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 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 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 > 1) clusters by infrared photodissociation spectroscopy combining with the time-of-flight mass spectrometer.