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Modeling Oil Shale Pyrolysis: High-Temperature Unimolecular Decomposition Pathways for Thiophene AnGayle K. Vasiliou,*,† Hui Hu,‡ Thomas W. Cowell,† Jared C. Whitman,† Jessica Porterfield,∥,§ and Carol A. Parish‡ †

Department of Chemistry and Biochemistry, Middlebury College, Middlebury, Vermont 05753, United States Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23713, United States § Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States

J. Phys. Chem. A 2017.121:7655-7666. Downloaded from pubs.acs.org by UNIV OF SYDNEY on 07/16/18. For personal use only.

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ABSTRACT: The thermal decomposition mechanism of thiophene has been investigated both experimentally and theoretically. Thermal decomposition experiments were done using a 1 mm × 3 cm pulsed silicon carbide microtubular reactor, C4H4S + Δ → Products. Unlike previous studies these experiments were able to identify the initial thiophene decomposition products. Thiophene was entrained in either Ar, Ne, or He carrier gas, passed through a heated (300−1700 K) SiC microtubular reactor (roughly ≤100 μs residence time), and exited into a vacuum chamber. The resultant molecular beam was probed by photoionization mass spectroscopy and IR spectroscopy. The pyrolysis mechanisms of thiophene were also investigated with the CBS-QB3 method using UB3LYP/6-311++G(2d,p) optimized geometries. In particular, these electronic structure methods were used to explore pathways for the formation of elemental sulfur as well as for the formation of H2S and 1,3-butadiyne. Thiophene was found to undergo unimolecular decomposition by five pathways: C4H4S → (1) SCCH2 + HCCH, (2) CS + HCCCH3, (3) HCS + HCCCH2, (4) H2S + HCC−CCH, and (5) S + HCC−CH CH2. The experimental and theoretical findings are in excellent agreement. levels should not exceed 75 ppb.10 To comply with these regulations, sulfur must be removed from fuels during the refining process. The most commonly used industrial process to reduce sulfur content in petroleum is hydrodesulfurization (HDS).15 Aromatic organosulfur compounds are less effectively catalyzed under HDS conditions and often require higher pressures and temperatures to be effectively removed, making them a nuisance to the refining process.16 Thiophene is one of the more abundant aromatic organosulfur contaminants found in fuels such as petroleum and shale gas.3,16 Thiophene is also the simplest molecule in the class of thiophenic compounds, one of the four main chemical motifs that make up organosulfur fuel contaminants (sulfides, disulfides, thiols, and thiophenic). As a result, understanding the thermal decomposition of thiophene is especially important as thiophene is not only present in fuels but also acts as a model compound for an entire class of thiophene derivatives. Notably, thiophene is an aromatic organosulfur compound of high innate stability, making it more difficult to remove during typical desulfurization methods, such as hydrodesulfurization.15 For all of these reasons, it is important to better understand thiophene

1. INTRODUCTION Energy production is one of the most challenging issues of modern times. Fossil fuels are still the most widely used source of energy in the world, accounting for 81% of the world energy share, half of which comes from petroleum.1 As global energy demand is expected to rise 35% by 2035, the utilization of alternate forms of energy, such as shale oil, will also need to increase.1 In order to obtain fuels that will burn cleanly, sulfur compounds must be removed from gaseous, liquid, and solid products.2 Shale oils are typically rich in both sulfur and organic compounds, as sulfates accumulate in sedimentary basins along with oil producing planktons. Organic matter in shale oil can be up to 3.1% sulfur by weight, while mineral components can be up to 2.6% sulfur by weight.3 Combustion emissions of sulfur oxides strongly influence the chemistry of the atmosphere, which adversely affects air quality and human health.4−8 Air pollution in the United States is regulated by the Clean Air Act, which enables the EPA to set air quality standards for six criteria of air pollutants, one of which is sulfur dioxide, SO2.4 Human exposure to the airborne pollutant SO2 has unfavorable effects on immune capable cells and airway responsiveness. The health effects from sulfurous air pollution have been well documented.9−14 All population subgroups are affected by SO2, including the most vulnerable: children, adolescents, cardiac- and respiratory-compromised individuals, and asthmatics.5 As such the EPA maintains that sulfur dioxide © 2017 American Chemical Society

