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 + CO{sub 2} reaction represents the final step for the production of CO{sub 2} 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 + CO{sub 2} in our discharge source, but cis-HOCO appears to be roughly two times more abundant when starting from H + CO{sub 2}. Using isotopically labelled precursors, the OH + C{sup 18}O reaction predominately yields HOC{sup 18}O for both isomers, but H{sup 18}OCO is observed as well, typically at the level of 10%-20% that of HOC{sup 18}O; the opposite propensity is found for the {sup 18}OH + CO reaction. DO + C{sup 18}O yields similar ratios between DOC{sup 18}O and D{sup 18}OCO as those found for OH + C{sup 18}O, suggesting that some fraction of HOCO (or DOCO) may be formed from the back-reaction H + CO{sub 2}, which, at the high pressure of our gas expansion, can readily occur. The large {sup 13}C Fermi-contact term (a{sub F}) for trans- and cis-HO{sup 13}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.