Structure and conformations of N-(chloroformyl) iminosulfur dichloride, ClC(O)NSCl2

Vibrational spectra [FT IR (gas and liquid); FT Raman (liquid)] and quantum chemical calculations at different levels of theory (HF, B3LYP and MP2 using 6-31G(d), 6-311 + G(2df) and cc-pVTZ base sets) demonstrate that N-(chloroformyl) iminosulfur dichloride (ClC(O)NSCl2) is present in the fluid phases as a mixture of syn–syn and syn–anti forms (syn orientation of the CN bond with respect to the ClSCl bisector; syn or anti orientation of the carbonyl group with respect to the NS bond). From the relative IR intensities of the CO stretching bands the contribution of the syn-anti form is calculated to be 6(2)% in the gas, corresponding to a Gibbs free energy difference ΔG° = G°syn–anti − G°syn–syn = 1.6(3) kcal/mol. This value is reproduced very well by quantum chemical calculations which include electron correlation effects (ΔG° = 1.59–1.91 kcal/mol). The HF approximation overestimates this energy difference (ΔG° = 4.45 kcal/mol). On the basis of theoretical calculations and reported data for other iminosulfur dichlorides, a complete assignment of the fundamental modes for ClC(O)NSCl2 is proposed.