Received: July 31, 2017 Revised: September 13, 2017 Published: September 14, 2017 7655

DOI: 10.1021/acs.jpca.7b07582 J. Phys. Chem. A 2017, 121, 7655−7666

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The Journal of Physical Chemistry A

Figure 1. Schematic of the pulsed microtubular reactor used for thiophene thermal cracking. Samples were collected in a 5 K cryogenic matrix for analysis by infrared absorption spectroscopy or photoionized with fixed frequency vacuum ultraviolet (VUV) light.

Torr. Upon shock heating, acetylene was found to be the major product at all temperatures; however, ethene, ethanethiol, hydrogen sulfide, carbon disulfide, and several oligomers were also detected.19 Based on the experimental findings, Ur Rahman Memon et al. suggested the thermal decomposition of thiophene was initiated by the homolytic cleavage of the C− S bond followed by ring opening and isomerization. Hore et al. conducted a laser pyrolysis study on thiophene and other fivemembered ring compounds.20 They suggested that a 1,2 hydrogen transfer is the most probable initiation step in thiophene decomposition.20 In this study, the thermal decomposition of thiophene was investigated using a pulsed microtubular reactor and two different methods of detection. The products of the reaction were monitored using 118.2 nm (10.487 eV) vacuum ultraviolet photoionization mass spectrometry (PIMS) and matrix isolation IR spectroscopy. Within the microtubular reactor, thiophene seeded in an inert carrier gas is rapidly heated to 1000−1300 K and decomposition is initiated. The relatively short residence time in the reactor (50−100 μs) ensures that the observed chemistry emphasizes the unimolecular processes excluding all but the most rapid bimolecular chemistry. High-level ab initio methods were used to identify and confirm decomposition pathways associated with the observed experimental spectra.

and its decomposition mechanism, as it is expected to play a critical role in the combustion and processing of fuels. Despite the important role of thiophene in industrial processes and combustion, a detailed pyrolysis mechanism is still unclear. To our knowledge, the current mechanism for the thermal decomposition of thiophene is based on end point chemistry, meaning that the reaction mechanisms have been proposed without direct evidence of reactive intermediates. This knowledge gap hinders any progress in desulfurization methodology since current efforts for improving thiophene removal technologies are done with an incomplete picture of the molecular level chemistry. The results from this study can be used in engineering simulations in order to model and predict the success of new or improved methodologies for desulfurization. One of the earliest studies of the decomposition of thiophene was conducted by Wynberg and Bantjer in a continuous flow reactor. A Vycor glass tube was heated to 1073−1123 K, and a stream of thiophene was passed through at a rate of 5 mL/h.17 From this they identified three isomeric dithiophenes as well as carbon disulfide, free carbon, hydrogen sulfide, and other hydrocarbons. Although this experiment identified many of the potential products formed from the decomposition of thiophene, the bimolecularity inherent to the liquid phase allows for the formation of many additional compounds, making it difficult to determine the initial decomposition products. More recent continuous flow reactor studies by Winkler et al. carried out the pyrolysis of thiophene in a quartz continuous flow reactor heated to 1373 K at atmospheric pressure with a reactor residence of 20 s. Under these conditions a variety of products were identified, including methane, benzene, hydrogen sulfide, and hydrogen.18 Winkler et al. concluded that the thermal decomposition of thiophene was initiated by a C−H bond cleavage. Thiophene decomposition has also been studied using shock tube methods. Ur Rahman Memon et al. used a single pulse stainless steel shock tube filled with 50 Torr of 0.5% thiophene in argon separated from the helium driver gas at 1598−2022

