Measurement of Negative Ions Formed by Electron Impact. II. The Ionization Efficiency Curves of Negative Nitro, Oxygen Atom and Nitromethylene Ions from Nitroalkanes

Abstract

The ionization efficiency (IE) curves of NO2−, O− and CH2NO2− ions from nitroalkanes (nitro-methane, nitroethane, 1-nitropropane and 2-nitropropane) have been measured with a Hitachi RMU-6D mass spectrometer. Based on each authorized value of IP(CH3, C2H5, C3H7, O and H), D(C–NO2, C–H, C–C, NO–O and N–O), EA(NO2 and O) and the excitation energy of CH3, the respective onsets observed for the electron energies above ∼3 eV on the IE curves were suggested to be interpreted as appearance potentials of the following reactions. For NO2− ions, RNO2+e→R*+NO2−, RNO2+e→R+NO2−+e; for O− ions, RNO2+e→RNO+O−, RNO2+e→RN+O+O−, RNO2+e→RN++O+O−+e;for CH2NO2− ions, CH3NO2+e→H+ CH2NO2−+e, RNO2+e→H+R1+CH2NO2−. On the other hand, the strong peak of NO2− ions due to the dissociative electron capture process (RNO2+e→R+NO2−) was observed at the lower electoron energy. Furthermore, a value of EA(CH2NO2) was presented to be ∼0.5 eV.

Electron induced dissociation in condensed-phase nitromethane I: desorption of ionic fragments

“…3(f)]. Three resonance features are also seen in the production of NO 2 À ions from the gas-phase nitromethane as reported by Tsuda et al, 12 at similar energies i.e. 4.6, 8.5 and 12 eV.…”

“…Most gas phase studies [11][12][13][14][15][16][17][18][19] were performed at incident electron energies below 10 eV and have identified NO 2 À as the major anionic product of DEA, especially at the lowest energies approaching 0 eV. 14,15,18,19 Indeed, a recent comparison of the partial cross section for NO 2 À production and the total cross section for DEA to gas phase NM show that the latter is dominated by the contribution of the nitro fragment ion in the low energy range (i.e., below 3 eV).…”

“…The O À yield functions displays a broad resonance structure with a maximum at the energy of 10 eV. According to previous studies, 12,19 possible reaction channels for production of O À include:…”

“…, as regards the energy position and relative intensity of resonances, were observed for larger nitroalkane molecules such as nitroethane, 1-nitropropane and 2-nitropropane. 12 In general the incident electron energy required to form a specific anionic fragment via DEA to a condensed molecule is expected to be lower than in the gas phase, due to the polarization interaction of the excess electron with the surrounding medium and supporting substrate. 5,20 Nevertheless, threshold energies for desorption and/or the energy of DEA maxima in ESD yield functions are not inevitably lower in energy than their homologues in gas phase experiments.…”

“…NO 2 À + CH 3 + , is estimated to lie at 8.3 eV by taking the values of the relevant bond energy, electron affinity of nitro group and ionization energy of methyl group [D(C-NO 2 ) = 2.3 eV, EA(NO 2 ) = 3.8 eV and IP(CH 3 ) = 9.8 eV]. 12 The most intense desorbed anion signal from d 3 -NM is mass 16 which a priori may be attributed to O À or ND À fragments [Fig. 3(b)].…”