THERMAL ELECTRON-ATTACHMENT TO C6F5X AND C6H5X (X=I,BR,CL, AND F)

Rate constants have been measured for thermal electron attachment to C 6F5X (X = I, Br, Cl, F, and H) and C6H5X (X = I, Br, Cl, and F) at roo m temperature in N2 buffer gas (1-100 Torr) using the pulse-radiolysis microwave cavity method. For all the compounds studied, the rate cons tants are of the two-body type. Unexpectedly, the values for C6F5X exc ept C6F5H are all the same (approximately 2 x 10(-7) cm3 molecule-1 s- 1), which are higher than most of the previous values, while that for C6F5H, measured in Xe and Ar buffer gases, is very low (7 x 10(-12) cm 3 molecule-1 s-1). For C6H5X, the value decreases dramatically with va rying X from I to Br to Cl as 1.0 x 10(-8) to 6.5 x 10(-12) to 3 x 10( -14) cm3 molecule-1 s-1, and that for C6H5F must be much lower than 10 (-13) cm3 molecule-1 s-1. These results for the magnitude of the rate constant are rationalized by the variation in the energy of a transien t negative-ion state of each molecule, which results from a combinatio n of the electron affinities of constituents (halogen atom X and C6F5 radical) and the strength of the C6F5-X (or C6H5-X) bond.