KINETICS OF INTRAMOLECULAR CARBON-ATOM EXCHANGE IN KETENE

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Intramolecular carbon atom exchange in highly vibrationally excited ketene was studied by monitoring the carbon monoxide fragments (CO)-C-12 and (CO)-C-13 from the photodissociation of (CH2)-C-12 (CO)-C-13 and (CH2)-C-13 (CO)-C-12. Two experimental techniques were employed. In one set of experiments the IR transient absorptions of (CO)-C-12 and (CO)-C-13 were measured following pulsed excimer excitation of ketene C-13 isotopomers, giving carbon atom exchange yields at 351 and 308 nm in a low pressure gas cell. In the other set of experiments, jet-cooled ketene C-13 isotopomers were excited with tunable near-UV radiation, and the CO products were detected by monitoring their VUV laser-induced fluorescence. Carbon atom exchange yields were measured for energies extending from below the triplet decomposition threshold (CH2CO --> CH2 (X 3B1) + CO(X 1-SIGMA+)) to about 4000 cm-1 above the singlet threshold (CH2CO --> CH2 (a 1A1) + CO(X 1-SIGMA+)). The exchange yields range from 4 to 19%, and the energy dependence of the yield exhibits pronounced structure, with maxima at the triplet and singlet decomposition thresholds. Kinetic measurements of the appearance of the CO products were also performed. The time constant for the appearance of the exchanged CO (e.g., (CO)-C-13 from (CH2)-C-13 (CO)-C-12) is significantly longer than that for the direct CO fragment (e.g., (CO)-C-12 from (CH2)-C-13 (CO)-C-12). All the experimental observations are consistent with a simple reaction mechanism involving ketene isomerization, (CH2)-C-13 (CO)-C-12 reversible (CH2)-C-12 (CO)-C-13, and dissociation, (CH2)-C-13 (CO)-C-12 --> (CH2)-C-13 + (CO)-C-12 and (CH2)-C-12 (CO)-C-13 --> (CH2)-C-12 + (CO)-C-13. The isomerization rate was determined by analyzing the CO kinetics and the carbon atom exchange yields in terms of the simple isomerization mechanism. A fit of the energy dependence of the isomerization rate constant to the results of tunneling-corrected Rice-Ramsberger-Kassel-Marcus (RRKM) calculations gave the threshold (28360 +/- 60 cm-1) for the isomerization process.
Publisher
AMER INST PHYSICS
Issue Date
1991-09
Language
English
Article Type
Article
Keywords

LASER-INDUCED FLUORESCENCE; ELECTRONIC STATES; RATE CONSTANTS; OXIRENE INTERMEDIATE; PHOTOFRAGMENTATION; REARRANGEMENT; DISSOCIATION; CH2(X3B1); ABINITIO; CO

Citation

JOURNAL OF CHEMICAL PHYSICS, v.95, no.6, pp.4081 - 4093

ISSN
0021-9606
DOI
10.1063/1.460764
URI
http://hdl.handle.net/10203/65940
Appears in Collection
CH-Journal Papers(저널논문)
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