Kinetic Effects on the Cycloaddition of 1,3-Cyclohexadiene to the 3C-SiC(001)-3 × 2 Surface Studied via Ab Initio Molecular Dynamics

Silicon carbide (SiC) surfaces are often the semiconductor material of choice for applications under extreme conditions or with biocompatibility requirements. The SiC(001)-3 × 2 surface has a top Si tilted dimer that should react with π bonds in organic molecules, potentially forming a well-ordered semiconductor-organic interface. Ab initio molecular dynamics simulations of a prototype cycloaddition system, 1,3-cyclohexadiene (CHD) + SiC(001)-3 × 2, reveal that four products form via a two-step carbocation mechanism:  [4 + 2] intradimer adduct, [2 + 2] intradimer adduct, H abstraction, and [4 + 2] subdimer adduct. The longer distance between dimers eliminates interdimer adducts that form on the Si(100)-2 × 1 system. CHD can wander more than 100 Å or 20 ps before finding the proper reactive environment. The intermediate lifetime ranges from 50 fs, when CHD is perfectly oriented, to more than 18 ps, when the CHD repeatedly visits the unstable [2 + 2] subdimer adduct. The reorientation caused by the [2 + 2] subdimer adduct favors hydrogen abstraction. Unfortunately, the [4 + 2] subdimer adduct destroys the reconstruction by creating an unsaturated Si in the third layer, thereby preventing cycloaddition reactions from creating a well-defined hybrid interface on this surface.