Chemical response of aldehydes to compression between (0001) surfaces of α-alumina

First-principles molecular dynamics simulations are used to investigate the chemical response of acetaldehyde molecules (MeCHO) to compression and decompression between (0001) surfaces of α-alumina (Al₂O₃), with pressures reaching approximately 40 GPa. The results demonstrate that the MeCHO molecules are transformed into other chemical species through a range of chemical processes involving the formation of C–O and C–C bonds between MeCHO monomers as well as proton transfer. The mechanistic details of a representative set of the observed reactions are elucidated through analysis of maximally localized Wannier functions. Analysis of the changes in structure demonstrates that the main role of compression is to reduce the distances between MeCHO molecules to facilitate the formation of C–O bonds. Additional examination of the electronic structure demonstrates that the surface plays a role in facilitating proton transfer by both rendering hydrogen atoms in adsorbed MeCHO molecules more acidic and by acting as a proton acceptor. In addition, adsorption of the MeCHO molecules on the surface renders the sp² carbon atoms in these molecules more electrophilic, which promotes the formation of C–C and C–O bonds. It is suggested that the reaction products may be beneficial in the context of wear inhibition. Comparison of the surface structure before compression and after decompression demonstrates that the aldehydes and reaction products are capable of inhibiting irreversible changes in the structure as long as there is at least a monolayer coverage of these species. As a whole, the study sheds light on the chemical behavior of the aldehydes in response to uniaxial compression in nanoscopic contacts that likely applies to other molecules containing carbonyl groups and other metal oxide surfaces.