Article pubs.acs.org/JPCA
Polarized Matrix Infrared Spectra of Cyclopentadienone: Observations, Calculations, and Assignment for an Important Intermediate in Combustion and Biomass Pyrolysis Thomas K. Ormond,†,‡ Adam M. Scheer,§ Mark R. Nimlos,† David J. Robichaud,† John W. Daily,∥ John F. Stanton,⊥ and G. Barney Ellison*,‡ †
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 § Combustion Research Facility, Sandia National Laboratory, P. O. Box 969, Livermore, California 94551-0969, United States ∥ Center for Combustion and Environmental Research, Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309-0427, United States ⊥ Institute for Theoretical Chemistry, Department of Chemistry, University of Texas, Austin, Texas 78712, United States ‡
S Supporting Information *
ABSTRACT: A detailed vibrational analysis of the infrared spectra of cyclopentadienone (C5H4O) in rare gas matrices has been carried out. Ab initio coupled-cluster anharmonic force field calculations were used to guide the assignments. Flash pyrolysis of o-phenylene sulfite (C6H4O2SO) was used to provide a molecular beam of C5H4O entrained in a rare gas carrier. The beam was interrogated with time-of-flight photoionization mass spectrometry (PIMS), confirming the clean, intense production of C5H4O. Matrix isolation infrared spectroscopy coupled with 355 nm polarized UV for photoorientation and linear dichroism experiments was used to determine the symmetries of the vibrations. Cyclopentadienone has 24 fundamental vibrational modes, Γvib = 9a1 ⊕ 3a2 ⊕ 4b1 ⊕ 8b2. Using vibrational perturbation theory and a deperturbation−diagonalization method, we report assignments of the following fundamental modes (cm−1) in a 4 K neon matrix: the a1 modes of X̃ 1A1 C5H4O are found to be ν1 = 3107, ν2 = (3100, 3099), ν3 = 1735, ν5 = 1333, ν7 = 952, ν8 = 843, and ν9 = 651; the inferred a2 modes are ν10 = 933, and ν11 = 722; the b1 modes are ν13 = 932, ν14 = 822, and ν15 = 629; the b2 fundamentals are ν17 = 3143, ν18 = (3078, 3076) ν19 = (1601 or 1595), ν20 = 1283, ν21 = 1138, ν22 = 1066, ν23 = 738, and ν24 = 458. The modes ν4 and ν6 were too weak to be detected, ν12 is dipole-forbidden and its position cannot be inferred from combination and overtone bands, and ν16 is below our detection range (CO bond (Scheme 2) leaves four π electrons delocalized in the planar ring, inducing an antiaromatic instability.19,20
II. EXPERIMENTAL SECTION A convenient thermal source of cyclopentadienone is ophenylene sulfite, as has been described previously.30 Only a brief description of the synthesis is given here. Catechol, thionyl chloride, CS2, and pyridine were obtained from Sigma-Aldrich. No purification of the reagents was done. The thionyl chloride and catechol were combined in CS2 solvent with pyridine to remove the HCl byproduct. The CS2 was removed from the phenylene sulfite product by vacuum distillation, and the product was stored in refrigeration. o-Phenylene sulfite is a yellow liquid at room temperature with a low vapor pressure. To ensure a sufficient gas-phase concentration in the rare gas carrier (He, Ne, and Ar), the sample was placed in a small quartz capillary and inserted into a sample heater located immediately before the pulsed valve and maintained at 45−55 °C. The valve and other internal component temperatures were monitored and kept warm to avoid condensation. The thermal decomposition mechanism of o-phenylene sulfite is shown in Scheme 3. The matrix isolated cyclopentadienone was generated by flash pyrolysis of o-phenylene sulfite, Scheme 3. A microtubular reactor31−37 has been adopted as a furnace to pyrolyze complex organic molecules. This unique experimental apparatus features a small (1 mm i.d. × 3.75 cm length) SiC pulsed reactor equipped with advanced diagnostics. The reactor can be heated
Scheme 2
