Experimental investigations of beta-hydrogen elimination from alkoxy and al
kyl 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. Previou
s studies have shown that fluorination increases the activation barrier (De
lta 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 (RCH2CH
2,(ad) --> RCH=CH2,(ad) + H-(ad), where R = CH3, CF3) on the Cu(111) surfac
e. The increase in barrier height with increasing fluorination was attribut
ed 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 fluori
nation does not alter the mechanism for beta-hydrogen elimination from etho
xy groups. Furthermore, the DKIE measurements confirm that the effects of f
luorine on the kinetics of beta-hydrogen elimination do not result from the
change in mass when hydrogen is substituted by fluorine. A systematic stud
y of fluorine substitution of surface-bound isopropoxy groups reveals combi
ned steric and electronic effects. An excellent correlation is found betwee
n the Delta E-act for beta-hydrogen elimination in adsorbed alkoxy groups a
nd the calculated reaction energetics (Delta H-rxn) for gas-phase dehydroge
nation of fluorinated alcohols in trans antiperiplanar conformations (e.g.,
RCH2OH(g) --> RCH=O-(g) + H-2,H-(g),H- where the hydroxyl hydrogen is anti
periplanar to a carbon and the oxygen is antiperiplanar to a fluorine). Ham
mett plots for P-hydrogen elimination give a reaction parameter of rho = -2
6. These correlations both suggest that the transition state for beta-hydro
gen 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 el
imination.