM4, with relative energies of 34.04, 44.27, 43.36, and 34.23 kcal mol−1, respectively. Further rearrangement pathways for the four adducts are summarized in Schemes 3−5. The adduct mIM1 is the most stable of the four initial adducts, and it can further react via two channels (Scheme 3). mIM1 can rearrange into another diradical intermediate mIM5 via the shifting of a H atom from the methyl group to the terminal O atom, forming a C2C6···H···OO five-memberedring structure (mTS9 shown in Figure S3, Supporting Information). The relative energy of mTS9 is 70.79 kcal mol−1, which is 36.75 kcal mol−1 above mIM1. mIM5 can react further by CO bond fission to produce HO2 and the 2thienylmethyl radical (mP1)the same products produced from H-abstraction via a lower energy pathway. The CO bond

cleavage in mIM5 is via transition state mTS10 with a relative energy of 60.06 kcal mol−1. A second rearrangement pathway for mIM1 is via OO bond breaking and simultaneous CO bond formation to produce the oxirane-like product mP5s + O3P via mTS11. This process is endothermic by 32.40 kcal mol−1 and must surmount an energy barrier of 70.23 kcal mol−1 (Scheme 3). Oxygen addition to C3 of methylthiophene forms mIM2 which has four rearrangement pathways leading to four types of products (Scheme 4). mIM2 can undergo a H-shift via mTS12 to form a new diradical intermediate mIM6, which is approximately 16 kcal mol−1 higher than mIM2 in energy. mIM6 undergoes further reaction either by HO2 elimination (mTS13) or by H2O elimination (mTS14). mTS13 leads to the 2-thienyl methyl radical mP1 + HO2, and mTS14 leads to 2methylenethiophen-3(2H)-one (mP6s) + H2O in a highly exothermic process (−95.64 kcal mol−1). mIM2 can also rearrange to diradical mIM7 by shifting an O atom via transition state mTS15. Unfortunately, mTS15 could not be located with MP2 but it was identified at the B3LYP/6-311+ +G(d, p) level. Therefore we used the optimized geometry obtained at the B3LYP level to calculate the G4MP2 energy shown in Scheme 4. The relative energy of mTS15 is calculated to be 76.33 kcal mol−1. Subsequent CC bond breaking in mIM7 I



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Scheme 6. Pathways for the Reaction of 2-Methylthiophene + O2(singlet)

Figure 7. Structures along the mTS2→mIM2s→mTS8s →mP6s + H2O pathway.

oxygen may not play an important role in the low and midtemperature ranges. 2-Methylthiophene + O 2(Singlet) Reaction. The potential energy surface for the reaction of O2(singlet) with 2-methylthiophene is shown in Figure 6. The various reaction pathways identified in this study are illustrated in Scheme 6. The corresponding optimized geometries are listed in the Supporting Information (Figure S4). Due to the detailed discussion of the thiophene + O2(singlet) reaction above, here we will only discuss aspects of Figure 6 that differ from Figure 3. The lowest energy channel in Figure 6 is the 2 + 4 cycloaddition via mTS1s to form a bicyclic compound (mIM1s) that contains a nonplanar thiophene, similar to IM3s described above for thiophene. The second lowest channel is via mTS2s (Figures 6 and 7), which is a new addition channel unique to 2-methylthiopene; i.e., it was not seen in the

leads to the linear (3-oxoprop-1-enyl)ethanethioate-like product mP7s. The linear singlet mP7s is much more stable than the diradical mIM7s and the process is exothermic by 66.45 kcal mol−1 relative to the reactant energies. Finally, mIM2 can also form a bicyclic oxirane-like product mP5s by releasing an O3P atom via mTS17. The rearrangement of mIM3 and mIM4 are relatively simple with both adducts forming an oxirane-like product mP8s + O3P via high energy transition states mTS18 and mTS19, respectively (Scheme 5). A comparison of each of the pathways on the triplet surface indicates that, although there are two exothermic channels both originating from the 3-peroxy substituted adduct (mIM2 → mTS15 → mIM7 → mTS16 → mP7s and mIM2 → mTS12 → mIM6 → mTS14 → mP6s), the barrier heights for both Habstraction and O2 addition are all more than 30 kcal mol−1. Therefore, oxidation of methylthiophene by ground state J



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Figure 8. Structures along the mIM2s → mTS9s → mIM5s pathway.

Figure 9. Structures along the addition pathways for the singlet O2 + 2-methylthiophene reaction.

