Letter pubs.acs.org/JPCL
Chirped-Pulse Fourier Transform Microwave Spectroscopy Coupled with a Flash Pyrolysis Microreactor: Structural Determination of the Reactive Intermediate Cyclopentadienone Nathanael M. Kidwell,† Vanesa Vaquero-Vara,†,§ Thomas K. Ormond,‡,∥ Grant T. Buckingham,‡,∥ Di Zhang,† Deepali N. Mehta-Hurt,† Laura McCaslin,¶ Mark R. Nimlos,‡ John W. Daily,⊥ Brian C. Dian,† John F. Stanton,*,¶ G. Barney Ellison,*,∥ and Timothy S. Zwier*,† †
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, United States National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States ∥ Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, United States ⊥ Center for Combustion and Environmental Research, Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309-0427, United States ¶ Institute of Theoretical Chemistry, Department of Chemistry, University of Texas, Austin, Texas 78712, United States ‡
S Supporting Information *
ABSTRACT: Chirped-pulse Fourier transform microwave spectroscopy (CP-FTMW) is combined with a flash pyrolysis (hyperthermal) microreactor as a novel method to investigate the molecular structure of cyclopentadienone (C5H4O), a key reactive intermediate in biomass decomposition and aromatic oxidation. Samples of C5H4O were generated cleanly from the pyrolysis of o-phenylene sulfite and cooled in a supersonic expansion. The 13C isotopic species were observed in natural abundance in both C5H4O and in C5D4O samples, allowing precise measurement of the heavy atom positions in C5H4O. The eight isotopomers include: C5H4O, C5D4O, and the singly 13C isotopomers with 13C substitution at the C1, C2, and C3 positions. Microwave spectra were interpreted by CCSD(T) ab initio electronic structure calculations and an re molecular structure for C5H4O was found. Comparisons of the structure of this “anti-aromatic” molecule are made with those of comparable organic molecules, and it is concluded that the disfavoring of the “anti-aromatic” zwitterionic resonance structure is consistent with a more pronounced CC/CC bond alternation. SECTION: Spectroscopy, Photochemistry, and Excited States
T
wave spectroscopy to study small molecular intermediates with absorptions in the 63−102 GHz range. Fourier transform microwave (FTMW) spectroscopy has proven itself a powerful technique for the structural characterization of jet-cooled molecules, radicals, and molecular clusters, with its high resolution and generality in detecting polar molecules. With the advent of CP-FTMW spectroscopy pioneered by Pate and co-workers,4−6 rotational spectroscopy has undergone a revolutionary refinement. By sweeping a wide frequency range in each high microwave power pulse, the entire spectrum is recorded with each pulse, opening up new types of measurements on transient species created by laser excitation, electric discharge, or laser desorption.7−11 Figure 1a shows the flash pyrolysis microreactor used in the present study, which was originally developed by Chen and co-
he chemical complexity of hydrocarbon fuels and the fastexpanding list of potential plant-derived biofuels offers a challenge to the scientific community seeking to provide a molecular-scale understanding of their combustion. The development of accurate combustion models stands on a foundation of experimental data on the kinetics and product branching ratios of individual reaction steps. As models attempt to predict the combustion behavior of more complex fuels, more sophisticated spectroscopic tools and methodologies must be developed and refined to selectively detect and characterize the widening array of fuel components and the reactive intermediates they generate upon combustion.1,2 This Letter describes such a development with the combination of two state-of-the-art techniques using a heated microreactor for flash pyrolysis as a source for generating key pyrolysis intermediates and detecting them with high sensitivity and selectivity using chirped-pulse Fourier transform microwave spectroscopy. We present a complementary strategy to what the Field group3 has adopted in using chirped-pulse millimeter© XXXX American Chemical Society
Received: May 30, 2014 Accepted: June 2, 2014
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properties of the plant. One of the major bottlenecks to incorporating biomass-derived fuels into the current petroleum infrastructure is the difficulty of rupturing the strong linkages of lignin to extract the energy-rich cellulose.22 Despite much research in the thermal decomposition pathways of lignin, a detailed molecular understanding is still far from complete. C5H4O is a particularly important reactive intermediate that is not only ubiquitous in biomass decomposition, but is also abundant in the oxidation of aromatic compounds.17,23−28 However, the C5H4O molecule is of great interest in its own right because it violates the 4n + 2 rule of aromaticity. Figure 2a
Figure 1. (a) Flash pyrolysis microreactor assembly used to decompose parent samples entrained in rare gas. (b) Interrogation of the rotationally cold (2.5 K) parent and thermal products occurs downstream with chirped-pulse Fourier transform microwave spectroscopy.
