Dupont J, Suarez PA, Burrow RA, Kintzinger JP
Laboratory of Molecular Catalysis, Instituto de Quimica, UFGRS, Porto Alegre, RS, Brazil.
Chemistry. 2000 Jul 3;6(13):2377-81. doi: 10.1002/1521-3765(20000703)6:13<2377::aid-chem2377>3.0.co;2-l.
The crystal structure of 1-n-butyl-3-methylimidazolium tetraphenylborate molten salt (1) shows C-H-pi interactions between the hydrogens of the imidazolium cation and the phenyl rings of the tetraphenylborate anion. The imidazolium ring is surrounded by three tetraphenylborate anions that are connected with the same cation by C-H-pi (phenyl rings) interactions. The nearest inter-ion interaction is found between the N-CH-N proton of the cation and the B-phenyl centroid (2.349 A) with a nearly T-shaped geometry. The inter-ionic solution structure of 1 has been investigated by the detection of inter-ionic contacts in 1H NOESY NMR spectra between the protons of the cation and the anion. The 1H-NMR spectra of molten salt 1 is almost independent of its concentration in [D6]DMSO solution, the imidazolium proton chemical shifts are in the expected region and there are no observable NOE effects between the protons of the cation with those of the anion, indicating that 1 behaves in [D6]DMSO as a solvent-separated ion pair. In CDCl3 the 1H-NMR spectra of 1 are concentration dependent and all the imidazolium protons are shielded as compared with those observed in [D6]DMSO. Moreover, the 1H NOESY NMR spectra show all the peaks affected by the interaction between the protons of the imidazolium cation and those of the anion, indicating that in CDCl3 1 possesses a contact ion pair structure. The NCHN proton of the cation exhibits the greatest shielding (up to -4.5 ppm). an indication of the existence of C-H-pi interactions, even in solution. The calculated distance of this proton to the phenyl centroid is 2.3 A for a C-H -pi angle of 180 degrees. The apparent volumes for the cation and anion, calculated from the measured 13C-NMR relaxation times, increase from 38 and 140 A3 in [D6]DMSO to 360 and 600 A3 in CDCl3, respectively; this indicates the formation of floating aggregates of the type (1)(n) in CDCl3 via weak hydrogen bonds, with increasing concentration.
1-正丁基-3-甲基咪唑四苯基硼酸盐熔盐（1）的晶体结构表明，咪唑阳离子的氢与四苯基硼酸根阴离子的苯环之间存在C-H-π相互作用。咪唑环被三个四苯基硼酸根阴离子包围，这些阴离子通过C-H-π（苯环）相互作用与同一个阳离子相连。在阳离子的N-CH-N质子与B-苯基质心之间发现了最近的离子间相互作用（2.349 Å），其几何形状近似为T形。通过检测1H NOESY NMR光谱中阳离子和阴离子质子之间的离子间接触，研究了1的离子间溶液结构。熔盐1的1H-NMR光谱几乎与其在[D6]DMSO溶液中的浓度无关，咪唑质子化学位移在预期区域，阳离子质子与阴离子质子之间没有可观察到的NOE效应，这表明1在[D6]DMSO中表现为溶剂分隔离子对。在CDCl3中，1的1H-NMR光谱与浓度有关，与在[D6]DMSO中观察到的相比，所有咪唑质子都被屏蔽。此外，1H NOESY NMR光谱显示所有峰都受到咪唑阳离子质子与阴离子质子之间相互作用的影响，这表明在CDCl3中1具有接触离子对结构。阳离子的NCHN质子表现出最大的屏蔽（高达-4.5 ppm），这表明即使在溶液中也存在C-H-π相互作用。对于180度的C-H-π角，该质子到苯基质心的计算距离为2.3 Å。根据测得的13C-NMR弛豫时间计算的阳离子和阴离子的表观体积，分别从[D6]DMSO中的38和140 Å3增加到CDCl3中的360和600 Å3；这表明随着浓度增加，在CDCl3中通过弱氢键形成了（1）（n）型漂浮聚集体。
