THERMAL ELECTRON DETACHMENT RATE CONSTANTS FOR THE MOLECULAR ANION OF PERFLUOROBENZENE

Rate constants for thermal electron detachment (TED) from the molecular anion of hexafluorobenzene (HFB) have been measured over the temperature range 300-350 K. The rate of TED by C6F6- increases exponentially with increased temperature in close agreement with the prediction, k-1(s-1) = 4.8 x 10(15) T3/2 k1 exp[ - (DELTA-S/R + EA/RT)], where EA = 12.0 kcal/mol is the electron affinity of HFB, S = 7.4 cal/K mol is the entropy of negative ionization of HFB, and k1 = 1.0 x 10(-7) cm-3 s-1 is the temperature-independent rate constant for electron capture (EC) by HFB. It is shown in this study that TED from C6F6- is sufficiently rapid at temperatures in excess of 300 K as to negate conclusions drawn in earlier studies of EC by HFB in which the rate of TED was assumed to be negligible. In addition, it is shown that C6F6- is highly susceptible to reaction with trace levels of molecular oxygen in the buffer gas and that this fact has undoubtedly caused additional complications in previous studies of the negative ion chemistry of HFB by techniques using buffer gas pressures near one atmosphere.