Structural and spectral characteristics of the electrogenerated tetracyanoethylene dianion

Structural and spectral characteristics of the electrogenerated tetracyanoethylene dianion (TCNE2-) were experimentally and theoretically examined. Spectroelectrochemistry of TCNE gives the spectra of TCNE2- in CH3CN at 220 nm, and in CH2Cl2 at 300 nm. These spectral characteristics are well explained by CIS/6-31G(d) and semiempirical CNDO/S-CI calculations. The bands in CH3CN and in CH2Cl2 are assigned to the degenerate 1E<--1A1 transition at the D2d structure and the 1B2u<--1Ag transition at the D2h structure, respectively. The rotation barrier of the C=C bond in TCNE2- is estimated by Hartree-Fock (HF), second-order Møller-Plesset perturbation (MP2) and fourth-order MP (MP4) calculations with 6-31G(d), 6-31+G(d) and 6-311+G(d) basis sets as 42-51 kJ mol(-1). The D2d structure is most stable, and the D2h structure represents the transition state of the internal rotation. The calculations reveal that the two-electron addition to the antibonding LUMO of TCNE causes an easy rotation around the C=C bond of TCNE2- characterized by the formal single bond. These results show that TCNE2- preferentially adopts D2d and D2h structures in solvents depending upon the solvent nature by virtue of the easy rotation around the C=C bond.