Electron affinities of cyano-substituted ethylenes

Electron affinities of ethylene and six cyano-substituted ethylenes (cyanoe thylene, 1,1-dicyanoethylene, cis-1,2-dicyanoethylene, trans-1,2-dicyanothy lene, tricyanoethylene, and tetracyanoethylene) were determined using six d ifferent density functional or hybrid Hartree-Fock/ density functional meth ods. Equilibrium geometries and harmonic vibrational frequencies for each s pecies were determined with each density functional method. Experimental el ectron 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 0.10 eV (BLY P), 0.19 ev (BHLYP), 0.22 eV (B3LYP), 0.20 eV (BP86), 0.78 eV (B3P86), and 0.81 eV (LSDA). The electron affinities of gem-dicyanoethylene, cis-discyan oethylene, and tricyanoethylene are not known from experiment but are predi cted here to be 1.23 eV (gem-dicyanoethylene), 1.32 eV (cis-dicyanoethylene ), and 2.41 eV (tricyanoethylene). Contrary to earlier suggestions, tetracy anoethylene is predicted to be planar, rather than twisted. Density functio nal theory predicts that the B-2(1u) state of the ethylene anion lies lower than the B-2(2g) state, which is reported by experimentalists as the (tran sient) ground state, and lower than the (2)A(g) state. Coupled-cluster resu lts indicate that the (2)A(g) state is lower than either the B-2(2g) or B-2 (1u) states. The energetic stabilization of cyano substitution on ethylene results from the pi and pi* conjugation of multiple cyano groups. The HOMO- LUMO gap in ethylene decreases with increasing cyano substitution, from 7.2 eV in C2H4 to 3.8 eV in C-2 (CN)(4), explaining the extreme difference bet ween the electron affinities of ethylene (negative) and tetracyanoethylene (similar to 3.0 eV).