Rotational dependence of the proton-transfer reaction HBr⁺ + CO₂ → HOCO+ +Br. I. Energy versus angular momentum effects

Cross sections for the endothermic proton-transfer reactions of rotationally state-selected HBr+ and DBr+ ions with CO₂ were measured in a guided ion beam apparatus in order to determine the influence of rotational excitation and collision energy in the center of mass (c.m.) system on the cross section. Ab initio calculations were performed to obtain energetic information about reactants, intermediates, and products. In the experiment HBr⁺ and DBr⁺ ions were prepared with the same mean rotational quantum number but different mean rotational energies as the rotational constants differ by about a factor of two. The mean rotational energy was varied from 1.4 to 66.3 meV for HBr⁺ and from 0.7 to 43.0 meV for DBr⁺. Collision energies (E_c.m.) ranged from 0.32 to 1.00 eV. Under all conditions considered, an increase in the rotational excitation leads to a decrease in the cross section for both reactions. However, the effect is more pronounced for the higher collision energies. For E _c.m. =1.00 and 0.85 eV; a comparison between the results for HBr ⁺ and DBr⁺ indicates that the cross section is dominated by effects of rotational energy rather than angular momentum. For lower collision energies the cross sections for the deuteron transfer and the proton transfer are in best agreement if not compared for the same c.m. collision energy but for the same value of the difference between the collision energy and the reaction enthalpy.