Alkyl–metal surface complexes: Methyl and lithium

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Abstract
Clusters containing 4 and 10 atoms of lithium interacting with methyl groups were investigated as models for alkyl–lithium surface complexes. Pople’s ab initio Gaussian 70 procedure was used to optimize geometrical arrangements starting from sites having a C3v site symmetry and selected on the basis of known chemical and physical properties. Two sites were identified as rational possibilities for binding of methyl to lithium surfaces. One of the two sites was calculated to be about 40 kcal more favorable than the other. Both sites are prevalent on Li(111). The position of CH3 above Li(111) and orientation of CH3 relative to the Li surface network, as well as binding strengths, orbital parentage, and charge distributions were qualitatively the same on both 4 and 10 atom models of substrate. The change in quantitative factors among the 4 and 10 atom models suggest that 7 to 13 atoms in the substrate are sufficient to approximate properties of surface complexes of lithium with alkyl hydrocarbon groups. A potential surface for motion of CH3 on Li(111) estimated with 10 points suggests that an activation energy of 35–40 kcal is to be experienced for associative desorption of methyl to ethane.

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