Thermal Decomposition of Propargyl Alcohol: Single Pulse Shock

Article pubs.acs.org/JPCA

Thermal Decomposition of Propargyl Alcohol: Single Pulse Shock Tube Experimental and ab Initio Theoretical Study N. Sharath,† K. P. J. Reddy,† and E. Arunan*,‡ †

Department of Aerospace Engineering and ‡Department of Inorganic and Physical Chemistry, Indian Institute of Science, 560 012 Bangalore, India S Supporting Information *

ABSTRACT: Thermal decomposition of propargyl alcohol (C3H3OH), a molecule of interest in interstellar chemistry and combustion, was investigated using a single pulse shock tube in the temperature ranging from 953 to 1262 K. The products identified include acetylene, propyne, vinylacetylene, propynal, propenal, and benzene. The experimentally observed overall rate constant for thermal decomposition of propargyl alcohol was found to be k = 10(10.17±0.36) exp(−(39.70 ± 1.83)/RT) s−1. Ab initio theoretical calculations were carried out to understand the potential energy surfaces involved in the primary and secondary steps of propargyl alcohol thermal decomposition. Transition state theory was used to predict the rate constants, which were then used and refined in a kinetic simulation of the product profile. The first step in the decomposition is C−O bond dissociation, leading to the formation of two important radicals in combustion, OH and propargyl. This has been used to study the reverse OH + propargyl radical reaction, about which there appears to be no prior work. Depending on the site of attack, this reaction leads to propargyl alcohol or propenal, one of the major products at temperatures below 1200 K. A detailed mechanism has been derived to explain all the observed products.

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INTRODUCTION Previous studies on thermal decomposition of methanol, ethanol, and butanol1−3 were carried out as these biofuels have the potential to replace kerosene fuel. Propargyl alcohol (C3H3OH) can be considered to be a derivative of methanol with one of the methyl hydrogens replaced by an acetylene group.4 It has two important functional groups C3H3 and OH that play crucial roles in the combustion process. Propargyl radical (C3H3), a resonance stabilized radical is considered one of the precursor for benzene formation. The rate constant for propargyl radical dimerization leading to benzene formation has been well studied.5−8 The propargyl radical was produced using C3H3I,9 C3H3Br, and C3H3Cl.10 The bimolecular reaction of propargyl radical with oxygen atom has also been studied and results showed that OH formation is one of the important pathway for propargyl radical oxidation process.11 Silva et al. studied the fulvenallenyl radical oxidation process and suggested that oxidation of fulvenallenyl would proceed through reaction with OH and O radicals rather than with O2 itself.12 The propargyl radical reaction with OH has not been investigated to the best of our knowledge. Initial theoretical calculations on the bond dissociation energy, i.e., BDE, in propargyl alcohol showed that the BDE of C−O bond has the lowest value compared to the other bonds in the molecule. Propargyl alcohol is also a molecule of astrophysical interest.13 Recently, theoretical investigation into the decomposition pathways for C3H4O isomers of astrophysical interest like propenal, methyl ketene, and hydroxylcyclopropene were © 2014 American Chemical Society

carried out to obtain the ground state potential energy surface and to understand the importance of different product channels.14 Propenal is one of the isomers of propargyl alcohol whose experimental and theoretical decomposition pathways have been investigated.15,16 Surprisingly, to the best of our knowledge, thermal decomposition of propargyl alcohol has not been investigated so far. This manuscript reports the first thermal decomposition study of propargyl alcohol. Results from both shock tube experiments and ab initio theoretical calculations are reported.

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EXPERIMENTAL SETUP Details of the experimental setup used in this work have been given elsewhere.17−19 A brief description of the experimental arrangement will be given here. A Chemical Shock Tube-2 (CST2) has been used to carry out the thermal decomposition experiments of propargyl alcohol. The driver and driven sections of lengths 2 and 4 m, respectively were separated by an aluminum diaphragm. The diaphragm was scored before placing it between the driver and driven sections to obtain the required conditions. The driven section was further divided into two sections, 3 m tube after the diaphragm station followed by a ball valve connected to a 1 m tube. PCB pressure transducers (Model No. 113A24) were used to obtain the pressure profile inside the shock tube. A Tektronix TDS-2014B Received: May 26, 2014 Revised: July 17, 2014 Published: July 18, 2014 5927

dx.doi.org/10.1021/jp505145j | J. Phys. Chem. A 2014, 118, 5927−5938

The Journal of Physical Chemistry A

Article

Figure 1. Post shock chromatogram of propargyl alcohol obtained using GC−FID. (a) T5 = 1142 K: A, acetylene; B, propyne; C, vinyl acetylene; D, propynal; E, propenal. (b) T5 = 1221 K: A, vinyl acetylene; B, propynal.

