Dissociation Pathways of 2, 3-Dihydrofuran Measured by Chirped

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Dissociation Pathways of 2,3-Dihydrofuran Measured by Chirped-Pulse Fourier Transform Microwave Spectroscopy Chandana Karunatilaka, Amanda J. Shirar, Giana L. Storck, Kelly M. Hotopp, Erin B. Biddle, Rickie Crawley, Jr., and Brian C. Dian* Department of Chemistry, Purdue University, 560, Oval Drive, West Lafayette, Indiana, 47907-2084

ABSTRACT This experiment combines the use of an ultrabroadband chirpedpulse Fourier transform microwave (CP-FTMW) spectrometer with a pulsed discharge nozzle to study products formed during dissociation of 2,3-dihydrofuran (2,3-DHF). Molecules identified in the spectrum include cyclopropanecarboxaldehyde (CPCA), acrolein, crotonaldehyde (CA), formaldehyde, propene, propyne and cyclopropenylidene. Individual cis and trans isomers were detected for CPCA and acrolein, but only the trans isomer of CA was observed. Although cis forms of CPCA and CA would be the most likely structures produced from 2,3-DHF isomerization, our discharge provides enough energy for the molecules to convert into multiple conformers. The identification of formaldehyde in the spectrum supports a proposed mechanism to form propyne directly from 2,3-DHF ring-opening, a scheme that has been difficult to verify. Although acetylene cannot be detected because of the lack of a permanent dipole moment, the existence of cyclopropenylidene (C3H2) is indirect evidence of its presence in the discharge. SECTION Kinetics, Spectroscopy

E

with an argon bath. Hot discharge products were cooled to a few Kelvin in the supersonic expansion, and the signal was characterized by recording the 11 GHz broadband rotational spectrum using a digital oscilloscope with a 40 Gs/s digitizer. Unlike previously reported discharge experiments,1,11 our system has the capability of recording the rotational spectrum of several molecules simultaneously. According to a study by Lifshitz,9 21 products were proposed in the pyrolysis of 2,3-DHF. Altogether, seven molecules have been positively identified in our discharge spectrum: cyclopropanecarboxaldehyde (CPCA), acrolein (prop-2-enal), crotonaldehyde (CA, but-2-enal), formaldehyde, propene, propyne, and cyclopropenylidene. Of these products, three molecules have cis and trans conformers. Since rotational constants differ between conformers, our microwave spectrometer distinguishes between them. For CPCA and acrolein, both cis and trans conformers were observed, but only the trans form of CA was identified. Molecules containing sufficient rotational transitions within our spectral range were fit using JB95,12 otherwise molecular transitions were simply compared to published frequencies. As can be seen in Figure 1, the present experiment not only identified molecular products created in the discharge process, but also distinguished

lectric discharge sources are commonly used to produce exotic chemical species such as radicals and ions that are difficult, if not impossible, to prepare on a laboratory benchtop. These species are important intermediaries in combustion processes and play significant roles in interstellar chemistry.1,2 Despite the recent advances of experimental techniques,3 the unambiguous identification of important reaction intermediates has become increasingly difficult as target compounds increase in size. Structural isomers can have widely varying chemical reactivities,4 necessitating chemical identification to provide accurate comparison to kinetic models. The experiment of Taatjes et al.3 demonstrates the current experimental complexity in positively identifying multiple isomers of the same mass. Ultrabroadband chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy has continued to find new developments5-7 with recent advances to state-of-the-art high speed digital electronics. 2,3-Dihydrofuran (2,3-DHF) is used in the present experiment as a prototypical system to demonstrate applying a CP-FTMW spectrometer as a shapesensitive detector to explore various dissociation pathways. The cyclic ether 2,3-DHF was chosen as an appropriate molecule to study since its dissociation products have been studied extensively and are well documented.8-10 Our spectrometer employs a pulsed discharge source to generate an electric field across two electrodes. The sample was introduced using a pulsed valve located before the electrodes, and dissociation of 2,3-DHF was initiated via penning ionization

r 2010 American Chemical Society

Received Date: April 1, 2010 Accepted Date: April 22, 2010 Published on Web Date: April 28, 2010

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DOI: 10.1021/jz100426c |J. Phys. Chem. Lett. 2010, 1, 1547–1551

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Figure 2. Relative zero point corrected energies for the four conformers of CA. Conformers related to the carbon double bond or the aldehyde group are identified by d- and s- respectively. The lowest energy structure, the s-trans conformer of d-trans-CA, is set at 0 kJ/mol.