2. EXPERIMENTAL AND COMPUTATIONAL METHODS A high-temperature pulsed microtubular reactor (or hyperthermal nozzle) was used to decompose thiophene. The microtubular reactor is a version of the Chen-Ellison reactor that has been used for several years to produce reactive intermediates.21−28 The hyperthermal nozzle features a (1 mm ID × 3 cm long) SiC tube that can be heated up to 1700 K, with the temperature monitored by a type C thermocouple mounted to the outer wall of the SiC tube. A benefit of the microtubular reactor is the short residence time (50−100 μs).29 This short residence time eliminates problems one might face 7656


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were confirmed to have all real frequencies while transition states were confirmed to have only one imaginary frequency. All species were also geometry optimized using UB3LYP with the larger and more mathematically flexible 6-311++G(2d, p) basis set. Zero-point energies (ZPEs) were calculated at the same level of theory. In order to further improve the energetic data, the CBS-QB3 method40 was also employed to refine the relative energies for all the reactants and products. This composite method is based on the additivity of several correction terms that use larger basis sets at lower levels of theory and smaller basis sets at higher levels of theory. The CBS-QB3 method has been shown to be accurate in the determination of gas-phase energies, producing a mean absolute error of only 0.87 kcal/mol on the G2/97 test set.41,42 In this work, CBS-QB3 energies were determined using UB3LYP/6311++G(2d,p) optimized geometries. Scaled zero-point energy corrections are included in all CBS-QB3 energies reported herein.

in a typical vacuum pyrolysis experiment, including secondary reactions.30−36 Thermal cracking products are produced by pulsing thiophene seeded in an inert gas (roughly 1−2 atm) through the resistively heated SiC tube into a vacuum chamber. The valve can be fired at a nominal rate of 10−50 Hz. The thiophene/Ar (≤0.1%) mixture is injected into the SiC tube where it undergoes thermal decomposition and expands supersonically into a vacuum chamber (≤10−6 Torr). The rotational, vibrational, and translational temperatures drop rapidly within a few reactor diameters. The dynamics of the pyrolysis and gas transport in the reactor have been well characterized using computational fluid dynamics and will not be discussed here.29 Two independent spectroscopic techniques, matrix isolation IR spectroscopy and PIMS, are used to monitor and characterize the output of the microreactor. Figure 1 provides a schematic of the experimental apparatus. Matrix isolation infrared spectroscopy involves freezing the gas to be analyzed on a cold CsI window and then probing with infrared light. The microtubular reactor is mounted to the vacuum shroud of a Sumitumo CKW-21 two-stage closed-cycle helium cryostat. The nascent cracking products, seeded in Ar, that exit the reactor pass through a heat shield aperture plate and are deposited on a CsI window cooled to ∼5 K. The CsI window is approximately 2.5 cm from the exit of the reactor. Infrared spectra were measured using a Thermo Scientific Nicolet iS50 FTIR with a mercury/cadmium/telluride (MCTA) detector. IR spectra were collected within the frequency range of 4000 to 500 cm−1. The IR beam passes through a pair of CsI side windows attached to the Sumitumo vacuum shroud. A benefit to the matrix isolation apparatus is that it allows for accumulation of thermal decomposition products over time, promoting good signal-to-noise, while simultaneously isolating the nascent products from each other within the matrix to prevent secondary reactions. The photoionization time-of-flight mass spectrometer that was used in these experiments has been described in more detail elsewhere and will only be briefly discussed here.23,37 In contrast to the matrix isolation setup, as the nascent cracking products exit the microtubular reactor the supersonic jet is skimmed approximately 3−5 mm after exiting the SiC tube and intersects with a 10.487 eV (118.2 nm) laser beam. The 10.487 eV light is generated by the frequency tripling of the third harmonic (356.6 nm, 10 mJ/pulse) of a Nd:YAG. The new ions are injected by a positively biased repeller plate into a reflection time-of-flight tube and accelerated into the drift zone by a strong electric field. At the end of the flight tube the ions are reflected back down to the microchannel plate detector biased at negative voltage. The PIMS provides mass-to-charge (m/z) information that complements the vibrational frequencies obtained via the matrix isolation experiment. The PIMS experiment, unlike the matrix experiment, does not require an accumulation of products for detection and, therefore, the time duration for an experiment is shorter. A typical matrix experiment can take up to 4 h for a single temperature, whereas the PIMS experiments can accomplish several temperatures in 1 h. For this reason, the PIMS was used for quick determination of the optimal temperature for the thermal decomposition of thiophene. All ab initio and density functional calculations were carried out using the Gaussian 09 package (Revision D.01).38All possible intermediates and transition states were first calculated at the UB3LYP/6-31G(d, p) level of theory.39 Local minima