Figure 2. (a) Resonance structures of C5H4O illustrating four π electrons in the ring. (b) Equilibrium (re) structure of C5H4O derived from experimental rotational constants of eight isotopomers corrected at the CCSD(T) level of theory for vibrational effects.
workers12,13 and has been further refined.14 Recently, such a heated microreactor has been used to investigate the thermal decomposition pathways of model biomass and lignin compounds using a combination of vacuum ultraviolet photoionization time-of-flight mass spectroscopy and Fourier transform infrared interrogation of deposited samples from the microreactor in matrix isolation.15−17 Central to these studies are the attractive advantages that flash pyrolysis affords. First, high densities of reactive intermediate products may be generated and cooled in a pulsed supersonic expansion. Second, a short gas pulse residence time (50−200 μs) in the microreactor helps to suppress product recombination, in contrast to conventional pyrolysis procedures. Used in tandem, CP-FTMW spectroscopy and the flash pyrolysis microreactor embody a powerful scheme capable of providing simultaneous characterization of multiple stable molecules and free radical populations at well-defined reactor wall temperatures. In particular, we show its utility to determine a high-precision molecular geometry of the reactive intermediate cyclopentadienone (C5H4O), which is found pervasively in the unimolecular decomposition pathways of lignin.17 Lignin is the second-most abundant biopolymer found in nature, acting as a major component in the cell walls of plants to give them structural rigidity.18−21 Varying the relative abundances and linkages of the three monomers (e.g., pcoumaryl, coniferyl, and sinapyl alcohol), therefore, dictates the
shows the resonance hybrids that describe C5H4O.29 The zwitterionic structure will resemble cyclobutadiene in that it is a conjugated ring with 4π electrons. As a consequence, it was predicted29 that C5H4O would be very unstable and difficult to isolate. C5H4O has been the subject of theoretical30−32 and experimental33−35 interest due to its intriguing electronic structure. Recent experiments have detected and recorded the infrared spectrum of C5H4O under matrix isolation conditions, produced by the pyrolysis of model lignin compounds17 and o-phenylene sulfite.36 Additionally, the microwave spectrum and dipole moment (3.132 D) of the main 12C C5H4O isotopic species was recorded by Brown and co-workers.35 We extend significantly the isotopic spectroscopy of this important combustion intermediate by recording the rotational spectra of eight isotopomers, which allows for the determination of the precise equilibrium molecular structure shown in Figure 2b, with atomic positions determined to within ± 0.001 Å. Herein, we employed the novel combination of CP-FTMW spectroscopy and the flash pyrolysis microreactor as a means to generate C5H4O from o-phenylene sulfite (C6H4O2SO). The sample, entrained as a dilute mixture in high pressure argon, is pulsed into the microreactor assembly (Figure 1a), which is 2202
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transitions. Figure 3c demonstrates the superb agreement between the simulated and experimental results. The exceptional resolution afforded by the FTMW method (40 kHz linewidths at frequencies of 10 GHz) is easily sufficient to resolve transitions from multiple species, as illustrated in Figure 3c. Furthermore, as can be seen in Figure 4, rotational transitions attributable to the 13C isotopomers observed in
heated to 1000 K to initiate decomposition. The microreactor assembly was modeled after the Boulder design. Pyrolysis products subsequently depart the reactor and are immediately cooled to rotational temperatures of about 2.5 K by the supersonic expansion, as determined from fitting the rotational spectral intensities.37 The CP-FTMW instrument has been described in detail elsewhere.38 A chirped microwave pulse from 7.5−18.5 GHz interacts with the effluent from the microreactor, producing a net polarization in the sample that consequently undergoes free induction decay (FID), which is collected by a receiving horn (Figure 1b). A detailed account of the experimental methods is given in Supporting Information. Figure 3a and b show the broadband rotational spectra of the C5H4O and C5D4O isotopic species from the pyrolysis of
Figure 3. 7.5−18.5 GHz rotational spectra of (a) C5H4O and (b) C5D4O from the pyrolysis of different o-phenylene sulfite samples. The simulated spectra of C5H4O and C5D4O are shown as inverted sticks below the rotational spectra. (c) An approximate 40 MHz window of band B (202−101) from the C5H4O broadband spectrum, highlighting the quality of the fit. All other transitions in this frequency range are due to the precursor.