Figure 1. Comparison of the discharge spectrum of 2,3-DHF obtained from CP-FTMW spectrometer (top) and predicted spectrum using Pickett's SPCAT6 (bottom).

obtain the torsional potential for cis and trans isomers of CPCA. The torsional barrier is 26.22 kJ/mol, and the trans conformer is 0.68 kJ/mol more stable than the cis conformer. Microwave work by Volltrauer and Schwendeman18 confirms that stable forms of cis and trans CPCA exist with an energy difference of 0.121 kJ/mol and a barrier height of 18.4 kJ/mol. In regards to CA, the problem is more complicated. The energies and vibrations of the four possible structures have been calculated (B3LYP/6-31þG**), and the zero point corrected energies are presented in Figure 2. A recent quantumchemical study by Bokareva et al.19 calculated structures and torsional barriers of acrolein derivatives, and their labeling scheme of CA was adopted here (d- indicates isomers relating to the carbon double bond). It can be seen in Figure 2 that both the d-trans conformers are more stable than either d-cis conformer. Dubnikova and Lifshitz10 predicted that 2,3-DHF proceeds directly to the s-cis conformer of d-cis-CA, shown in the left panel of Figure 2. In the discharge spectrum, only the trans conformer of d-trans-CA was recorded. In order to convert from the predicted theoretical structure to the experimentally observed structure, two separate torsions would need to be applied: one to the aldehyde group and one around the carbon double bond. The torsional barrier for the aldehyde group is fairly straightforward to calculate, and Bokareva et al.19 published results of ∼25 kJ/mol for d-cis conformers and ∼36 kJ/mol for d-trans conformers. Experimentally, only the d-trans molecules have been studied, and vibrational analysis20 determined this torsional barrier to be ∼68 kJ/ mol. Unfortunately, calculating the torsion around the double bond is much more difficult and has not been studied in CA. However, many studies involving the double bond in stilbene have been conducted and the barrier to cis-trans isomerization in stilbene21 is ∼187 kJ/mol. This barrier height gives a rough estimate of the possible barrier to convert from the cis form of d-cis-CA to the trans form of d-trans-CA. Once CA has been created in the discharge, it likely contains an excess of internal energy that allows for torsion around a double bond to relax into a lower energy structure. A purely trans assignment was confirmed by obtaining the pure rotational spectrum of cis and trans CA using our spectrometer system. In the region of 14 GHz, a feature similar to cis CA was observed, but

between cis and trans conformers on the basis of their unique rotational frequencies and transition patterns. Two different pyrolysis experiments using 2,3-DHF have been reported in the literature. In the first study,8 both CPCA and CA molecules were identified by their melting points at various pyrolysis temperatures (670-820 K). In the late 1980s, Lifshitz9 revisited this study of 2,3-DHF pyrolysis behind reflected shocks in a single-pulse tube at increased temperatures (up to 1300 K). Major reaction pathways and rate constants for certain unimolecular reactions were determined, and additional pyrolytic products were detected. Identification of products was derived from retention times and gas chromatography/mass spectrometry (GC/MS), but they could not distinguish between individual conformers. Contrary to the experimental results obtained for tetrahydrofuran,13 furan14 and 2,5-dihydrofuran,15 Lifshitz discovered that the major reaction path of 2,3-DHF was the unimolecular isomerization process to yield CPCA. Another favorable unimolecular isomerization to form CA from CPCA at higher temperatures was also proposed, but the reaction rate could not be determined because of incomplete separation between GC signals. However, as can been seen in Figure 1, our spectrometer has the capability to isolate both CPCA and CA as well as the cis and trans conformers of CPCA since our detector is especially sensitive to the shape of the molecule. According to theory and experiment, the main thermal reaction of 2,3-DHF is the furan ring-opening to form two isomerization products, CPCA and CA. The original experimental studies9,16 focused on determining combustion products, but were unable to determine contributions from conformers of any species. Later, density functional theory calculations by Dubnikova and Lifshitz10 proposed that both CPCA and CA are formed by unimolecular isomerization processes and proceed via a concerted mechanism with a single transition state. These calculations follow a specific reaction coordinate that leads exclusively to the cis conformer for each reactant. Both cis and trans conformers of CPCAwere identified on our spectrum, but only the trans conformer of CA. Density functional theory calculations (B3LYP/6-31þG**) using the Gaussian 03 program suite17 were performed to


