Vibrational Spectroscopy of Nitroalkane Chains Using Electron Autodetachment and Ar Predissociation

Abstract

If the binding energy of an excess electron is lower than some of the vibrational levels of its host anion, vibrational excitation can lead to autodetachment. We use excitation of CH stretching modes in nitroalkane anions (2700-3000 cm(-1)), where the excess electron is localized predominantly on the NO2 group. We present data on nitroalkane anions of various chain lengths, showing that this technique is a valid approach to the vibrational spectroscopy of such systems extending to nitroalkane anions at least the size of nitropentane. We compare spectra taken by using vibrational autodetachment with spectra obtained by monitoring Ar evaporation from Ar solvated nitroalkane anions. The spectra of nitromethane and nitroethane are assigned on the basis of ab initio calculations with a detailed analysis of Fermi resonances of CH stretching fundamentals with overtones and combination bands of HCH bending modes.

Low-energy photoelectron imaging spectroscopy of nitromethane anions: Electron affinity, vibrational features, anisotropies, and the dipole-bound state

“…We note that all photon energies in this study were chosen carefully to avoid the influence of known vibrational autodetachment resonances of the CH 3 NO 2 − anion. 3,5 At this point, we can tentatively assign the three intense features as the main vibrational progression, with the vibrational 0-0 origin transition at the nominal peak position of E B = ͑170Ϯ 5͒ meV, and the features toward lower binding energies as hot bands. The energy spacing of ͑613Ϯ 56͒ cm −1 is in agreement with the value of ͑645Ϯ 70͒ cm −1 or ͑80Ϯ 9͒ meV reported by Compton et al 1 and corresponds to the 603 cm −1 frequency for the NO 2 wag mode of gas-phase CH 3 NO 2 reported by Gorse et al 30 The comparison of the spectrum of bare CH 3 NO 2 − and that of CH 3 NO 2 − ·Ar ͑see …”

“…The three CH stretch fundamental frequencies for the anion were taken from infrared spectroscopy experiments. 3,5 The other anion frequencies were estimated by scaling the calculated B3LYP/ 6-311+ + G͑2df ,2p͒ harmonic values by the ratio of the experimental fundamental to calculated harmonic frequency of the neutral for the similar normal mode. These frequencies are given in Table I.…”

“…The interaction of nitromethane ͑CH 3 NO 2 ͒ with lowenergy electrons and the properties of the CH 3 NO 2 − anion have been the subject of many studies, both experimentally [1][2][3][4][5] and theoretically. 6,7 One very remarkable feature of CH 3 NO 2 is that it has a sufficiently high dipole moment ͓ = 3.46 D ͑Ref.…”

“…1,2,6,8 The binding energy of the valencelike state is comparatively small, 1 below several of the fundamental vibrational transitions in the anion, making it an ideal model system for the investigation of the interaction of vibrational excitation and electron emission. 3,5 Interestingly, the signature of the dipolebound state of CH 3 NO 2 − has not been found in photoelectron ͑PE͒ spectroscopy so far. 1 The adiabatic electron affinity ͑AEA͒ of CH 3 NO 2 has been determined to be ͑260Ϯ 80͒ meV by Compton et al 1 by using the last discernible peaks on a very extended and congested vibrational progression as bracketing features.…”

Anion Resonances of para-Benzoquinone Probed by Frequency-Resolved Photoelectron Imaging

“…The additional information obtained through the PADs clearly identifies the involvement of resonances; similar changes in PADs have been observed in vibrational autodetachment of anions. 53,54 In principle, PADs can be calculated using, for example, the relevant Dyson orbitals. 61,62 However, when autodetachment occurs from resonances and interference can occur with direct detachment channels, these methods become more difficult to apply and additional development on the theoretical front is required to capture changes in the PADs.…”