Article Cite This: J. Phys. Chem. A 2019, 123, 6130−6143
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Infrared Emission from Photodissociation of Methyl Formate [HC(O)OCH3] at 248 and 193 nm: Absence of Roaming Signature Published as part of The Journal of Physical Chemistry virtual special issue “Hai-Lung Dai Festschrift”. Lucia Lanfri,† Yen-Lin Wang,‡ Tien V. Pham,‡,¶ Nghia Trong Nguyen,¶,§ Maxi Burgos Paci,† M. C. Lin,*,‡,§ and Yuan-Pern Lee*,‡,§,∇ †
Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Ala I - 2do Piso Ciudad Universitaria, Pabellón, Argentina Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan ¶ School of Chemical Engineering, Hanoi University of Science and Technology, Hanoi, Vietnam § Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu 30010, Taiwan ∇ Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan
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S Supporting Information *
ABSTRACT: Following photodissociation at 248 nm of gaseous methyl formate (HC(O)OCH3, 0.73 Torr) and Ar (0.14 Torr), temporally resolved vibration−rotational emission spectra of highly internally excited CO (ν ≤ 11, J ≤ 27) in the 1850−2250 cm−1 region were recorded with a step-scan Fourier-transform spectrometer. The vibration−rotational distribution of CO is almost Boltzmann, with a nascent average rotational energy (E0R) of 3 ± 1 kJ mol−1 and a vibrational energy (E0V) of 76 ± 9 kJ mol−1. With 3 Torr of Ar added to the system, the average vibrational energy was decreased to E0V = 61 ± 7 kJ mol−1. We observed no distinct evidence of a bimodal rotational distribution for ν = 1 and 2, as reported previously [Lombardi et al., J. Phys. Chem. A 2016, 129, 5155], as evidence of a roaming mechanism. The vibrational distribution with a temperature of ∼13000 ± 1000 K, however, agrees satisfactorily with trajectory calculations of these authors, who took into account conical intersections from the S1 state. Highly internally excited CH3OH that is expected to be produced from a roaming mechanism was unobserved. Following photodissociation at 193 nm of gaseous HC(O)OCH3 (0.42 Torr) and Ar (0.09 Torr), vibration− rotational emission spectra of CO (ν ≤ 4, J ≤ 38) and CO2 (with two components of varied internal distributions) were observed, indicating that new channels are open. Quantum-chemical calculations, computed at varied levels of theory, on the ground electronic potential-energy schemes provide a possible explanation for some of our observations. At 193 nm, the CO2 was produced from secondary dissociation of the products HC(O)O and CH3OCO, and CO was produced primarily from secondary dissociation of the product HCO produced on the S1 surface or the decomposition to CH3OH + CO on the S0 surface.
I. INTRODUCTION
Pereira and Isolani employed infrared multiphoton dissociation (IRMPD) of HC(O)OCH3 to detect CO, CO2, CH3OH, CH4, and C2H6.14 Based on the observed products, they proposed that the dissociation of HC(O)OCH3 proceeds via two channels to form CH3OH + CO and CH4 + CO2.14 Francisco employed the CCSD(T)/6-311G(2df,2p)//MP2/6311G(2df,2p) method to investigate the decomposition of HC(O)OCH3 and reported that the experimental products of decomposition (CH3OH, H2CO, and CO) can be explained by two competitive parallel reactions forming CH3OH + CO and H2CO + H2CO, followed by decomposition of H2CO to
Methyl formate, HC(O)OCH3, plays important roles in many aspects, including astrochemistry and organic chemistry. Since its discovery in the emission spectrum of Sgr B2 in 1975,1 methyl formate was observed in significant amounts in varied astrophysical environments,2−6 with reported column densities in the range of (2.6−19) × 1016 cm−2.7 To understand the observed abundance of methyl formate and its isotopic fractionation, reaction paths for the formation and destruction of methyl formate should be considered. Furthermore, HC(O)OCH3 is the simplest ester and is commonly employed in organic synthesis. Its catalytic conversion to synthetic gases8−11 or its pyrolysis to produce CO has been extensively explored.12,13 © 2019 American Chemical Society
Received: May 1, 2019 Revised: July 3, 2019 Published: July 3, 2019 6130
DOI: 10.1021/acs.jpca.9b04129 J. Phys. Chem. A 2019, 123, 6130−6143
Article
The Journal of Physical Chemistry A
CO was predicted from their simulation, but only up to ν = 4 was observed in their experiments. In this work, we investigated the infrared emission from products of HC(O)OCH3 irradiated at 248 or 193 nm with a step-scan Fourier-transform spectrometer. At 248 nm, we observed emission of CO up to ν = 11, but with no clear evidence of a small-J component from the roaming channel and no emission of CH3OH above 1800 cm−1. At 193 nm, an intense broad feature of CO2 with bimodal distributions and weaker CO emission with a smaller vibrational excitation (ν ≤ 4) were observed. These observations can be rationalized with quantum-chemical calculations and rate predictions.