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be formed via the dissociative recombination27 seen in reaction 3.

we were unable to reasonably fit these transitions. Splitting on the K1 bands of these transitions does suggest that this unassigned molecule has a methyl rotor. The presence of these additional conformers suggests that the discharge products contain enough internal energy to isomerize to these conformers subsequent to dissociation. This excess energy could be due to the discharge itself, or collisions with other excited species. Although we are unable to determine the origin of this additional energy, the presence of additional conformers implies the discharge environment provides enough energy to initiate numerous isomerization reactions. In addition to the aldehydes, both propyne and propene were observed. Several studies9,22 propose that propyne is produced from either propene or 2,3-DHF ring-opening. To form propyne directly from 2,3-DHF, a side product of formaldehyde is necessary (reaction 1).

Unfortunately acetylene, ethane, and allene cannot be detected by our CP-FTMW spectrometer because of their lack of a permanent dipole. The strong signal of this carbene may be from contributions from several sources and could be indirect evidence for these molecules. A few lines remain unidentified in our spectrum despite extensive search for different isomerization and decomposition products. Small organic molecules characterized in the previous pyrolysis study9 were not detected by our spectrometer because they either lack a permanent dipole moment or their rotational frequencies are located outside the bandwidth of our spectrometer. The open and closed shell transition states proposed in previous DFT calculations10 and other possible free radical species could also be responsible for unassigned peaks. We present here the use of 2,3-DHF as a prototypical system to demonstrate CP-FTMW as a shape-sensitive detection device. In addition to identifying seven distinct discharge products, our spectrometer was able to discriminate between cis and trans conformers of multiple species. Although previous theory10 predicted isomerization mechanisms that ended with only cis conformers of CPCA and CA, our results suggest that product species have sufficient internal energy after dissociation to convert between molecular conformations. Formaldehyde was observed and helps support a ring-opening mechanism to form propyne. An unpredicted discharge product, cyclopropenylidene, was detected and a mechanism derived from 2,3-DHF was proposed. Further experiments are needed to get rate information of the proposed reaction mechanisms by using relative peak intensities from experiments with varying concentrations of 2,3-DHF. Moreover, introducing oxygen into the discharge with different molecules would more accurately represent a combustion environment and should be explored. We believe combining a CP-FTMW spectrometer with a pulsed discharge source will continue to find new applications that will help to discover the rich chemistry behind these reaction mechanisms.

The authors were unable to verify this reaction, as no formaldehyde was observed in the postshock samples. Unlike those experiments, a strong formaldehyde signal exists in our discharge spectrum (14488.495 MHz). We are unable to distinguish between the two mechanisms, but there is now evidence to support direct formation of propyne from 2,3-DHF. Additionally, it has been proposed that propene is formed by secondary thermal decomposition of the aldehyde products instead of a direct decomposition of 2,3-DHF.9 The kinetics derived in the experiment did not agree with a single-step unimolecular reaction. However, such conclusions cannot be drawn from our data set without performing isotopic substituted experiments of 2,3-DHF. A product not reported in the pyrolysis study, but tentatively assigned in our spectrum, is cyclopropenylidene (C3H2). Possible precursors to this molecule could be propyne, allene, acetylene, or ethane, which are all significant products of 2,3DHF pyrolysis.9 Since the presence of propyne can be confirmed in the discharge, a mechanism initiating from this molecule is proposed as the primary source of cyclopropenylidene. With the excess of energy produced in the discharge, propyne can dissociate to form vinylidenecarbene23 (reaction 2).

