Collisional Energy Transfer from Highly Vibrationally Excited Radicals

Dec 11, 2012 - Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States. J. Phys. Chem. Lett. , 2013, 4 (1), pp 23â...
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Collisional Energy Transfer from Highly Vibrationally Excited Radicals Is Very Efficient Michael J. Wilhelm,†,§ Matthew Nikow,†,∥ Jonathan M. Smith,‡ and Hai-Lung Dai*,‡ †

Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States

‡

S Supporting Information *

ABSTRACT: Although highly vibrationally excited (HVE) radicals are ubiquitous in natural environments, the effect of collisional energy transfer (ET) on their reactivity has yet to be fully characterized. We have used time-resolved IR emission spectroscopy to characterize the vibrational-to-translational quenching of a small HVE radical, ketenyl (HCCO), by inert gases. Photolysis of ethyl ethynyl ether at 193 nm provides HVE HCCO in the X̃ 2A″ electronic ground-state, with a nascent internal energy of 2.2 ± 0.6 eV. IR emission is monitored as an indicator of vibrational energy, and spectral modeling allows direct determination of the average energy lost per collision as a function of the internal energy. Collisional deactivation of HVE HCCO is shown to be minimally an order of magnitude more efficient than closed-shell molecules of comparable size. Schwartz− Slawsky−Herzfeld−Tanczos (SSHT) theory, modified for HVE molecules, suggests that this ET enhancement is due to a strong attractive intermolecular interaction. SECTION: Kinetics and Dynamics

F

such as nitrogen dioxide (NO2) are not included in the discussion. The work of Damm et al., examining vibrational-totranslational (V−T) ET from the benzyl (C6H6−CH2) radical,18,19 represents the only known prior study with the specific goal to characterize V−T ET from a HVE radical. Collisional ET from benzyl radicals, initially prepared with 20 560 cm−1 of internal excitation, was shown to be highly efficient. However, due to the size of the radical, the deactivation rate was comparable to, though somewhat larger than, that of closed-shell aromatic molecules.20 In principle, delocalized aromatic electrons are more readily polarizable and therefore more susceptible to dipole-induced attractive interactions with collider species. For the benzyl radical, the concurrent presence of aromatic and radical electrons likely obscured any radical-specific ET effect.18,19 The influence of an unpaired radical electron in intermolecular interactions (and likewise ET) should be more apparent for small nonaromatic species. In fact, in a study primarily aimed at understanding the photochemistry of halogenated methane, as a side note, Baughcum and Leone reported remarkably efficient V−T ET from HVE iodomethyl radical (CH2I) quenched with Ar.21 Here, we report the very efficient V−T collisional quenching of a small nonaromatic HVE radical, the ketenyl (HCCO) radical. Ketenyl has been identified as a key intermediate in combustion.22 Specifically, hydrocarbons produce acetylene as

or environments such as combustion chambers or atmospheric photochemical systems, where highly vibrationally excited (HVE) molecules are routinely generated, molecular energy content and, subsequently, reactivity are greatly influenced by collision induced energy transfer (ET) with ambient species. Collisional quenching of HVE molecules in energy regions where level density is high, the energy gap small, and intramolecular coupling strong, is expected to be substantially different from that of molecules excited in low vibrational levels. Experimental characterization of the collisional quenching of HVE molecules has been actively pursued in the last few decades due to advancements in experimental methods. Indeed, the quenching behavior of HVE molecules has been found to exhibit qualitatively different characteristics, such as significantly larger collisional cross sections (e.g., σexcited ∼ 5−20 σhard‑sphere) and a larger dependency upon long-range interactions, compared with molecules in lower energy states.1−9 Systems where HVE molecules are found often contain high concentrations of radicals.10 However, collisional quenching studies so far have focused almost exclusively on molecules. Examination of excited radicals has generally been limited to diatomic species in their lowest vibrational levels.11−17 This is primarily due to the fact that radicals are experimentally more challenging to investigate10 as they: (1) are generally produced in small quantities, (2) exhibit short lifetimes, and (3) are often spectroscopically less well characterized for detection. In this discussion, radicals are defined, following the Herzberg definition, as species that have open-shell electronic structure and are chemically unstable.10 In this way, for example, species © 2012 American Chemical Society