Earlier theoretical studies21−23 of a series of conjugated, cyclic ketones predicted that the lowest energy spin allowed transition of C5H4O would be ππ*, X̃ 1A1 → à 1B2 in the ring, rather than the typical nπ* X̃ 1A1 → à 1A2 in the carbonyl group. The UV/vis spectrum24,25 of C5H4O showed UV band maxima at 195 and 360 nm, in good agreement with the predicted ππ* low-energy transitions. The photoelectron spectrum26,27 of C5H4O has been reported and assigned the symmetry of the ground state cation as X̃ 2A2 (π), while the first excited state was assigned as à 2B2 (n). The rotational28 and vibrational24,25,29 spectroscopies of C5H4O have been studied using various pyrolytic/photolytic precursors and detection/isolation schemes. Comparative analyses confirmed the presence of C5H4O. However, an assignment of the vibrational frequencies of C5H4O has not previously been reported. In the following, the infrared absorbance spectra of C5H4O frozen in solid rare gas matrices are reported, and high-level ab initio anharmonic force field calculations are used to help assign the fundamental vibrations, Fermi resonances, and selected overtone and combination bands. Furthermore, we
Scheme 3
B
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deposition to identify background matrix absorptions. Roughly 100−300 Torr of the mixture was deposited for both Ne and Ar matrix isolation IR studies. The initial source pressure behind the pulsed valve was 1000 Torr. Spectral comparison of a temperature series of the pyrolysis products, the room temperature parent, and heated blank neon/argon absorption features served to identify absorption features arising from cyclopentadienone. Polarized laser light for photoorientation experiments was produced by the third harmonic of a Surelite Continuum Nd:YAG laser at 355 nm. Polarized IR radiation was generated using a Nicolet rotatable ZnSe crystal IR polarizer. Defocused UV irradiation (355 nm) bathed the CsI window diameter for 1−5 h with 5−25 mJ pulse−1 Y-polarized (horizontal) light, which photooriented the sample. An infrared spectrum was then collected using Y-polarized IR radiation and then again using Z-polarized IR radiation. The difference of these spectra was calculated and used to analyze the symmetry of the vibrational bands.
to about 1700 K without sustaining damage. The residence time in the reactor is short, 50−200 μs, allowing for study of the initial pyrolysis products. The diagnosticsmatrix isolation/infrared spectroscopy and photoionization mass spectrometryare used together to identify species in the reactor output. This includes organic radicals and other highly reactive intermediates as well as closed-shell, unreactive species.38,39 Fixed-Frequency 118.2 nm (10.487 eV) Photoionization Mass Spectrometry. Photoionization mass spectrometry was used to monitor the extent of pyrolysis of the o-phenylene sulfite in the heated microtubular reactor. Molecular beam optimization was done via fixed-frequency 118.2 nm (10.487 eV) time-of-flight photoionization mass spectrometry (PIMS) using a Parker General solenoid actuated pulsed valve (30 Hz, 1.4 ms open time) upstream from the 1 mm i.d. SiC heated microtubular reactor. Typical precursor concentrations through the pulsed valve were 0.1% in He carrier gas,37 and the pressure behind the pulsed valve was 1000 Torr in a 3 L reservoir. The pyrolyzed He molecular beam containing fragments of the ophenylene sulfite decomposition emerged from the SiC reactor and expanded into 10−6 Torr. A skimmer plate was used to select the component of the molecular beam with unidirectional velocity to quench collisions. The ninth harmonic of a Nd:YAG (30 Hz, 10 ns pulse width) was generated by frequency tripling the third harmonic in a Xe/Ar gas mixture.40 Ions were generated in a constant static electric field and accelerated into a Jordan reflectron time-of-flight tube. The molecular beam pulse was synchronized to the laser Q-switch, and the detector signal was time gated. Spectra were typically composite averages of 1000 scans. Time-of-flight to mass calibration was done with propylene and NO. The typical full width at half-maximum (fwhm) of our mass spectra was 0.070 amu. The 118 nm radiation has sufficient energy to ionize most molecules of interest; however, acetylene41 and carbon monoxide42 have ionization potentials above our photon energy of 10.487 eV. Infrared Spectroscopy in Rare Gas Matrices. For infrared studies the molecular beam containing the parent ophenylene sulfite was diluted in argon or neon and pulsed into the SiC microtubular reactor, and the products were deposited onto a cryogenic CsI window. A solenoid pulsed valve (Parker General) before the reactor was used to inject a gas