Electronic effects in a prototype push-pull ethylene: A study of rotational barriers in C4H4N4 isomers

Quantum mechanical calculations of the rotational barriers in the prototype push-pull ethylene 1,1-diamino-2,2-dicyanoethylene (DADCE) and its non-push-pull analogues diaminofumaronitrile (DAFN) and diaminomaleonitrile (DAMN) have been used quantitatively to assess the influence of the push-pull effect. Results for DADCE indicate an in vacuo barrier for C=C rotation of approximate to 28 kcal mol(-1) which decreases in solvents of increasing dielectric strength. The barrier for C-N rotation in DADCE is found to be approximate to 8 kcal mol-l and increases with solvent dielectric strength. The comparable C=C barrier connecting DAMN and DAFN is calculated to be > 20 kcal mol(-1) higher than the DADCE barrier. Analysis of the conformational energies of DADCE, DAMN, and DAFN gives further evidence for the increased dipolar character and nitrogen lone pair conjugation in DADCE as compared with DAMN and DAFN.