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Solvent and protonation effects on nitrogen NMR shieldings of isoamides (o -aminoethers) 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 a-carbon on the solute-to-solvent interactions as evidenced by nitrogen NMR shieldings. In addition to comparing the behaviour of the imino moiety in compound 1 (Fig. 1) with that of simple imines in a variety of solvents, we wish to study the effects ofsolute-to-solvent hydrogen-bond formation on nitrogen shieldings. This is feasible for compound 2 owing to the presence of an NH moiety. A further aspect of this work is estimation of the influence of the effects of 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). In this work we use the sign convention in which an increase in nuclear shielding has a plus sign [1-3]. Thus we employ the term 'nuclear shielding,' rather than 'chemical shift'. These two terms are equivalent in magnitude but are of opposite sign. 2. Experimental The compounds studied were prepared by previously published procedures [4,5]. Particular care was taken in the NMR measurements to use only very pure and dry solvents as previously reported[3]. All solutions were prepared and handled in glove bags. The 14N shielding measurements were taken on a Bruker AM500 Spectrometer at 35 _+0.2°C, as maintained by a VT unit, at a frequency of 36.14MHz. Random and systematic errors were reduced to below 0.1 ppm for the solute-nitrogen shieldings in different solvents. External neat liquid nitromethane was used as a reference by means of 10 mm/4 mm o.d. coaxial tubes. The inner tube contained 0.3 M nitromethane in acetone-d6; the nitrogen shielding of this solution is + 0.77 ppm from that of neat liquid nitromethane [3]. This value is obtained from measurements using concentric spherical sample/reference containers in order to eliminate bulk susceptibility effects. The value of + 0.77 ppm is used as a conversion constant. Thus the contents of the inner tube act both as a reference, with respect to neat nitromethane as standard, and as a deuterium lock for the NMR Spectrometer. The exact resonance frequency of the 14N signal of neat nitromethane is 36.141524 MHz, from which a value of 36.136826 MHz is obtained for the bare nitrogen nucleus [3]. This latter value is used in conjunction with the relevant resonance frequency differences to calculate the nitrogen shieldings relative to that of neat nitromethane. Lorentzian line-shape fitting of the 14N signals was used to produce values for the precise resonance frequencies of both the samples used and of the external standard. Dilute solutions were used in the present study hence their susceptibilities are assumed to be equal to those of the corresponding solvent at 35°C. |
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jurkat.1640
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4楼2008-05-23 08:04:03
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溶剂和质子的影响，氮核磁共振shieldings的isoamides （邻aminoethers ） 精度高，溶剂诱导14 N核磁共振shieldings介绍了一些isoamides和他们的N -质子形式。结果表明，该的主要来源，溶质-溶剂的相互作用，从而产生氮屏蔽的变化，是溶剂t osolute氢键而孤对电子的亚氨基氮作为氢键受体中心的地位。这个结果在一个显着的屏蔽增幅为亚氨基氮原子。另一种互动，可重要的是，当信息NH moieties都在场，是溶质-溶剂氢键而信息NH哮吼是从事作为一个捐助者。在本研究中这种作用是发现做一个谦虚的一和产生的氮deshielding 。影响非特异性的相互作用，所产生的溶剂极性的影响，比较软弱，比较，对于类似的亚胺系统。方向和规模效应，这是正确的复制分子轨道计算，纳入solvaton模型的非特异性solutesolvent互动。质子化效应对氮shieldings的isoamides十分可观，生产增加，在屏蔽约80 ppm的。这是在与观察为溶剂-溶质的氢键。 关键字“ 。氢键; isoamide （ 〜 -亚胺醚） ;氮核磁共振shieldings ;质子效应;溶剂效应 1 。导言 结构isoamides ，或〜 - iminoethers ，图。一，是同分异构体与尊重，以相应的酰胺。该isoamides包含一亚胺型基元，并从不同的亚胺在这一个或哮吼是附加到C 〜碳，发现在isoamides 。我们以往的研究对亚胺[ 1 ]及其类似物的芳香[ 2 ]表明，氮核磁共振shieldings是非常敏感的分子相互作用的解决办法。双方具体的和非特异性溶质-溶剂的相互作用发挥重要作用，在变化中氮的屏蔽作为一个功能的溶剂。 在一般唯一对电子对亚胺氮原子法作为一种强有力的受体网站为溶剂tosolute氢键。这种氢键通常是负责显着增加，氮核屏蔽。这提供了一个衡量相对碱度的各种氮原子所涉及的氢键。此外，非特异性的相互作用，如因溶剂极性似乎也产生显着增加，在亚氨基氮屏蔽作为溶剂极性增加。这屏蔽增强源自移民电子电荷对亚氨基氮原子。 在目前的工作就isoamide系统，我们所关注的影响，该基元或在1 -碳对溶质-溶剂的相互作用，证明氮核磁共振shieldings 。此外，以比较的行为，该基元在亚氨基化合物1 （图1 ）与简单的亚胺在各种溶剂中，我们要研究的影响ofsolute到溶剂氢键的形成 氮shieldings 。 这是可行的化合物2 ，由于在场的一个信息NH基元。另一个方面，这项工作是估计的影响，影响质子的亚氨基基元就其氮屏蔽。 有关化合物为这方面的调查是号码3和第4款（图1 ） 。 在这项工作中，我们使用的标志公约，其中增加在核屏蔽有一个加号[ 1-3 ] 。因此，我们聘请一词核屏蔽， ' ，而非'化学位移' 。这两个名词是相等的幅度，但相反的迹象。 2 。实验 化合物的研究，编写了先前公布的程序[ 4,5 ] 。特别是采取了照顾，在核磁共振测量只使用非常纯净，干燥的溶剂如以前所报告[ 3 ] 。所有的解决办法，准备和处理在手套袋。该14 N屏蔽测量所采取的一布鲁克am500谱仪在35 _ 0.2 ° C时，保持一个佛蒙特州单位，在一个频率36.14mhz 。随机和系统误差减少到低于0.1百万分之一 Powered |
5楼2008-05-23 10:34:38
wang17152
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6楼2008-05-23 10:47:47
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