The cis- and trans-formylperoxy radical: fundamental vibrational frequencies and relative energies of the 2A′′ and à 2A′ states

Abstract

Acylperoxy radicals [RC(O)OO˙] play an important catalytic role in many atmospheric and combustion reactions. Accordingly, the prototypical formylperoxy radical [HC(O)OO˙] is characterized here using high-level ab initio coupled-cluster theory. Important experiments have been carried out on this system, but have not comprehensively described the properties of even the ground electronic state. We report cis and trans geometries for the ground ( ²A′′) and first excited (à ²A′) state equilibrium conformers and the torsional saddle point on the ground state surface at the CCSD(T)/ANO2 level of theory. Relative energies of these ground- and excited-state stationary points were obtained using coupled cluster theory with up to perturbative quadruple excitations, extrapolated from the sextuple zeta basis set to the complete basis set limit. These methods predict conformational energy differences ΔE(trans- → cis-) = 2.35 kcal mol⁻¹ and ΔE(trans-à → cis-Ã) = −2.95 kcal mol⁻¹. On the surface, the transition state for the conformational change lies 8.42 kcal mol⁻¹ above the trans ground state minima. The adiabatic electronic excitation energies from the ground state isomers are predicted to be 18.17 ± 0.10 (trans) and 13.03 ± 0.10 kcal mol⁻¹ (cis). The former is in excellent agreement with the 18.1 ± 1.4 kcal mol⁻¹ transition found by Lineberger and coworkers. Additionally, transition properties between the ²A′′ and à ²A′ states are reported for the first time, using the equation of motion (EOM)-CCSD method, which predicts lifetimes for trans-à ²A′ HC(O)OO˙ of 5.4 ms and cis-à ²A′ HC(O)OO˙ of 20.5 ms. Second-order vibrational perturbation theory was utilized to determine the fundamental frequencies at the CCSD(T)/ANO2 level of theory for the cis and trans conformers of the and à states and five ground state isotopologues of both conformers: H¹³C(O)OO˙, HC(¹⁸O)OO˙, HC(O)¹⁸O¹⁸O˙, DC(O)OO˙, and DC(O)¹⁸O¹⁸O˙. This study provides high accuracy predictions of vibrational frequencies, helping to resolve large uncertainties and disagreements in the experimental values. Furthermore, we characterize experimentally unassigned vibrational frequencies and transition properties.