Isotopic studies of trans- and cis-HOCO using rotational spectroscopy: Formation, chemical bonding, and molecular structures.

HOCO is an important intermediate in combustion and atmospheric processes because the OH + CO → H + CO₂ 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 + C¹⁸O reaction predominately yields HOC¹⁸O for both isomers, but H18OCO is observed as well, typically at the level of 10%-20% that of HOC¹⁸O; the opposite propensity is found for the 18OH + CO reaction. DO + C¹⁸O yields similar ratios between DOC¹⁸O and D18OCO as those found for OH + C¹⁸O, 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 large ¹³C Fermi-contact term (aF) for trans- and cis-HO¹³CO 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]