Conformational Analysis Using Infrared and Vibrational Circular Dichroism Spectroscopies: The Chiral Cyclic Sulfoxides 1-Thiochroman-4-one S-Oxide, 1-Thiaindan S-Oxide and 1-Thiachroman S-Oxide

We report the conformational analysis of three chiral cyclic sulfoxides, 1-thiochroman-4-one S-oxide (1), 1-thiaindan S-oxide (2), and 1-thiochroman S-oxide (3). Vibrational unpolarized absorption (IR) and vibrational circular dichroism (VCD) spectroscopies are employed, in combination with ab initio density functional theory (DFT). DFT calculations predict two, two, and three conformations with energies spanning < 1 kcal/mol in 1, 2, and 3, respectively. Harmonic IR and VCD spectra of the conformations of 1-3, predicted using DFT, are a sensitive function of conformational structure, and conformationally averaged IR and VCD spectra are Consequently more complex than the spectra of individual conformations. Predicted IR and VCD spectra are compared to experimental spectra of CCl4 and CS2 solutions of 1-3 in the mid-IR spectral region. Calculated and experimental spectra are in good agreement, permitting assignment of very nearly all experimental bands. The analysis unambiguously confirms the presence of the conformations of 1-3 predicted by DFT