Molecular structures of cis- and trans-S-ethyl thiocrotonate. A combined vibrational spectroscopic and ab initio SCF-MO study

Ab initio 6-31G* SCF-MO calculations have been carried out for cis- and trans-S-ethyl thiocrotonate [cis- and trans- CH₃-CH=CH-C(=O)SCH₂CH₃]. Fully optimized geometries, relative stabilities, dipole moments and harmonic force fields for several conformers of these molecules have been determined and the results compared with those of similar molecules. Combined with FTIR spectroscopic data, the theoretical results demonstrate that trans-S-ethyl thiocrotonate exists in two different confromations about the C_α-C bond (the s-cis and s-trans forms, with C=C-C=O dihedral angles equal to 0° and 180°, respectively), the s-cis conformation being more stable than the s-trans form by ca. 7 kJ mol^-1 for the isolated molecule situation, while the cis-s-ethyl thiocrotonate molecule adopts only the s-cis conformation about this bond. A comparison of the experimental and theoretical vibrational spectra also shows that the existence of the less stable s-trans isomer of an α,β-unsaturated thioester can be successfully monitored by the IR band at ca. 1165 cm^-1, ascribed to the C_α-C stretching mode of this form. Additional conformationally sensitive bands are also identified, but these are difficult to use in the IR spectrum because of overlap with other features.