Sulfur Dioxide–Pyridine Dimer. FTIR and Theoretical Evidence for a Low-Symmetry Structure

Sulfur dioxide–pyridine complex formation was reinvestigated using Fourier transform infrared (FTIR) spectroscopy and computational methods. The SO2–pyridine dimer has been proposed to have a v-shaped, C s -symmetric structure based on the microwave spectrum; however, recent research showing the occurrence of X···H–C hydrogen bonds in noncovalent complexes suggested that the structure of the complex should be re-examined. The FTIR spectrum of the dimer was obtained by numerical analysis of the spectra of pyridine–SO2 mixtures in CCl4. The spectrum showed ortho C–H stretching modes consistent with a C 1-symmetric structure containing a S–O bond oriented approximately coplanar with the pyridine ring and adjacent to an ortho C–H moiety. The C 1 structure, which was identified as the global minimum by various density functional theory and correlated ab initio calculations, is also consistent with the out-of-plane second moment (P bb ) value previously determined by microwave spectroscopy. The complex is converted to its mirror image via three possible C s -symmetric transition states: v-shaped, bisected, and flat. At the M06–2X/6–311++G­(2d,p) level of theory, the rotational barriers (ΔG o‡) are 1.40, 1.87, and 3.63 kcal mol–1, respectively. Natural bond order analysis indicated the asymmetric complex is stabilized both by N→S donation and back-donation from O to antibonding orbitals on pyridine. Atoms in molecules calculations identified a bond critical point within the O···H−C gap consistent with a normal, albeit weak, hydrogen bond. Theoretical studies also identified a high-energy sandwich-type dimer with Cs symmetry, and a C 2-symmetric SO2–pyridine2 trimer.