Received: October 31, 2012 Accepted: December 11, 2012 Published: December 11, 2012 23

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a combustion intermediate,23 which generates HCCO after reaction with atomic oxygen O(3P).24 Additionally, reaction of quartet-state methylidyne CH(4Σ−) with ground-state carbon monoxide has been predicted to yield quartet-state HCCO(4A″),25 which can be converted to HVE HCCO(2A″) via collision-induced intersystem crossing (CI-ISC).26 Subsequently, quantitative understanding of the collisional deactivation process of HCCO(2A″) is essential for characterizing combustion processes. Production of HCCO(2A″) through the 193 nm photolysis of ethyl ethynyl ether (CH3CH2OCCH, EEE) was verified in a photofragment translational energy spectroscopy (PTS) study by Krish et al.27 We have previously characterized the infrared (IR) emission bands of this radical with submicrosecond timeresolved Fourier transform IR (FTIR) emission spectroscopy.28 Consistent with the PTS study,27 it was observed that the photolysis produces HCCO with near unit quantum efficiency. IR emission from nascent HCCO(2A″) displayed a significant anharmonic red-shift (ca. 500 cm−1) in the ν1 CH stretch mode, indicating the production of HCCO(2A″) with high vibrational excitation. It has previously been established that highly anharmonic IR emission, described with the normal mode picture, can be used for determining the internal energy content of HVE molecules.29−32 The time-resolved IR emission spectra, recorded following the production of HVE HCCO(2A″), can thus be used to deduce the internal energy content as a function of the number of collisions, and subsequently the collisional quenching rate. In this report, we examine V−T ET from HVE HCCO(2A″) radicals to three rare-gas (Rg) collisional partners: He, Ne, and Ar. Modeling of the time-resolved IR emission spectra, using ab initio calculated anharmonic constants, permits a direct measure of the HCCO(2A″) average vibrational energy ⟨E⟩, as a function of the number of Rg collisions (ZRg), and determination of the average energy lost per collision ⟨ΔE⟩. Our measurements show that V−T ET from HVE radicals, as compared with closed-shell molecules, is a significantly more effective process due to a stronger attractive interaction between radical and collider. Figure 1 depicts representative time-resolved IR emission spectra at early and late times following the 193 nm photolysis of EEE in Ar. The spectra shown exhibit three main emission bands: one centered near 2000 cm−1, and the other two spanning the CH stretching region. The PTS study27 indicated that photolysis of EEE results in nascent photoproducts consisting primarily of vibrationally excited ketenyl (i.e., 63% in the excited 4A″ state and 37% in the ground 2A″ state) and vibrationally cool (i.e., vk ≤ 1) ethyl (CH3CH2) radicals. Correspondingly, the majority of the observed emission bands are assignable to ketenyl. Note that the sample pressures of the current experiments ensured the complete unimolecular dissociation of HCCO(4A″) prior to spectral acquisition,26 hence the measured HCCO signal arises solely from the ground-state species. Emission assignable to the ν1 CH stretch of HCCO(2A″) exhibits substantial vibrational anharmonicity, initially spanning 2700−3300 cm−1 and shifting to the fundamental (3,232 cm−1) at later time, indicating that the nascent radicals were generated with a large amount of internal energy. Other key features observed in the spectra include vibrationally excited CO (2,143 cm−1 fundamental), generated from the secondary dissociation of excited HCCO(2A″) following CI-ISC from HCCO(4A″),26 as well as electronically excited ethynyl HCC(2Π) radical.

Figure 1. Time-resolved IR emission spectra collected at 0.9 (blue) and 17.9 (red) μs following the photolysis of EEE. For times shown, the photofragments experienced ca. 30 and 700 collisions, respectively. The vertical gray line shows the low energy cutoff of the detector.

