The electron affinities of the perfluorocarbons C2Fn, n=1–6

Abstract

Several independent density functional theory (DFT) methods have been employed to determine the molecular structures and total energies of C2Fn and C2Fn− (n=1–6). Three measures of neutral-anion separation are reported: the adiabatic electron affinity, the vertical electron affinity, and the vertical detachment energy of the anion. The first fluorine atom and fluoride ion dissociation energies as well as the harmonic vibrational frequencies of C2Fn and C2Fn− are also reported. Self-consistent Kohn–Sham orbitals were obtained using various exchange correlation functionals and a double-ζ plus polarization basis set augmented with diffuse s-type and p-type functions. Previously observed trends in the prediction of bond lengths by the DFT methods continue for the C2Fn/C2Fn− series. The BHLYP method, based upon the Becke half-and-half exchange functional and the Lee–Yang–Parr correlation functional, continues to predict the shortest and most accurate bond lengths of all of the DFT methods. More generally, the Hartree–Fock/DFT hybrid methods predict shorter and more accurate bond lengths than the pure DFT methods, as expected. As in previous studies, the BHLYP method predicts the lowest electron affinity values which are also those generally closest to available experiments. Agreement to within a few tenths of an electronvolt is typically achieved. The BHLYP adiabatic electron affinities are: C2F, 2.97 eV; C2F2, 1.26 eV; C2F3, 2.25 eV; C2F4, 0.21 eV; C2F5, 1.77 eV; and C2F6, <0.0 eV. Further, C2F6− is found to have a negative vertical detachment energy, while C2F2, C2F4, and C2F6 are predicted to have negative vertical electron affinities.