Theoretical investigation of germane and germylenedecomposition kinetics

Abstract

The dissociation kinetics of germane and its decomposition products were studied determining microcanonical kinetic constants with RRKM theory and integrating the master equation using a stochastic approach. Relevant reaction parameters were calculated through first principles calculations. Structures of reactants and transition states were determined at the B3LYP/aug-cc-pvtz level while energies were computed at the CCSD(T) level and extended to the complete basis set limit. Though similar for many aspects to the kinetics of decomposition of SiH₄, GeH₄ has some peculiar features that indicate a different chemical reactivity. It was found that the main decomposition channel leads to the formation of germylene, GeH₂, which rapidly decomposes to atomic Ge and H₂. The dissociation of GeH₂ to Ge and H₂ is a formally spin forbidden reaction characterized by an activation energy of 160.3 kJ mol⁻¹ calculated at the minimum energy crossing point between the singlet and triplet states. The intersystem crossing probability was explicitly included in the microcanonical simulations through Landau–Zener theory. It was found that its effect on the reaction rate is almost negligible, both because of the large spin–orbit coupling between the singlet and triplet states and for the fall off conditions prevailing in the examined pressure and temperature ranges. Kinetic constants of the main decomposition channels were determined as a function of pressure and temperature between 0.0013 and 10 bar and 1100 and 1700 K. The high and low pressure kinetic constants for GeH₄ decomposition are 6.4 × 10¹³ (T/K)^0.272 exp(−26 700 K/T) and 2.7 × 10⁴⁸ (T/K)^−9.05 exp(−31 600 K/T), while those for GeH₂ are 6.02 × 10¹² (T/K)^0.203 exp(−19 660 K/T) and 1.6 × 10²⁶ (T/K)^−3.06 exp(−21 121 K/T), respectively. A quantitative agreement with experimental data for GeH₄ decomposition could be obtained adopting a downward energy transfer parameter of 340 × (T/298 K)^0.85 cm⁻¹ in the collisional model, and assuming that atomic Ge can react fast with GeH₄ to form Ge₂H₂ and H₂, thus enhancing the germane decomposition rate and suggesting that a fast kinetic route leading to the Ge₂H₂ production can be active in the gas phase.