(ZPE), we used the frequencies calculated at MP2/6-31G* level instead of the standard HF/6-31G* level used in the G3 calculations. Hence, a scaling factor of 0.9427 was used instead of 0.895722 for ZPE. On the basis of the products observed, a detailed mechanism was derived. Kinetic simulation was performed using CHEMKIN23 to derive the concentration profiles of reactant, intermediates, and products. For most of the chemical species in the mechanism, experimental data on thermodynamic parameters were not available. Hence in CHEMKIN calculations, thermodynamic polynomials for every species were obtained using the VIBE fitdat utility. The enthalpy of formation and entropy at 298 K were calculated at the G3 level of theory. For these calculations, frequencies obtained at the B3LYP/6-311+G(d,p) level of theory were used, as these are expected to be more accurate than results calculated at HF or MP2 level of theories.

oscilloscope was used to record the pressure signals. An AdixenATP 400 turbo molecular vacuum pump was used to evacuate the driven section of the shock tube. We have used pressure traces obtained from three pressure sensors mounted in the driven section of the shock tube to obtain the shock Mach number. The Mach number was then used in one-dimensional shock equation to get temperature and pressure. The reaction time was obtained from the pressure sensor mounted at a distance of 5 mm from the end wall. Propargyl alcohol obtained from Spectrochem was distilled before use and confirmed to be 99.5% pure using gas chromatography (GC). Post shock mixtures were analyzed quantitatively using a GC with a flame ionization detector (FID). Another GC−MS and an FTIR were used for positive qualitative identification of all products. An HP-5 cross-linked (29 m × 0.53 mm × 1.53 μm) capillary column was used in the GC−FID and an HP-5MS (30 m × 0.25 mm × 0.25 μm) capillary column was used in the GC− MS. The flow rate in the GC−FID and the GC−MS were maintained at 8 mL/min and 0.5 mL/min, respectively. For a few cases GC−MS was also operated at a flow rate of 3 mL/ min. The oven was maintained at 65 °C. Because the vapor pressure for propargyl alcohol is substantially high at room temperature, 10 Torr of sample was loaded into a separate chamber and diluted with argon. The required amount was then fed into sample chamber of the shock tube. Blank runs were taken to ensure no previous sample exists in the shock tube. A sample cell of volume 200 mL was used to extract sample from the shock tube for analysis. The sample cell was also evacuated before each run and a blank run with argon was taken to ensure that it was free of previous sample.

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EXPERIMENTAL RESULTS Experiments were carried out in the temperature range 953− 1262 K. The reaction time was 1.36−1.67 ms as measured from the pressure trace. The pressure (P5) was above 10 bar in all experiments and all unimolecular reactions were in the high pressure limit. Acetylene, propyne, vinylacetylene, propynal, propenal, benzene, carbon monoxide, and water were the observed products from thermal decomposition of propargyl alcohol. Acetylene was observed in all temperature range. Propyne and propynal, which were minor products in the thermal decomposition reached their maximum concentrations near 1200 K. The production of benzene was observed after 1150 K. A gas chromatogram of the post shock propargyl alcohol mixture obtained using GC−FID is shown in Figure 1a,b. Figure 1a shows the chromatogram where benzene just started to form, and Figure 1b shows the chromatogram where benzene accounted for 2% of overall concentration. Carbon monoxide and water were detected only using the FTIR. No quantitative information regarding the concentrations of carbon monoxide and water could be obtained. However, kinetic simulation has been used to obtain information regarding the concentrations of carbon monoxide and water. The peaks for acetylene and propyne in GC−FID were overlapping and they

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COMPUTATIONAL DETAILS The Gaussian-0920 package was used to optimize the structure of reactants, products, and transition states. Frequency calculations were used to confirm the nature of optimized structures. Initial optimization was carried out at the B3LYP/6311+G(d,p) level of theory. Calculations at the G3 level were then used to improve the accuracy of results. For reactants, intermediates, and products, G3 calculations could be done using the Gaussian09 package directly. For transition states, we followed the procedure described by Curtiss et al.21 manually to estimate the G3 energies. For calculating zero point energies 5928



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were refined using a standard curve fitting method given in eq 1 to get area of respective peaks. 2 2 A y = y0 + × e−2(x − xc) / w π w (1) 2

The initial mole fraction of the propargyl alcohol was obtained as the sum of reactant and all product concentrations obtained after the shock using eq 2, following our earlier work.19 The concentration of C4H4 and C6H6 are multiplied by 2 as two reactants are consumed in producing one each of these molecules; see the section on the Chemical Kinetic Mechanism. [C3H3OH]t gives the concentration of the unreacted propargyl alcohol whereas rest of the terms account for the reacted propargyl alcohol. [C3H3OH]0 = [C3H3OH]t + [C2H 2] + [C3H4] + [C3H 2O] + [C2H3CHO] + 2 × [C4H4] + 2 × [C6H6]

(2)

Figure 3. Arrhenius plot for the overall decomposition of propargyl alcohol.

The initial mole fraction of propargyl alcohol varied from 2.35 × 10−4 to 9 × 10−4. The variation of propargyl alcohol concentration with temperature has been shown in Figure 2.

Experimental conditions and observed product concentrations have been given in Tables 1 and 2, respectively. The maximum uncertainty in the calculated Mach number has been found to be ±1.5%. The variation in product concentrations with temperature has been given in Figure 4. For reasons described in the Theoretical section, attempts were made to detect formaldehyde in the post shock mixture. However, it could not be detected in either GC−MS or FTIR. Though Table 1. Summary of Experimental Condition at Which Present Studies Were Carried Out

Figure 2. Propargyl alcohol concentration at various temperatures. Line: Mechanism. Triangles: Experiment.

The overall rate constant for thermal decomposition of propargyl alcohol for each case was obtained using eq 3. Variation of ln k with 1/T5 has been shown in Figure 3. Using the plot, we obtained the Arrhenius parameters for the overall rate constant and it has been given in eq 4. It should be remembered that these parameters are not for any elementary reaction but for the overall decomposition of propargyl alcohol in our experiments. Hence, A and Ea cannot be interpreted in terms of a specific transition state. k=−

[PA]t 1 × ln t [PA]0

(3)

Here, t is the test time/dwell time obtained using the pressure signal mounted near end of the shock tube. The reaction time was measured for every run. [PA]0 and [PA]t correspond to propargyl alcohol concentration before and after shock, respectively. ⎛ −39.70 ± 1.83 ⎞ ⎟ k = 10(10.17 ± 0.36) exp⎜ ⎝ ⎠ RT

(4) 5929

exp no.