3. RESULTS: THERMAL DECOMPOSITION OF THIOPHENE Scheme 1 depicts the five product channels for the unimolecular thermal decomposition of thiophene found in this work. Scheme 1. Unimolecular Pathways for the Thermal Decomposition of Thiophene

In Figure 2 are shown the PIMS spectra of the decomposition products of thiophene in the high-temperature microtubular reactor. The bottom trace in Figure 2 is the mass spectrum that results when thiophene seeded in Helium (0.021% C4H4S in He) was pulsed through the microtubular reactor at room temperature (300 K). The ionization energy of thiophene is 8.874 ± 0.005 eV.43 Photoionization with 118.2 nm (10.487 eV) VUV light produced the parent ion, m/z C4H4S+, and a 13C1 isotopomer peak at m/z 85 and a 34S1 isotopomer peak at m/z 86. As the reactor is heated to 1400 K product ions 45 and 58 are observed, which arise from the ionization of thioformyl radical, HCS+ (I.E. 7.412 ± 0.007 eV)44 and thioketene, CH2CS+ (I.E. = 8.89 eV).45 Increasing the reactor temperature to 1500 K results in increased yield of m/z 45 and 58, and new features at m/z 32, 39, 40, 50, 51, and 52. The PIMS spectra in Figure 3 provides a more detailed look at produced ions. The bottom trace in Figure 3 is the room temperature mass spectrum of thiophene. As the reactor is heated to 1300 K product ions 39, 45, and 58 are observed which arise from the ionization of propargyl 7657


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Figure 2. PIMS spectra of the decomposition products of thiophene in a high-temperature microreactor. The bottom trace is the mass spectrum that results when thiophene (0.021% C4H4S in He) transits the reactor at room temperature (300 K).

Figure 3. PIMS spectra for the appearance of decomposition products from thiophene as the temperature of the microreactor is increased.

radical, CH2CCH+ (I.E. = 8.674 ± 0.001 eV),46 HCS+ and CH2CS+. As the reactor temperature is increased to 1400 K new product ions at m/z 40 and 52 arise from methylacetylene, CH3CCH+ (I.E. = 10.36744 ± 0.00012 eV)47 and vinylacetylene, CH2CHCCH+ (I.E. = 9.58 eV).48 As the temperature of the reactor is further increased to 1500 K additional product ions m/z 32 and 50 are detected from the ionizations of sulfur atom, S+ (I.E. = 10.36001 eV)49 and 1,3− butadiyne (or diacetylene), CHC−CCH+ (I.E. = 10.17 eV).48 The feature observed at m/z 51 results from the dissociate ionization of thermally excited vinylacetylene:

