Thermochemistry for hydrocarbon intermediates chemisorbed on metal surfaces: CHn-m(CH3)(m) with n=1, 2, 3 and m <= n on Pt, Ir, Os, Pd, Ph, and Ru

To provide insight and understanding of the thermochemistry underlying hydr ocarbon rearrangements on transition metal surfaces, we report systematic s tudies of hydrocarbon radicals chemisorbed on metal clusters representing t he closest packed surfaces of the six second and third row group VIII trans ition metals. Using first principles quantum mechanics [nonlocal density fu nctional theory with exact HF exchange (B3LYP)], we find that (i) CH3-m(CH3 )(m) forms one bond to the surface, preferring the on-top site (eta(1)), (i i) CH2-m(CH3)(m) forms two bonds to the surface, preferring the bridge site (eta(2)), and (iii) CH1-m(CH3)(m) forms three bonds to the surface, prefer ring the 3-fold site (eta(3)). For all six metals, the adiabatic bond energ y is nearly proportional to the number of bonds to the surface, but there a re dramatic decreases in the bond energy with successive methyl substitutio n. Thus from CH3 to CH2CH3, CH(CH3)(2), and C(CH3)(3), the binding energy d ecreases by 6, 14, and 23 kcal/mol, respectively (out of similar to 50). Fr om CH2 to CHCH3 and C(CH3)(2), the binding energy decreases by 8 and 22 kca l/mol, respectively (out of similar to 100). These decreases due to methyl substitution can be understood in terms of steric repulsion with the electr ons of the metal surface. For CH to C(CH3), the bond energy decreases by 13 kcal/mol (out of similar to 160), which is due to electronic promotion ene rgies. These results are cast in terms of a thermochemical group additivity framework for hydrocarbons on metal surfaces similar to the Benson scheme so useful for gas-phase hydrocarbons. This is used to predict the chemisorp tion energies of more complex adsorbates.