T5 (K)

P5 (atm)

dwell time (ms)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

1180 1160 1220 1218 1161 1180 995 1107 1142 980 1073 953 1124 1142 1107 1010 1221 1073 1262 1262 1143 1219 1123 1073 1056 1009 1040 1240 1107

12.08 12.35 13.35 13.87 12.05 12.45 10.23 11.71 11.78 9.5 12.29 9.44 12.71 13.99 13.11 11.8 14.26 12.5 16.94 15.21 11.89 11.45 12.72 11.41 11.66 11.65 11.65 15 14.49

1.48 1.65 1.64 1.67 1.5 1.57 1.63 1.44 1.44 1.51 1.5 1.38 1.55 1.42 1.5 1.66 1.51 1.4 1.47 1.48 1.57 1.5 1.5 1.6 1.42 1.42 1.36 1.53 1.49



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Table 2. Summary of Mole Fraction of Products Obtained in the Thermal Decomposition of Propargyl Alcohola exp no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 a

C2H2 3.28 3.79 3.96 3.88 2.69 3.93 1.26 1.14 1.37 1.78 6.35 2.77 1.32 1.40 7.91 1.18 3.84 1.10 5.45 5.01 2.56 4.40 1.43 7.60 3.52 1.01 2.65 4.02 6.24

× × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

10−01 10−01 10−01 10−01 10−01 10−01 10−02 10−01 10−01 10−02 10−02 10−03 10−01 10−01 10−02 10−02 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−02 10−02 10−02 10−02 10−01 10−02

C3H4 5.18 7.36 9.24 9.31 7.98 9.87 1.94 3.60 4.77 2.97 2.37 7.85 3.98 4.41 1.88 2.02 1.01 2.97 8.11 8.05 6.66 9.78 4.72 2.47 9.11 2.74 7.87 9.99 2.45

× × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

10−02 10−02 10−02 10−02 10−02 10−02 10−03 10−02 10−02 10−03 10−02 10−04 10−02 10−02 10−02 10−03 10−01 10−02 10−02 10−02 10−02 10−02 10−02 10−02 10−03 10−03 10−03 10−02 10−02

C4H4 4.40 3.53 4.27 3.98 2.56 4.31 0 1.39 1.29 0 0 0 1.13 4.20 5.43 0 3.98 0 4.50 4.11 2.21 4.22 1.28 9.96 0 0 0 3.97 7.65

× × × × × ×

C3H2O

10−02 10−02 10−02 10−02 10−02 10−02

2.32 2.59 2.41 2.58 2.87 2.77 1.37 3.58 3.98 3.49 1.17 1.18 2.08 3.62 1.44 1.80 3.13 1.33 1.04 1.56 3.16 2.17 3.10 3.87 8.09 1.87 1.03 3.08 2.28

× 10−02 × 10−02

× 10−02 × 10−02 × 10−03 × 10−02 × × × × × ×

10−02 10−02 10−02 10−02 10−02 10−03

× 10−02 × 10−03

× × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

10−02 10−02 10−02 10−02 10−02 10−02 10−03 10−02 10−02 10−03 10−02 10−03 10−02 10−02 10−02 10−03 10−02 10−02 10−02 10−02 10−02 10−02 10−02 10−02 10−03 10−03 10−02 10−02 10−02

C3H4O 4.97 1.61 1.85 1.57 2.14 2.00 1.59 1.24 1.74 2.16 8.13 5.37 1.57 1.75 1.13 1.70 1.92 1.03 1.07 1.27 2.22 1.75 1.93 8.02 6.15 2.29 5.01 2.09 1.26

× × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

10−02 10−01 10−01 10−01 10−01 10−01 10−02 10−01 10−01 10−02 10−02 10−03 10−01 10−01 10−01 10−02 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−02 10−02 10−02 10−02 10−01 10−01

C6H6 0 2.03 0 3.99 1.11 3.87 0 0 1.19 0 0 0 2.13 0 0 0 2.39 0 7.79 5.96 9.82 4.53 2.22 0 0 0 0 4.63 0

× 10

propargyl alcohol −03

× 10−03 × 10−03 × 10−03

× 10−04

× 10−04

× 10−03 × × × × ×

10−03 10−03 10−04 10−03 10−04

× 10−03

4.59 2.86 2.17 2.48 3.55 1.86 9.68 6.62 5.76 9.54 8.20 9.90 6.28 5.20 7.64 9.67 2.08 7.44 1.51 1.82 3.77 1.72 5.60 7.60 8.86 9.62 9.05 1.70 7.49

× × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01 10−01

All concentrations are normalized by initial propargyl alcohol (PA) concentration.

