Department of Chemistry, University of Pune, Ganeshkhind, Pune 411007, India.
J Phys Chem A. 2010 Feb 18;114(6):2338-43. doi: 10.1021/jp9090602.
Interactions of sulfur hexafluoride (SF(6)) with cucurbit[6]uril (CB[6]) have been investigated using the density functional calculations. An encapsulation of guest within CB[6] cavity as well as its binding to either exterior host protons or portal uredio oxygens have been analyzed. The present calculations predict that the complexes with the complete inclusion of SF(6) are favored over those possessing lateral or external interactions. The interactions between fluorine and ureido carbons (F---C) of the host contribute to lowering of energy of the complex. Normal mode analyses from the calculated vibrational spectra show a red-shifted stretching (approximately 928 cm(-1)) of S-F bonds perpendicular to the CB[6] cavity axis when SF(6) is encapsulated in the host cavity. On the other hand, S-F bonds parallel to the cavity axis exhibit a blue shift as compared to the corresponding vibration of the isolated guest. These frequency shifts of S-F bonds have further been analyzed by mapping the difference electron density on the bond critical point(s) (bcp) in molecular electron density (MED) topography and natural bond orbital (NBO) analyses. A depletion of electron density at the bcp along with an enhanced electron density in antibonding S-F* orbital engender weakening of the bond. Concomitant redistribution of electron density leads to the strengthening of S-F bonds parallel to the cavity axis (directing toward either portals). (1)H NMR reveals that the protons directing toward CB[n] portals are not influenced by encapsulation of the guest, which is in consonant with experimentally measured NMR spectra.
使用密度泛函计算研究了六氟化硫（SF(6)）与葫芦脲（CB[6]）的相互作用。分析了客体在 CB[6]腔体内的包合以及与主体外部质子或门户脲氧基的结合。目前的计算预测，完全包含 SF(6)的配合物比具有侧向或外部相互作用的配合物更有利。主体中氟原子和脲基碳原子（F---C）之间的相互作用有助于降低配合物的能量。从计算得到的振动光谱的正则模态分析表明，当 SF(6)被包封在主体腔体内时，垂直于 CB[6]腔轴的 S-F 键发生红移（约 928 cm(-1)）。另一方面，与空腔轴平行的 S-F 键与孤立客体的对应振动相比，出现蓝移。这些 S-F 键的频率位移进一步通过在分子电子密度（MED）地形和自然键轨道（NBO）分析的键临界点（bcp）上映射电子密度差进行分析。随着 bcp 处电子密度的耗尽和反键 S-F*轨道中电子密度的增加，键的强度减弱。伴随电子密度的重新分布，导致与空腔轴平行的 S-F 键（指向任一门户）增强。（1）H NMR 表明，指向 CB[n]门户的质子不受客体包合的影响，这与实验测量的 NMR 谱一致。
