Theoretical spectroscopic study of acetyl (CH3CO), vinoxy (CH2CHO), and 1-methylvinoxy (CH3COCH2) radicals. Barrierless formation processes of acetone in the gas phase

Abstract

Background: Acetone is present in the earth´s atmosphere and extra-terrestrially. The knowledge of its chemical history in these environments represents a challenge with important implications for global tropospheric chemistry and astrochemistry. The results of a search for efficient barrierless pathways producing acetone from radicals in the gas phase are described in this paper. The spectroscopic properties of radicals needed for their experimental detection are provided.   Methods: The reactants were acetone fragments of low stability and small species containing C, O and H atoms. Two exergonic bimolecular addition reactions involving the radicals CH3, CH3CO, and CH3COCH2, were found to be competitive according to the kinetic rates calculated at different temperatures. An extensive spectroscopic study of the radicals CH3COCH2 and CH3CO, as well as the CH2CHO isomer, was performed. Rovibrational parameters, anharmonic vibrational transitions, and excitations to the low-lying excited states are provided. For this purpose, RCCSD(T)-F12 and MRCI/CASSCF calculations were performed. In addition, since all the species presented non-rigid properties, a variational procedure of reduced dimensionality was employed to explore the far infrared region. Results: The internal rotation barriers were determined to be V3=143.7 cm-1 (CH3CO), V2=3838.7 cm-1 (CH2CHO) and V3=161.4 cm-1 and V2=2727.5 cm-1 (CH3COCH2).The splitting of the ground vibrational state due to the torsional barrier have been computed to be 2.997 cm-1, 0.0 cm-1, and 0.320 cm-1, for CH3CO, CH2CHO, and CH3COCH2, respectively. Conclusions: Two addition reactions, H+CH3COCH2 and CH3+CH3CO, could be considered barrierless formation processes of acetone after considering all the possible formation routes, starting from 58 selected reactants, which are fragments of the molecule. The spectroscopic study of the radicals involved in the formation processes present non-rigidity. The interconversion of their equilibrium geometries has important spectroscopic effects on CH3CO and CH3COCH2, but is negligible for CH2CHO.