Vibrational spectra and structures of halogencarbonyl alkyldisulfanes XC(O)SSR with X=F, Cl and R=CF3, CH3

The disulfides FC(O)SSCF3, ClC(O)SSCF3 and ClC(O)SSCH3 have been prepared by new synthetic routes. Infrared and Raman spectra have been recorded and assigned. From the vibrational spectra and from the gas electron diffraction experiment it was concluded that FC(O)SSCF3 exists as a mixture of syn- and anti-conformers (CO synperiplanar/antiperiplanar to SS bond). The main conformer (83(5)% and ΔG°=G°(anti)−G°(syn)=0.95(28) kcal mol−1) possesses syn-structure. The vibrational spectra of the chlorocarbonyl disulfanes are interpreted in terms of a single conformer, but small amounts (<5%) of a second conformer cannot be excluded. Quantum chemical calculations (HF, MP2, B3PW91 with 6-31G∗ basis sets) reproduce the experimental results (conformational properties, geometric structure and vibrational frequencies) satisfactorily. The predicted difference in Gibbs free energy, ΔG°, for FC(O)SSCF3 varies between 0.8 and 1.5 kcal mol−1, those for the chlorine derivatives are larger than 2.6 kcal mol−1.