Knowledge Bank

A hydrogen-bonded OH-CO complex has been identified along the reaction coordinate for the $OH+CO\leftrightarrow HOCO\to H+CO2$ reaction. The vibrational spectrum of the OH-CO complex has been examined in the OH overtone region at $1.4\mu m$ using infrared action spectroscopy, which relies on detection of $OH(v=1)$ fragments from vibrational predissociation by laser-induced fluorescence. The observed infrared spectrum of OH-CO consists of the pure OH overtone band at $6941.7cm-1$ and combination bands involving the simultaneous excitation of OH stretch and intermolecular vibrations, which appear 50 to $250cm-1$ higher in energy than the pure overtone. The rotational structure of the pure overtone band is indicative of a parallel transition of a linear OH-CO complex having a $P=3/2$ projection of the total angular momentum on the intermolecular axis, which arises from the unquenched electronic angular momentum of OH. The OH-CO binding energy, $D0\le 430cm-1$, is also established from the quantum state distribution of the OH fragments following pure overtone excitation. The strongest combination bands with 51.1, 57.1, and $247.3cm-1$ of intermolecular energy are attributed to geared bend and H-atom bend excitation, which are the modes that drive the transformation from OH-CO to HOCO. These combination bands exhibit rotational structures that are characteristic of perpendicular transitions $(\Delta P=\pm 1)$ to states with vibrational angular momentum from the bending motions of the complex. A complete analysis of the experimental results promises to yield a spectroscopic quality characterization of the $OH+CO\to HOCO$ reaction pathway. Permanant address: Department of Chemistry, Amherst College, Amherst, MA 01002-5000