Pinjari Rahul V, Gejji Shridhar P
Department of Chemistry, University of Pune, Ganeshkhind, Pune 411007, India.
J Phys Chem A. 2009 Feb 19;113(7):1368-76. doi: 10.1021/jp809293s.
Inverted cucurbit[n]uril (i(x)CB[n], x = 1, 2; n = 6-8), the enantiomers of cucurbit[n]uril (CB[n]) comprising one or more inverted glycouril units, show distinct selectivity in recognition toward the guest by the virtue of shape and dimensions of its cavity. The iCB[n] (x = 1 and n = 6, 7) are isolated as intermediates during the synthesis of CB[n]. In this work, density functional theory using the hybrid B3LYP functional has been employed to derive the electronic structure and the NMR chemical shifts in the i(x)CB[n] hosts. The present calculations have shown that the inversion of the glycouril unit of CB[6] and CB[7] engenders a destabilization by 4.2 and 5.7 kJ mol(-1), respectively, and, as opposed to this, the iCB[8] is favored by 18.6 kJ mol(-1) over the corresponding CB[8] host. Likewise, i2CB[7] possessing two inverted glycourils are highly destabilized over CB[7]. A large separation of the inverted glycouril units reduces the repulsion between methine protons inside the cavity, rendering the 1,4-i2CB[n] (n = 7 or 8) to be of lowest energy. Stabilization energies from the self-consistent reaction field (SCRF) theory are calculated with water, ethanol, and tetrahydrofuran (THF) as solvents. Unlike in gas phase and other solvents, the stabilization hierarchy iCB[6] < iCB[7] < iCB[8] has been predicted in THF. Molecular electrostatic potential (MESP) was used to gauge the cavity shape of these hosts. Consequently the iCB[6] reveals a half-sprocket-like cavity; an additional tooth for each glycouril in the succeeding iCB[n] homologue was noticed. In the case of the 1,5-i2CB[8] enantiomer, the cavity turns out to be rectangular. The deeper MESP minima near the ureido oxygens suggest strong electrostatic interactions with the guest at the iCB[6] portals. The electron-rich region within the cavity explains the large affinity of CB[n] toward the electron deficient guests. The electronic distribution and shape and size of the cavity thus derived provide insights for the inclusion of guests of different shapes in a variety of i(x)CB[n] hosts. NMR chemical shifts have shown that the methylene protons near the inverted glycouril and the methine protons those are directing toward the cavity yield distinct signals, consistent with those observed in experiments. The protons within the cavity are less affected by solvation.
葫芦[n]脲的对映异构体（i(x)CB[n]，x = 1, 2；n = 6 - 8），包含一个或多个倒置甘脲单元，凭借其腔的形状和尺寸对客体表现出独特的识别选择性。iCB[n]（x = 1且n = 6, 7）在CB[n]的合成过程中作为中间体被分离出来。在这项工作中，采用了使用杂化B3LYP泛函的密度泛函理论来推导i(x)CB[n]主体的电子结构和NMR化学位移。目前的计算表明，CB[6]和CB[7]的甘脲单元的倒置分别导致4.2和5.7 kJ mol⁻¹的不稳定，与此相反，iCB[8]比相应的CB[8]主体更稳定18.6 kJ mol⁻¹。同样，具有两个倒置甘脲的i2CB[7]相对于CB[7]高度不稳定。倒置甘脲单元的大间距减少了腔内次甲基质子之间的排斥，使得1,4 - i2CB[n]（n = 7或8）具有最低能量。使用水、乙醇和四氢呋喃（THF）作为溶剂，根据自洽反应场（SCRF）理论计算稳定化能。与气相和其他溶剂不同，在THF中预测了稳定化顺序为iCB[6] < iCB[7] < iCB[8]。分子静电势（MESP）用于衡量这些主体的腔形状。因此，iCB[6]显示出半链轮状的腔；在后续的iCB[n]同系物中，每个甘脲都有一个额外的齿被注意到。在1,5 - i2CB[8]对映体的情况下，腔原来是矩形的。脲基氧附近更深的MESP最小值表明在iCB[6]入口处与客体有强烈的静电相互作用。腔内富电子区域解释了CB[n]对缺电子客体的大亲和力。由此得出的电子分布以及腔的形状和大小为在各种i(x)CB[n]主体中包含不同形状的客体提供了见解。NMR化学位移表明，倒置甘脲附近的亚甲基质子和指向腔的次甲基质子产生不同的信号，与实验中观察到的一致。腔内的质子受溶剂化的影响较小。