Partition functions were calculated using rotational constants and vibrational frequencies obtained at MP2/6-31G* level of theory. The frequencies used for above calculations were scaled by 0.9427. Rate constants obtained using theoretical calculations have been given in Table 3. This will help in comparing the theoretical rate constants with the ones used in kinetic simulation. Though propargyl alcohol can be considered as acetylenesubstituted methanol, the CO bond energies in these two molecules are very different. The experimental CO bond energy in propargyl alcohol is not known. The theoretical value calculated at the B3LYP/6-311++G(d,p) level for CO BDE (bond dissociation energy) in methanol (90.4 kcal mol−1)25 can be compared to our results for propargyl alcohol (70.4 kcal mol−1) at the B3LYP/6-311+G(d,p) level of theory. This can be compared to the activation energy used in the simulation, which is 78 kcal mol−1. The CC and CH BDE in propyne (124 kcal mol−1 and 92.5 kcal mol−1)26 also have higher barriers when compared to CO BDE in propargyl alcohol. The C1C2 and C2C3 BDE at B3LYP/6-311+G(d,p) are 114.5 and 213.1 kcal mol−1, respectively. The C1H6, C3− H7, and O4H8 BDE at the G3 (B3LYP/6-311+G(d,p)) level of theory were found to be 81.6(77), 132.4(129.4), and 105(99.2) kcal mol−1, respectively. Clearly, the primary decomposition pathway for propargyl alcohol is CO bond dissociation. The recombination of OH and propargyl radical has more possibilities. The OH can add to the CH2 group to form propargyl alcohol or it can add to the CC group. The

there has been no prior work on thermal decomposition of propargyl alcohol, Jaman et al. reported propynal production using low pressure propargyl alcohol flow in DC glow discharge tube.24 Our experiments show that it is a minor product in the thermal decomposition of propargyl alcohol. Jaman et al.’s interest was to record the microwave spectrum of propynal, and they did not investigate pyrolysis of propargyl alcohol in a DC discharge.24 Hence no comparison could be made in this paper.

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THEORETICAL RESULTS In this section we will be describing the possible pathways through which propargyl alcohol could dissociate to form products. Some of the pathways that might not contribute in the present experimental temperature range (953−1262 K) will also be described to give a clear picture on all possible pathways for propargyl alcohol dissociation. As described in the previous section, calculations were carried out at B3LYP/6-311+G(d,p) and G3 level of theories. Theoretical rate constants were obtained using transition theory rate equation shown in eq 5. k=l

⎛ −E ⎞ KBT Q# × × exp⎜ 0 ⎟ ⎝ RT ⎠ h QR

(5)

Here, l is reaction path degeneracy, QR and Q# correspond to partition functions for reactant and activated complex, respectively. E0 is the activation energy for the reaction. kB is the Boltzmann constant, R is the universal gas constant, and T is temperature. 5930



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Figure 4. Concentration profile of all products from propargyl alcohol decomposition at various temperatures. VA: vinylacetylene. PA: propargyl alcohol. Line: mechanism. Triangles: experiment.

In a related study, Chin and Lee have reported theoretical investigations on possible pathways for dissociation and isomerization of propenal, an isomer of propargyl alcohol.14 There appears to be no work yet on propynal dissociation pathways. Both propynal and propenal are products in propargyl alcohol pyrolysis, and some of their decomposition pathways are included in this investigation. Pathways that cannot contribute in the present working temperature range (where kinetic fit has been performed for experimental data) include formation of formaldehyde and acetylene from propargyl alcohol, formation of carbon

resultant diradical can easily isomerize to propenal as the barrier is calculated to be 46.2 kcal mol−1 only. It should be noted that propargyl alcohol can exist in trans and gauche conformers. For the sake of completeness, both structures were optimized and a potential energy scan along the torsional coordinate, ∠C2C1O4H8, which leads to interconversion was carried out. These results are presented in the Supporting Information, Figure S1. Experimental observation has only confirmed the existence of the gauche conformer.27 We consider only the gauche conformer in all our calculations for reaction rates. 5931



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monoxide and acetylene from propynal, and formation of vinyldiene and formaldehyde from propenal. The barriers for these reactions are above 85 kcal mol−1 and they have preexponential factors that are typically orders of magnitude less than that for the bond dissociation reactions.19 Hence we are not considering these reactions in the kinetic mechanism described later in this paper. Structures of molecules and intermediates involved in decomposition of propargyl alcohol have been given in Figure 5. The Cartesian coordinates for these structures have been given in the Supporting Information. All transition state(TS) structures described below have been shown in Figure 6, and their Cartesian coordinates have been given in the Supporting Information. The schematic diagram depicting potential energy changes involved in reactions mentioned in this paper is shown in Figure 7. The following description will help in understanding Figure 7. Symbols used in the following lines are same as those used in the caption of Figure 7. All the pathways leading from

Table 3. Transition State Theory Parameters for Reactions Calculated Using Eq 5a reactant and product(s)

log A

E0

(a) and (5) (g) and (13) (b) and (c) (b) and (9) (d) and (e) (1) and (3) (2) and (4) (11) and (12) (f) and (14) (c) and (7)

13.53 14.66 13.52 13.64 13.19 12.41 13.81 14.07 14.32 14.23

88.7 86.31 24.25 33.31 56.36 24.51 25.15 16.48 34.64 41.16

a

Terms used for reactant and products are the same as those used in Figure 7.