mixture of approximately 0.1% o-phenylene sulfite in carrier gas at a deposition rate of roughly 1 Torr min−1 from a 3 L reservoir. The valve was operated at 10 Hz with 500 μs opening time. Gas exiting the reactor underwent free expansion that cooled the beam and quenched reactive chemistry allowing the preservation of radicals and other reactive intermediates. The cooled salt window was interfaced with a two-stage closed-cycle helium cryostat (APD) for argon matrix isolation (20 K) or interfaced with a two-stage closed-cycle helium cryostat (Janis Research) for neon matrix isolation (4 K). The temperature was monitored by a silicon diode thermostat and controlled by a 50 Ω Nichrome heater for annealing of the matrix, which was done after every deposition (roughly 12 K). The decomposition products were interrogated with infrared radiation from a Nicolet 6700 FTIR spectrometer. An MCT B detector with a range of 400−4000 cm−1 was used. The FTIR spectra were composite averages of 1000 scans and were taken with 0.125 cm−1 resolution. About 50−100 Torr of the carrier gas was deposited from a 3 L reservoir at the pyrolysis temperature of the experiment prior to cyclopentadienone
III. CALCULATIONS The equilibrium structure and harmonic force field of C5H4O were calculated at the CCSD(T) level of theory43 with the ANO1 basis44 of Taylor and Almlöf.45 A similar calculation was then done with the smaller, computationally tractable, ANO0 truncation44 of the ANO basis. Using rotational constants, harmonic frequencies, and Coriolis coupling constants from the ANO1 calculation and cubic and quartic force constants from the ANO0 computation (which were obtained by numerical differentiation of analytically computed gradients), second-order vibrational perturbation theory46,47 (VPT2) was used to compute the positions and infrared absorption intensities of the vibrational levels. This hybrid procedure using two basis sets has been used by us before and proved quite effective in assigning the infrared spectrum of methyl nitrate.48 States participating in Fermi resonances were unambiguously identified using the harmonic derivatives approach of ref 49, and construction and subsequent diagonalization of the deperturbed Hamiltonian followed standard approaches.50 As it is known that the principal source of error in an anharmonic molecular force field in dimensionless normal coordinates is in the harmonic contribution, it might be expected that the difference between experimental and calculated VPT2 frequencies is well approximated by a corresponding error in the associated harmonic frequencies. That is, the contribution of anharmonicity to the observed level position is more accurate than the harmonic frequencies themselves, which can be used to advantage in the following way. Suppose the calculated position of a particular two quantum transition, νl + νm, is αlm which is the sum of anharmonic and harmonic contributions, δlm and ωl + ωm, respectively. If the exact positions of the fundamentals νl and νm are known, then the actual position of the combination level can be estimated as δlm + [ωl + νlobs − νlVPT] + [ωm + νmobs − νmVPT], where the last two bracketed terms represent improved estimates to the harmonic frequencies of modes l and m. Such procedures have been used successfully in the past, both by others51 and by our group,46 and provided a means both for estimating overtone and combination positions (as demonstrated) and, by a simple construction, for inferring the position of a fundamental, given information regarding the positions of another fundamental and the associated combination level. We C
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will term such estimates as harmonically adjusted VPT2 (HAVPT2), as was done in ref 46.
IV. RESULTS Fixed-Frequency 118.2 nm (10.487 eV) Photoionization Mass Spectrometry. Figure 2 is the 118.2 nm PIMS
Figure 3. Neon matrix infrared spectrum of the pyrolysis products of C5H4O generated from the flash pyrolysis of o-phenylene sulfite at the temperatures shown, confirming the production of CO as well as the production of vinylacetylene and acetylene at high temperatures. Green traces are of neon passed through the SiC microtubular reactor at 1500 K and deposited onto the 4 K CsI window.
