Recently, we reported STM images of the methylated Si(111) surface [prepared through chlorination−alkylation of the Si(111)−H surface] taken at 4.7 K, indicating that the torsion angle of the methyl group with respect to the subsurface silicon layer is φ = 23 ± 3°. Repulsions between H atoms in adjacent methyl groups are minimized at 30°, while repulsions between H atoms and second layer Si atoms are minimized at 60°. The experimental result of 23° is surprising because it suggests a tendency of the methyl group toward the eclipsed configuration (0°) rather than staggered (60°). In contrast, extensive fully periodic quantum mechanical Density Functional Theory studies of this surface give an equilibrium torsion angle of 37.5°, indicating a tendency toward the staggered configuration. This discrepancy can be resolved by showing that the CH_3 on the step edges and etch pits interacts repulsively with the CH_3 on the surface terraces unless a stacking fault is introduced between the first and second silicon layers of the Si(111)−CH_3 surface terraces. We propose that this could occur during the chlorination−alkylation of the Si(111)−H surface. This stacking fault model predicted φ = 22.5° measured with respect to the bulk (corresponding to φ = 37.5° with respect to the second layer Si atoms). This model can be tested by measuring the orientation of the CH_3 within the etch pits, which should have φ = 37.5°, or by making a surface without etch pits, which should have φ = 37.5°