Emission from the ethynyl radical was initially observed from Σ ←2Π electronic transitions spanning 3600−3800 cm−1, followed by a near constant presence of the ground-state HCC(2Σ+) ν1 CH stretch fundamental33 at 3290 cm−1. Additionally, the fundamental34 of the ν10 asymmetric CH2 stretch of CH2CH3 is observed at 3129 cm−1. The low energy cutoff of the InSb detector at 1850 cm−1 (Figure 1) limits collection of red-shifted emission from the intense ν2 asymmetric CCO stretch of HVE HCCO(2A″) (2,022.644 cm−1 fundamental).35 This limitation, coupled with the concurrent appearance of overlapping emission from vibrationally excited CO(v ≫ 1),26 renders any signal arising from the ν2 mode of HCCO(2A″) as unsuitable for quantitative analysis. Consequently, we rely solely on emission from the highly anharmonic ν1 CH stretch transitions as a probe of the vibrational energy of HCCO(2A″). Note that, while the analysis is focused on a single vibrational mode, due to effects of collisions and intramolecular vibrational redistribution, the measured energy is indicative of the total vibrational energy. Spectral fitting permits determination of the time-dependent population distribution, from which ⟨E⟩ can be calculated as a function of ZRg (defined by Lennard-Jones (LJ) collisional cross sections; see Supporting Information (SI)). HCCO(2A″) was modeled as a near-prolate symmetric top, and the energies of the thermodynamically accessible ro-vibrational levels were calculated using the six nondegenerate normal mode vibrations and three rotational constants.36 Vibrational anharmonic constants have not been measured for HCCO(2A″), hence we use the calculated χij of Simmonett et al.37 Emission spectra from each ro-vibrational level can be generated individually with harmonic scaling of normal mode quantum numbers and symmetric rotor selection rules. These synthetic spectra form a basis for fitting the observed spectra and extracting the population distribution over the ro-vibrational levels (see SI for details). A representative best fit to the HCCO(2A″) ν1 emission spectrum is shown in Figure 2A. To account for the presence of fundamental CH stretch transitions of HCC33 and CH2CH3,34 Lorentzian functions (with variable amplitude and full width at half-maximum (fwhm)) centered on each fundamental were 2 +

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evolution of ⟨E⟩ as a function of ZRg. The exponential fit results for all three data sets are presented in Table S1 (SI). As the calculated energy levels were modeled using ab initio spectral constants, there is inherent uncertainty linked to the determination of ⟨E⟩, particularly for extrapolation to high vibrational levels. Nevertheless, a 10% variation in the calculated spectral constants resulted in only a modest (ca. ± 500 cm−1) change in the measured ⟨E⟩. Estimates of the nascent average vibrational energy ⟨E0⟩vib can be extracted from the fit results through back extrapolation to the zero-collision point. For all three collider systems, an ⟨E0⟩vib of 17 500 ± 4515 cm−1 (2.2 ± 0.6 eV) is obtained. For our experimental conditions, ⟨ΔE⟩ for HCCO(2A″) + Rg can be determined as

Figure 2. Time-resolved spectral fit results: (a) Representative best fit (black broken line) to the emission band (solid red line) of the HCCO ν1 mode, recorded at 4.3 μs (ZAr = 170) following photolysis. The 0← 1 CH stretch transitions for C2H and CH2CH3 are shown as Lorentzian functions (dotted blue lines). The fit residual (Δ) is shown above. (b) Contour plot of the internal energy distribution of HCCO(2A″) as a function of time and ZAr. The ⟨E⟩ of the distribution are plotted (red circles) over the contour.

⟨ΔE⟩ =

(2)

The inset of Figure 3 depicts the series of ⟨ΔE⟩ as a function of ⟨E⟩, derived from the exponential decay fit results of the ⟨E⟩ versus ZRg data using eqs 1 and 2. Surprisingly, it is observed that Ne is the most efficient collider, followed next by He, and finally Ar. Furthermore, ⟨ΔE⟩ for HCCO(2A″) are observed to be minimally an order of magnitude larger than similar sized stable ground-state HVE molecules (e.g., for E < 5000 cm−1; SO2, CS2, and NCNO all exhibit ⟨ΔE⟩ < 1 cm−1).38−40 In an attempt to understand the origin of efficient ET from HVE HCCO(2A″), we modified the Schwartz−Slawsky− Herzfeld−Tanczos (SSHT) theory,41,42 well-established for describing collisional ET from low vibrational levels, to treat HVE species. Within the formulation of SSHT theory, V−T ET is induced through the repulsive asymptote of the interaction, modeled as a LJ potential. Attraction is treated empirically via an exponential coefficient containing the associated LJ well depth (De),42,43 which provides acceleration in the translational motion of the approaching colliders. Subsequently, SSHT theory can be used to derive an expression for ⟨ΔE⟩ composed of separate repulsive and attractive components (see SI for details). Briefly, for a HVE species, the probability (Pj←i) of a collision-induced transition from initial state |φi⟩ to final state |φj⟩ can be described as the product of a repulsive and attractive interaction, i.e.,