Figure 5. Structures of reactant, intermediates, and products optimized at the G3 level of theory. The chemical formula written below each structure is the same as that used in the kinetic mechanism in Table 6. (a), (e), (g), and (h) are the stable molecules, propargyl alcohol, propenal, propynal and propyne, respectively. 5932



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Figure 6. Transition states optimized at the G3 level of theory. Details regarding the reactant and product state for each transition state have been described in Figure 7.

level as a reference for the reaction. For example, the schematics showing energy level of (2), which represents, propargyl alcohol + H [(a) + H] is shown at the same level as that of propargyl alcohol [(a)]. The relative energy showing E0 and ΔE for the reaction is always clearly represented in Figure 7. Explanations for propargyl alcohol + OH [(1)] and propenal + H [(11)] are similar to the above. The following procedure was employed if the reaction of one of the products formed from dissociation has to be represented. Consider reaction C3H3O [(b)] to propynal + H [(9)]. The reactant [(b)] is formed along with a H atom from dissociation of propargyl

propargyl alcohol [(a)] are straightforward. The pathways given include (a) to acetylene + formaldehyde [(5)], (a) to C3H3 + OH, and (a) to C3H3O + H [(6)]. The pathways showing C3H3 + OH recombination to form CH2CCHOH [(d)] followed by isomerization to form propenal [(e)] is then depicted in Figure 7. Then the schematics for propenal reaction which include (e) to vinyldiene + formaldehyde [(8)] and (e) to C3H3O(B) + H [(10)] is depicted. The schematics showing the potential energy for radical reactions, which needs explanation will be discussed below. To represent the radical reaction with molecule, we have used the molecular energy 5933



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calculations underestimate barriers for HCl molecular elimination reactions as noted earlier.28 Unless specified otherwise, transition states described in the following section were optimized at MP2/6-31G* level of theory as part of the G3 calculations. At the transition state, C1C2C3 reduces to 79.9° from 177.7° and the C1−C2 bond distance increases to 2.20 Å from 1.48 Å. The newly forming C2−H8 bond distance is 1.54 Å. The O4− H8 bond distance increases to 1.14 Å from 0.97 Å. The C1−O4 bond length reduces to 1.29 Å from 1.43 Å. The value of the same at the product state is 1.22 Å. The C2−C3 bond distance increases to 1.26 Å from 1.22 Å. The C2−C3 bond distance at the product state is 1.22 Å. Hence the bending motion brings the C2H group of propargyl alcohol near to the OH hydrogen atom, facilitating its abstraction by the former to form acetylene. The IRC (intrinsic reaction coordinate) scan carried out starting from this transition state has been shown in Figure 8, and it indeed connects the reactant (PA) and products (acetylene and CH2O).

Figure 7. Schematic diagram showing relative energies of reactants, products, and transition states for the various reactions considered. Dotted lines show results from the B3LYP/6-311+G(d,P) level and the solid lines show results from the G3 level. Labels (a), (b), (c), ... correspond to molecules described in Figure 5. TS1, TS2, TS, ... correspond to transition state structures described in Figure 6. Each number in the scheme represents two chemicals as given next: (1) = (a) + (OH), (2) = (a) + H, (3) = (b) + water, (4) = (b) + H2, (5) = acetylene + formaldehyde, (6) = (b) + H, (7) = acetylene + (k), (8) = vinyldiene + formaldehyde, (9) = (g) + H, (10) = (f) + H, (11) = (e) + H, (12) = (f) + H2, (13) = acetylene + CO, (14) = (j) + CO. The schematic diagram is not to scale except for reactions starting from the same reactants.

alcohol. Because (b) can further react, to show the reaction involving (b), the energy level next to C3H3O + H [(6)] is taken as reference for (b). As above, E0 and ΔE for the reaction is clearly depicted. Similar explanations hold for propynal [(g)] to acetylene + CO [(13)] and C3H3O(B) [(f)] to C2H3 + CO [(14)]. The isomerization from C3H3O [(b)] to C3H3O(A) [(c)] followed by dissociation to acetylene and CHO needs no explanation. We discuss all these pathways in more detail below.

Figure 8. IRC scan for decomposition of propargyl alcohol to formaldehyde and acetylene.

Propynal. Propynal, HCCCHO, the structure of which has been shown in Figure 5g, is an unsaturated aldehyde, work on which rarely exists in the literature. Because propynal is one of the products of propargyl alcohol thermal decomposition, attempts were made to find out its dissociation pathways. The molecular elimination pathway leading to acetylene and carbon monoxide has a transition state structure shown in Figure 6b. At the transition state, the ∠C1C2C3 changes from 178.6° to 151.6°. The ∠H5C1O4 of the aldehyde group changes from 121.2° to 168.6°. The C2−C1 bond distance varies from 1.45 to 2.13 Å. The C1−O4 bond distance reduced to 1.16 Å from 1.23 Å. The value of the same at product state is 1.15 Å. The C2−C3 bond distance remains nearly constant, changing from 1.24 to 1.22 Å. The C2−C3 bond distance at the product state is 1.22 Å. The C2−H5 bond distance at the transition state is 1.71 Å. ∠C2C1H5 changes from 115.6° in the reactant to 53° in the transition state. Thus, the dissociating C2C3H6 molecular group abstracts the hydrogen atom H5 from H5C1O4 to form acetylene and carbon monoxide. The IRC calculation carried out for this transition state connects the reactant to product state, and results are shown in Figure 9.