EXPERIMENTAL SECTION The spectrometer is divided into three distinct regions: (1) pulse generation, (2) sample interaction, and (3) molecular detection. A 1 μs, chirped microwave pulse was generated using an arbitrary waveform generator (Tektronix, AWG7101) covering the frequency ranges of 1.875-4.625 GHz. The bandwidth of this pulse was then increased with an x4 frequency multiplier (Phase One PS06-0161) to create an 11 GHz bandwidth pulse (7.5-18.5 GHz). After increasing the power of the signal with a 200 W amplifier (Amplifier Research 200T8G18A), the pulse was broadcast and received through a vacuum chamber using a set of microwave horns

Since the most stable conformer of C3H2 is cyclopropenylidene,24,25 vinylidenecarbene will rearrange to that configuration. Likewise allene, an isomer of propyne, can also form vinylidenecarbene,23 and experiments have shown that allene in a discharge26 generates cyclopropenylidene. Another mechanism that could be occurring begins with a stable cyclopropenyl cation, C3H3þ, that can be created from acetylene and ethane.27-29 Cyclopropenylidene could then


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(Amplifier Research AT4004) where it interacted perpendicularly with the sample. The sample was introduced into the chamber at 30 psi through a pulsed nozzle located prior to the discharge. The electric discharge nozzle (Amy Facility, Purdue University) consisted of a set of aluminum electrodes separated by a 1.5 mm Delrin spacer and fit in a Delrin holder. Using a highvoltage pulse modulator (DEI model PVM-4210), an electric pulse of þ300 V and -500 V was applied to the electrodes. The high voltage pulse modulator was controlled using a discharge timing control box with a pulse duration range from 1 to 100 μs that was initiated after opening the pulsed valve. The discharge initiated Penning ionization in Ar to induce chemical dissociation. Discharge conditions were optimized by monitoring the depletion of the 000-101 ground state rotational transition of 2,3-DHF. After the discharge, the sample entered a diffusion-pumped (Varian, VHS-10), low pressure (∼10-5 Torr) vacuum chamber backed by a roots blower and a two-stage mechanical pump. Molecules entering the vacuum chamber expanded supersonically and as a result, the molecules were at very low temperatures. We estimated the rotational temperature to be 2.5 K by fitting the intensities of the b-type Q-branch transitions of cis CPCA using SPCAT.30 After interaction with the electric field in the chamber, the molecular free induction decay was detected, amplified, (Miteq, AMF-6F-06001800-15-10P) and mixed down (Miteq, TB0440LW1) with an 18.9 GHz phase-locked dielectric resonant oscillator (PDRO, Microwave Dynamics PLO-2000-18.9) to allow collection and digitization with a 12 GHz oscilloscope (Tektronix TDS6124C 12 GHz, 40 Gs/s). The low-noise amplifier was protected from the high power polarization pulse by a PIN-diode limiter (Advanced Control Components, ACLM 4619F-C36-1K) and a single-pole singlethrow microwave switch (Advanced Technical Materials, S1517D) operating in series. The rotational free inductive decay was digitized for 10 μs, yielding an experimental resolution of 40 kHz after zero-padding of the time spectrum. Molecular emission was signal averaged in the time domain before applying a Kaiser-Bessel Digital filter and Fourier transforming to the frequency domain. Molecules with a sufficient number of rotational lines present in the spectrum, such as with CPCA, were fit using published rotational constants. Molecules, such as formaldehyde, that only contained a single transition were compared directly to frequencies from literature values. Some of the difference between experimental and published values could be due to unresolved A-E methyl rotor splitting or the lack of centrifugal distortion terms when fitting the data.

