Thermal electron attachment to C6F5X and C6H5X (X=I, Br, Cl, and F)

Abstract

Rate constants have been measured for thermal electron attachment to C6F5X (X=I, Br, Cl, F, and H) and C6H5X (X=I, Br, Cl, and F) at room temperature in N2 buffer gas (1–100 Torr) using the pulse-radiolysis microwave cavity method. For all the compounds studied, the rate constants are of the two-body type. Unexpectedly, the values for C6F5X except C6F5H are all the same (∼2×10−7 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 (7×10−12 cm3 molecule−1 s−1). For C6H5X, the value decreases dramatically with varying X from I to Br to Cl as 1.0×10−8 to 6.5×10−12 to 3×10−14 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 transient negative-ion state of each molecule, which results from a combination of the electron affinities of constituents (halogen atom X and C6F5 radical) and the strength of the C6F5–X (or C6H5–X) bond.