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MOLECULAR ELIMINATION PATHWAYS Two molecular elimination pathways for propenal leading to the formation of acetylene have been described in the paper by Chin and Lee.14 Formation of acetylene and formaldehyde from propargyl alcohol and of acetylene and carbon monoxide from propynal will be presented here. Propargyl Alcohol. Formation of acetylene and formaldehyde from propargyl alcohol is one of the interesting reactions in which the bending motion of the molecule comes into play. The transition state structure (TS1) for the formation of formaldehyde and acetylene from the propargyl alcohol has been shown in Figure 6a. The reaction coordinate includes bending motion, which brings the C2 carbon in the vicinity of the OH hydrogen atom, allowing it to be abstracted by the C2H group forming acetylene and formaldehyde. The energy barrier of 75.9 kcal mol−1, obtained at the B3LYP/6-311+G(d,p) level of theory might be an underestimate, as revealed by G3 calculations that give a barrier of 87.3 kcal mol−1. The B3LYP 5934



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compared to that of abstraction of the same by OH radical. At the transition state, the distance between H8−H9 and O4−H8 atoms are 0.86 and 1.27 Å, respectively. At the reactant state, the distance O4−H8 is 0.97 Å. At the transition state, ∠O4H8H9 is 171.4°. The C1−O4 bond distance remains almost constant during the course of reaction. It reduces to 1.41 Å from 1.43 Å. The C1−O4 bond distance at the product state is 1.38 Å. A similar transition state for H abstraction by the hydrogen atom could be observed in the case of propenal. The transition state structure (TS8) for H abstraction belonging to the CHO group of propenal by the hydrogen atom has been shown in Figure 6h. At the transition state, the C1−H5 increases to 1.35 Å from 1.11 Å. The newly formed H5−H9 bond distance and ∠C1H5H9 are 0.98 Å and 178.9°, respectively. The ∠C2C1O4 increases to 127.2° from 124°. The value of the same at the product state is 179.6°. The C1−O4 bond length reduces to 1.20 Å from 1.23 Å. The value of C1−O4 bond length at product state is 1.18 Å. Isomerization of C3H4O and C3H3O. Apart from H abstraction, the dissociated OH radical can also recombine with the C3H3 radical at C1 to form a structure shown in Figure 5d. The CH2CCHOH molecule can isomerize to form propenal. The transition state structure (TS5) for this isomerization reaction has been shown in Figure 6e. The activation energy for this reaction is 56.4 (46.2) kcal mol−1 at the G3(B3LYP/ 6311+G(d,p)) level and 50 kcal mol−1 was used in the kinetic simulation. Comparison among bond lengths, angles, and dihedral angles of CH2CCHOH, the transition state structure (TS5), and propenal are given in Table 4.

Figure 9. IRC scan for decomposition of propynal to carbon monoxide and acetylene.

Hydrogen Abstraction by Radicals. Hydrogen abstraction reaction by an H or an OH radical was found to be the most important propagating reaction. The radicals formed from the removal of hydrogen atom will have a lower BDE to form CO or acetylene molecule. The main initiator for this process is formation of the OH radical from propargyl alcohol. The OH thus formed can abstract a hydrogen atom from propargyl alcohol and there are three possibilities. The H from OH, CH2, or CH can be abstracted. The transition state for abstracting the H atom from OH could be optimized and it is shown in Figure 6f. Attempts to optimize the TS for H abstraction from CH2 and CH did not succeed. However, the C1−H6 bond is weaker than the O−H bond in propargyl alcohol and so H abstraction from the CH2 group was included in the kinetic mechanism with typical kinetic parameters. The structure of the radical (C3H3O(C)) formed by H abstraction from the CH2 group of the propargyl alcohol has been shown in Figure 5l. C3H3O(C) can isomerize to C3H3O(A). The transition state for the above isomerization reaction could not be optimized, and hence the activation energy for the C3H3O(C) to C3H3O(A) isomerization reaction was approximated to that of the C3H3O to C 3 H 3 O(A) isomerization and included in the kinetic mechanism. Hydrogen abstraction from the OH group leads to the C3H3O radical. The C3H3O radical readily dissociates to hydrogen and propynal. The hydrogen dissociated from C3H3O can abstract another hydrogen from propargyl alcohol giving back C3H3O radical and the chain can continue. Hydrogen abstraction by the OH radical from propargyl alcohol will lead to formation of water and the C3H3O radical. At the transition state (TS6) for this reaction, the O4−H8 bond distance increases to 1.11 Å from 0.97 Å. The oxygen atom O9 of the OH group, which is abstracting a H atom from propargyl alcohol will be at a distance of 1.22 Å from H8, the hydrogen atom corresponding to OH of propargyl alcohol. The value of the same at the product state is 0.97 Å. ∠O4H8O9H10 and ∠H8O9H9 at the transition state are 33.3° and 103.9°, respectively. In the course of the reaction the variation in C1− O4 bond distance was found to be small. It was reduced to 1.42 Å from 1.43 Å. The value of the same at product state was 1.38 Å. The transition state structure (TS7) for hydrogen abstraction from the OH group of the propargyl alcohol by an H atom has been shown in Figure 6g. It has a 3 kcal mol−1 higher barrier

Table 4. Comparison among Various Parameters of Reactant, Transition State, and Product State Corresponding to CHOHCCH2 Isomerization Reaction to Propenal parameter

reactant

transition state

product

C2−C3 C1−O4 O4−H8 C2−H6 ∠C1C2C3 ∠04C1C2C3

1.31 Å 1.38 Å 0.97 Å 2.45 Å 179.8° −170.4°

1.35 Å 1.29 Å 1.21 Å 1.59 Å 125.8° −145.8

1.34 Å 1.23 Å 2.65 Å 1.08 Å 120.5° 180°

The C3H3O radical formed from the hydrogen abstraction reaction could isomerize to CHCHCHO. The transition state for this isomerization has been optimized at the B3LYP/6311+G(d,p) level of theory. Attempts to optimize the same at the MP2/6-31G* level of theory were not successful. The optimized transition state structure (TS3) has been shown in Figure 6c. At the transition state, the C1−H6, C2−H6, and C1−O4 bond distances are 1.38, 1.41, and 1.25 Å, respectively. The dihedral angles ∠C3C2C1H5 and ∠C2C1H6 at the transition state are 10.2° and 59.3°, respectively. Other structural parameters for the reactant, transition state, and product are compared in Table 5. Radical Dissociation. Radicals formed from hydrogen abstraction have lower BDE and dissociate to a stable molecule and a radical. Radical dissociation reactions include dissociation of CHCCH2O to CHCCHO and H atom, dissociation of CHCHCHO to acetylene and CHO, and dissociation of CH2CHCO to CH2CH and CO. The reaction coordinate in each of these reactions is simple bond dissociation of the C−H or C−C bond. Because one of the final products is a stable 5935



