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Abstract High-precision solvent-induced 14N NMR shieldings are presented for some isoamides and their N-protonated forms. The results indicate that the major source of solute-solvent interactions, giving rise to nitrogen shielding variations, is solvent-tosolute hydrogen bonding where the lone pair electrons of the imino nitrogen act as the hydrogen-bond acceptor centre. This results in a remarkable shielding increase for the imino nitrogen atom. Another interaction which could be important, when NH moieties are present, is solute-to-solvent hydrogen bonding where the NH croup is engaged as a donor. In the present study this effect is found to be a modest one and produces nitrogen deshielding. The influence of non-specific interactions, arising from solvent polarity effects, is rather feeble compared with that for analogous imine systems. The direction and magnitude of this effect is correctly reproduced by molecular orbital calculations incorporating the Solvaton model for non-specific solutesolvent interactions. Protonation effects on the nitrogen shieldings of the isoamides are substantial, producing an increase in shielding of about 80 ppm. This is in accord with that observed for solvent-to-solute hydrogen bonding. Keywords." Hydrogen bonding; Isoamide (~-imino ether); Nitrogen NMR shieldings; Protonation effects; Solvent effects 1. Introduction The structures of isoamides, or ~-iminoethers, Fig. 1, are isomeric with respect to the corresponding amides. The isoamides contain an imino-type moiety and differ from imines in that an OR croup is attached to the c~ carbon found in isoamides. Our previous studies on imines [1] and their aromatic analogues [2] show that nitrogen NMR shieldings are very sensitive to molecular interactions in solution. Both specific and non-specific solute-solvent interactions play significant roles in the variation of nitrogen shielding as a function of solvent. In general the lone pair electrons on the imino nitrogen atom act as a strong acceptor site for solvent-tosolute hydrogen bonding. Such hydrogen bonding is usually responsible for a significant increase in nitrogen nuclear shielding. This offers a measure of the relative basicity of the various nitrogen atoms involved in hydrogen bonding. In addition, nonspecific interactions such as those arising from solvent polarity also appear to produce a significant increase in imino nitrogen shielding as the solvent polarity increases. This shielding enhancement arises from the migration of electronic charge towards the imino nitrogen atom. In the present work on isoamide systems we are concerned with the effects of the OR moiety at the protonation of the imino moiety on its nitrogen shielding. The relevant compounds for this aspect of the investigation are numbers 3 and 4 (Fig. 1). |
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chengcj.2008
至尊木虫 (知名作家)
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1. 引言 异酰胺或~-亚氨基醚的结构如 Fig 1所示, 与相应的酰胺互为异构体. 异酰胺含有亚胺形式, 但它与亚胺的不同在于, 异酰胺中的烷氧基(OR)与碳相连. 我们以前对亚胺[1]和其芳香类似物[2]的研究结果表明, 氮原子 NMR 屏蔽对于溶液中分子间相互作用非常敏感. 无论是特定还是非特定的溶质溶剂相互作用, 对于作为溶剂函数的氮原子屏蔽的变化都非常重要. 一般来说, 亚胺基氮原子上的孤对电子作为强的溶剂对溶质的氢键受体. 这种氢键通常使得氮原子的屏蔽大大增强. 这提供了一种测定形成氢键的氮原子的相对碱性的方法. 此外, 随溶剂极性增加, 非特定相互作用, 如溶剂极性产生的作用, 也能使氮原子的屏蔽显著增强. 这种屏蔽的增强源于电子向亚氨基氮原子的迁移. 本文对于异酰胺体系的研究, 我们侧重于烷氧基(OR)对质子化的亚胺氮原子屏蔽的影响. 这方面所研究的相关化合物是3号与4号(Fig. 1). |
3楼2008-05-22 18:11:47
chengcj.2008
至尊木虫 (知名作家)
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摘要 本文对异酰胺及其质子化形式提供了高精度的溶剂诱导的 14N NMR 屏蔽. 结果表明, 溶质与溶剂之间的相互作用主要源于溶剂对溶质氢键的形成, 其中亚胺基氮原子的孤对电子作为氢键的受体中心．溶质与溶剂之间的相互作用，使得氮原子的屏蔽作用增强．这导致了对亚胺基氮原子的屏蔽作用明显增强．当 NH 存在时,另一种重要的相互作用是溶质对溶剂氢键的形成, 这里 NH 作为供体. 目前的研究表明, 这种效应具有中等的作用, 并造成氮原子的去屏蔽. 与类似的亚胺体系相比, 由溶剂极性所产生的非特殊相互作用的影响很弱. 运用分子轨道计算并结合非特殊溶质溶剂相互作用的溶剂化模型, 这种效应的方向与强度能够被正确地重复. 质子化效应对异酰胺氮原子的屏蔽是显而易见的, 致使屏蔽增加了 80 ppm. 这与观察到的溶剂对溶质的氢键作用是一致的. 关键词: 氢键; 异酰胺 (~-亚氨基醚); 氮原子 NMR 屏蔽; 质子化效应; 溶剂效应 |
2楼2008-05-22 16:25:06