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-SO, 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 (Delta G(o double dagger)) 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 C, symmetry, and a C-2-symmetric SO2-pyridine(2) trimer.