Knowledge Bank

The structure of $HBr-CO3$ has recently been reported to be T-shaped from IR absorption spectroscopy measurements.1 This geometry differs considerably from the linear or near-linear structures of $CO2-HF$ and $CO3\cdot HCl$. Presumably, the lower gas-phase acidity of HBr and the higher polarizability of Br results in a stronger bromine-carbon interaction in the $HBr-CO5$ complex. Here, we report further details of the structure and dynamics of $HBr-CO2$ from a FT-microwave investigation. A number of b-type transitions have been measured for $H79Br-CO2$ and $H81Br-CO2$. Using an asymmetric rotor, centrifugal distortion Hamiltonian with first order nuclear-electric quadrupole interaction terms, we report the following rotational and hyperfine constants. [FIGURE] Calculations of the geometry of the complex indicate a planar T-shaped structure with a OC-Br angle of $\approx 90\circ$. The bromine-carbon bond distance is 3.586 {\AA} and the hydrogen is positioned at an angle of $66\Delta$ (or $114\circ$) from the a axis of the complex. The $Ka=1-0$ separation observed here differ substantially ($\approx 1200MHz$) from that calculated from the constants of Sharpe et al1, suggesting hydrogen tunneling between the two equivelent C, structures with a $C2$, transition state. We are currently searching for $Ka=1\to Ka=2$ transitions to confirm this assertion.