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The radical formed from the hydrogen abstraction from propenal has been shown in Figure 5f. It can decompose to C2H3 and CO molecule and the reaction coordinate is dominated by C2C1 bond stretching. The transition state structure (TS9) has been shown in Figure 6i. At the transition state the C2C1 bond distance increases to 2.17 Å from 1.33 Å and ∠C3C2C1 reduces to 112.4° from 124.4°. The C1−O4 bond distance reduces to 1.16 Å from 1.18 Å. The product C O bond distance is 1.11 Å.

Table 5. Comparison among Various Parameters of Reactant, Transition State, and Product State Corresponding to CHCCH2O Isomerization to CHCHCHO parameter

reactant

transition state

product

C1−O4 C1−H6 ∠C1C2C3 ∠C2C1H6

1.36 Å 1.11 Å 179.6° 110.4°

1.25 Å 1.38 Å 179.7° 59.3°

1.2 Å 2.19 Å 122.3° 26.7°

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molecule, there exists a barrier between the reactant and the product. The transition state for the above-mentioned reactions will be described in the following paragraphs. The potential energy changes along their coordinates are given in the Supporting Information. The reaction coordinate for propynal formation is characterized by dissociation of the C1−H5 bond from the molecular structure shown in Figure 5b. The optimized transition state structure (TS4) has been shown in Figure 6d. At the transition state, the C1−H5 bond distance increases to 1.78 Å from 1.09 Å and the dihedral angle ∠C3C2C1H5 increases to 117.4° from 58.4°. The C1−O4 bond distance reduces to 1.23 Å from 1.39 Å. The C1−O4 bond distance in propynal is also 1.23 Å. Formation of acetylene from CHCHCHO (Figure 5c) involves lengthening of the C2−C1 bond and changes in the ∠C3C2C1 angle. This has a small barrier of 41.2 kcal mol−1. The transition state structure (TS10) for the above-mentioned reaction has been shown in Figure 6j. At the transition state, the C2−C1 bond distance increases to 1.98 Å from 1.52 Å and ∠C3C2C1 decreases to 101.3° from 119.8°. The C2−C3 bond distance reduced to 1.21 Å from 1.28 Å. The value of the same at the product is 1.22 Å.

CHEMICAL KINETIC MECHANISM The kinetic mechanism (Table 6) for thermal decomposition of propargyl alcohol was derived using rate constants obtained from the above-mentioned theoretical calculations and available literature values. Because most of the rate constants are not available and few reactions compete with each other, theoretically calculated rate constants were taken as first approximations and later pre-exponential factors (except for reaction R3) were modified to fit experimentally observed product concentrations. As seen from the mechanism, CO, H2O, and H2 will be formed along with the other molecules that are detectable by GC−FID. The initial OH dissociation rate parameters were obtained with a combination of present and literature values. The activation energy was obtained at the G3 level theory. The preexponential factor was approximated to that of C−O dissociation from methanol. The pre-exponential factor for the CH3OH = CH3 + OH reaction has been reported by the different groups and they are on the order of 1017−1018.29−31 Moreover, the pre-exponential factor for the C2H5OH = C2H5 + OH reaction has also been reported to be of the same

Table 6. Kinetic Mechanism Used To Explain Thermal Decomposition of Propargyl Alcohola Sl no. R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22

reactionb

A

C3H3OH = C3H3 + OH C3H3 + OH = CH2CCHOH CH2CCHOH ⇒ C2H3CHO C3H3OH + OH = C3H3O + H2O C3H3OH + H = C3H3O + H2 C3H3O = C3H3O(A) C3H3O = C3H2O + H C2H3CHO + H = C3H3O(B) + H2 C2H3CHO + OH = C3H3O(B) + H2O C3H3O(A) = C2H2 + CHO C3H3O(B) = C2H3 + CO C3H3 + H = C3H4 C2H3 = C2H2 + H C2H2 + C2H3 = C4H4 + H C3H3 + C3H3 = C6H6 CHO = CO + H C3H3OH + H = C3H3O(C) + H2 C3H3OH + OH = C3H3O(C) + H2O C3H3O(C) = C3H3O(A) C2H3 + H = C2H2 + H2 H + H + M = H2 + M H + OH + M = H2O + M

2 × 10 5.0 × 1014 1 × 1015 1 × 1014 9.18 × 1014 5 × 1013 6.00 × 1016 8.0 × 1014 1.00 × 1015 1.71 × 1014 2.10 × 1014 5.00 × 1013 3.50 × 1014 2.00 × 1013 5.00 × 1013 4.82 × 1013 1.5 × 1013 1.5 × 1013 5 × 1013 2 × 1013 1 × 1010 2 × 1010 18

n

Ea

refc

0.0 −0.1 0.0 0.25 0.25 00.0 0.0 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 00.0 0.0 0.0 0.0

