Establishment of the C(2)H(5)+O(2) reaction mechanism: a combustion archetype.

The celebrated C(2)H(5)+O(2) reaction is an archetype for hydrocarbon combustion, and the critical step in the process is the concerted elimination of HO(2) from the ethylperoxy intermediate (C(2)H(5)O(2)). Master equation kinetic models fitted to measured reaction rates place the concerted elimination barrier 3.0 kcal mol(-1) below the C(2)H(5)+O(2) reactants, whereas the best previous electronic structure computations yield a barrier more than 2.0 kcal mol(-1) higher. We resolve this discrepancy here by means of the most rigorous computations to date, using focal point methods to converge on the ab initio limit. Explicit computations were executed with basis sets as large as cc-pV5Z and correlation treatments as extensive as coupled cluster through full triples with a perturbative inclusion of quadruple excitations [CCSDT(Q)]. The final predicted barrier is -3.0 kcal mol(-1), bringing the concerted elimination mechanism into precise agreement with experiment. This work demonstrates that higher correlation treatments such as CCSDT(Q) are not only feasible on systems of chemical interest but are necessary to supply accuracy beyond 0.5 kcal mol(-1), which is not obtained with the "gold standard" CCSD(T) method. Finally, we compute the enthalpy of formation of C(2)H(5)O(2) to be Delta(f)H degrees (298 K)=-5.3+/-0.5 kcal mol(-1) and Delta(f)H degrees (0 K)=-1.5+/-0.5 kcal mol(-1).