Energy and structure of the transition states in the reaction OH + CO → H + CO2
Two, quite different, experimental studies have been carried out on the reaction between OH and CO at low total pressures. In the first, rate constants have been determined at 82 and 106 K. At the lower temperature, measurements were made at 2 and 5 Torr total pressure, yielding k=(1.0 ± 0.12)× 10–13 cm3 molecule–1 s–1 and (0.91 ± 0.1)× 10–13 cm3 molecule–1 s–1, respectively. At 106 K and 4 Torr, k=(0.98 ± 0.08)× 10–13 cm3 molecule–1 s–1. Theoretical considerations show that the reaction must be in its low-pressure limit, yielding H + CO2, and that the vibrational ground-state adiabatic barrier to formation of HOCO must be <200 cm–1, significantly lower than estimated previously.
In the second series of experiments, a tunable diode laser has been used to observe transient absorptions on transitions in the ν3 infrared bands of the CO2 product of the reaction, when it is initiated by flash photolysis at room temperature. There is no excitation of the ν3 mode, and the overall vibrational distribution corresponds to an averaged vibrational energy yield of only 6%. It is concluded that energy is released largely as repulsion following passage through a transition state in which the OCO angle is ca. 171° and the O—C, C—O bond distances are very similar to those in isolated CO2.