EFFECT OF FLUORINATION ON THIOL REACTIVITY - REACTION OF 2,2,2-TRIFLUOROETHANETHIOL ON MO(LLO)

The reactions of 2,2,2-trifluoroethanethiol on Mo(110) were studied us ing temperature-programmed reaction, Auger electron, and infrared spec troscopies. The chemistry of trifluoroethanethiol on Mo(110) is quite complex and significantly different than that observed for ethanethiol . Most significant is the evolution at 265 K of trifluoroethyl radical from a saturation coverage of CF3CH2S-. Ethyl radical was not detecte d in the reaction of ethanethiol on Mo(110). The strong coverage depen dence for trifluoroethyl radical evolution and models depicting triflu oroethyl thiolate orientation at saturation coverage strongly suggest that surface crowding plays a significant role in radical formation. T he stability of the radical and the steric inhibition to finding an ad sorption site explain the evolution of the radical into the gas phase. C-S bond hydrogenolysis, yielding trifluoroethane, and defluorination , yielding difluoroethylene, are of nearly equal importance in the rea ction of trifluoroethyl thiolate, whereas C-S bond hydrogenolysis of e thyl thiolate to form ethane predominates. The C-S bond hydrogenolysis pathway is similar for the two thiols, occurring at approximately 300 K in both cases. Dehydrogenation and alkene elimination from CH3CH2S- occur at approximately 340 K, as the supply of surface hydrogen is de pleted through hydrogen recombination. In contrast, defluorination and fluoroalkene elimination from CF3CH2S- occur over a wide temperature range, 200-520 K. The relative facility of difluoroethylene formation is rationalized in thermodynamic terms. The formation of difluoroethyl ene on Mo(110) is nearly thermoneutral, due to the comparable strength s of the C-F and Mo-F bonds and the stability of difluoroethylene.