MOLECULAR-STRUCTURE OF S-ETHYL THIOACRYLATE - COMBINED VIBRATIONAL SPECTROSCOPIC AND AB-INITIO SCF-MO STUDY

Ab initio 6-31G SCF-MO calculations have been carried out on S-ethyl thioacrylate [CH2=CHC(O)SCH2CH3]. Fully optimized geometries, relative stabilities, dipole moments and harmonic force fields for several con formers of this molecule have been determined and the results compared with those for similar molecules. Together with FTIR and Raman spectr oscopic data, the theoretical results demonstrate that S-ethyl thioacr ylate exists in two different conformations about the C-alpha-C bond ( the s-cis and s-trans forms, with C=C-C=O dihedral angles equal to 0 d egrees and 180 degrees, respectively); the s-cis conformation being mo re stable than the s-trans form by ca. 6 kJ mol(-1) for the isolated m olecule. Comparison of the experimental and theoretical vibrational sp ectra confirms that, as concluded from our previous study on the analo gous trans-S-ethyl thiocrotonate molecule (R. Fausto, P. J. Tonge and P. R. Carey, J. Chem. Sec., Faraday Trans., 1994, 90, 3491), the prese nce of the s-trans isomer of an alpha,beta-unsaturated thioester can b e successfully monitored by the IR band at ca. 1170 cm(-1), ascribed t o the C-alpha-C stretching mode of this form. In addition, we were als o able to identify some IR bands sensitive to the conformation of the ethyl group that may be used as spectroscopic probes to study conforma tional equilibria associated with this internal degree of freedom in m ore complex S-ethyl thioesters.