Negative Ion Mass Spectra, Electron Affinities, Gas Phase Acidities, Bond Dissociation Energies, and Negative Ion States of Cytosine and Thymine

Abstract

Negative ion Morse potential energy curves for cytosine (C) and thymine (T) consolidate electron affinity, gas-phase acidity, bond dissociation energy, photoelectron spectrum, electron impact, transmission, and transfer data. We have previously published experimental adiabatic electron affinities for guanine, 1.51 ± 0.05; adenine, 0.95 ± 0.05; C, 0.56 ± 0.05; uracil, 0.80 ± 0.05; and T, 0.79 ± 0.05 (all in eV). These values were obtained from half-wave reduction potentials in aprotic solvents and verified by AM1 semiempirical multiconfiguration configuration interaction (AM1-MCCI) calculations. We present a computer simulation of thermal charge-transfer experiments to support these values. Unpublished negative chemical ionization mass spectrometry data support our gas-phase acidities: for A, G, and T, 14.1; for U, 14.2; and for C, 14.3 (all in eV). We use dipole-bound and valence-state electron affinities previously obtained from photoelectron spectra (Schiedt, J.; Weinkauf, R.; Neumark, D. M.; Schlag, E. W. Chem. Phys. 1998, 239, 511). We interpret monoenergetic electron beam spectra reported in the literature to obtain vertical electron affinities, negative ion distributions, and limits to bond dissociation energies for C and T (Huels, M. A.; Hahndorf, I.; Illenberger, E.; Sanche, L. J. Chem. Phys. 1998, 108, 1309).