Examining the ground and first excited states of methyl peroxy radical with high-level coupled-cluster theory

¹Dedicated to Professor Sourav Pal.

Abstract

Peroxy radicals (RO₂) are intermediates in fuel combustion, where they engage in efficiency-limiting autoignition reactions. They also participate in atmospheric chemistry leading to the formation of unwanted tropospheric ozone. Advances in spectroscopic techniques have allowed for the possibility of employing the lowest () electronic transition of RO₂ as a tool to selectively monitor these species, enabling accurate kinetic values to be obtained. Herein, high-level ab initio methods are employed to systematically refine spectroscopic predictions for the methyl peroxy radical (CH₃O₂), one of the most abundant peroxy radicals in the atmosphere. In particular, vibrationally corrected geometries and anharmonic vibrational frequencies for both the ground () and first excited () state are predicted using coupled-cluster theory with up to perturbative triples [CCSD(T)] and large atomic natural orbital basis sets. Equation-of-motion coupled-cluster theory is utilised to compute vertical transition properties; a radiative lifetime of 4.7 ms is suggested for the excited state. Finally, we predict the adiabatic excitation energy (T₀) via systematic extrapolation to the complete basis limit of coupled-cluster with up to full quadruples (CCSDTQ). After accounting for several approximations, and including an anharmonic zero-point vibrational energy correction, we match experiment for this transition to within 9 cm⁻¹.

Keywords:

• methyl peroxy radical
• equation-of-motion
• coupled-cluster
• transition energy
• anharmonic frequencies

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplemental data

Supplemental data for this article can be accessed at http://dx.doi.org/10.1080/00268976.2015.1063729.

Notes

¹ Dedicated to Professor Sourav Pal.

Additional information

Funding

This research was supported by the Department of Energy, Office of Basic Energy Sciences [grant number DE-FG02-97-ER14748].