Pulsed-nozzle Fourier-transform microwave investigation of the large-amplitude motions in HBr–CO2

Abstract

Microwave spectra of H79Br–CO2 and H81Br–CO2 and their D and 18O isotopomers have been measured using a pulsed‐nozzle Fourier‐transform microwave spectrometer. The spectra are consistent with a T‐shaped Br–CO2 geometry, as concluded previously by Zeng et al. [Y. P. Zeng, S. W. Sharpe, S. K. Shin, C. Wittig, and R. A. Beaudet, J. Chem. Phys. 97, 5392 (1992)] from an investigation of the rotationally resolved infrared spectrum of the asymmetric C=O stretching vibration of the complex. Only b‐type K a =1←0 transitions are observed, with the symmetry‐allowed a‐type ΔK a =0 transitions being too weak to be detected. The absence of a strong a‐type spectrum implies that the HBr axis is nearly parallel to the b‐inertial axis of the complex, which itself is parallel to the C∞ axis of the CO2. The K a =1←0 energy level spacing is approximately 1.2 GHz larger than that predicted from the infrared rotational constants due to an additional contribution to the splitting arising from the hindered‐rotation tunneling of the HBr through a C s or C2v transition state. Because the Bose–Einstein statistics of the spin‐zero oxygen nuclei allow only symmetric tunneling states for K a even and antisymmetric tunneling states for K a odd, no doubling of the lines is observed. No evidence was obtained for this tunneling motion in the infrared spectrum of Zeng et al., since the tunneling state selection rules are symmetric↔symmetric and antisymmetric↔antisymmetric for the band studied. A dynamical modeling of the 79Br and 81Br nuclear quadrupole coupling constants gives an equilibrium ∠CBrH angle of ∼103° and an HBr zero‐point bending amplitude of ∼24°. The implication of this study on the interpretation of experiments on the photoinitiated reaction of H atoms with CO2 using an HBr–CO2 precursor are discussed.