X-ray crystallography, variable temperature H-1 NMR and nuclear Overhauser effect experiments, and ab initio calculations provided evidence of the s-trans preferential conformation of captodative olefin 1-acetylvinyl p-nitrobenzoate (la) in solid, solution, and gas phases. The reactivity of la in cycloaddition reactions was rationalized under the basis of experimental ionization energy and electron affinity parameters. Correlation of the latter with the frontier molecular orbitals, calculated at the ab initio 3-21G and 6-31G* levels, suggests a nonsignificant effect of the electron-donor group upon control of the reactivity and regioselectivity of these olefins in Diels-Alder reactions.