Structural effects influencing cis-trans isomerisation in methoxy and cyano substituted stilbene derivatives

Three substituted stilbenes were synthesised, and their photochemical E-Z isomerisation behaviour in solution studied. A rationalisation of the composition of the equilibrium mixture through the ab initio calculated energy difference between the electronic ground states of the isomers proved to be not possible. In contrast, the calculated energy differences agree quite well with the E-Z compositions obtained from the thermodynamic equilibrium, reached by thermal isomerisation in solution with a trace of iodine present. Additionally, X-ray structural information on the compounds studied is presented, and theoretical NMR shifts, calculated at the Hartree-Fock and density functional levels, have been used to assign the experimental C-13 NMR spectra of the new compounds.