absorption features grew in. The ν3(HCCH) was observed with its well known Dennison−Darling resonance,56,57 and the acetylenic C−H stretch, ν1(HCCHCHCH2), was also observed as a doublet in neon. Careful annealing revealed that the splitting of ν1(HCCHCHCH2) was a matrix site effect. The absorption features here were compared to an authentic sample of vinylacetylene, and the splitting observed was identical to that of the authentic sample. It is observed that the relative intensities of vinylacetylene and acetylene changed as the temperature was increased. These data indicate that the ratio of acetylene/vinylacetylene increases with increasing temperature. The product branching ratios of cyclopentadienone pyrolysis as a function of temperature is a topic of ongoing research. Linear Dichroism in an Argon Matrix. Linear dichroism experiments exploit two facts: the C5H4O molecules are fixed, or otherwise unable to freely translate or rotate in space, and the probability of photon absorption is dependent upon the angle between the irradiation electric field vector and the transition dipole moment of the molecule as ⟨( μ⃗ ·E ⃗ )2⟩, where the brackets indicate spatial averaging. If the polarization of the photon and the molecular transition dipole moment are aligned parallel, then the probability amplitude for the radiation field coupling to the normal mode of vibration is at a maximum, while if they are orthogonal to each other, the probability amplitude is at a minimum. Linear dichroism experiments bridge these two ideas by photoorienting a spatially and rotationally fixed cryogenically frozen matrix sample with linearly polarized UV radiation. The polarization selective absorption of the oriented sample is interrogated with polarized IR radiation from 650 to 4000 cm−1, and relative band
Figure 2. PIMS spectra at 10.487 eV resulting from pyrolysis of ophenylene sulfite at the temperatures shown, confirming the pyrolysis mechanism in Scheme 3.
results for the thermal decomposition of o-phenylene sulfite. Initial scans at 125 °C (400 K) of the precursor confirm the identity of o-phenylene sulfite (m/z 156). At low pyrolysis temperatures (900−1100 K) the appearance of m/z 48 and m/z 80 indicate the conversion of o-phenylene sulfite to SO (m/z 48) + CO (m/z 28) + C5H4O (m/z 80). The measured ionization energy52 (IE) of SO is 10.294 ± 0.004 eV, while that of C5H4O has been measured at 9.49 ± 0.02 eV26 and 9.40 eV.27 At higher pyrolysis temperatures (>1100 K), the C5H4O signal is attenuated and m/z 52, vinylacetylene53 (HCCCHCH2), grows. Even though 118.2 nm is almost 1 eV above the IE(C5H4 O), no obvious dissociative ionization products were observed at 10.487 eV. Production of acetylene and CO was not observed in the 10.487 eV PIMS but was identified in the infrared, as discussed in the next subsection. Infrared Spectroscopy in Rare Gas Matrices. Cryogenic matrix isolation was done with both argon at 20 K and neon at 4 K. Figure 3 is consistent with Scheme 3. The green traces in Figure 3 show neon passed through the SiC reactor at 1500 K (background spectrum). The black traces are of o-phenylene sulfite with the SiC tube held at the temperatures indicated. It is clear that at lower pyrolysis temperature there was no acetylene or vinylacetylene production, and observation of the SO fundamental at 1137 cm−1 (not shown in Figure 3) at 1000 K and higher indicates54,55 the formation of C5H4O. At higher temperature (>1100 K) HCCH and HCCCHCH2 IR D
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intensities are highlighted by spectral subtraction. The permanent electric dipole of cyclopentadienone has been measured and is known28 to be 3.132 ± 0.007 D. Molecules belonging to many symmetry point groups, including the C2v point group of cyclopentadienone, will have transition dipole moments that are restricted to lie along one of the three mutually orthogonal principal molecular axes. In the case of cyclopentadienone, the transition dipole moment lies along the molecular x (out of the molecular plane, b1), molecular y (in plane, b2), or molecular z (in plane along the carbonyl bond, a1) axes. In principle this allows for the determination of the symmetry of any molecular absorption or emission. The HOMO of C5H4O is a π orbital of a2 symmetry and is localized on the ring, as shown in the orbital diagram in Figure 4, corresponding to the antisymmetric combination of
Figure 5. Linear dichroism and Ar matrix absorbance vs wavenumber. Linear dichroism experiments use IR light polarized parallel to and perpendicular to the irradiating polarized UV light. The results show b2 vibrational mode depletion (ν20, ν21, and ν23). The a1 (ν3 and ν5) and b1 (ν14 and ν15) vibrational modes show positive dichroism, confirming that the upper electronic state is accessed via the ycomponent (B2) of the transition dipole moment. The results also confirm the symmetries of many of the assigned vibrational modes. The red trace is the linear dichroism difference spectrum (∥ − ⊥), and the black trace is the depleted spectrum following 355 nm irradiation, and collected using unpolarized IR light.
the next section. Positive dichroism was observed for the a1 vibrational transitions ν3 and ν5, as well as for the b1 transitions ν14 and ν15. These results are consistent with the proposal that the electronic transition at 360 nm is allowed via the ycomponent (B2) of the transition dipole moment. A summary of the linear dichroism results is in Table 1.