incorporated into the fitting. This approach is valid as the latter two radicals were generated ro-vibrationally cold (i.e., vk ≤ 1), hence their emission transitions were not anharmonically shifted. In Figure 2B, the excited HCCO(2A″) population distribution and the series of ⟨E⟩’s are plotted as a function of time and ZRg. At early times, before many collisions have occurred, the HCCO(2A″) population distribution is broad with a maximum peaked near 20 000 cm−1. As time proceeds and the number of collisions increases, the distribution narrows, and the maximum shifts toward the HCCO(2A″) zero-point, set to zero on this scale. The evolution of ⟨E⟩ can be well described by an exponential decay: ⟨E⟩ = Ω 0 + Ω1 × exp(−kRgZ Rg)

d⟨E⟩ dZ Rg

(1)

where Ω0 is an energy offset, Ω1 is a scaling factor, and kRg is a decay constant. Figure 3 highlights a comparison of the

⎛ −De ⎞ j←i P j ← i = PRep (ΔEj ← i , μ) × exp⎜ ⎟ ⎝ kBT ⎠

(3)

where μ is the reduced mass of the colliders. The attractive (exponential) component is independent of ⟨E⟩, and behaves as a scaling factor enhancing the probability of a transition and increasing the magnitude of ⟨ΔE⟩. By partitioning the vibrational manifold into discrete energy bins, representative ⟨ΔE⟩ for each energy bin can be determined by summing over the repulsive-probability (Pj←i Rep) weighted transitions: j←i ⟨ΔEi⟩Rep = N (Ei)−1 ∑ PRep ΔEj ← i j

(4)

and normalizing by the total number of allowed transitions. Combination of the attractive and repulsive components yields a comprehensive expression for ⟨ΔE⟩ for each energy bin as

Figure 3. ⟨E⟩ of HCCO plotted as a function of ZRg. The inset depicts ⟨ΔE⟩ as a function of ⟨E⟩. Error bars, determined from spectral fittings, have been included for early and late time points. Average values of the nascent vibrational energy, derived from the PTS27 and current results, are shown on the bottom of the inset. The broken lines are visual guides.

⎛ −D ⎞ ⟨ΔEi⟩ = ⟨ΔEi⟩Rep × exp⎜ e ⎟ ⎝ kBT ⎠ 25

(5)

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generation of accurate potential energy surfaces. Just as Blitz et al. resolved their SSH incompatible results12 using the interaction potential energy surface of Alexander et al.,48 the observation that the HCCO/Rg series exhibit large De’s demands further scrutiny from the theoretical community. Intuitively, De ratios should scale relative to the associated atomic polarizabilities. For the series (He, Ne, Ar), the polarizability ratios with respect to He are determined to be 1:2:8. The series of Rg atoms, in complexes with closed-shell species (e.g., methane49 and benzene50), exhibit De ratios of ca. 1:2:4, in qualitative agreement with the measured HCCO/Rg De ratios of 1:2:3. Given the presence of the unpaired radical electron in HCCO, it is prudent to anticipate that there are additional interactions, beyond van der Waals forces (e.g., electron occupancy effects51), contributing to the attractive potential. For repulsive force dominated ET, smaller (ground-state) molecules typically exhibit ⟨ΔE⟩ ≪ 1 cm−1, regardless of ⟨E⟩.38−40 Table 1 depicts a comparison of ⟨ΔE⟩ for aromatic

As depicted in Figure 4A, a purely repulsive ET model (i.e., De = 0) incorrectly predicts a reduced mass trend in which He