CH 2 CHCCH* + ℏω 1 1 8 n m → [CH 2 CHCCH + ]* → CHCHCCH+. Dissociate ionization of vinylacetylene has been observed in earlier PIMS studies.24,50 The PIMS spectra in Figures 2 and 3 are very revealing but provide an incomplete picture of the thiophene thermal cracking mechanism. For example, because product species HCCH (I.E. = 11.4006 ± 0.00012 eV)51 and CS (I.E. = 11.33 ± 0.01 eV)52 have ionization potentials larger than the energy of the PIMS laser at 118.2 nm (10.487 eV) these species are not detected. Matrix IR spectroscopy is able to confirm the products from the PIMS and detect HCCH and CS. Thiophene 7658


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Figure 4. (a) Matrix IR absorption spectra from 3360 to 3270 cm−1 of room temperature C4H4S (bottom trace) and authentic samples of HCCH (green spectrum) and CH3CCH (blue spectrum). The top trace is the IR spectrum resulting from heating a 0.021% C4H4S/Ne mixture to 1500 K. IR bands of the pyrolysis products HCCH (ν3), CH3CCH (ν1), and HCC−CCH (ν4) are observed. (b) Vibrational bands for pyrolysis products HCCH (ν5) and CH3CCH (ν3) in the IR region of 2300 cm−1 to 2000 and 1000 cm−1 to 500 cm−1.

Figure 5 is the infrared spectra of the thermal decomposition of C4H4S in the regions of 1800−1700 cm−1, 1350 to 1300 cm−1, and 1000−850 cm−1. Distinctive bands for thioketene, CH2CS and vinylacetyelene, CH2CHCCH are clearly observed as the microreactor is heated from 300 to 1500 K. The bands observed in Figure 5 at 1758 (ν2) and 1330 (ν3) cm−1 confirm the presence of CH2CS. These bands closely match those previously assigned in the literature.56,57 An IR transition for CH2CS at 689 cm−1 for ν5 was also observed. The IR bands at 974 (ν14) and 926 (ν15) cm−1 in Figure 5 affirm the presence of CH2CHCCH as a thermal decomposition product. These bands closely match those previously assigned for CH2CHCCH in an argon matrix.50 In addition to ν14 and ν15, vibrational modes ν7 = 1412 cm−1, ν11 = 637 cm−1, and ν17 = 618 cm−1 were also observed for CH2CHCCH.50 The 1500 K matrix IR spectra in Figure 6 reveal the presence of hydrogen sulfide, H2S, carbon monosulfide, CS, and propargyl radical, HCCCH2, as thermal decomposition products of thiophene. The absorption at 1179 cm−1 in the heated C4H4S spectrum is unmistakably from the ν2 vibrational mode in H2S. The vibrational mode ν3 = 2635 cm−1 was also observed for H2S. Figure 6 confirms the presence of HCCCH2, as the most well studied mode in the radical ν6 is observed at

entrained in Argon or Neon (0.021% C4H4S/Ar or Ne) was pyrolyzed in the microreactor and the product gas was deposited on a 5 K CsI salt window. Figure 4(a) compares the matrix IR spectrum of C4H4S heated to 1500 K with neat samples of HCCH (green trace) and CH3CCH (blue trace) that were deposited at room temperature. The bottom trace in Figure 4(a) is a spectrum of room temperature C4H4S, where all bands for HCCH and CH3CCH are absent. In the 1500 K spectrum in Figure 4(a) new bands at 3301 and 3287 cm−1 are associated with the wellknown Darling-Dennison mixing of ν3 and ν2 + ν4 + ν5 in acetylene.53 The intense IR band observed at 3335 cm−1 in the decomposition of C4H4S is assigned to the ν1 transition of CH3CCH. Additionally, in Figure 4(a) is shown the vibrational mode ν4 = 3335 cm−1 for product 1,3-butadiyne, HCC−CCH. A band at 628 cm−1 was also observed for ν5 of HCC−CCH. Vibrational modes ν4 and ν5 for HCC−CCH are in very good agreement with previously assigned literature values.54,55 Figure 4(b) shows additional vibrational bands for pyrolysis products at 736 cm−1 (ν5) for HCCH and 2140 cm−1 (ν3) for CH3CCH. In the case of acetylene, ν3 and ν5 are the only active modes in the IR. In addition to ν1 and ν3 for CH3CCH, vibrational modes ν7 = 1483 cm −1 and ν9 = 630 cm−1 were also observed. 7659


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Figure 5. Infrared spectrum of decomposition products of thiophene at 1500 K. Both thioketene and vinylacetylene are clearly present as indicated by the ν2 (1758 cm−1) and ν3 (1330 cm−1) bands for CH2CS and the ν14 (974 cm−1) and ν15 (926 cm−1) bands for CH2CHCCH.

