1. Isotopic studies of trans- and cis-HOCO using rotational spectroscopy: Formation, chemical bonding, and molecular structures.
- Author
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McCarthy, Michael C., Martinez, Jr., Oscar, McGuire, Brett A., Crabtree, Kyle N., Martin-Drumel, Marie-Aline, and Stanton, John F.
- Subjects
CARBOXYL group ,SPECTRUM analysis ,MOLECULAR structure ,CHEMICAL bonds ,FOURIER transform spectroscopy - Abstract
HOCO is an important intermediate in combustion and atmospheric processes because the OH + CO → H + CO
2 reaction represents the final step for the production of CO2 in hydrocarbon oxidation, and theoretical studies predict that this reaction proceeds via various intermediates, the most important being this radical. Isotopic investigations of trans- and cis-HOCO have been undertaken using Fourier transform microwave spectroscopy and millimeter-wave double resonance techniques in combination with a supersonic molecular beam discharge source to better understand the formation, chemical bonding, and molecular structures of this radical pair. We find that trans- HOCO can be produced almost equally well from either OH + CO or H + CO2 in our discharge source, but cis-HOCO appears to be roughly two times more abundant when starting from H + CO2 . Using isotopically labelled precursors, the OH + C18 O reaction predominately yields HOC18 O for both isomers, but H18OCO is observed as well, typically at the level of 10%-20% that of HOC18 O; the opposite propensity is found for the 18OH + CO reaction. DO + C18 O yields similar ratios between DOC18 O and D18OCO as those found for OH + C18 O, suggesting that some fraction of HOCO (or DOCO) may be formed from the back-reaction H + CO2 , which, at the high pressure of our gas expansion, can readily occur. The large13 C Fermi-contact term (aF) for trans- and cis-HO13 CO implicates significant unpaired electronic density in a σ-type orbital at the carbon atom, in good agreement with theoretical predictions. By correcting the experimental rotational constants for zero-point vibration motion calculated theoretically using second-order vibrational perturbation theory, precise geometrical structures have been derived for both isomers. [ABSTRACT FROM AUTHOR]- Published
- 2016
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