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 S=O 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) and 448 cm(-1) (Ce) of (II) in melt were employed in Van't Hoff plot and gave the values DeltaH degrees (Ca-Ce) = 380 +/- 40 cal mol(-1) (I) and DeltaH degrees (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.