687 cm−1. This band is in very good agreement with both previously reported gas phase (687.17603 ± 0.00062 cm−1)58 and argon matrix (686 cm−1)21 values. The authentic room temperature spectra of HCCCH2 were generated by the thermal decomposition of propargyl bromide, HCCCH2Br, using the experimental protocol outlined by Jochnowitz et al.21 Production of CS from pyrolysis of thiophene is shown clearly by the growth of the CS stretch at 1272 cm−1, the only vibrational mode in the diatomic.59 Table 1 provides a summary of all observed thermal decomposition products of thiophene in both IR and PIMS.

level of theory, the barriers associated with hydrogen migration are significantly lower, ranging between 67 and 87 kcal/mol. The experimental findings of this study support the theoretical predictions by Parish and co-workers and suggest that the major thermal fragmentation processes of thiophene ensue following hydrogen migration rather than CH, CS, or CC cleavage. The mechanistic pathways will be discussed in order of energetic feasibility. 4.1. Reaction Pathways 1 and 2. The pathways for formation of HCCH + SCCH2 (Scheme 1a), CS + HCCCH3 (Scheme 1b), and HCS + HCCCH2 (Scheme 1c) following a 1,2-hydrogen migration. Figure 7 depicts reaction pathways 1 and 2. Both pathways are initiated by a 1,2hydrogen migration following rearrangement to either the αcarbene (which directly fragments to HCCH + SCCH2) or the β-carbene. The β-carbene isomerizes to intermediate buta-2,3-dienethial, CH2CCH−CHS, which decomposes to CS + CH3CCH. The propargyl radical (CH2CCH, 2B1) and HCS are also produced from buta-2,3-dienethial. The C−C bond energy of buta-2,3-dienethial is 78.60 kcal mol−1 (3.4 eV) at the CBS-QB3 level of theory, suggesting that the CH2C CH−CHS → CH2CCH + HCS pathway is feasible. The mechanism presented in Figure 7 is analogous to the wellestablished mechanism for the decomposition of furan (C4H4O) in a high-temperature microreactor.27,61 It is also of note that the CBS-QB3 barrier for simultaneous β-carbene formation and CS bond cleavage is 74.59 kcal mol−1, lower than that of direct decomposition.60 4.2. Reaction Pathway 3. The pathway for formation of H2S + CHC−CCH (Scheme 1d) following a 1,2-

4. DISCUSSION: DECOMPOSITION MECHANISM OF THIOPHENE Thermal cracking of thiophene can take place via four initiation processes. Three of the four observed reaction pathways are initiated by hydrogen migration reactions. The electronic structure calculations by Parish and co-workers60 have shown that initial unimolecular decomposition based on CH, CS, and CC cleavage is unlikely due to high energy barriers. For instance, the energies associated with CC and CS bond cleavage were estimated to be 133.67 and 88.77 kcal/mol at the CBS-QB3 level of theory. These are the upper bounds determined using calculations on the triplet state of thiophene. The triplet state was utilized because CS and CC bond fission in the singlet state generates biradicals that necessitate highly correlated multireference approaches for proper characterization. The energies for CH bond cleavage were determined to be 116.87 (alpha) and 114.44 (beta) kcal/mol.60 At the same 7660


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Figure 6. Matrix IR absorption spectrum identifying H2S, CS, and CH2CCH as thermal cracking products of thiophene.

