Thermal rate constants, energy dependence and isotope effect for laser initiated halogen--hygrogenhalide reactions

Abstract

The laser initiated chemical reaction method has been used to determine the thermal reaction rate constants for the reactions Y + HX ..-->.. HY(v greater than or equal to 0) + X, Y = Cl or Br and X = Br or I, in the temperature range 220 to 400/sup 0/K. The Y atoms are produced by Y/sub 2/ photolysis. The reaction rates vary slowly with temperature. For Cl + HI the effective cross section reaches a maximum of 31 A/sup 2/ near 300/sup 0/K. For Cl + HBr, although no maximum is observed within the limited temperature range, the rate is increasing much less rapidly at high temperature than at low temperature. For Br + HI, the cross section decreases slightly with increasing temperature. The dependence of the rate on reagent translational, rotational energy and H-D substitution has been measured for the reaction Cl + HI. It is found that a factor of 3.9 increase in translational velocity over room temperature actually decreases the cross section for Cl + HI and DI by a factor of 9 and 6.6, respectively. However, at 13.8 kcal/mole collisional energy, the cross section increases by 1.2 as the rotational temperature increases by 1.2 as the rotational temperature increases from 223 to 295/sup 0/K. The isotope effect increases in favor of HI from 1.5 to 2.7 as the temperature decreases from 400 to 223/sup 0/K. A reaction model is proposed for thermal reaction in which the attacking halogen atom is attracted to the halogen end of the hydrogenhalide and then rotation of the hydrogen completes the reaction. The reaction at 13.8 kcal/mole translational energy results mostly from head-on collinear encounters of Cl with H of HI. The isotope effect is thought to be a result of the difference in rotational velocity between HI and DI and perhaps of the existence of a barrier along the rotational coordinate.