Electron affinities from electron transfer equilibria in the gas phase and the electron affinity of SO2

Abstract

Measurements of gas phase electron transfer equilibria [Eq. (1)] A−+B=A+B− were performed with a pulsed electron beam high ion source pressure mass spectrometer. The equilibrium constants K1 were used to evaluate ΔG01=−RT ln K1. The ΔG01 were found to change little with temperature such that ΔG01≊ΔH01. Good agreement was observed between the present results and earlier measurements by Fukuda and McIver, which used ion cyclotron resonance and worked at much lower pressures. Absolute electron affinities were obtained by anchoring the relative electron affinities (ΔH1 values) to known absolute E.A.’s from the literature. The relative results agree well with differences predicted by absolute measurements. The resulting E.A. are in eV: CH3NO2, 0.49; CS2, 0.54; (C6H5)CO, 0.64; 2, 4, 6-trimethyl nitrobenzene 0.68; 2, 3-butanedione 0.71; 2, 3-dimethyl nitrobenzene, 0.83; 2-methyl nitrobenzene, 0.88; 3-methyl nitrobenzene, 0.94; nitrobenzene, 0.97; 1-fluoro-4-nitrobenzene, 1.06; SO2 1.11; 1-chloro-3-nitrobenzene, 1.23; maleic anhydride, 1.42; 1,3-dinitrobenzene, 1.6; 2, 5-dimethyl-p-benzoquinone, 1.72; methyl-p-benzoquinone, 1.78; p-benzoquinone, 1.86. The assignment of the present absolute values disagrees with that by Fukuda and McIver based on ICR bracketing experiments. It is suggested that the bracketing experiments were misleading due to presence of excitation in the ionic reactants used.