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

Abstract

Peroxy radicals (RO 2 ) 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 (à ←X) electronic transition of RO 2 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 (CH 3 O 2 ), one of the most abundant peroxy radicals in the atmosphere. In particular, vibrationally corrected geometries and anharmonic vibrational frequencies for both the ground (X 2 A ) and first excited (à 2 A ) 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 verticalà 2 A ←X 2 A transition properties; a radiative lifetime of 4.7 ms is suggested for the excited state. Finally, we predict the adiabatic excitation energy (T 0 ) 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 .

Synthesis, Electronic Spectroscopy, and Photochemistry of Methacrolein Oxide: A Four-Carbon Unsaturated Criegee Intermediate from Isoprene Ozonolysis

“…For comparison, the vibrational frequency of the O–O stretch (harmonic) is predicted to be ca. 900 cm –1 for CH 2 OO in the excited 1 1 ππ* state, which is in accord with typical O–O stretches in the excited states of peroxide molecules. − The oscillatory structure in the MACR-oxide spectrum likely has contributions from several vibrational modes including the O–O stretch (dissociation coordinate) and/or multiple conformers. This could explain the greater breadth and variability of the oscillatory features in the MACR-oxide spectrum compared to the average breadth (ca.…”

Photodissociation dynamics of the simplest alkyl peroxy radicals, CH3OO and C2H5OO, at 248 nm

“…Similar computational work examining CH 3 OO has been performed by Jafri and Phillips 8 in which the O-O bond has been shown to be repulsive in the Franck-Condon accessible region of several excited states, but the authors do not address the likelihood of forming the two electronic states of atomic oxygen. However, generalized valence bond diagrams for peroxy radicals in theB 2 A state show that the cleavage of the O-O bond correlates with O( 1 D) formation, 15,66,67 suggesting that the CH 3 OO excited state surface mimics that of HO 2 . This, in combination with the parallel angular distribution observed in our experiment, is consistent with an electronic transition to theB 2 A state to yield O( 1 D) + CH 3 O products with small amounts of curve crossing leading to O( 3 P) formation.…”

Vacuum ultraviolet photodynamics of the methyl peroxy radical studied by double imaging photoelectron photoion coincidences

“…The neutral CH 3 O 2 at the X 2 A″ ground state has a Cs symmetry with electronic configuration of (1a′) 2 …(9a′) 2 (2a″) 2 (10a′) 2 (3a″) 1 . 8 Removing an electron from the 10a′ orbital in photoionization can lead to two spin cationic states, the X 3 A″ ground electronic state and the a 1 A′ first excited state. Theoretical calculations with a complete basis set method using the atomic pair natural orbital expansion (CBS-APNO) at the QCISD(T)/CBS level of theory were performed by Meloni et al and predicted the adiabatic ionization energy (AIE) of CH 3 O 2 at 10.21 eV.…”