An investigation of the lowest reaction pathway of propene + BCl₃ decomposition in chemical vapor deposition process

The lowest reaction pathway or one of the favored possible paths in the CVD process of preparing boron carbides with BCl₃-C₃H₆(propene)-H₂ precursors was searched theoretically, which involves 95 transition states and 103 intermediates. The geometries of the species were optimized by employing the B3PW91/6-311G(d,p) method. The transition states as well as their linked intermediates were confirmed with frequency and the intrinsic reaction coordinates analyses. The energy barriers and the reaction energies were evaluated with the accurate model chemistry method at G3(MP2) level after a non-dynamical electronic correlation detection. The heat capacities and entropies were obtained with statistical thermodynamics, and the heat capacities were fitted into analytical equations. The Gibbs free energies at 298.15 K for all of the reaction steps were reported. The energies at any temperature could be derived classically by using the analytical heat capacities. All the possible elementary reactions, including both direct decomposition and the radical attacking dissociations, for each reaction step were examined, and the one with the lowest energy or energy barrier was further studied in the next step. It was found that there are 19 reaction steps in the lowest path to produce the final BC₃ cluster including two steps of initializing the reaction chain of producing H and Cl radicals. The highest energy in the lowest reaction pathway is 215.1 kJ/mol at 298.15 K and that for 1,200 K is 275.1 kJ/mol. The results are comparable with the most recent experimental observation of the apparent activation energy 208.7 kJ/mol.