Thermochemistry and Thermal Decomposition of the Chlorinated Disilanes (Si2HnCl6-n, n = 0−6) Studied by ab Initio Molecular Orbital Methods

Abstract

The thermochemistry and thermal unimolecular decomposition reactions of the chlorinated disilanes have been characterized using ab initio molecular orbital techniques. Silylene, chlorosilylene, dichlorosilylene, and hydrogen elimination reactions and their reverse insertions were considered. Reactant, product, and transition-state geometries and vibrational frequencies were calculated at the MP2/6-31G(d,p) level. Energetics were obtained at the MP2/6-31+G(2df,p), MP4/6-31+G(2df,p), G2(MP2), and/or G2 levels of theory, depending on the number of chlorine atoms in the molecule. In addition to the expected insertion reactions, direct reaction paths for SiHCl + SiHnCl4-n h SiH2 + SiHn-1Cl5-n and SiHCl + SiHnCl4-n h SiCl2 + SiHn+1Cl3-n were observed, with energetic barriers lying a few kcal/mol above the insertion reactions. To our knowledge, these concerted, two-atom exchange reactions have not previously been observed or predicted. They appear to represent a new type of elementary reaction for these compounds. Heats of formation for the chlorinated disilane reactants and chlorinated silylsilylene products of hydrogen elimination were calculated using isodesmic reactions. Energy barriers and conventional transition state theory rate constants for all of the reactions are presented. These can provide a basis for the construction of a detailed mechanism for the multistep thermal decomposition of the chlorinated silanes, which plays an important role in the chemical vapor deposition of epitaxial silicon from the chlorinated silanes.