title: Sulfur Dioxide-Pyridine Dimer. FTIR and Theoretical Evidence for a Low-Symmetry Structure.
abstract: 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, Cs-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 C1-symmetric structure containing a S-O bond oriented approximately coplanar with the pyridine ring and adjacent to an ortho C-H moiety. The C1 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 (Pbb) value previously determined by microwave spectroscopy. The complex is converted to its mirror image via three possible Cs-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 C2-symmetric SO2-pyridine2 trimer.
authors.affiliation: Department of Chemistry and Biochemistry, University of Alaska Fairbanks , Fairbanks, Alaska 99775-6160, United States.
authors.forename: John W
authors.initials: JW
authors.lastname: Keller
mesh_terms: D019281:Dimerization; D015394:Molecular Structure; D011725:Pyridines / Q000737:chemistry*; D011789:Quantum Theory; D017550:Spectroscopy, Fourier Transform Infrared; D013458:Sulfur Dioxide / Q000737:chemistry*
chemical_list: D011725:Pyridines; D013458:Sulfur Dioxide; C023666:pyridine