J. Kua et al., 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, J AM CHEM S, 122(10), 2000, pp. 2309-2321
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.