cis and trans conformations in 3-substituted thietane-1-oxide

For a series of 3-substituted-thietane-1-oxide (3-R-TOX) derivatives (R=chloro, methyl, ethyl, acetate, t-buthyl, phenyl and p-chlorophenyl), the cis⇔trans isomerization reactions have been theoretically studied in the frame of molecular orbital theory. Optimized geometries have been obtained at HF/6-31G** level, whereas the energetics and thermodynamics were calculated with basis sets that include polarization, diffuse functions and electron correlation at the second-order Møller-Plesset perturbation theory. The size and nature of the substituent seems to exert no influence on the structure of the thietane-1-oxide ring. Gas phase thermodynamics predict the cis isomers to be the preferred structure in these compounds as a result of the H10⋯O9 non-bonded interaction. The exception to this trend are the chloro and acetate derivatives where a 14 and 46% of the trans isomer, respectively, is predicted. In CCl4 solution, the isomerization reaction would take place in large extent. The calculated 1H-NMR chemical shifts correlate well with the experimental data and demonstrate that the β-protons (H10) are magnetically sensible enough to allow the cis and trans isomeric forms to be distinguished. The β-protons chemical shifts calculated in CCl4 do not differ appreciably from the gas phase values. The 17O-NMR signals have also been derived and the potentiality of this nucleus to be used to assign the preferred geometry as well as to follow the isomerization reaction, in this kind of compounds is pointed out. The present work also indicates that the SO stretching mode can be used to assign either a cis or trans conformation in these species, though this vibrational mode is strongly coupled with some ring and H-C-H angle deformation modes.