Competing Mechanisms in the Acetaldehyde Functionalization of Positively Charged Hydrogenated Silicene.

Density functional theory calculations were used to elucidate the mechanism of the addition reaction of acetaldehyde to positively charged hydrogenated silicene (H-silicene). We found that the positively charged H-silicene plane could be partially restructured to form a vacant Si site, which enabled an additional nucleophilic addition reaction. After attachment of the acetaldehyde molecule to the H-silicene plane, two competing pathways were found to be involved in the hydrogen-abstraction process: a random-reaction mechanism and a chain-reaction mechanism. The theoretical results provided detailed information about stable structures and thermodynamic parameters of the reaction pathways, such as equilibrium geometries, Gibbs free energies, and the evolution of the spin densities and atomic charges. Our results reveal that the existence of a positive charge can significantly activate the grafting of unsaturated species on hydrogenated silicene, even if no silicon dangling bond is created proactively. The simulated Raman spectra of the two products were analyzed to elucidate the features of the competing mechanisms.