Figure 4. 200 MHz portion of the 11 GHz spectrum of (a) C5H4O and (b) C5D4O using o-phenylene sulfite as the precursor. Calculated frequencies and intensities of the (a) 202−101 and (b) 313−212 transitions are due to the 12C and 13C isotopic species in natural abundance shown as sticks below the spectra. All other transitions in this frequency range are due to the precursor.
o-phenylene sulfite and its deuterated analog, respectively, with measured frequencies in Supporting Information. The experimental spectra have contributions from the precursor and from C5H4O 13C isotopic species. Therefore, the spectra were deconvoluted by using ab initio rotational constants of each isotopomer as input into the JB95 spectral fitting program37 in order to simulate each isotope’s rotational spectrum. The simulated spectra of C5H4O and C5D4O are given as inverted sticks below their experimental spectra. All other transitions in the frequency range belong to the corresponding precursor. The prominent bands belonging to C5H4O and C5D4O are within a factor of 2 in intensity of their parent
natural abundance are also readily observed, with correct frequencies and relative intensities consistent with their assignment to each isotopomer. A single rotational transition of the 12C isotopomer of both C5H4O and C5D4O is identified in red (Figure 4a) and purple (Figure 4b) in the computed spectrum below experiment, respectively. Their relative intensities are 100-fold greater than the transitions marked as belonging to the 13C isotopomers. There are three unique carbon atom sites in C5H4O, which are color-coded in Figure 4. Thus, one expects approximately 1% intensity of 2203
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Table 1. Spectroscopic Constantsa of Isotopic C5H4Ob
a
In MHz. bThe color coding of the molecular structures matches the individual 13C positions from Figure 4. cExperimental data from ground vibrational state, “Set B”. dWith vibrational contributions calculated with CCSD(T)/ANO0, “Set I”. eWith electronic contribution. fNo electronic contribution. gInertial defect in amu Å2.
