Theoretical ab initio study of TiCl₄ ammonolysis: Gas phase reactions of TiN chemical vapor deposition

The mechanism of TiCl₄ ammonolysis has been studied theoretically at ab initio Hartree-Fock, B3LYP, MP2 and CCSD(T)//B3LYP levels using effective core potentials for Ti and Cl and 6-31G* basis sets for N and H. TiCl₄ and products of its ammonolysis form five- and six-coordinated complexes with ammonia, which intermediate substitution of Cl atoms by NH₂ groups. Transition state energies for the subsequent steps of ammonolysis decrease with increasing number of NH₂ groups bound to Ti. The energy of the transition state for the first step of ammonolysis is 19 kcal mol^-1 above the energy of the reactants (TiCl₄ + NH₃) and 8 kcal mol^-1 above the products (TiCl₃NH₂ + HCl). The following steps have transition states energetically located below the products, indicating weak hydrogen bonded complex formation as intermediate between transition state and product. A thermodynamic estimation shows the last step of ammonolysis to be endothermic, while the first three steps are exothermic if the adduct formation energy is taken into account.