Electron affinities, gas phase acidities, and potential energy curves: Benzene

The experimental electron affinities of benzene, E_a(Bz), 0.4 to -4.8 eV, are evaluated. Multiple negative ion states are proposed to account for different electron affinities. The semi-empirical procedure known as "configuration interaction or unrestricted orbitals to relate experimental quantities to self-consistent field values by estimating electron correlation" (CURES-EC) has several advantages: (i) supports multiple E_a(Bz), (ii) supports the E_a(phenyl) and the D(C-H,Bz), (iii) supports the gas phase acidity of benzene from the latter, (iv) predicts the singlet-triplet split for the phenyl anion of 1.2(2) eV, and (v) predicts the existence of an excited quartet state of the benzene anion with an E _a(Bz), -2.5(2) eV. Nine ionic Morse curves are calculated from CURES-EC properties and experimental data. These are compared with quantum mechanical crossing "c" potentials obtained using a subroutine in commercial software and ab initio and density functional theory (DFT) procedures. Curves are calculated for the proposed quartet state of the benzene anion.