Measurement of Negative Ions Formed by Electron Impact. VIII. Ionization Efficiency Curves of Negative Ions from Methyl and Ethyl Cyanides

The ionization efficiency (I.E) curves of m/e 26(CN−), 27(HCN−), 38(C2N−) and 39(CHCN−) ions from methyl and ethyl cyanides, besides 40(CH2CN−) ions from methyl cyanide, and 50(C3N−) ions from ethyl cyanide were measured to the extent of about 25 eV electron energies under the pressure of ∼10−6 mmHg by a Hitachi RMU-6D mass spectrometer. Schemes of plausible reactions expected to appear at each onset value observed in the respective IE curves were sought thermochemically by using ΔHf values of the reactants and products. The estimation method of unknown ΔHf values is described also. It should be noted that for CN− ions one process (probably, R+CN−) was observed in lower energies than 2 eV; C2N− ions are expected to be formed with excess energy; also, for CN− and HCN− ions rather strange phenomena of the decrement of their signal intensities with increasing electron energies were observed in the range of higher energies. Moreover, the IE curves of C3N− ions in the higher energy region were well interpreted in terms of the overlapping phenomena of dissociative capture process with ion pair formation process. It was found from this work that EA(HCN) 1 eV, ΔHf(CHCN−) ∼3.2 eV, EA(CHCN) 1.1 eV and EA(CH2CN) 1.6 eV. For the electron affinity of C2N a value 2.3 eV is tentatively introduced on the basis of EA(C3N)=2.4 eV, EA(C5N)=2.3 eV, and EA(CN)=3.4 eV.