Slow Photoelectron Velocity-Map Imaging of the i-Methylvinoxide Anion

Theoretical spectroscopic study of acetyl (CH 3CO), vinoxy (CH 2CHO), and 1-methylvinoxy (CH 3COCH 2) radicals. Barrierless formation processes of acetone in the gas phase

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 CH 3, CH 3CO, and CH 3COCH 2, were found to be competitive according to the kinetic rates calculated at different temperatures. An extensive spectroscopic study of the radicals CH 3COCH 2 and CH 3CO, as well as the CH 2CHO 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 V 3=143.7 cm -1 (CH 3CO), V 2=3838.7 cm -1 (CH 2CHO) and V 3=161.4 cm -1 and V 2=2727.5 cm -1 (CH 3COCH 2).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 CH 3CO, CH 2CHO, and CH 3COCH 2, respectively. Conclusions: Two addition reactions, H+CH 3COCH 2 and CH 3+CH 3CO, 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 CH 3CO and CH 3COCH 2, but is negligible for CH 2CHO.

Infrared Spectra of the 1-Methylvinoxide Radical and Anion Isolated in Solid Argon

The 1-methylvinoxy radical (1-MVO) is an important intermediate in the combustion and tropospheric reaction of OH. However, the vibrational structures of this species and its anionic form, 1-methylvinoxide anion (1-MVO^-), are not fully known. Thus, in this study, we obtained the infrared (IR) absorption spectra of 1-MVO and 1-MVO^- trapped in a solid Ar matrix. 1-MVO^- anions were produced by electron bombardment during matrix deposition of Ar containing a small amount of acetone. The anions were destroyed upon irradiation at 675, 365, and 160 nm, although the formation of 1-MVO was only observed upon irradiation at 675 nm. The assignment of the IR bands of 1-MVO^- and 1-MVO was based on the expected chemistry upon photoexcitation and comparison of line wavenumbers, relative IR intensities, and D-isotopic shift ratios with those predicted at the B3LYP/aug-cc-pVTZ level of theory.

Photodissociation Dynamics of the i-Methylvinoxy Radical at 308, 248, and 225 nm Using Fast Beam Photofragment Translational Spectroscopy

The photodissociation dynamics of the i-methylvinoxy (CH₃COCH₂) radical have been studied by means of fast beam coincidence translational spectroscopy. The radical was produced by photodetachment of the i-methylvinoxide anion at 700 nm, followed by dissociation at 225 nm (5.51 eV), 248 nm (5.00 eV), and 308 nm (4.03 eV). At all three dissociation energies, the major products were found to be CH₃ + CH₂CO, with a small amount of CO + C₂H₅ produced at the higher dissociation energies. Photofragment mass distributions and translational energy distributions were recorded for each wavelength. Comparison of the mass distributions with dissociation of fully deuterated i-methylvinoxy aided the assignment of the observed channels. Electronic structure calculations were performed to determine the relative energies of minima and transition states involved in the dissociation and to aid interpretation of the experimental results. The proposed dissociation mechanism involves internal conversion from the initially excited electronic state, followed by dissociation over a barrier on the ground state.