the 12C parent peak for the carbonyl 13C transition (labeled blue) and 2% intensity for the other two 13C rotational transitions (green and brown). Note the excellent agreement between the simulated and experimental frequencies and relative intensities. Due to the fact that we were able to observe 13C in natural abundance at all positions on the ring, a considerable amount of information is now at hand regarding the isotopic spectroscopy of C5H4O. Consequently, one can attempt a structural determination by means of fitting the rotational constants of the eight isotopic species observed to principal axis moments of inertia. The simplest way to do this is to make the approximation that the rotational constants are inversely proportional to the principal moments of inertia of the corresponding isotopomer and then adjust the molecular structure to best fit the latter in a least-squares sense. A structure determined in this way is called an “effective” (r0) geometry, but such a structure does not have a rigorous quantum-mechanical definition (see Supporting Information). Another approach tied to rotational spectroscopy is the socalled rs structure that results from determining the principal axis Cartesian coordinates of an atom by monitoring the change in rotational constants brought about when that atom is isotopically substituted. However, these “substitution” structures also lack a well-defined and rigorous geometrical interpretation. Perhaps the most appealing definition of molecular structure is that associated with the equilibrium (re) geometry, which corresponds to the positions of the nuclei at the minimum of the Born−Oppenheimer (BO) potential energy surface. Such geometries have an unambiguous interpretation and moreover are not isotope-dependent; these serve as the best structures to associate with a particular molecule as well as to allow fairly detailed comparisons of structures to those of chemically related species. However, there is no way to measure such a structure directly. Nevertheless, the re structure may be inferred by means of perturbation theory applied to the experimentally determined rotational constants. The information needed for this determinationin addition to the ground state rotational constantsare the corrections to the ground state constants that arise from effects of zero-point motion. In the absence of (Coriolis) resonances, this can be determined to lowest order in perturbation theory from the rotation−vibration interaction
constants (αi(A,B,C)), which describe the effect of excitation in the vibration mode νi on the rotational constants (A,B,C). However, an experimental determination of these constants (all are needed) is possible only for very small molecules, and it is now widely recognized that the vibrational correction to the rotational constants can be computed quite accurately with high-level quantum chemical methods. The procedure for doing such a calculation is fully documented39 and has been followed here. The vibrational contributions to the rotational constants of the isotopic species of C5H4O studied here were computed at the CCSD(T) level of theory,40 using the ANO0 basis set of Almlöf and Taylor.41,42 The electronic contributions to the corrections were computed at the same level of theory using the theoretical framework outlined by Flygare.43 In addition, the equilibrium geometries of C5H4O were computed at the CCSD(T) level of theory with a number of basis sets (ANO0, ANO1, ANO2, cc-pCVTZ, and cc-pCVQZ; the first three in the frozen core approximation, and the last two in all-electron calculations) to monitor convergence and core correlation effects. Those obtained with the cc-pCVTZ and cc-pCVQZ are the most accurate. All electronic structure calculations were done with the CFOUR suite of programs.44 Estimated equilibrium rotational constants are listed in Table 1, along with the uncorrected experimental constants and the associated inertial defects. It is particularly striking that the inertial defects (which are nonzero for the planar C5H4O molecule only because of vibrational contributions to the rotational constants) drop very close to the value of zero that must correspond to the exact equilibrium structure when the calculated corrections are applied. This behavior is typically seen in well-behaved systems, where the rotational constants of a fairly rigid planar species are corrected for vibrational effects in the manner done here.45 The fact that the calculated inertial defects are so small is a good indication of the quality of the calculations and, correspondingly, the fidelity of the equilibrium rotational constants (see Supporting Information). Efforts to refine the structure based on the observed isotopic species were ultimately not successful without the imposition of constraints because of strong parameter correlation between the CH distances and HCC bond angles. Although isotopic spectroscopy of monodeuterated species would likely alleviate this problem, the low natural abundance of deuterium and the 2204
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Table 2. Molecular Structure Parameters of C5H4Oa
a
comment
r(CO)
r(OC C)
r(C C)
CH fixedb CH fixedc HCC angles fixedb HCC angles fixedc ANO0/CCSD(T)d ANO1/CCSD(T)d ANO2/CCSD(T)d cc-pCVTZ/CCSD(T)d cc-pCVQZ/CCSD(T)d
1.2087 1.2083 1.2088 1.2084 1.22 1.213 1.211 1.211 1.2083
1.5016 1.5019 1.5015 1.5018 1.517 1.509 1.507 1.507 1.5038
1.3385 1.3389 1.3389 1.339 1.354 1.344 1.342 1.342 1.3389
r(OCC H)
1.0758 1.0762 1.085 1.079 1.078 1.0776 1.0766
r(CCC H)
∠(OC C)
∠(CC C)
∠(HC CO)
∠(HC CC)
107.38 107.35 107.38 107.35 107.5 107.4 107.3 107.4 107.36
123.6 123.59
126.76 126.73
1.0789 1.0785 1.087 1.08 1.08 1.0796 1.0787
127.03 127.03 127.02 127.02 127.1 127.1 127 127.1 127.05
123.8 123.7 123.7 123.7 123.71
126.9 126.8 126.8 126.8 126.76
Given in Å and degrees. bFit to “Set B”. cFit to “Set I”. dre structure.
