INFRARED-ABSORPTION SPECTROSCOPY OF CO2-HX COMPLEXES USING THE CO2 ASYMMETRIC STRETCH CHROMOPHORE - CO2HF(DF) AND CO2HCL(DCL) LINEAR AND CO2HBR BENT EQUILIBRIUM GEOMETRIES

Infrared absorption spectra associated with the CO2 asymmetric stretch vibration have been recorded for weakly bonded gas-phase complexes of CO2 with HF, DF, HCl1 DCl, and HBr, using tunable diode laser spectroscopy and a pulsed slit expansion (0.15x38 mm2) that provides > 20 MHz overall resolution. Results obtained with CO2-HF are in agreement with earlier studies, in which the HF-stretch region near 3900 cm -1 was examined. In both cases, broad linewidths suggest subnanosecond predissociation. With CO2-DF, the natural linewidths are markedly narrower than with CO2-HF (e.g., 28 vs 182 MHz), and this difference is attributed to slower predissociation, possibly implicating resonances in the case of CO2-HF. Both CO2-HF and CO 2-DF exhibited overlapping features: simple P and R branches associated with a linear rotor, and P and R branches containing doublets. As in earlier studies, the second feature can be assigned to either a slightly asymmetric rotor with Ka = 1, or a hot band involving a low-frequency intermolecular bend mode. Results obtained with CO2-HCl are in excellent agreement with earlier microwave measurements on the ground vibrational state, and the vibrationally excited state is almost identical to the lower state. Like CO2-DF, linewidths of CO2-HCl and CO2-DCl are much sharper than those of CO2-HF, and in addition, CO2-HCl and CO2-DCl exhibited weak hot bands, as were also evident with CO2-HF and CO2-DF. Upon forming complexes with either HF or HCl, the asymmetric stretch mode of CO2 underwent a blue shift relative to uncomplexed CO2. This can be understood in terms of the nature of the hydrogen bonds, and ab initio calculations are surprisingly good at predicting these shifts. Deuteration of both HF and HCl resulted in further blue shifts of the band origins. These additional shifts are attributed to stronger intermolecular interactions, i.e., deuteration lowers the zero-point energy, and in a highly enharmonic field this results in a more compact average structure. While both HF and HCl complexes exhibit nearly linear geometries,CO2-HBr is asymmetric, with the Br-C symmetry line essentially perpendicular to the CO2 axis, and the H atom probably localized near one of the oxygens. Although the moments of inertia are insensitive to the location of the H atom in CO2-HBr, Bose-Einstein statistics require that odd K″a states are missing for C2v, symmetry, as is observed with T-shaped CO 2-(rare gas) complexes. However, we observe a full complement of odd and even Ka states, indicating that the H atom is not located symmetrically about the C2v, axis on the time scale of the measurement. With CO2-HBr, the low gas-phase acidity of HBr and the high Br-atom polarizability encourage a qualitative change in the geometry relative to CO2-HCl and CO2-HF. This has valuable implications for photoinitiated reactions in such complexes. © 1990 American Institute of Physics.