Figure 4. Modified SSHT model calculation of ⟨ΔE⟩ as a function of ⟨E⟩, plotted as lines over the experimental results (markers), based upon (a) setting De = 0 and (b) variation of De in a fit.

is calculated to be the most effective quencher. Moreover, the calculated values of ⟨ΔE⟩ are all observed to be grossly underestimated. The only characteristic of the experimental data reproduced by the repulsive model is the linear relationship between ⟨ΔE⟩ and ⟨E⟩. By contrast, inclusion of an attractive interaction (i.e., |De| > 0) increases the probability of V−T ET, suggesting that attractive interactions enhance ⟨ΔE⟩. By setting De as a variable parameter, it is possible to fit the experimental ⟨ΔE⟩ versus ⟨E⟩ curves (Figure 3 inset) and extract values of De. Figure 4B shows the resulting best fits obtained for each of the ⟨ΔE⟩ versus ⟨E⟩ data sets. Indeed, both the reduced mass trend and the magnitudes of each of the ⟨ΔE⟩ data sets are reproduced for De of −173(He), −359(Ne), and −433(Ar) cm−1. In this study, a few features of the V−T ET of HCCO(2A″) stand out as remarkable. Similar to Baughcum’s observations for HVE CH2I + Ar,21 for all quenchers considered, ⟨ΔE⟩ is minimally an order of magnitude greater than similarly sized closed-shell molecules. Furthermore, the reduced mass trend is unique in that Ne, rather than He, is found to be the most effective collider. While this is not the first observation of a disordered reduced mass trend,12 as before, it suggests that the pure SSH model would be insufficient. In an attempt to understand the physical origin of these observations, we examine the results from the modified SSHT theory as applied to HVE HCCO(2A″). To our knowledge, values of De for the HCCO/Rg series have not been reported previously. We note that the size of each De deduced here, particularly for He, are larger than what is typically observed for stable closed-shell molecules of comparable size (e.g., methane-Rg:44 De[Ne] = 43 cm−1, De[Ar] = 109 cm−1, De[Kr] = 132 cm−1, De[Xe] = 149 cm−1). This is not unreasonable considering that HCCO possesses an unpaired electron. Indeed, Parmenter noted that excited secbutyl (CH3CHCH2CH3) radicals exhibit a large self-interaction well depth.45 Furthermore, the deduced De’s exhibit a monotonic increase from He to Ar, consistent with expectations. The literature contains various examples of radical/Rg systems that exhibit De’s that far exceed typical van der Waals binding energies, often by an order of magnitude, but still weaker than a chemical bond.46,47 The vast majority focus on open-shell diatomic hydrides (e.g., OH, SH) with the series of Rg atoms, as these comparatively simple systems allow application of robust quantum chemical calculations for the

Table 1. Comparison of ⟨ΔE⟩ for V−T ET from Representative Aromatic and Nonaromatic Molecules, Collisionally Deactivated with Ar, for ⟨E⟩ of 5000 and 10 000 cm−1 donor + Ar NCNO (ref 40) CS2 (ref 38) SO2 (ref 39) ketenyl iodomethyl (ref 21) benzyl (ref 18) pyridine (ref 20) toluene (ref 20) azulene (ref 20)

