Through Stronger Hindrance to Higher Reactivity: Influence of the Alkyl Chains on the Activation Energy of Ether Cleavage on Silicon

Reactivity in surface chemistry is often discussed in terms of the interaction between surface states and the functional groups of the reacting molecule. Herein, we demonstrate that for finite submonolayer coverage, i.e., surface coverage at which the molecular adsorbates cannot be treated as isolated molecules anymore, the seemingly innocent side chains of the adsorbate can also play a decisive role. For the example of ether cleavage on Si(001), which represents the surface analogue of an <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>S</mml:mi> <mml:mi>N</mml:mi></mml:msub> <mml:annotation>${\rm S}_{\rm N}$</mml:annotation></mml:semantics> </mml:math> 2-type reaction, we show both experimentally and based on ab initio calculations that steric hindrance by the side chains determines the activation energy for C-O dissociation into the final state. In contrast to a simple expectation, the stronger steric hindrance of the butyl group in butyl methyl ether leads to a lower activation energy for ether cleavage on Si(001) when compared to diethyl ether. This effect was traced back to different degrees of destabilization of the precursor and the transition state. Given the fact that for almost all technologically relevant processes the surface coverage is typically above the isolated-molecule limit, our findings are of general importance for surface functionalization and can help to properly design future experiments.