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

Abstract

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, 4νOH, produces at least an eightfold excess of OD over OH, reflecting the preferential cleavage of the vibrationally excited bond.

Communication: Equivalence between symmetric and antisymmetric stretching modes of NH3 in promoting H + NH3 → H2 + NH2 reaction

“…Due to the mass difference between H and D, the stretching vibrations become manifestly local and their excitations lead to selective bond breaking. Such bond selectivity has been experimentally demonstrated for the H/Cl + HOD 46,47 and H/Cl + CDH 3 /CHD 3 reactions. 48,49 Similar bond selective behaviors can be expected for the title reaction as well.…”

Communication: State-to-state dynamics of the Cl + H2O → HCl + OH reaction: Energy flow into reaction coordinate and transition-state control of product energy disposal

“…This reaction and its reverse are of great importance in atmospheric chemistry 21 and have also served as a prototype for understanding mode specificity in reaction dynamics. 1,5 Experimental studies found that the excitation of the local OH bond to v OH = 4 in HOD enhances the O-H cleavage by eightfold, 22 and the released energy is mostly deposited into the internal excitation of the HCl product with internally cold OD. 23 These results suggest that the non-reactive OD moiety serves essentially as a spectator.…”

More on femtosecond bond formation

“…controlled the bimolecular reaction between semi-deuterated water and chlorine. 4 Those experiments took advantage of the fact that overtone excitation of either the OH or OD bond remained localized long enough to permit selective chemistry to occur following a collision with a chlorine atom. The femtosecond photoassociation and its chirp enhancement-the work highlighted in Wilson's report-bypass the need for long-lived intermediates and promise a fertile new field of chemical investigation.…”