Fragmentation of two dimethylaluminum peroxy radical isomers formed by the hypergolic reaction of trimethyl aluminum with O2

Abstract

We have investigated computationally the kinetics and mechanisms for the fragmentation of the (CH3)2Al(O)O and CH3Al(O)OCH3 radicals formed by different O2-for-CH3 substitution reactions of (CH3)3Al with O2 at the CCSD(T)//6-311++G(3df,2p)//B3LYP/6-311++G(3df,2p) level of theory. The fragmentation of these radicals may play a key role in the chain-propagation of hypergolic combustion of trimethyl aluminum in air under ambient conditions. Rate constants for various product channels have been calculated under different pressures between 100 and 7600 Torr at temperatures from 300 to 2000 K. Among the many fragmentation processes of the CH3Al(O)OCH3 radical, produced by the predominant (CH3)3Al + O2 reaction, its exothermic isomerization to CH2Al(OH)OCH3 (−26.7 kcal/mol) with a small barrier (9.7 kcal/mol), to be followed by decomposition giving a reactive product pair CH3AlOH + CH2O with a small endothermicity of 15.8 kcal/mol is believed to be most important. The ensuing exothermic and barrierless reaction of CH3AlOH with O2 gives rise to a ring intermediate, cy_CH3Al(O2)OH (−70.5 kcal/mol), which readily fragments to HOAlO2 + CH3 with an overall exothermicity of 21.7 kcal/mol. The (CH3)2Al(O)O radical isomer, which is effectively the association complex of (CH3)2Al and O2, on the other hand, is stable and has no low-energy exit paths to reactive products including its isomer CH3Al(O)OCH3. Rate constants for the three key reactions leading to the ultimate formation of CH3 + HOAlO2 + CH2O have been calculated for combustion modeling applications.