fact that C5H4O is a reactive intermediate requiring laboratory synthesis would make such an investigation difficult and expensive. However, this parameter correlation was completely removed by fixing either the CH distances or the HCC angles to distances calculated at high levels of ab initio theory. As can be seen in the table, the calculations done here are sufficient to suggest an accuracy of at least 0.001 Å in the equilibrium CH distances, or 0.1 degrees in the HCC angles. By constraining either of these two parameter pairs, the structural refinement was straightforward and resulted in the two sets of distances and angles shown in Table 2. The quoted values and error bars in Figure 2 represent our best estimate of the actual equilibrium structure and a (conservative) estimate of the remaining uncertainty. Hence, the equilibrium structure of the C5H4O molecule, a reactive intermediate of considerable interest in combustion science, is now among the most precisely known, a situation similar to that recently achieved for the H2COO “Criegee intermediate”.9 Indeed, the combination of rapid advances in microwave spectroscopy and computational chemistry suggests that highly accurate equilibrium structures will become increasingly common for such “unstable” molecules and radicals in the future, a development which potentially will provide a wealth of information for structural chemists to analyze. Armed with this accurate equilibrium structure, it is appropriate to return to the issue of this molecule’s instability, commonly associated with its “anti-aromaticity.”29 The notion of aromaticity of conjugated rings is deeply rooted in organic chemistry. Rings with (4n + 2) π electrons are termed46 aromatic, whereas those with 4n π electrons are deemed antiaromatic47 or nonaromatic; the differences between these classes is not so precise.46 Aromatic compounds are conjugated rings with a special number of π electrons, (4n + 2), that confer an unusual stability on the molecule. Aromatic species (such as benzene) are relatively unreactive and tend to have equal CC bond lengths. Antiaromatic compounds48 (such as cyclobutadiene) with 4n π electrons are exceptionally unstable in contrast to nonaromatic molecules (such as cyclooctatetraene), which also have 4n π electrons. One might say that the π bonds “repel each other” in the antiaromatic molecules, whereas those of nonaromatic species do not interact. The structure of C5H4O can be compared to other organic species (Figure 5). The structures shown there are taken from a number of sources, and are a collection of re, r0, and rs geometries. It is difficult to draw conclusions about the effects of “anti-aromaticity” on structure due to the rather small effects that arise. For example, the CC bond lengths in C5H4O are essentially the same as the CC bond in cyclopentent-2-
Figure 5. Comparison of the experimental structures (Å) and electric dipole moments of C5H4O and several related structures. The nature of the structural determinations (re, rs, or r0) as well as the references, can be found in the Supporting Information.
