Adsorption of Structural and Stereoisomers of Cyclohexanediamine at the Ge(100)-2 x 1 Surface: Geometric Effects in Adsorption on a Semiconductor Surface

The effects of structural isomerism and stereoisomerism in molecular adsorption at surfaces are studied through the reaction of four cyclohexanediamine isomers-cis-1,2-, trans-1,2-, cis-1,4-, and trans-1,4-cyclohexanediamine-at the Ge(100)-2 x 1 semiconductor surface. Using a combination of X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy, we find that all isomers primarily adsorb by reacting with both functional groups, either forming an N-Ge dative bond to the surface or undergoing N-H dissociation to form Ge-N and Ge-H ordinary covalent bonds. Among the four isomers, differences are observed in the relative amounts of dative bonded versus dissociated amines. Density functional theory calculations explain these results by showing that strain of the surface-adsorbate bonds and in the cyclohexane backbone of the adsorbate give rise to differences in the reaction energetics. In particular, trans-1,4-cyclohexanediamine is the only isomer that must adopt the twist-boat conformation to interact with the surface via both functional groups, and the energetic penalty of this requirement leads to its distinct product distribution in which N dative bonding is more prevalent than for the other three isomers studied.