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Observation of the simplest Criegee intermediate CH2OO in the gas-phase ozonolysis of ethylene.

Womack CC, Martin-Drumel MA, Brown GG, Field RW, McCarthy MC - Sci Adv (2015)

Bottom Line: Although numerous studies have confirmed a 1,3-cycloaddition mechanism that generates a Criegee intermediate (CI) with form R1R2COO, no small CIs have ever been directly observed in the ozonolysis of alkenes because of their high reactivity.Nine other product species of the O3 + C2H4 reaction were also detected, including formaldehyde, formic acid, dioxirane, and ethylene ozonide.The presence of all these species can be attributed to the unimolecular and bimolecular reactions of CH2OO, and their abundances are in qualitative agreement with published mechanisms and rate constants.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.

ABSTRACT
Ozonolysis is one of the dominant oxidation pathways for tropospheric alkenes. Although numerous studies have confirmed a 1,3-cycloaddition mechanism that generates a Criegee intermediate (CI) with form R1R2COO, no small CIs have ever been directly observed in the ozonolysis of alkenes because of their high reactivity. We present the first experimental detection of CH2OO in the gas-phase ozonolysis of ethylene, using Fourier transform microwave spectroscopy and a modified pulsed nozzle, which combines high reactant concentrations with rapid sampling and sensitive detection. Nine other product species of the O3 + C2H4 reaction were also detected, including formaldehyde, formic acid, dioxirane, and ethylene ozonide. The presence of all these species can be attributed to the unimolecular and bimolecular reactions of CH2OO, and their abundances are in qualitative agreement with published mechanisms and rate constants.

No MeSH data available.


The fundamental rotational line (10,1–00,0) of the simplest CI, CH2OO, detected in the O3 + C2H4 reaction, acquired after 4.3 hours of integration, or 93,000 sample injections.The peak is split into two Doppler components, a result of the alignment of the molecular beam with the two traveling waves of the Fabry-Pèrot cavity. A more intense peak is visible, about 200 kHz higher in energy, the carrier of which remains unidentified.
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Figure 3: The fundamental rotational line (10,1–00,0) of the simplest CI, CH2OO, detected in the O3 + C2H4 reaction, acquired after 4.3 hours of integration, or 93,000 sample injections.The peak is split into two Doppler components, a result of the alignment of the molecular beam with the two traveling waves of the Fabry-Pèrot cavity. A more intense peak is visible, about 200 kHz higher in energy, the carrier of which remains unidentified.

Mentions: In total, nine product species were detected in our experiment at room temperature and pressure. All required the presence of both O3 and C2H4 and were sensitive to the flow rates and relative O3 and C2H4 concentrations. The signal attributed to CH2OO, shown in Fig. 3, was scrutinized carefully. Although weak, the frequency of this line matches the published high-resolution data (16, 17) to within 3 kHz, and we ensured that it was not an oscilloscope artifact or image by shifting the probe frequency. Additionally, the line vanished when a second microwave source, aligned perpendicularly to the cavity axis in a double resonance scheme, excited the 20,2–10,1 transition (16, 17). For these reasons, the line can be unequivocally attributed to CH2OO. Its signal intensity corresponds to about 5.9 × 109 molecules/pulse, which is close to the detection limit of the microwave spectrometer. Other detected species are listed in Table 1. All can be attributed to established secondary chemistry of the O3 + C2H4 reaction, as illustrated in Figs. 4 and 5, and discussed below. A small number of the product species in the figures were not detected, because they either could not be detected in our spectrometer, were too reactive, or do not have published rotational lines.


Observation of the simplest Criegee intermediate CH2OO in the gas-phase ozonolysis of ethylene.

Womack CC, Martin-Drumel MA, Brown GG, Field RW, McCarthy MC - Sci Adv (2015)

The fundamental rotational line (10,1–00,0) of the simplest CI, CH2OO, detected in the O3 + C2H4 reaction, acquired after 4.3 hours of integration, or 93,000 sample injections.The peak is split into two Doppler components, a result of the alignment of the molecular beam with the two traveling waves of the Fabry-Pèrot cavity. A more intense peak is visible, about 200 kHz higher in energy, the carrier of which remains unidentified.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4643816&req=5

Figure 3: The fundamental rotational line (10,1–00,0) of the simplest CI, CH2OO, detected in the O3 + C2H4 reaction, acquired after 4.3 hours of integration, or 93,000 sample injections.The peak is split into two Doppler components, a result of the alignment of the molecular beam with the two traveling waves of the Fabry-Pèrot cavity. A more intense peak is visible, about 200 kHz higher in energy, the carrier of which remains unidentified.
Mentions: In total, nine product species were detected in our experiment at room temperature and pressure. All required the presence of both O3 and C2H4 and were sensitive to the flow rates and relative O3 and C2H4 concentrations. The signal attributed to CH2OO, shown in Fig. 3, was scrutinized carefully. Although weak, the frequency of this line matches the published high-resolution data (16, 17) to within 3 kHz, and we ensured that it was not an oscilloscope artifact or image by shifting the probe frequency. Additionally, the line vanished when a second microwave source, aligned perpendicularly to the cavity axis in a double resonance scheme, excited the 20,2–10,1 transition (16, 17). For these reasons, the line can be unequivocally attributed to CH2OO. Its signal intensity corresponds to about 5.9 × 109 molecules/pulse, which is close to the detection limit of the microwave spectrometer. Other detected species are listed in Table 1. All can be attributed to established secondary chemistry of the O3 + C2H4 reaction, as illustrated in Figs. 4 and 5, and discussed below. A small number of the product species in the figures were not detected, because they either could not be detected in our spectrometer, were too reactive, or do not have published rotational lines.

Bottom Line: Although numerous studies have confirmed a 1,3-cycloaddition mechanism that generates a Criegee intermediate (CI) with form R1R2COO, no small CIs have ever been directly observed in the ozonolysis of alkenes because of their high reactivity.Nine other product species of the O3 + C2H4 reaction were also detected, including formaldehyde, formic acid, dioxirane, and ethylene ozonide.The presence of all these species can be attributed to the unimolecular and bimolecular reactions of CH2OO, and their abundances are in qualitative agreement with published mechanisms and rate constants.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.

ABSTRACT
Ozonolysis is one of the dominant oxidation pathways for tropospheric alkenes. Although numerous studies have confirmed a 1,3-cycloaddition mechanism that generates a Criegee intermediate (CI) with form R1R2COO, no small CIs have ever been directly observed in the ozonolysis of alkenes because of their high reactivity. We present the first experimental detection of CH2OO in the gas-phase ozonolysis of ethylene, using Fourier transform microwave spectroscopy and a modified pulsed nozzle, which combines high reactant concentrations with rapid sampling and sensitive detection. Nine other product species of the O3 + C2H4 reaction were also detected, including formaldehyde, formic acid, dioxirane, and ethylene ozonide. The presence of all these species can be attributed to the unimolecular and bimolecular reactions of CH2OO, and their abundances are in qualitative agreement with published mechanisms and rate constants.

No MeSH data available.