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

A high resolution rovibrational absorption spectrum 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: Rcm=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: RCBr=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 ROH=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.