IM2 readily isomerizes to IM2a which can undergo a second C−S bond cleavage forming H2S and butadiyne. 4.3. Reaction Pathway 4. Proposed pathway for formation of S + CH2CHCCH (Scheme 1e). CBS-QB3 results suggest that the formation of elemental S can be obtained via four different pathways as shown in Scheme 2. The first mechanism shown in Scheme 2 involves excitation to the nonplanar triplet state followed by ring opening via CS bond cleavage. Ring opening is relatively facile, costing just 13.13 kcal/mol; however, desulfurization is endothermic by 127.92 kcal/mol from the linear-thiophene biradical intermediate, •SCHCHCHCH•. Pathways to elemental sulfur can also be identified from the H-migration/ring opened intermediates reported in the earlier theoretical work of Parish and co-workers, i.e., buta-2,3-dienethial, but-3-ynethial, and IM2a.60 Buta-2,3-dienethial and but-3-ynethial can decompose directly via a barrierless, ever increasing energy function converging to maximum value at the formation of elemental sulfur (Scheme 2). These are very high energy pathways; the CBS-QB3 heats of reaction are 205.66 and 217.21 kcal/mol, respectively. The most energetically feasible pathway to elemental sulfur originates from IM2a.60 This species is formed by α-H migration from C2 to S in thiophene forming the alpha

Table 1. Observed Products for Thermal Decomposition of C4H4S Species

Method of Detection

Thiophene: C4H4S Acetylene: HCCH

IR, PIMS IR

Thioketene: CH2CS Carbon monosulfide: CS

IR, PIMS IR

Methylactylene: CH3CCH Thioformyl Radical: HCS Propargyl radical: HCCCH2 Hydrogen Sulfide: H2S 1,3-Butadiyne: CHC−CCH Sulfur atom: S Vinylacetylene: CH2CHCCH

IR, PIMS PIMS PIMS, IR IR PIMS, IR PIMS IR, PIMS

Notes IP above PIMS laser IP above PIMS laser

Inactive in IR

hydrogen migration. The highest energy barrier along this pathway is 127.96 kcal/mol (Figure 8). However, the products H2S and CHC−CCH are the most stable species formed following 1,2-hydrogen migration. This pathway begins with an α-H migration from C2 to S, followed by C2−S bond cleavage, forming intermediate IM2 46.84 kcal/mol above thiophene. 7661


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Figure 7. Thermal decomposition mechanism of reaction pathways 1 and 2 for formation of HCCH + SCCH2, CS + HCCCH3, and HCS + HCCCH2..

Figure 8. CBS-QB3 potential energy surface for the formation of H2S and butadiyne. (Energies are reported in kcal/mol relative to thiophene; TS5 was not located at the CBS-QB3 level but was found on the MP2/6-311++G(2d,p) surface 126.58 kcal/mol above thiophene.)60

of TS6 with two adjacent CC bonds relative to the adjacent single and triple bonds found in either IM4 or IM2a. From the IM4 intermediate, rupture of the three-center C−C−S ring produces elemental sulfur and H2CCHCCH. The second channel for the formation of elemental sulfur from IM2a is a higher energy pathway. Hydrogen migration from the S atom to C4 proceeds through transition state TS7 (129.44 kcal/mol), generating HCCH + SCCH2. The geometry of TS7 shows the hydrogen migrating from sulfur to

carbene intermediate (IM1) which rearranges to IM2a. From IM2a there are two channels leading to elemental sulfur (Figure 9). In the minimum energy pathway, the H atom attached to S migrates to the adjacent carbon atom (C2) via transition state TS6 (97.25 kcal/mol). This process reduces the C2−C3 bond order from 2 to 1 while forming a three-center ring (IM4; 50.98 kcal/mol; geometry optimized structures are shown in Figure 10). The C−S bond distance increases from 1.751 Å (IM2a) to 1.825 Å (IM4). It is worth noting the more delocalized nature 7662


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Scheme 2. Minimum Energy Structures along the Possible Pathways for Forming Elemental Sulfur from the Decomposition of Thiophene

Figure 9. CBS-QB3 potential energy surface for the formation of elemental sulfur. (Energies are reported in kcal/mol relative to thiophene.)