⟨ΔE⟩E=5000/cm−1 0.1 0.5 23.0 25.9 45.9

⟨ΔE⟩E=10 000/cm−1 0.44 0.5 2.0 49.0 58.4 78.6 63.0 64.0 68.0

and nonaromatic molecules quenched with Ar. All comparisons were purposely limited to ⟨E⟩ ≤ 10 000 cm−1 to avoid ⟨ΔE⟩ enhancements due to vibronic coupling.52,53 HCCO(2A″) displays highly effective V−T ET with ⟨ΔE⟩ minimally an order of magnitude greater than comparably sized stable closedshell molecules. For comparison with this work, we reanalyzed the benzyl data to extract ⟨ΔE⟩ values.18,19 As shown in Table 1, V−T ⟨ΔE⟩ from benzyl is found to be ca. 15% larger than that of comparably sized aromatic species, in particular the structurally identical toluene. Given the aromatic nature of benzyl, enhanced V−T ET is not surprising. For instance, all aromatic systems listed exhibit ET efficiencies comparable to that of HCCO(2A″). Efficient deactivation of aromatic molecules is presumably due to a larger De. For example, Oliver et al.54 calculated the De for toluene−Ar to be −397.7 cm−1. The ⟨ΔE⟩ enhancement for benzyl may originate from a stronger De resulting from the unpaired radical electron. It has been observed that ⟨ΔE⟩ can be greatly increased through vibronic coupling.52,53 Strong vibronic coupling results in a substantial enhancement of the transition dipole moment, which in turn enhances ET.52,53 Consequently, ⟨ΔE⟩ versus ⟨E⟩ curves display an enhancement at the energetic onset of the coupling. As depicted in Figure 5, ground-state NO2 exhibits unremarkable ET until ⟨E⟩ is sufficient to couple to excited electronic states. Alternatively, for HCCO(2A″), iodomethyl,21 26

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attractive interactions. Furthermore, this study suggests that for modeling combustion and photochemical systems, the collisional relaxation rate for excited radicals needs to be properly considered. Specifically, it should be anticipated that faster quenching rates may significantly affect the radical reactivity in those systems.

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MATERIALS AND METHODS EEE is available commercially (Acros Organics, ≥50% weight stabilized in hexanes). The sample was degassed with several freeze−pump−thaw cycles before use. At 193 nm, EEE has a relatively strong absorption cross section of σ193 = 7 × 10−18 cm2 molecule−1, as compared with the negligible σ193 5000 cm−1, and well over 100 cm−1 for E > 10 000 cm−1. Furthermore, it is found that HCCO(2A″)/Rg ET exhibits a distinct reduced mass trend in which Ne is the most effective quencher, followed by He and then Ar. The unusually efficient ET and the unique reduced mass dependence can be accurately reproduced with SSHT theory modified for the treatment of HVE molecules. Our analysis suggests that a large attractive interaction between the radical and the collider, exhibiting a well-behaved trend De(HCCOHe) < De(HCCO-Ne) < De(HCCO-Ar), is germane for explaining the highly efficient ET as well as the unique mass dependence. Our observations show that collisional ET from HVE radicals is distinct from similar sized closed-shell molecules. This study, combined with both Damm et al.’s examination of benzyl18,19 and Baughcum’s study of iodomethyl,21 demonstrates that ET from HVE radicals is enhanced due to stronger radical-collider

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ASSOCIATED CONTENT

S Supporting Information *

Details regarding the calculation of the ro-vibrational manifold of ground-state HCCO, spectral fitting analysis, modified SSHT analysis, and the photodissociation dynamics of HCCO at 193 nm. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses §

Department of Chemistry, Temple University, Philadelphia, PA 19122, USA. ∥ Agilent Technologies, Inc., Wilmington, DE 19808, USA. 27