en-1-one and perhaps slightly shorter than those in fulvene and cyclopentadiene. Similarly, the carbonyl distance is not sufficiently different from those in acetone, cyclopentanone and cyclopentent-2-en-1-one to be significant in the context of this imperfect comparison. However, there is one clear structural marker that can be identified from the comparison. In Figure 2a, the antiaromatic zwitterionic resonance structure might be expected to lead to a 5-membered ring with roughly equal CC bond lengths commensurate to those in C5H5+ but perturbed by the carbonyl group. On the other hand, the conjugated ketone in Figure 2a would be expected to lead to a more pronounced CC/CC bond alternation, and this alternation is indeed apparent in Figure 5. Specifically, this perspective appears to effectively rationalize patterns of the CC and C-C distances in C5H4 O, fulvene, and cyclopentadiene seen in Figure 5. The latter two molecules have bond alternations of 0.121 and 0.124 Å for the CC and CC bond opposite the carbon that lies on the C2 symmetry axis, whereas the corresponding alternation in C5H4O is 0.158, a difference that is significant even given the different types of “structures” being compared. Although this seems to be a clear, qualitative indicator, any efforts to draw more quantitative conclusions would require that accurate re structures be obtained for all species in the comparison set. In summary, we have demonstrated the novel combination of CP-FTMW spectroscopy with a flash pyrolysis microreactor, which is a powerful method for the rapid and sensitive 2205
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copy for Studies of Pyrolysis Reactions. Phys. Chem. Chem. Phys. 2014, DOI: 10.1039/C1033CP55352C. (4) 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. (5) Steber, A. L.; Harris, B. J.; Neill, J. L.; Pate, B. H. An Arbitrary Waveform Generator Based Chirped-Pulse Fourier-Transform Spectrometer Operating from 260 to 295 GHz. J. Mol. Spectrosc. 2012, 280, 3−10. (6) Neill, J. L.; Harris, B. J.; Steber, A. L.; Douglass, K. O.; Plusquellic, D. F.; Pate, B. H. Segmented Chirped-Pulse FourierTransform Submillimeter Spectroscopy for Broadband Gas Analysis. Opt. Express 2013, 21, 19743−19749. (7) Prozument, K.; Shaver, R. G.; Ciuba, M. A.; Muenter, J. S.; Park, G. B.; Stanton, J. F.; Guo, H.; Wong, B. M.; Perry, D. S.; Field, R. W. A New Approach Toward Transition State Spectroscopy. Faraday Discuss. 2013, 163, 33−57. (8) Karunatilaka, C.; Shirar, A. J.; Storck, G. L.; Hotopp, K. M.; Biddle, E. B.; Crawley, R.; Dian, B. C. Dissociation Pathways of 2,3Dihydrofuran Measured by Chirped-Pulse Fourier Transform Microwave Spectroscopy. J. Phys. Chem. Lett. 2010, 1, 1547−1551. (9) McCarthy, M. C.; Cheng, L.; Crabtree, K. N.; Martinez, O.; Nguyen, T. L.; Womack, C. C.; Stanton, J. F. The Simplest Criegee Intermediate (H2CO-O): Isotopic Spectroscopy, Equilibrium Structure, and Possible Formation from Atmospheric Lightning. J. Phys. Chem. Lett. 2013, 4, 4133−4139. (10) Sanz, M. E.; Lessari, A.; Pena, M. I.; Vaquero, V.; Cortijo, V.; Lopez, J. C.; Alonso, J. L. The Shape of b-Alanine. J. Am. Chem. Soc. 2006, 128, 3812−3817. (11) Bird, R. G.; Vaquero-Vara, V.; Zaleski, D. P.; Pate, B. H.; Pratt, D. W. Chirped-Pulsed FTMW Spectra of Valeric Acid, 5-Aminovaleric Acid, and d-Valerolactam: A Study of Amino Acid Mimics in the Gas Phase. J. Mol. Spectrosc. 2012, 280, 42−46. (12) Blush, J. A.; Clauberg, H.; Kohn, D. W.; Minsek, D. W.; Xu, Z.; Chen, P. Photoionization Mass and Photoelectron-Spectroscopy of Radicals, Carbenes, and Biradicals. Acc. Chem. Res. 1992, 25, 385−392. (13) Kohn, D. W.; Clauberg, H.; Chen, P. Flash Pyrolysis Nozzle for Generation of Radicals in a Supersonic Jet Expansion. Rev. Sci. Instrum. 1992, 63, 4003−4005. (14) Zhang, X.; Friderichsen, A. V.; Nandi, S.; Ellison, G. B.; David, D. E.; McKinnon, J. T.; Lindeman, T. G.; Dayton, D. C.; Nimlos, M. R. Intense, Hyperthermal Source of Organic Radicals for MatrixIsolation Spectroscopy. Rev. Sci. Instrum. 