Hydrogen abstraction from n-butyl formate by H˙ and HO₂˙

The combustion chemistry of esters has been elucidated in the past through the study of smaller formates and acetates. Hydrogen abstraction from the fuel as an initiation step is mostly modeled based on estimations for similar abstractions from nonoxygenated hydrocarbons. This study reports computed ab initio rates for abstractions by H˙ and HO₂˙ radicals from the recently proposed biofuel candidate n-butyl formate. The energies are evaluated with a double hybrid density functional that performs especially well for barrier heights (B2KPLYP/aug-cc-pvtz). Hindered rotation of HO₂˙ with respect to n-butyl formate is treated using accurate eigenvalue summation and shows large influence on the rates. Transition states at the γ and δ positions are still influenced by the formate group. The abstraction from the γ carbon by HO₂˙ is slowest, although proceeding over the lowest barriers, due to the important influence of transition state entropies. A comparison with smaller esters and n-butanol shows that estimated rates deviate within 1 order of magnitude from the ab initio computations for similar groups in n-butyl formate.