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Notes

(18) Damm, M.; Deckert, F.; Hippler, H.; Rink, G. Specific Rate Constants for the Fragmentation of Vibrationally Excited Benzyl Radicals. Phys. Chem. Chem. Phys. 1999, 1, 81−90. (19) Damm, M.; Deckert, F.; Hippler, H. Collisional Deactivation of Vibrationally Highly Excited Benzyl Radicals. Ber. Bunsenges. Phys. Chem. 1997, 101, 1901−1908. (20) Fay, N.; Luther, K. Temperature Dependence of Collisional Deactivation of Highly Vibrationally Excited Biphenylene. Z. Phys. Chem. 2000, 214, 839−864. (21) Baughcum, S. L.; Leone, S. R. Photofragmentation Infrared Emission Studies of Vibrationally Excited Free Radicals CH3 and CH2I. J. Chem. Phys. 1980, 72, 6531. (22) Fenimore, C. P.; Jones, G. W. Destruction of Acetylene in Flames with Oxygen. J. Chem. Phys. 1963, 39, 1514. (23) Miller, J. A.; Kee, R. J.; Westbrook, C. K. Chemical Kinetics and Combustion Modeling. Annu. Rev. Phys. Chem. 1990, 41, 345−387. (24) Osborn, D. L.; Mordaunt, D. H.; Choi, H.; Bise, R. T.; Neumark, D. M.; Rohlfing, C. M. Photodissociation Spectroscopy and Dynamics of the HCCO Free Radical. J. Chem. Phys. 1997, 106, 10087−10098. (25) Hu, C.-H.; Schaefer, H. F.; Hou, Z.; Bayed, K. D. The Attractive Quartet Potential Energy Surface for the CH(a4Σ−) + CO Reaction: A Role for the a4A″ State of the Ketenyl Radical in Combustion? J. Am. Chem. Soc. 1993, 115, 6904−6907. (26) McNavage, W.; Wilhelm, M. J.; Dai, H.-L. The Lowest QuartetState of the Ketenyl (HCCO) Radical: Collision-Induced Intersystem Crossing and the ν2 Vibrational Mode. Chem. Phys. 2012, submitted for publication. (27) Krisch, M. J.; Miller, J. L.; Butler, L. J.; Su, H.; Bersohn, R.; Shu, J. Photodissociation Dynamics of Ethyl Ethynyl Ether: A New Ketenyl Radical Precursor. J. Chem. Phys. 2003, 119, 176−186. (28) Wilhelm, M. J.; McNavage, W.; Groller, R.; Dai, H.-L. The ν1 CH Stretching Mode of the Ketenyl (HCCO) Radical. J. Chem. Phys. 2008, 128, 064313. (29) Hartland, G. V.; Qin, D.; Dai, H.-L.; Chen, C. Collisional Energy Transfer of Highly Vibrationally Excited NO2: The Role of Intramolecular Vibronic Coupling and the Transition Dipole Coupling Mechanism. J. Chem. Phys. 1997, 107, 2890−2902. (30) Nikow, M.; Wilhelm, M. J.; Smith, J. M.; Dai, H.-L. Strong Combination-Band IR Emission from Highly Vibrationally Excited Acetylene. Phys. Chem. Chem. Phys. 2010, 12, 2915−2922. (31) Pibel, C. D.; Sirota, E.; Brenner, J.; Dai, H.-L. Nanosecond Time-Resolved FTIR Emission Spectroscopy: Monitoring the Energy Distribution of Highly Vibrationally Excited Molecules During Collisional Deactivation. J. Chem. Phys. 1998, 108, 1297−1300. (32) Wilhelm, M. J.; Nikow, M.; Letendre, L.; Dai, H.-L. Photodissociation of Vinyl Cyanide at 193 nm: Nascent Product Distributions of the Molecular Elimination Channels. J. Chem. Phys. 2009, 130, 044307. (33) Tarroni, R.; Carter, S. Theoretical Calculation of Vibronic Levels of C2H and C2D to 10000 cm−1. J. Chem. Phys. 2003, 119, 12878. (34) Davis, S.; Uy, D.; Nesbitt, D. J. Laser Spectroscopy of JetCooled Ethyl Radical: Infrared Studies in the CH2 Stretch Manifold. J. Chem. Phys. 2000, 112, 1823−1834. (35) Unfried, K. G.; Curl, R. F. Infrared Flash Kinetic Spectroscopy of ν2 of Ketenyl Radical. J. Mol. Spectrosc. 1991, 150, 86−98. (36) Endo, Y.; Hirota, E. The Submillimeter-Wave Spectrum of the HCCO Radical. J. Chem. Phys. 1987, 86, 4319. (37) Simmonett, A. C.; Stibrich, N. J.; Papas, B. N.; Schaefer, H. F.; Allen, W. D. Barrier to Linearity and Anharmonic Force Field of the Ketenyl Radical. J. Phys. Chem. A 2009, 113, 11643−11650. (38) Chimbayo, A.; Toselli, B. M.; Barker, J. R. Deactivation of Highly Excited CS2 and SO2 by Rare Gases. J. Chem. Phys. 1998, 108, 2383. (39) Qin, D.; Hartland, G. V.; Chen, C. L.; Dai, H.-L. Collisional Deactivation of Highly Vibrationally Excited SO2: A Time-Resolved FTIR Emission Spectroscopy Study. Z. Phys. Chem. 2000, 214, 1501.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part through the US Department of Energy, Basic Energy Sciences, Grant No. DEFG 02-86ER 134584. M.J.W. and M.N. acknowledge support from Temple University.

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