The determination of molecular electron affinities and the relationship with molecular complex formation



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The literature concerning the various methods for the determination of electron affinities is reviewed and the results obtained for atoms, molecules and radicals are tabulated. A study of the parameters characterizing the electron capture detector operated in the pulse sampling mode was carried out. The pulse width, -.4 psec, applied voltage -50 volts and the pulse period =1,000 psec, necessary to collect all of the electrons and to achieve a steady state when argon-10% methane is used as a carrier gas were determined. It was assumed that the electrons acquired a thermal distribution when no potential was applied to the cell and that the results were independent of the pulse potential examined up to 80 volts. A kinetic model of the processes occurring within the electron capture detector operated in the pulse sampling mode has been proposed. For the case in which the electron capturing species is capable of forming a stable negative ion (in contrast to dissociative electron capture), the system of differential equations has been solved using the steady state approximation. From this solution, one can obtain the previously defined electron capture coefficient in terms of the rate constants for the processes proposed in the model. In certain cases one can obtain values for the rate constants and/or the electron affinity of the molecule from the temperature dependence of this electron capture coefficient. Evidence is given for the validity of the proposed model, and the magnitude of the rate constants and the electron affinities are given for several aromatic hydrocarbons and aromatic carbonyl compounds. The experimental electron affinities obtained above are correlated with the half wave reduction potentials, the energy of the electronic transitions, and the Huckel coefficients of the lowest unfilled molecular orbitals for the compounds. The experimental values are compared with the calculated values of the electron affinities. The electronegativity of the aromatic hydrocarbons has been calculated and is found to be constant and equal to 4.17-.10 ev. The change in the spectra of the three and four ring aromatic hydrocarbons upon addition of methylbenzenes in an inert solvent has been interpreted as being due to complex formation. The observed increase in the stability of the complex with decreasing ionization potential of the methylbenzenes is considered as evidence for at least some charge transfer stabilization. The correlation of increased stability with increasing electron affinity of the aromatic hydrocarbon adds further proof to the charge transfer interpretation. In turn it also adds support to the interpretation of the electron capture coefficients in terms of the electron affinities.