Ethylperoxy radical: approaching spectroscopic accuracy via coupled-cluster theory

Abstract

Interest in peroxy radicals derives from their central role in tropospheric and low-temperature combustion processes; however, their transient nature limits the scope of possible experimental characterization. As a result, theoretical methods (notably, coupled-cluster theory) have become indispensable in the reliable prediction of properties of such ephemeral open-shell systems. Herein, the and à state conformers of ethylperoxy radical (C₂H₅O₂) have been structurally optimized at the CCSD(T)/ANO2 level of theory. Relative enthalpies at 0 K [including à ← transition origins (T₀)] are reported, incorporating CCSD(T) electronic energies extrapolated to the complete basis set limit via the focal point approach. Higher-level computations, employing basis sets as large as cc-pV5Z and post-HF methods up to CCSDT(Q), prove essential in achieving predictions to within 10 cm⁻¹ for experimental T₀; we predict 7363 and 7583 cm⁻¹ for the trans and gauche conformers, respectively. Furthermore, predictions of state fundamental transitions incorporating CCSD(T)/ANO0 anharmonic contributions are given. For each conformer, all 21 modes were characterized, improving upon the 16 modes reported in the experimental literature, and providing predictions for the 5 remaining modes.