Dehydrogenation of 1,4-cyclohexadiene on Si(001): A first-principles study

Abstract

The adsorption and reaction of 1,4-cyclohexadiene on the Si (001) surface are investigated by first-principles density-functional calculations. We find that there are two kinds of single di-sigma bonding configurations: One is the on-top (OT) structure on top of a single Si dimer and the other is the end-bridge (EB) structure across the ends of two adjacent Si dimers in the same dimer row. Formation of the OT and EB structures is kinetically facilitated through the pi-complex precursor state in which the electron-deficient down Si atom of the buckled dimer attracts the pi bond of 1,4-cyclohexadiene. However, there exists a drastic difference in dehydrogenation kinetics of the two single di-sigma bonding configurations. Dehydrogenation of the OT structure hardly takes place because of the existence of a high activation barrier of 1.34 eV, whereas that of the EB structure is kinetically feasible at room temperature with a relatively lower activation barrier of 0.68 eV. Thus, we can say that the recently observed dehydrogenation process in the single di-sigma bonding configuration would be associated with the EB structure rather than the OT structure. We also discuss the recent experimental data about the desorption of produced benzene and the formation of double di-sigma bonding configuration.