Intermolecular vibrations of the hydrogen bonded OH–CO reactant complex

Abstract

Intermolecular vibrations of the linear OH–CO reactant complex have been observed as combination bands in the OH overtone region using infrared action spectroscopy. Rotational analyses and simulations of the band structures have been carried out for transitions to geared bend, excited spin–orbit, and H-atom bend states with 50–250 cm−1 of intermolecular excitation. The projection quantum number associated with each of these upper states is identified through the intensity profile of the band contour, missing rotational lines, and/or parity splitting of individual rotational lines. Intermolecular states with projection quantum numbers P=1/2 and 5/2 are observed for each of the two bending modes, arising from coupling of the unquenched angular momentum of OH with the vibrational angular momentum associated with the bending motion of the complex. An additional P=1/2 state is attributed to spin–orbit excitation, which shifts to higher energy than in free OH and gains infrared transition strength through the spin-decoupling interaction. The intermolecular energy level pattern is also examined in the context of the Renner-Teller interaction and spin–orbit coupling. The intermolecular bends of the OH–CO complex are of special interest because they probe portions of the reaction path leading to trans-HOCO formation.