yield CO + H2;15 however, the channel leading to CH4 + CO2 has a large barrier. Lee employed a molecular beam to investigate the photodissociation of DC(O)OCH3 at 193 nm and reported four dissociation channels to form CH3O + DCO (X ∼ 2A′), CH3O + DCO (A ∼ 2A″), CH3OCO + D, and CH3OD + CO, with estimated branching factors 0.73, 0.06, 0.13, and 0.08, respectively;16 products DCO (A ∼ 2A″) and C(O)OCH3 further decomposed to D + CO and CH3 + CO2, respectively. Chao et al. studied the photodissociation of HC(O)OCH3 at 234 nm using velocity ion imaging coupled with (2 + 1) resonance-enhanced multiphoton ionization (REMPI) to detect the CO fragment.17 They selected varied rotational levels of CO (ν″ = 0) to evaluate the translational and internal energy disposals in the fragments. Two distinct distributions of translational energy of CO were reported: for J″ ≥ 24, a broad distribution resulted from the CH3OH + CO channel with an average translational energy of 96 kJ mol−1, extending to 293 kJ mol−1, was reported; for smaller J″, a bimodal distribution having an additional sharp, low-energy component, with an energy of 4−8 kJ mol−1, that increased with decreasing J″ was observed. These authors ascribed this additional low-energy channel to a roaming mechanism via the radical channel CH3O + HCO to form CH3OH + CO. de Wit et al. employed the phase-space theory (PST) to investigate the photolysis of methyl formate and indicated that the dynamical signature of the roaming donor fragment is similar to that of the triple fragmentation.18 These authors indicated that, although the small energy distribution of CO might arise from a roaming reaction, CO from triple fragmentation cannot be discounted; therefore, finding an unambiguous way to determine the true mechanism is prevailing. Tsai et al.19 further investigated the system with photodissociation wavelengths of 225−250 nm to encompass the theoretical threshold for the triple fragmentation channel lying near 480.9 kJ mol−1 or 248 nm. With the help of trajectory dynamic simulations, these authors deconvoluted their observed kinetic distribution of CO into three components to correlate a slow component due to the opening of the triple fragmentation channel, an intermediate one associated with the roaming mechanism, and a rapid one from the minimal energy path of the molecular channel. To confirm these results, Lombardi et al.20 studied the photolysis of HC(O)OCH3 at 248 nm with a time-resolved Fourier-transform infrared (FTIR) spectrometer to record the emission spectrum of vibration−rotationally excited CO (ν ≤ 4) and reported two distinct rotational distributions with rotational temperatures of ∼1000 and 450 K in the ν = 1 and ν = 2 vibrational states; these two rotational distributions were assigned to the direct molecular channel and the roaming path, respectively, according to their dynamical simulations. Although all reports from these groups of Lin and Aquilanti17,19,20 seem to point consistently to the existence of a roaming mechanism for the formation of CO, truly unambiguous experimental results to support this mechanism, such as an observation of highly internally excited CH3OH and experimental data with improved ratios of signal-to-noise to identify clearly a bimodal distribution of CO, are lacking. The rotational temperatures estimated from ion-imaging experiments (∼200 K for roaming and ∼420 K for a molecular channel) are distinct from those derived from the IR emission experiments (∼450 K for roaming and ∼1000 K for molecular channel).21 Furthermore, significant vibrational excitation of
II. METHODS II.A. Experiments. To acquire time-resolved IR emission spectra, we employed a step-scan FTIR spectrometer coupled to a vacuum chamber containing a set of Welsh mirrors to maximize the efficiency of collection of the emission induced from the ultraviolet (UV)-irradiated flowing gaseous samples. Since this system has been described with greater details in previous publications,22−24 only a summary is given here. The mildly focused photolysis beam, emitted from a KrF excimer laser (Coherent Model COMPexPro-102F, 248 nm, 17 Hz, 160 mJ pulse−1) or an ArF excimer laser (Coherent Model COMPexPro-50, 193 nm, 50 Hz, 60 mJ pulse−1), entered the chamber and propagated in front of the two Welsh mirrors near the FTIR spectrometer side. The laser beam has dimensions ∼2.0 × 15.0 mm2 at the photolysis center. An InSb detector was employed to detect the transient signal, which was preamplified and further amplified by a factor of 50 with a voltage amplifier (bandwidth = 1 MHz) before being recorded with a data-acquisition board (12-bit, 25 ns). Survey spectra were obtained at a resolution of 6 cm−1 in the spectral region of 1800−4900 cm−1. To detect the emission of CO at a resolution of 0.3 cm−1, a filter allowing the passage of light in the region of 1670−2325 cm−1 was employed; this filtering enables undersampling to decrease the number of data points and, hence, the acquisition period. The CO emission was recorded effectively in the region of 1850−2325 cm−1, because the lower bound was limited by the detectivity of the InSb detector. Similarly, to investigate the emission in other spectral regions, we employed other filters. Temporal profiles at each mirror step of the FTIR analysis were typically averaged over 30 laser pulses. For analysis with a satisfactory signal-to-noise (S/N) ratio, 40 consecutive time slices obtained at 25 ns intervals were typically summed to yield spectra at intervals of 1 μs. In addition, the S/N ratio was improved by averaging 4− 6 sets of spectra recorded under similar experimental conditions. The spectra were corrected for the transmission of the filter employed and the instrumental spectral-response function that was obtained on calibration with blackbody radiation. Methyl formate (Alfa Aesar, 97%) was used without purification, except for degassing through freeze−pump− thaw cycling. The sample was injected into the vacuum chamber as a diffusive beam through a slit-shaped inlet. Typically no buffer gas was added except Ar (AGA Specialty Gases, 99.999%) to a small extent, which was flowed near the entrance of the photolysis port to avoid a solid deposit on the quartz window. The partial pressure of the gases inside the chamber was maintained typically at 0.42−0.73 and 0.09−0.14 Torr for HC(O)OCH3 and Ar, respectively, so that satisfactory spectra could be obtained with a minimal pressure. In one set 6131
DOI: 10.1021/acs.jpca.9b04129 J. Phys. Chem. A 2019, 123, 6130−6143
Article
The Journal of Physical Chemistry A
Information; the order of the power dependence was determined to be 1.02 ± 0.06. The laser fluence employed in most experiments was
