Keller John W
Department of Chemistry and Biochemistry, University of Alaska Fairbanks , Fairbanks, Alaska 99775-6160, United States.
J Phys Chem A. 2015 Oct 15;119(41):10390-8. doi: 10.1021/acs.jpca.5b06122. Epub 2015 Oct 6.
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.
利用傅里叶变换红外（FTIR）光谱和计算方法对二氧化硫 - 吡啶配合物的形成进行了重新研究。基于微波光谱，已提出SO₂ - 吡啶二聚体具有V形、Cs对称结构；然而，最近的研究表明在非共价配合物中存在X···H - C氢键，这表明该配合物的结构应重新审视。通过对CCl₄中吡啶 - SO₂混合物光谱的数值分析获得了二聚体的FTIR光谱。该光谱显示邻位C - H伸缩模式与C₁对称结构一致，该结构包含一个与吡啶环大致共面且与邻位C - H部分相邻的S - O键。通过各种密度泛函理论和相关的从头算计算确定为全局最小值的C₁结构，也与先前通过微波光谱测定的面外二阶矩（Pbb）值一致。该配合物通过三种可能的Cs对称过渡态转变为其镜像：V形、平分形和平坦形。在M06 - 2X/6 - 311++G(2d,p)理论水平下，旋转势垒（ΔG(o‡)）分别为1.40、1.87和3.63 kcal mol⁻¹。自然键序分析表明，不对称配合物通过N→S供体作用以及从O到吡啶上反键轨道的反馈作用而稳定。分子中的原子计算确定了O···H - C间隙内的一个键临界点，这与一个正常的（尽管较弱）氢键一致。理论研究还确定了一种具有Cs对称性的高能夹心型二聚体，以及一种C₂对称的SO₂ - 吡啶₂三聚体。
