Infrared spectroscopy of CO2–D(H)Br: Molecular structure and its reliability

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Abstract
A high resolution rovibrational absorptionspectrum of the weakly bonded CO2–DBr complex has been recorded in the 2350 cm−1 region by exciting the CO2 asymmetric stretch vibration with a tunable diode laser. The CO2–DBr band origin associated with this mode is 2348.2710 cm−1, red‐shifted by 0.87 cm−1 from uncomplexed CO2. The position of the hydrogen atom is determined from differences in moments‐of‐inertia between CO2–DBr and CO2–HBr, i.e., by using the Kraitchman method. From this, we conclude that ground state CO2–H(D)Br has an average geometry that is planar and inertially T‐shaped, with essentially parallel HBr and CO2 axes. Average values of intermolecular parameters are: R cm=3.58 Å, θBrCO=79.8°, and θHBrC=93.1°. The validity of using the Kraitchman method, which was designed for use with rigid molecules, with a floppy complex like CO2–HBr is discussed. The experimental structure is corroborated qualitatively by results from Mo/ller–Plesset second‐order perturbation calculations, corrected for basis set superposition errors. The theoretical equilibrium geometry for the inertially T‐shaped complex is planar with structural parameters: R CBr=3.62 Å, θBrCO=89°, and θHBrC=86°. A number of cuts on the four dimensional intermolecular potential surface confirm large zero‐point amplitudes, which are known to be characteristic of such systems, and these cuts are used to estimate tunneling splittings. Tunneling is shown to occur by out‐of‐plane rotation of the H atom, in accord with the experimental observations of Rice et al. There is no significant in‐plane tunneling. A quasilinear hingelike isomer (OCO–HBr) with R OH=2.35 Å at equilibrium is calculated to be as stable as the T‐shaped complex; however, this species has yet to be observed experimentally. Photoinitiated reactions in CO2–HX complexes are discussed.