Rotational Isomerism of Vinyl Ethers and Sulfides

Abstract

Abstract The investigations of rotational isomerism of p,π-conjugated systems discussed in the present review have revealed unexpected and, in many respects, conspicuous features in their molecular and electronic structure. The most stable conformation of ethenol, ethenethiol, methyl vinyl ether and methyl vinyl sulfide is the s-cis form exhibiting the largest steric strain. The stabilization of s-cis rotamers in these compounds is caused by not only the energy difference of p,π-interactions in planar s-cis and s-trans conformations, but also by a significant contribution to the potential of internal rotation about the Csp2 -X bond (X = O, S) from a third-order barrier. The molecules of compounds of a general formula CH2[dbnd]CHXR and CH2[dbnd]CHXCH[dbnd]CH, (X = O, S) show two stable conformers of quite different types. The conformers of the first type are characterized by a certain torsional angle and slight oscillations relative to the position with minimal potential energy. The conformers of the second type display motion of large amplitude and potential well with a wide flattened bottom. This motion may involve not one but several minima on the potential curve of internal rotation, and the number of stable conformers may exceed that obtained by considering only static rotamer forms. Key Words: Vinyl ethersvinyl sulfidesrotational isomerism.