PULSED-NOZZLE FOURIER-TRANSFORM MICROWAVE INVESTIGATION OF THE LARGE-AMPLITUDE MOTIONS IN HBR-CO2

Microwave spectra of (HBr)-Br-79-CO2 and (HBr)-Br-81-CO2 and their D a nd O-18 isotopomers have been measured using a pulsed-nozzle Fourier-t ransform microwave spectrometer, The spectra are consistent with a T-s haped Br-CO2 geometry, as concluded previously by Zeng et al. [Y. P. Z eng, S. W. Sharpe, S. K. Shin, C. Wittig, and R. A. Beaudet, J. Chem. Phys. 97, 5392 (1992)] from an investigation of the rotationally resol ved infrared spectrum of the asymmetric C=O stretching vibration of th e complex. Only b-type K-a = 1<--0 transitions are observed, with the symmetry-allowed a-type Delta K-a = 0 transitions being too weak to be detected. The absence of a strong a-type spectrum implies that the HB r axis is nearly parallel to the b-inertial axis of the complex, which itself is parallel to the C-infinity axis of the CO2. The K-a = 1<--0 energy level spacing is approximately 1.2 GHz larger than that predic ted from the infrared rotational constants due to an additional contri bution to the splitting arising from the hindered-rotation tunneling o f the HBr through a C-s or C-2v transition state. Because the Bose-Ein stein statistics of the spin-zero oxygen nuclei allow only symmetric t unneling 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., sin ce the tunneling state selection rules are symmetricttsymmetric and an tisymmetric<-->antisymmetric for the band studied. A dynamical modelin g of the Br-79 and Br-81 nuclear quadrupole coupling constants gives a n equilibrium angle CBrH angle of similar to 103 degrees and an HBr ze ro-point bending amplitude of similar to 24 degrees. The implication o f this study on the interpretation of experiments on the photoinitiate d reaction of H atoms with CO2 using an HBr-CO2 precursor are discusse d.