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The transition state for -hydride elimination in alkyl groups on the Pt(111) surface has been probed by studying the effects of fluorine substitution on the barriers to -hydride elimination, %@mt;sys@%%@ital@%E%@rsf@%%@sx@%H%@be@%‡%@sxx@%%@mx@%. Four different fluoroalkyl groups have been formed on the Pt(111) surface by dissociative adsorption of four fluoroalkyl iodides:  RCH2CH2−I (R CF3, CF3CH2, and CF3CF2) and (CF3)2CHCH2−I. In the absence of preadsorbed hydrogen, fluoroalkyl groups on the Pt(111) surface dehydrogenate via -hydride elimination to form unsaturated fluorocarbons and deposit hydrogen atoms onto the surface. Those hydrogen atoms then hydrogenate the remaining fluoroalkyl groups to produce fluoroalkanes that desorb rapidly from the surface. The kinetics of hydrogenation and fluoroalkane desorption are rate limited by the -hydride elimination step and thus serve as measures of the kinetics of -hydride elimination. The field effects of the fluorinated substituents increase the barriers to -hydride elimination with a reaction constant of F 19 ± 2 kJ/mol. The interpretation of this effect is that the -carbon atom in the transition state is cationic, RC···H‡, with respect to the reactant. The field effect of the fluorinated substituent energetically destabilizes the electron deficient -carbon atom in the transition state. This is consistent with observations made on the Cu(111) surface; however, the substituent effect is significantly smaller on the Pt(111) surface. On the Pt(111) surface, the transition state for -hydride elimination is less polarized with respect to the initial state alkyl group than on the Cu(111) surface. [ABSTRACT FROM AUTHOR]

Photodissociation of fluorine-substituted benzenes, including 1,3-difluorobenzene, 1,2,4-trifluorobenzene, 1,2,4,5-tetrafluorobenzene, and pentafluorobenzene, at 193 nm under collision-free conditions has been studied in separate experiments using multimass ion imaging techniques. HF elimination was found to be the major dissociation channel for all of these molecules. Small amounts of photofragments of C6H3F2 and C6H2F3 from 1,3-difluorobenzene and 1,2,4-trifluorobenzene, respectively, were also observed. They correspond to the minor dissociation channel of hydrogen elimination. Dissociation rates and fragment translational energy distributions obtained from experimental measurements suggest that HF and hydrogen elimination reactions occur in the ground electronic state. The potential energy surface obtained from ab initio calculations indicates that the four-center reaction in the ground electronic state is the major dissociation mechanism for the HF eliminations. A comparison with the RRKM calculation has been made. [ABSTRACT FROM AUTHOR]