Controlling bimolecular reactions: Mode and bond selected reaction of water with translationally excited chlorine atoms

Vibrational overtone excitation prepares water molecules in the ‖13〉⁻, ‖04〉⁻, ‖02〉⁻‖2〉, and ‖03〉⁻ local mode states for a study of the influence of reagent vibration on the endothermic bimolecular reaction Cl+H₂O→OH+HCl. The reaction of water molecules excited to the ‖04〉⁻ vibrational state predominantly produces OH(v=0) while reaction from the ‖13〉⁻ state forms mostly OH(v=1). These results support a spectator model for reaction in which the vibrational excitation of the products directly reflects the nodal pattern of the vibrational wave function in the energized molecule. Comparison of relative OH product yield from Cl+H₂O(‖04〉⁻) and the calibration reaction O(³P)+CH₃OH shows that vibrational excitation of water to the ‖04〉⁻ state leads to a near gas kinetic reaction rate (k(‖04〉⁻)=2×10⁻¹⁰ cm³ molecule⁻¹ s⁻¹). Relative rate measurements for the two vibrational states ‖03〉⁻ and ‖02〉⁻‖2〉, which have similar total energies but correspond to very different distributions of vibrational excitation, demonstrate the control that initially selected vibrations exert on reaction rates. The local mode stretching state ‖03〉⁻ promotes the Cl+H₂O reaction much more efficiently than the state having part of its energy in bending excitation (‖02〉⁻‖2〉). The localized character of the vibrational overtone excitation in water permits the observation of bond selected bimolecular reaction using this approach. The reaction of chlorine atoms with HOD molecules excited in the region of the third overtone of the O–H stretching vibration, 4ν_OH, produces at least an eightfold excess of OD over OH, reflecting the preferential cleavage of the vibrationally excited bond.