Figure 4. Calculated molecular orbitals and corresponding energies, suggesting that the first spin allowed electronic transition of C5H4O is ππ* (1A1 → 1B2). It also confirms that the HOMO of C5H4O is a2, while the LUMO is b1.
V. DISCUSSION Straightforward Assignments. Assignments for the fundamentals ν3, ν5, ν8, ν9, ν14, ν15, ν21, ν23, and ν24 are uncomplicated; all of them are clearly seen in the neon matrix spectrum, and none of them deviates from the predicted VPT2 band positions by more than 8 cm−1 (see Figure 6). The remaining fundamentals are more elusive, and their assignment is a challenge. In particular, the CH stretching region of the spectrum in any hydrocarbon molecule with a reasonable number of atoms invariably involves Fermi resonances, and a large number of fundamentals in the fingerprint region are either very weak (four have predicted intensities of less than 0.1 km mol−1) or also involve resonances. Before engaging in a discussion of the remaining assignments, it is constructive to return to the easily assigned bands and analyze the level of agreement between VPT2 and observation. For the nine fundamentals listed above, differences between the observed band positions and the calculation are as follows (VPT2 − experiment, in cm−1): −4, −8, −7, −5, −3, −1, −8, −6, and −5. In addition to the small magnitude of the errors noted in the preceding sentence, the behavior is also entirely systematic: the VPT2 calculations tend to undershoot
ethylene-like π orbitals. The LUMO of C5H4O is a π* orbital of b1 symmetry and is delocalized over the entire molecule. It is therefore likely that the lowest energy electronic transition is ππ*, X̃ 1A1 → Ã 1B2, which is allowed by the ycomponent (B2) of the transition dipole moment. The linear dichroism experiments photooriented the 20 K C5H4O sample diluted in an argon matrix on a CsI window along the laboratory horizontal axis. After the ca. 1−5 h irradiation with polarized 355 nm radiation, accessing the ππ* B2 polarized transition, the C5H4O molecules with their b2 axis aligned with the field were depleted in signal. Infrared radiation polarized parallel to the UV radiation is expected to show a resultant depletion in the vibrational modes of b2 symmetry (∥), and infrared radiation polarized perpendicular to the UV radiation is expected to show a relative depletion in vibrations of a1 and b1 symmetry (⊥). Taking the difference (∥ −⊥) yields a linear dichroism spectrum where the b2 vibrations have negative intensity, and the a1 and b1 vibrations have positive intensity (see Figure 5). Negative dichroism was clearly observed for ν20, ν21, and ν23all vibrational transitions of b2 symmetry, as assigned in E
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Table 1. Tabulated Experimental and Calculated Vibrational Frequencies of C5H4O (Γvib = 9a1 ⊕ 3a2 ⊕ 4b1 ⊕ 8b2) VPT2 a1
a2
b1
b2
mode
neon matrix (cm−1)
ν1 ν2 2ν19 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11 νl2 ν13 ν14 ν15 ν16 ν17 ν18 ν4 + ν19 ν19 ν20 ν8 + ν24 ν21 ν22 ν12 + ν15 ν23 ν24
3107 3099, 3100 3196 1735a
⊥
1333a
⊥
952a 843a 651 933b 722b 932b 822 629
exp pol
⊥
⊥ ⊥
3143 3076, 3078 1601, 1595 1283a 1306 1138 1066a 738 458
|| || || ||
lit. (Ar)
energy (cm−1)
intensity (km mol−1)
3120 (0.97)c 3096 (0.97)c 3188 (0.90)c 1739 1524 1325 1072 944 836 646 926 718 446 929 819 628 203 3142 (0.61)c 3071 (0.53)c 3098 (0.43)c 1592 1274 (0.62)c 1297 (0.38)c 1130 1084 (0.52)c 1062 (0.48)c 732 453