78.0 0.0 50.4 24.5 25.2 22.6 33.3 16.5 16.5 41.2 34.6 0.0 33.0 04.9 12.0 15.8 10.0 15.0 32.5 0.0 0.0 0.0

27 37, d pw pw pw pw pw pw pw pw pw 38 39 40, e fitted 41 pw pw pw 42 fittedf fittedf

Units: A in cm3 mol s, temperature in K, Ea in kcal mol−1. bThe reactions with lower energy barriers can proceed after test time. Hence, all the reactions were simulated for test conditions given in Table 1 and then for 10 ms with temperature 300 K and pressure 2 atm to consume radicals after test time. cpw: present work. Values were approximated using theoretical rate constant obtained at G3 level of theory. Only the forward reaction for reaction R3 was considered. Fitted: rate constant was varied to fit the experimental concentration. dRead text. ePre-exponential factor was increased by one order. fReactions were included to account for radical recombination after the test time. a

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order.32 The alternative molecular elimination pathway for propargyl alcohol, reaction R23, was not considered as the rate constant is 5 orders of magnitude smaller than that for the C− O bond dissociation reaction. The rate constant for reaction R23, k23, at 1260 K is 0.0118 s−1 whereas k1 at 1260 K is 57 934 s−1. C3H3OH = C2H 2 + CH 2O

isomerization. Propenal has been observed to be a major product. Acetylene, propyne, vinyl acetylene, propynal and benzene were the other products of propargyl alcohol decomposition. The dominant propagation pathway in pyrolysis of propargyl alcohol include H and OH radical reaction with molecules and radical dissociation to form another radical and a stable molecule. The C3H3 dimerization process was observed to be a viable pathway for benzene formation. The rate constant k = 5.00 × 1013T0.1 exp(−12.00/ RT) for benzene formation was observed to explain the experimental profile. Theoretical calculations were carried out to find minimum energy pathways for all the observed products.

⎛ 87.27 ⎞ ⎟ k = 1.66 × 1013 exp⎜ − ⎝ RT ⎠ (R23)

Reaction R2 has been assumed to be barrierless and the initial guess of pre-exponential factor was taken from OH recombination with CH3 to form methanol. This OH recombination rate constant can be justified as reactions R25, R26, and R27 have approximately similar values. CH3OH = CH3 + OH

⎛ 68.36 ⎞ ⎟ (ref 30) k = 2.0 × 1017 exp⎜ − ⎝ RT ⎠

(R24) CH3OH = CH3 + OH

⎛ 70.94 ⎞ ⎟ (ref 31) k = 1.37 × 1017 exp⎜ − ⎝ RT ⎠

(R24)

C2H5 + OH = C2H5OH C4 H 9 + OH = C4 H 9OH C3H7 + OH = C3H7OH C3H 2O = C2H 2 + CO

k = 7.69 × 1013 (ref 33) 13

k = 2.41 × 10 (ref 34) 13

k = 2.41 × 10 (ref 35)

ASSOCIATED CONTENT

■

AUTHOR INFORMATION

Cartesian coordinates, figures of dihedral angle and bond distance scans, normal-mode frequencies of molecules, intermediates, and transition states, and their energies calculated at B3LYP/6-311+G(d,p) and G3 levels of theory, pressure profiles, and thermochemical data. Full citation for ref 20. This material is available free of charge via the Internet at http://pubs.acs.org/.

(R24) CH3OH = CH3 + OH

■

S Supporting Information *

⎛ 94.39 ⎞ ⎟ (ref 29) k = 2.0 × 1018 exp⎜ − ⎝ RT ⎠

Corresponding Author

*E. Arunan. E-mail: [email protected].

(R25)

Notes

(R26)

The authors declare no competing financial interest.

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(R27)

ACKNOWLEDGMENTS We acknowledge funding from Defense Research and Development Organization, Aeronautics Research, and Development Board and IISc-ISRO Space Technology Cell.

⎛ 84.02 ⎞ ⎟ k = 1.75 × 1014 exp⎜ − ⎝ RT ⎠

(R28)

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The rate parameters for reaction R9 were approximated to those of reaction R8 as attempts to optimize transition state for H abstraction in propenal by OH radical were not successful. Available data on C3H3 dimerization gave a range of rate constants varying from a factor of 1010 to 1013.6,8,36 We have used rate parameters (A, n, and Ea) 5.0 × 1013, 0.1, and 12 kcal mol−1 to fit the experimental data for temperatures ranging from 953 to 1262 K. Reaction R23 will not be important at any temperature compared to the C−O dissociation reaction. The rate constants for propynal decomposition to acetylene and carbon monoxide, reaction R28, k28, at 1260 K is 0.45 s−1 and hence not considered in the kinetic mechanism presented in this paper. Product concentration simulated from the mechanism has been given in Figure 4. It is very clear that the mechanism could not reproduce vinyl acetylene concentration. Also, increasing the pre-exponential factor for reaction R14 by several orders of magnitude (to 1017) was not useful in explaining the observed experimental concentration.

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CONCLUSION Thermal decomposition experiments of propargyl alcohol have been carried out for temperatures ranging from 953 to 1262 K. As the primary step is CO dissociation, the OH + propargyl radical reaction could be investigated as well. Both OH and propargyl radicals are important intermediates in combustion. The reaction between OH and propargyl radical can happen via two pathways, one giving back propargyl alcohol and another giving propenal via OH addition at the CC following an 5937



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Thermal Decomposition of Propargyl Alcohol: Single Pulse Shock

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