Experimental and theoretical study of the reaction K+HCl

Abstract

The reaction K+HCl→KCl+H was studied by the pulsed photodissociation 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 2P3/2–4 1S1/2)]. The HCl concentration was monitored by absorption spectroscopy at 184.9 nm. This reaction exhibits non-Arrhenius behavior, with the rate coefficient being given by k(252 K<T<780 K)=(1.69±0.52)×10−10 exp[−(15.21±2.00) kJ mol−1/RT]+(1.51±0.12)×10−11 exp[−(4.94±1.72) kJ mol−1/RT] cm3 molecule−1 s−1. The quoted uncertainties are 2σ. This result is in very good accord with several molecular beam studies, whose relative reaction cross sections can now be put onto an absolute basis. Ab initio calculations were then employed to determine the saddle points on the reaction potential energy hypersurface as a function of the K–Cl–H angle. There is a marked steric effect, with the reaction proceeding through a linear transition state or one that is strongly bent (θKClH=49.1°). The reaction is also characterized by a late barrier, in accord with the observed enhancement of the reaction cross section by vibrational excitation of the HCl. Transition state theory calculations with the linear transition state 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 contribution to the reaction from vibrationally excited HCl at higher temperatures. Finally, the influence of the reverse reaction on the chemistry of meteor-ablated potassium in the upper atmosphere is discussed.