On the hypergolicity of trimethyl aluminum in air

A theoretical study on the mechanisms and kinetics of the reaction of (CH₃)₃Al (TMA), an important industrial compound, with O₂ has been carried out at the CCSD(T)/6-311++G(3df,2p)//B3LYP/6-311++G(3df,2p) level in conjunction with the conventional transition state theory (TST) calculations. The potential energy surface (PES) of the reaction indicates that the TMA + O₂ system has two pathways leading to different product pairs: the first one passes through a tight transition state with a high energy barrier, 16.9 kcal/mol, producing (CH₃)₂AlO₂ + CH₃ and the other one goes via a loose roaming-like transition state with a much lower energy barrier, 6.8 kcal/mol, yielding CH₃Al(O)OCH₃ + CH₃. The barrier predicted for the former was found to be too high for combustion initiation under the ambient condition. The latter, however, may play a key role in initiating the hypergolic reaction of TMA in the air. Rate constants for both channels have been calculated for the temperature range of 300–2000 K. The pressure-independent rate constant for the TMA + O₂ reaction via the loose transition state is predicted to be k(T) = 3.05 × 10⁻²¹T^3.003 exp(−3226.1/T) cm³ molecule⁻¹ s⁻¹; this result gives the half-life of TMA in air under the ambient condition to be as short as 1.7 × 10⁻² s, which is sufficiently short for the hypergolic combustion initiation without even considering the ensuing rapid radical chain reactions.