Figure 10. UB3LYP/CBSB7 optimized geometries of IM2a, TS6, IM4, TS7, and TS8 (Unit: Å).

C4, forming a five-membered-ring structure, while the hydrogen attached to C2 simultaneously migrates from C2 to C3.

Subsequent S−H and C−C bond cleavage results in two species, SCCH2 and acetylene. SCCH2 then under7663


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Figure 11. Detailed mechanism of the five product channels of the unimolecular decomposition of thiophene.

energy of H2S (I.E. = 10.4607 ± 0.0026 eV)65 is below that of the PIMS laser, it was only observed in the IR. This may be due to the fact that the matrix isolation apparatus allows for accumulation of nascent thermal decomposition products over time, promoting good signal-to-noise and thus increased sensitivity for H2S detection.

goes a unimolecular rearrangement forming cyclized SCHCH. This cyclic species then ruptures, producing a sulfur atom and two molecules of acetylene. This pathway is calculated to be 176.04 kcal/mol endothermic. This is significantly higher than the minimum energy pathway described above that produces S + CH2CHCCH (136.71 kcal/mol).

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5. CONCLUSION We have studied the thermal decomposition of thiophene in a pulsed microtubular reactor and by ab initio electronic structure calculations. The reactions were monitored with matrix isolation spectroscopy and VUV photoionization mass spectrometry. The experimental data collected are explained by the unimolecular decomposition of thiophene to its pyrolysis products. CBS-QB3 calculations were used to interpret the experimental results and to visualize the decomposition pathways. A summary of the thermal decomposition mechanism of thiophene based on experimental and electronic structure calculations is presented in Figure 11. We have found that reaction pathways 1 and 2 are analogous to those found for furan pyrolysis but reaction pathways 3 and 4 are unique to thiophene decomposition.27,61 The study by Hore et al. reports allene,20 CH2CCH2 (m/z 40), as a pyrolysis product of thiophene. It is possible the PIMS signal at m/z 40 could be evidence of allene rather than of CH3CCH. However, the IR spectrum of allene is well-known21 and no authentic IR bands for allene were observed in the thermal cracking of thiophene. To our knowledge, thioformyl radical, HCS, has been studied using photoionization mass spectroscopy44 and its rotational parameters identified from microwave spectra but experimental assignments of vibrational modes are lacking. For this reason, further investigation is needed before confident assignment can be made for HCS in the matrix. However, in contrast to the formyl radical, HCO, which has been shown to easily dissociate in the microtubular reactor (ΔH298(H−CO) = 15.6 ± 0.1 kcal mol−1),62 HCS is more stable (ΔDe(H−CS) = 45.7 kcal mol−1)63,64 and consequently should be observed. Although the ionization

AUTHOR INFORMATION

Corresponding Author

*Tel.: 802-443-5517. E-mail: [email protected]. ORCID

AnGayle K. Vasiliou: 0000-0001-5963-4014 Present Address ∥

Harvard Smithsonian Center for Astrophysics, Cambridge, MA 02138, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.K.V. acknowledges support by grants provided by the National Science Foundation (CHE-1566282) and Donors of the American Chemical Society Petroleum Research Fund for support of this research. C.A.P. acknowledges support from the Department of Energy (Grant DE-SC0001093), NSF RUI (Grant CHE-1213271), and the Donors of the American Chemical Society Petroleum Research Fund. Computational resources were provided, in part, by the MERCURY supercomputer consortium under NSF grants CHE-0116435, CHE0521063, CHE-0821581, and CHE-1229354.

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REFERENCES

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