INFRARED-SPECTROSCOPY OF CO2-D(H)BR - MOLECULAR-STRUCTURE AND ITS RELIABILITY

Abstract

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: R(cm)=3.58 angstrom, theta(BrCO)=79.8-degrees, and theta(HBrC)=93.1-degrees. 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 Moller-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 angstrom, theta(BrCO) = 89-degrees, and theta(HBrC) = 86-degrees. 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 angstrom 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.