2003, 74, 3077−3086. (15) Vasiliou, A. K.; Piech, K. M.; Reed, B.; Zhang, X.; Nimlos, M. R.; Ahmed, M.; Golan, A.; Kostko, O.; Osborn, D. L.; David, D. E.; Urness, K. N.; Daily, J. W.; Stanton, J. F.; Ellison, G. B. Thermal Decomposition of CH3CHO Studied by Matrix Infrared Spectroscopy and Photoionization Mass Spectroscopy. J. Chem. Phys. 2012, 137, 164308. (16) Vasiliou, A. K.; Kim, J. H.; Ormond, T. K.; Piech, K. M.; Urness, K. N.; Scheer, A. M.; Robichaud, D. J.; Mukarakate, C.; Nimlos, M. R.; Daily, J. W.; Guan, Q.; Carstensen, H. H.; Ellison, G. B. Biomass Pyrolysis: Thermal Decomposition Mechanisms of Furfural and Benzaldehyde. J. Chem. Phys. 2013, 139, 104310. (17) Scheer, A. M.; Mukarakate, C.; Robichaud, D. J.; Nimlos, M. R.; Ellison, G. B. Thermal Decomposition Mechanisms of the Methoxyphenols: Formation of Phenol, Cyclopentadienone, Vinylacetylene, and Acetylene. J. Phys. Chem. A 2011, 115, 13381−13389. (18) Boerjan, W.; Ralph, J.; Baucher, M. Lignin Biosynthesis. Annu. Rev. Plant Biol. 2003, 54, 519−546. (19) Dorrestijn, E.; Laarhoven, L. J. J.; Arends, I. W. C. E.; Mulder, P. The Occurrence and Reactivity of Phenoxyl Linkages in Lignin and Low Rank Coal. J. Anal. Appl. Pyrol. 2000, 54, 153−192. (20) Ralph, J. Hydroxycinnamates in Lignification. Phytochem. Rev. 2010, 9, 65−83. (21) Parthasarathi, R.; Romero, R. A.; Redondo, A.; Gnanakaran, S. Theoretical Study of the Remarkably Diverse Linkages in Lignin. J. Phys. Chem. Lett. 2011, 2, 2660−2666.
characterization of pyrolysis products at well-defined temperatures. In particular, we employ the method as a means to structurally characterize a key reactive intermediate, cyclopentadienone, which is ubiquitous in the unimolecular decomposition of lignin and in aromatic oxidation. As the only requirement for product detection is for the species of interest to possess a permanent dipole moment, the technique shows promise for application to a wider range of relevant areas including biofuel pyrolysis dynamics and other systems relevant to combustion science. Many pyrolysis species typically have substantial dipole moments, therefore making them amenable to sensitive detection by microwave spectroscopy, and the exceptional resolution afforded by the method avoids spectral overlap.
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ASSOCIATED CONTENT
S Supporting Information *
Includes a table of the measured rotational lines of isotopic C5H4O, experimental methods, footnotes, full citation for reference 44, and references for Figure 5. This material is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Authors
*
[email protected] *
[email protected] *
[email protected] Present Address §
(V.V.-V.) Laboratoire Interactions, Dynamique et Lasers (LIDYL), CEA, Saclay, 91191 Gif-sur-Yvette, France.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS N.M.K., V.V.V., D.Z., and T.S.Z. gratefully acknowledge support from the Department of Energy Basic Energy Research, Chemical Sciences Division under Grant No. DE-FG0296ER14656. N.M.K., V.V.V., D.Z., and T.S.Z. also thank the Jonathan Amy Facility for Chemical Instrumentation at Purdue University for personnel and equipment support. J.W.D., J.F.S., and G.B.E. would like to acknowledge support from the National Science Foundation (CHE-0848606 and CHE1112466). J.F.S. also acknowledges support from the Robert A. Welch Foundation (Grant F-1283) and the United States Department of Energy, Basic Energy Sciences (DE-FG0207ER15884). M.R.N. is supported by the United States Department of Energy’s Bioenergy Technology Office, under Contract No. DE-AC36-99GO10337 with the National Renewable Energy Laboratory.
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REFERENCES
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