Ethylperoxy radical: approaching spectroscopic accuracy via coupled-cluster theory

Area of Science:

• Atmospheric Chemistry
• Combustion Science
• Quantum Chemistry

Background:

• Peroxy radicals are crucial in atmospheric and combustion processes.
• Their transient nature complicates experimental studies.
• Theoretical methods, especially coupled-cluster theory, are vital for characterizing these species.

Purpose of the Study:

• To accurately predict the properties of ethylperoxy radical (C2H5O2) conformers.
• To determine the transition origins (T0) for the X and à states.
• To characterize the fundamental vibrational transitions of the X state.

Main Methods:

• Structural optimization using coupled-cluster theory with the augmented correlation-consistent basis set ANO2 (CCSD(T)/ANO2).
• Extrapolation of electronic energies to the complete basis set limit using the focal point approach.
• High-level computations including basis sets up to cc-pV5Z and post-Hartree-Fock methods up to CCSDT(Q).
• Inclusion of anharmonic contributions for vibrational transition predictions.

Main Results:

• Predicted transition origins (T0) of 7363 cm⁻¹ (trans) and 7583 cm⁻¹ (gauche) for the à ← X transition, with accuracy within 10 cm⁻¹.
• Characterization of all 21 vibrational modes for each conformer, surpassing the 16 modes previously reported experimentally.
• Predictions for the 5 unassigned vibrational modes.

Conclusions:

• High-level theoretical calculations are essential for precise characterization of transient peroxy radicals.
• The study provides accurate spectroscopic data for ethylperoxy radical conformers, aiding experimental interpretation.
• This work advances the understanding of peroxy radical chemistry in atmospheric and combustion environments.