PRESSURE-DEPENDENCE OF METAL-CATALYZED POLYMERIZATION IN THE GAS-PHASE - A STUDY OF RADIATIVE AND COLLISIONAL RELAXATION

In the gas phase Fe+-catalyzed polymerization of chlorobenzene Fe(C6H4 )(3)(+) (1) reacts with chlorobenzene to give (C6H4)(3)(+) (2) and Fe( C6H4)(4)(+) (3) in a ratio that depends on the chlorobenzene pressure, Increasing the chlorobenzene pressure favors formation of 2. Decreasi ng the pressure of chlorobenzene or increasing the pressure of collisi on gases Ar or C6F6 favors the formation of 3. The branching ratio for the formation of 2 and 3 changes 12-fold between the high- and low-pr essure behaviors. The results suggest that the loss of Fe+ from 1 to f orm 2 is nor a process analogous to collision-induced dissociation but involves a more active participation from the neutral chlorobenzene. A mechanism accounting for the pressure dependence is suggested, such that 1 is formed with considerable internal energy, which strongly aff ects the relative rate constants in the two reaction channels. At lowe r pressure 1 relaxes to the ground state mainly through a radiative pr ocess during the relatively longer time between collisions, whereas at higher pressure the shorter time between collisions does not allow fo r significant radiative relaxation, and hence, the reactions at higher pressure depict the behavior of 1 in an excited state. Incorporating a buffer gas at the higher pressure allows 1 to relax through collisio nal cooling and exhibit ground-state behavior. Comparison of the exper imentally observed behavior of the reaction is consistent with all the important features of the behavior predicted by the proposed mechanis m and yields a value for the radiative relaxation rate constant of 1.7 +/- 0.6 s(-1).