The vibrational spectra of 1,3-dithiane-1-oxide and 1,3-dithia-1-oxocyclohept-5-ene

The IR spectra of 1,3-dithiane-1-oxide (I) and 1,3-dithia-1-oxocyclohept-5-ene (II) were recorded in solution, solid and liquid phase over 4000–400 cm−1 spectral range. It was found that both (I) and (II) in liquid phase and solutions exist in two conformations: (I) chair-e (Ce) and chair-a (Ca) with equatorial and axial positions of the SO bond, respectively, and (II) chair-e (Ce) and boat-e (Be). The intensity variations with temperature (300–180 K) of the bands 632 (Ca) and 644 cm−1 (Ce) of (I) in acetone-d6 and the bands 482 (Be) и 448 cm−1 (Ce) of (II) in melt were employed in Van’t Hoff plot and gave the values ΔH°(Ca − Ce) = 380 ± 40 cal mol−1 (I) and ΔH° (Be − Ce) = 400 ± 100 cal mol−1 (II).Ab initio calculations were carried out with the Gaussian 98 program using the basis set 6-31G(d) for (I) and 6-311++G(d,p) for (II). The energy difference between Ca and Ce conformations for (I) and Be and Ce for (II) are in a good agreement with experimental results. Vibrational frequencies for both conformations (I) and (II) were calculated. After appropriate scaling a reasonably good agreement between the experimental and calculated wave numbers was obtained.