addition of a methyl group to the thiophene ring decreases the barrier for 2 + 2 cycloaddition. These pathways lead to two bicyclic structures (mIM5s and mIM6s), which are exothermic by 22.92 and 21.68 kcal mol−1, respectively. (Note that mIM5s is also formed by a lower energy rearrangement pathway originating at mIM2s; vide supra.) The lowest energy 2 + 2 cycloaddition transition state for methylthiophene (mTS3s, 15.38 kcal mol−1) corresponds to singlet O2 addition to the SC bond, indicating that this process is more favorable than addition to either the C2C3 or C4C5 double bond. mTS3s leads to mIM3s, a bicyclic intermediate which can further rearrange to form sulfine-like mIM7s, an exothermic process which releases 74 kcal mol−1 of energy. Singlet oxygen addition on methylthiophene can also occur via direct O2 attack on the S atom via mTS4s to produce mIM4s endothermically as shown in Figures 6 and 10. (mTS4s corresponds to TS8s in the thiophene system.) A comparison of the barriers associated in common between 2-methylthiophene and thiophene suggests that methyl substitution reduces the barriers for all of the addition pathways. Regarding H-abstraction reactions on the singlet surface, from the results shown in Scheme 2 we know that it is more difficult to abstract a hydrogen atom from the thiophene ring than from the methyl group. Therefore, for the singlet surface,

reactions of thiophene described above. In mTS2s, a sixmembered ring is forming, caused by the interaction of one terminal O atom of the singlet O2 attacking the C3 ring atom while the other O atom is abstracting a hydrogen from the methyl group (Figure 7). This exothermic concerted process leads to mIM2s, which is the lowest intermediate lying 26.14 kcal mol−1 below the reactants. mIM2s can further react via a four-center ring transition state (mTS8s) to produce 2methylenethiophen-3(2H)-one (mP6s) and H2O. The barrier for this rearrangement pathway, relative to mIM2s, is approximately 41 kcal mol−1 whereas the overall pathway is highly exothermic relative to the reactants (−95.64 kcal mol−1). mIM2s can also rearrange to mIM5s via the migration of a H atom from the peroxy group to the methylene site while the terminal O atom is approaching the C2 site (mTS9s; Figure 8). The relative energy of mTS9s is 25.06 kcal mol−1 above the original reactants whereas the bicyclic mIM5s adduct lies 22.92 kcal mol−1 below the original reactants. Three transition states (mTS3s, mTS5s, and mTS6s) are all 2 + 2 cycloadditions that correspond to 2 + 2 side-on addition (TS4s) in the thiophene system. In mTS5s and mTS6s, O2 attacks the C2C3 and C4C5 double bonds, respectively (Figure 9). The barrier heights of both mTS5s (36.42 kcal mol−1) and mTS6s (34.94 kcal mol−1) are slightly lower than TS4s (38.46 kcal mol−1) in the thiophene system. This suggests that the K



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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Postdoctoral (X.S.) and PI summer support (C.A.P.) was provided by the Department of Energy (Grant CHE-0239664). Undergraduate summer stipends and supplies were provided by NSF RUI (Grant CHE-0809462) and the Henry Dreyfus Teacher Scholar Award program, as well as the Floyd D. and Elisabeth S. Gottwald Endowment. Support for supplies is also acknowledged from the Donors of the American Chemical Society Petroleum Research Fund. We are indebted to Dr. René Kanters of the University of Richmond for detailed comments on a preliminary version of the manuscript.

Figure 10. Structures along the addition pathways for the singlet O2 + 2-methylthiophene reaction.

we only considered H-abstraction from the methyl group. The corresponding H-abstraction transition state is mTS7s, which is 29.32 kcal mol−1 above the 2-methylthiophene + O2(singlet) reactants (Figure 6). mTS7s leads to the same 2-thienylmethyl radical (mP1) + HO2 as does mTS1 on the triplet surface. For the reaction between singlet O2 and 2-methylthiophene, the 2 + 4 cycloaddition reaction via mTS1s is the most favorable. This channel only needs to overcome a 3.72 kcal mol−1 barrier and is exothermic by 22.98 kcal mol−1. A second channel involving O2 addition via C3 and the terminal methyl moiety is also likely competitive with a corresponding barrier height of 9.62 kcal mol−1 and the release of 26.14 kcal mol−1 upon formation of the resulting intermediate (mIM2s).

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CONCLUSIONS A detailed quantum mechanistic study at the G4MP2 level of theory using MP2/6-311++G(d,p) geometries has been performed on reactions of singlet and triplet oxygen with thiophene and 2-methylthiophene. Two types of reaction mechanisms are possible, namely, direct hydrogen abstraction and addition/elimination. Our computational results show that reaction with singlet oxygen is likely the dominant channel for both thiophene and 2-methylthiophene. The 2 + 4 cycloaddition of singlet O2 to the C2 and C5 sites on thiophene is exothermic by 18.98 kcal mol−1 and leads to an endoperoxide intermediate with a barrier of only 7.70 kcal mol−1. For 2methylthiophene, a 2 + 4 cycloaddition, similar to that seen for thiophene, is possible. Our results show that, competitive with this 2 + 4 addition, is a second channel that follows a unique pathway whereby one oxygen atom adds to C3 while the second oxygen atom abstracts a H from the methyl group eventually forming 2-methylenethiophen-3-one. This highly exothermic pathway must overcome a barrier of only 9.62 kcal mol−1. All of the initial channels for reaction with triplet O2 have significant barriers, suggesting that it is difficult to oxidize thiophene and 2-methylthiophene by ground state molecular oxygen directly in the low to midtemperature regimes.

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ASSOCIATED CONTENT

S Supporting Information *

MO images, resonance structures, optimized geometries, and energies for 2-methylthiophene reacting with O2 singlet and triplet obtained at the G4MP2 level of theory along with XYZ coordinates for all species. Full ref 24. This information is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (804) 484-1548. Fax: (804) 287-1897. L



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