Abstract
Electron affinities of ethylene and six cyano-substituted ethylenes (cyanoethylene, 1, 1-dicyanoethylene, cis-1, 2-dicyanoethylene, trans-1, 2-dicyanothylene, tricyanoethylene, and tetracyanoethylene) were determined using six different density functional or hybrid Hartree-Fock/density functional methods. Equilibrium geometries and harmonic vibrational frequencies for each species were determined with each density functional method. Experimental electron affinities exist for three of the six systems studied (cyanoethylene, trans-1, 2-dicyanoethylene, and tetracyanoethylene); for the three systems, the absolute average EA errors for the different methods are 0.10eV (BLYP), 0.19ev (BHLYP), 0.22eV (B3LYP), 0.20eV (BP86), 0.78eV (B3P86), and 0.81eV (LSDA). The electron affinities of gem-dicyanoethylene, cis-discyanoethylene, and tricyanoethylene are not known from experiment but are predicted here to be 1.23eV (gem-dicyanoethylene), 1.32eV (cis-dicyanoethylene), and 2.41eV (tricyanoethylene). Contrary to earlier suggestions, tetracyanoethylene is predicted to be planar, rather than twisted. Density functional theory predicts that the 2B1u state of the ethylene anion lies lower than the 2B2g state, which is reported by experimentalists as the (transient) ground state, and lower than the 2Ag state. Coupled-cluster results indicate that the 2Ag state is lower than either the 2B2g or 2B1u states. The energetic stabilization of cyano substitution on ethylene results from the π and π∗ conjugation of multiple cyano groups. The HOMO-LUMO gap in ethylene decreases with increasing cyano substitution, from 7.2eV in C2H4 to 3.8eV in C2(CN)4, explaining the extreme difference between the electron affinities of ethylene (negative) and tetracyanoethylene (∼T3.0eV).