Transition state for β-elimination of hydrogen from alkoxy groups on metal surfaces

Experimental investigations of beta-hydrogen elimination from alkoxy and alkyl groups bound to a Cu(111) surface have been coupled with computational studies of gas-phase analogues to provide insight into the transition state for catalytic hydrogenation and dehydrogenation on metal surfaces. Previous studies have shown that fluorination increases the activation barrier (Delta E-act) to beta-hydrogen elimination in ethoxy groups (RCH2O(ad) --> RCH=O-(ad) + H-(ad), where R = CH3, CFH2, CHF2, CF3) and propyl groups (RCH2CH2,(ad) --> RCH=CH2,(ad) + H-(ad), where R = CH3, CF3) on the Cu(111) surface. The increase in barrier height with increasing fluorination was attributed to the inductive influence of fluorine, which energetically destabilizes a hydride-like transition state of the form [RCdelta+...Hdelta-](not equal). In this paper, deuterium kinetic isotope effects (DKIE) show that fluorination does not alter the mechanism for beta-hydrogen elimination from ethoxy groups. Furthermore, the DKIE measurements confirm that the effects of fluorine on the kinetics of beta-hydrogen elimination do not result from the change in mass when hydrogen is substituted by fluorine. A systematic study of fluorine substitution of surface-bound isopropoxy groups reveals combined steric and electronic effects. An excellent correlation is found between the Delta E-act for beta-hydrogen elimination in adsorbed alkoxy groups and the calculated reaction energetics (Delta H-rxn) for gas-phase dehydrogenation of fluorinated alcohols in trans antiperiplanar conformations (e.g., RCH2OH(g) --> RCH=O-(g) + H-2,H-(g),H- where the hydroxyl hydrogen is antiperiplanar to a carbon and the oxygen is antiperiplanar to a fluorine). Hammett plots for P-hydrogen elimination give a reaction parameter of rho = -26. These correlations both suggest that the transition state for beta-hydrogen elimination develops a greater partial positive charge on the carbinol carbon than is found in the adsorbed reactant. Furthermore, the transition state is energetically late in the reaction coordinate for beta-hydrogen elimination.