ACKNOWLEDGMENT This material was supported by the ACS-PRF #47285-G6 and Purdue University. Special thanks to Dr. Josh Newby and Dr. Tim Zwier for consultation regarding the discharge nozzle.

REFERENCES (1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

(14)

SUPPORTING INFORMATION AVAILABLE A full list of molecules characterized in the discharge experiment and their assigned frequencies. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author: (17)

*To whom correspondence should be addressed. E-mail: dianb@ purdue.edu.


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Kasai, Y.; Obi, K.; Ohshima, Y.; Endo, Y.; Kawaguchi, K. Pure Rotational Spectrum of FeCO. J. Chem. Phys. 1995, 103, 90–95. Travers, M. J.; McCarthy, M. C.; Gottlieb, C. A.; Thaddeus, P. Laboratory Detection of the Ring-Chain Molecule C5H2. Astrophys. J. 1997, 483, L135–L138. Taatjes, C. A.; Hansen, N.; McIlroy, A.; Miller, J. A.; Sensiain, J. P.; Klippenstein, S. J.; Qi, F.; Sheng, L.; Zhang, Y.; Cool, T. A.; et al. Enols Are Common Intermediates in Hydrocarbon Oxidation. Science 2005, 308, 1887–1889. Miller, J. A.; Kippenstein, S. The Recombination of Propargyl Radicals: Solving the Master Equation. J. Phys. Chem. A 2001, 105, 7254–7266. Brown, G. G.; Dian, B. C.; Douglass, K. O.; Geyer, S. M.; Shipman, S. T.; Pate, B. H. A Broadband Fourier Transform Microwave Spectrometer Based on Chirped Pulse Excitation. Rev. Sci. Instrum. 2008, 79, 053103 (053101-053113). Dian, B. C.; Brown, G. G.; Douglass, K. O.; Pate, B. H. Measuring Picosecond Isomerization Kinetics via Broadband Microwave Spectroscopy. Science 2008, 320, 924–928. Pajski, J. J.; Logan, M. D.; Douglass, K. O.; Brown, G. G.; Dian, B. C.; Pate, B. H.; Suenram, R. D. Chirped-Pulse Fourier Transform Microwave Spectroscopy: A New Technique for Rapid Identification of Chemical Agents. Int. J. High Speed Electron. Syst. 2008, 18, 31–45. Wilson, C. L. Reactions of Furan Compounds. VII. Thermal Interconversion of 2,3-Dihydrofuran and Cyclopropane Aldehyde. J. Am. Chem. Soc. 1947, 69, 3002–3004. Lifshitz, A.; Bidani, M. Thermal Reactions of Cyclic Ethers at High Temperatures. 5. Pyrolysis of 2,3-Dihydrofuran Behind Reflected Shocks. J. Phys. Chem. 1989, 93, 1139–1144. Dubnikova, F.; Lifshitz, A. Isomerization of 2,3-Dihydrofuran and 5-Methyl-2,3-Dihydrofuran: Quantum Chemical and Kinetics Calculations. J. Phys. Chem. A 2002, 106, 1026–1034. Kukolich, S. G.; Tanjaroon, C.; McCarthy, M. C.; Thaddeus, P. Microwave Spectrum of o-Benzyne Produced in a Discharge Nozzle. J. Chem. Phys. 2003, 119, 4353–4359. Plusquellic, D. F.; Suenram, R. D.; Mate, B.; Jensen, J. O.; Samuels, A. C. The Conformational Structures and Dipole Moments of Ethyl Sulfide in the Gas Phase. J. Chem. Phys. 2001, 115, 3057–3067. Lifshitz, A.; Bidani, M.; Bidani, S. Thermal Reactions of Cyclic Ethers at High Temperatures. Part 3. Pyrolysis of Tetrahydrofuran Behind Reflected Shocks. J. Phys. Chem. 1986, 90, 3422–3429. Lifshitz, A.; Bidani, M.; Bidani, S. Thermal Reactions of Cyclic Ethers at High Temperatures. III. Pyrolysis of Furan Behind Reflected Shocks. J. Phys. Chem. 1986, 90, 5373–5377. Lifshitz, A.; Bidani, M.; Bidani, S. Thermal Reactions of Cyclic Ethers at High Temperatures. 4. Pyrolysis of 2,5-Dihydrofuran Behind Reflected Shocks. J. Phys. Chem. 1986, 90, 6011–6014. Lifshitz, A.; Laskin, A. Isomerization of 2-Methyl-4,5-Dihydrofuran. Studies with a Single-Pulse Shock Tube. J. Phys. Chem. 1994, 98, 2341–2345. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.;


