Quantum mechanics calculations of the thermodynamically controlled coverage and structure of alkyl monolayers on Si(111) surfaces

The heat of formation, ΔE, for silicon (111) surfaces terminated with increasing densities of the alkyl groups CH₃- (methyl), C ₂H₅- (ethyl), (CH₃)₂CH- (isopropyl), (CH₃)₃C- (terf-butyl), CH₃(CH ₂)₅- (hexyl), CH₃(CH₂) ₇(octyl), and C₆H₅- (phenyl) was calculated using quantum mechanics (QM) methods, with unalkylated sites being H-terminated. The free energy, ΔG, for the formation of both Si-C and Si-H bonds from Si-Cl model componds was also calculated using QM, with four separate Si-H formation mechanisms proposed, to give overall ΔGs values for the formation of alkylated Si(111) surfaces through a two step chlorination/ alkylation method. The data are in good agreement with measurements of the packing densities for alkylated surfaces formed through this technique, for Si-H free energies of formation, ΔG_H, corresponding to a reaction mechanism including the elimination of two H atoms and the formation of a C=C double bond in either unreacted alkyl Grignard groups or tetrahydrofuran solvent.