Knowledge Bank

The microwave spectrum of $HBr-CO2$ has been analyzed using a model developed by Baiochi and $Klemperer1$ to estimate both the equilibrium position of the H atom in the complex and the HBr bending amplitude. We find that the complex has a T-shaped heavy atom frame with the H atom directed away from the $CO2$ subunit, as initially inferred by $Zengetal.2$ from the infared spectrum. The equilibrium C-BrH angle is-$103\circ$, where $180\circ$ coresponds to a linear C-BrH configuration. Modeling the large-amplitude HBr bending vibration allows us to estimate zero-point probabilities for various values of the C-BrH angle. These probabilities are compared with results from dynamical $calculations3$ undertaken to interpret the OH product state $distributions4$ resulting from the photolysis of HBr in the $HBr-CO2$ cluster. We find that the probabilities for C-BrH angles less than $25\circ ,50\circ ,75\circ$, and $100\circ$ are < 0.001%, 0.14%, 6%, and 44%, respectively. From combined ab initio and quasiclassical trajectory calculations, Kudla and $Schatz3$ estimate that the reaction probability is only $\sim 3$ for C-BrH angles between 0 and $28\circ$, with larger angles giving even smaller probabilities. These results suggest that the quantum yield for the photoinitiated $H+CO2$ reaction in $HBr-CO2$ must be extremely small or that the conditions of the precursor limited reaction are still not fully understood. The difficult measurement of the absolute quantum yield in the photodissociation experiments is important to more fully understand the mechanism for the formation of OH radicals from a presumably very unfavorable initial reactant orientation.