EXPERIMENTAL AND THEORETICAL-STUDY OF THE REACTION K+HCL

The reaction K + HCl --> KCl + H was studied by the pulsed photodissoc iation at 193.3 nm of KCl vapor to produce K atoms in an excess of HCl and He bath gas, followed by time-resolved laser induced fluorescence spectroscopy of atomic K at 766.5 nm [K(4 P-2(3/2) - 4 S-1(1/2))]. Th e HCl concentration was monitored by absorption spectroscopy at 184.9 nm. This reaction exhibits non-Arrhenius behavior, with the rate coeff icient being given by k(252 K < T < 780 K) = (1.69+/-0.52) x 10(-10) e xp[-(15.21 +/- 2.00) kJ mol-1/RT] + (1.51 +/- 0.12) X 10(-11) exp[-(4. 94 +/- 1.72) kJ mol-1/RT]cm3 molecule-1 s-1. The quoted uncertainties are 2sigma. This result is in very good accord with several molecular beam studies, whose relative reaction cross sections can now be put on to an absolute basis. Ab initio calculations were then employed to det ermine the saddle points on the reaction potential energy hypersurface as a function of the K-Cl-H angle. There is a marked steric effect, w ith the reaction proceeding through a linear transition state or one t hat is strongly bent (theta(KClH) = 49.1-degrees). The reaction is als o characterized by a late barrier, in accord with the observed enhance ment of the reaction cross section by vibrational excitation of the HC l. Transition state theory calculations with the linear transition sta te are shown to be in excellent accord with the experimental results, and indicate that the non-Arrhenius behavior of the reaction is caused by a very loose transition state, rather than a significant contribut ion to the reaction from vibrationally excited HCl at higher temperatu res. Finally, the influence of the reverse reaction on the chemistry o f meteor-ablated potassium in the upper atmosphere is discussed.