pubs.acs.org/JPCL

(18)

(19)

(20)

(21)

(22) (23)

(24)

(25)

(26)

(27)

(28)

(29)

(30)

Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. Volltrauer, H. N.; Schwendeman, R. H. Microwave Spectra, Dipole Moments, and Torsional Potential Constants of cisand trans- Cyclopropanecarboxaldehyde. J. Chem. Phys. 1971, 54, 260–267. Bokareva, O. S.; Bataev, V. A.; Godunov, I. A. Structures and Conformational Dynamics of Monomethylated Derivatives of Acrolein: A Quantum-Chemical Study. J. Mol. Struct.: THEOCHEM 2009, 913, 254–264. Durig, J. R.; Brown, S. C.; Kalasinsky, V. F. Low-Frequency Vibrations of Crotonaldehyde. Spectrochim. Acta, Part A 1976, 32A, 807–813. Han, W.; Lovell, T.; Liu, T.; Noodleman, L Density Functional Studies of the Ground- and Excited-State Potential-Energy Curves of Stilbene cis-trans Isomerization. ChemPhysChem 2002, 3, 167–178. Burcat, A. Cracking of Propylene in a Shock Tube. Fuel 1975, 54, 87–93. Mebel, A. M.; Jackson, W. M.; Chang, A. H. H.; Lin, S. H. Photodissociation Dynamics of Propyne and Allene: A View From Ab Initio Calculations of the C3Hn (n = 1-4) Species and the Isomerization for C3H2. J. Am. Chem. Soc. 1998, 120, 5751–5763. Hehre, W. J.; Pople, J. A.; Lathan, W. A.; Radom, L.; Wasserman, E.; Wasserman, Z. R. Molecular Orbital Theory of the Electronic Structure of Organic Compounds. 28. Geometries and Energies of Singlet and Triplet States of the C3H2 Hydrocarbons. J. Am. Chem. Soc. 1976, 98, 4378–4383. Lee, T. J.; Bunge, A.; Schaeffer, H. F., III. Toward the Laboratory Identification of Cyclopropenylidene. J. Am. Chem. Soc. 1985, 107, 137–142. Bogey, M.; Demuynck, C.; Destombes, J. L.; Dubus, H Molecular Structure of Cyclopropenylidene, H-CCC-H from the Millimeter Wave Spectra of Its Isotopomers. J. Mol. Spectrosc. 1987, 122, 313–324. Thaddeus, P.; Vrtilek, J.; Gottlieb, C. Laboratory and Astronomical Identification of Cyclopropenylidene, C3H2. Astrophys. J. 1985, 299, L63–L66. Herbst, E.; Adams, N.; Smith, D. Theoretical Reinvestigation of Hydrocarbon and Cyanoacetylene Abundances in TMC-1. Astrophys. J. 1984, 285, 618–621. Vasile, M. J.; Smolinsky, G. The Chemistry of the Radiofrequency Ethane Discharge. Int. J. Mass Spectrom. Ion Phys. 1976, 21, 263–277. Pickett, H. M. The Fitting and Prediction of VibrationRotation Spectra with Spin Interactions. J. Mol. Spectrosc. 1991, 148, 371–377.


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