A Study of the Temperature Dependence of Ion-Molecule Association Reactions. Halide Ion Addition to a Selected Group of Lewis Acids.

Recent evidence has indicated that the ion-molecule association reaction BF$\sb 3$ + F$\sp -$ - thinspace- BF$\sb 4\sp -$ has both collisional stabilization and radiative stabilization pathways. Preliminary studies indicated that other boron trihalide systems (BF$\sb 3$ + Cl$\sp -$, Br$\sp -$, and BCl$\sb 3$ + Cl$\sp -$, Br$\sp -$) behave similarly. We have studied the temperature dependence of these systems in an effort to learn more about the individual rate coefficients that constitute the standard association mechanism modified to include a radiative stabilization pathway. In order to determine properties of a system which play an important role in the opening of a radiative stabilization channel, we extended our studies to include other halogen ion-Lewis acid addition reactions: SiF$\sb 4$ + F$\sp -$, Cl$\sp -$, and Br$\sp-$; and SF$\sb4$ + F$\sp-$, Cl$\sp-$, and Br$\sp -$. These allow us to assess the role of number of degrees of freedom, exoergicity, and complex lifetime in influencing the association rate. These systems were studied using a selected-ion flow tube (SIFT) in the temperature region 219 to 408 K for pressures in the region 0.2 to 0.9 torr. Our results yield overall rates which have a negative dependence upon temperature (k$\sb{\rm obs} = {\rm AT}\sp{\rm -n}).$ This temperature dependence is consistent with a theoretical model in which rotation and vibration are included for calculation of n, the magnitude of the temperature dependence. The magnitude of n ranges from 1.8 for the BF$\sb 3$ + F$\sp -$ system to 3.3 for the SiF$\sb 4$ + Cl$\sp -$ system. In addition all of the boron systems radiate with the possible exception of BF$\sb 3$ + Br$\sp -$ where the unimolecular decomposition back to reactants is very fast. The SiF$\sb 4$ and SF$\sb 4$ systems appear not to possess a radiative stabilization pathway