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 prot otype push-pull ethylene 1,1-diamino-2,2-dicyanoethylene (DADCE) and i ts non-push-pull analogues diaminofumaronitrile (DAFN) and diaminomale onitrile (DAMN) have been used quantitatively to assess the influence of the push-pull effect. Results for DADCE indicate an in vacuo barrie r for C=C rotation of approximate to 28 kcal mol(-1) which decreases i n solvents of increasing dielectric strength. The barrier for C-N rota tion in DADCE is found to be approximate to 8 kcal mol-l and increases with solvent dielectric strength. The comparable C=C barrier connecti ng 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.