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

Direct observation on argon tagging nitrobenzene radical anion in gas phase: Infrared photodissociation spectroscopy and theoretical calculation

The infrared photodissociation spectroscopy was applied to characterize nitrobenzene radical anion (NB^-). NB^- tagged by argon [NB(Ar)^-] was prepared by a mixture of nitrobenzene/Ar through a supersonic ion source and then selected by a time-of-flight mass spectrometer. Eight strong peaks observed at 977.9, 999.6, 1059.8, 1275.7, 1309.7, 1339.7, 1367.6 and 1581.7 cm^-1 in the fingerprint region were assigned to NB(Ar)^-, corresponding to CC bending, CC stretching, CC bending + symmetric O-N stretching vibration, antisymmetric O-N and CC stretching vibration, antisymmetric O-N stretching and CH rocking vibration, CC stretching + antisymmetric O-N stretching vibration, C-N stretching vibration, and symmetric CC stretching vibration. Most interestingly, the distinguishable CH stretching vibrations were observed at 3006.5, 3048.6 and 3084.5 cm^-1 absorptions. Combined with density functional theoretical (DFT) calculation, five tagging argon NB^- isomers were optimized and analyzed with no imaginary frequency. The results indicated that most bond lengths in NB^- become longer than those of neutral NB, except for the C1-C2/C4-C5 bonds, which are only slightly shorter than those of neutral NB, and the C-N bond, which is 0.085 A shorter in the anion. The NB^- tagged by argon located on the nitro group had no change on bond parameters with Ar-tagging or not theoretically. Natural population analysis (NPA) show that the negative natural charges are mainly distributed on both oxygen atoms. And the one electron resonates between the nitro group and benzene ring. N-O bonds in NB^- become much more polar than those of the neutral NB. This paper proved the usefulness to characterize NB(Ar)^- and further explore the structures of NB_n^- (n > 1) clusters by infrared photodissociation spectroscopy combining with the time-of-flight mass spectrometer.

IR absorption spectra of aniline cation, anilino radical, and phenylnitrene isolated in solid argon

Electron bombardment of aniline (PhNH₂) in an Ar matrix mainly generated the aniline cation (PhNH₂⁺), anilino (PhNH) and phenyl (Ph) radicals, and phenylnitrene (PhN). Further irradiation of the electron-bombarded matrix sample at 365 nm depleted PhNH₂⁺ and PhN, and resulted in the formation of PhNH₂, PhNH, and Ph. In separate experiments, irradiation of the PhNH₂/Ar matrix samples at 265 or 160 nm mainly generated PhNH and Ph radicals, but without the formation of PhNH₂⁺ and PhN. According to the observed photochemical behaviors, quantum-chemically predicted harmonic vibrational wavenumbers of each species, and the information reported in previous photodissociation studies, we unambiguously characterized the IR features of the aromatic species. The information of the vibrational fundamentals of PhNH is new and the formation mechanism is discussed.

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.

IR absorption spectra of hexafluorobenzene anions and pentafluorophenyl radicals in solid argon

Hexafluorobenzene anions (HFB^-) and pentafluorophenyl radicals (PFP) were generated by the electron bombardment of a HFB/Ar sample during matrix deposition. Further irradiation of the matrix sample at 365 nm detached the electron from HFB^- and produced its neutral counterpart HFB in solid Ar. Secondary photolysis of the matrix sample at 160 nm destroyed HFB and generated HFB^- and PFP. In a separate experiment, the photolysis of the HFB/Ar matrix at 160 nm produced PFP, Dewar-C₆F₆, and various neutral fluorocarbon species, but without the production of HFB^-. The infrared (IR) lines of HFB^- and PFP were assigned on the basis of the observed photochemical behaviors, as well as by comparing the predictions of the vibrational wavenumbers with the corresponding IR intensities and the relative stabilities of the related species predicted via the B3LYP/aug-cc-pVTZ theory.