LANTHANIDE SHIFT REAGENTS IN NUCLEAR MAGNETIC-RESONANCE SPECTROSCOPY
LANTHANIDE SHIFT REAGENTS IN NUCLEAR MAGNETIC-RESONANCE SPECTROSCOPY
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DOI：
10.1039/cs9730200049
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发表时间：
1973-01-01
影响因子：
46.2
通讯作者：
MAYO, BC
中科院分区：
文献类型：
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作者：
MAYO, BC
核磁共振波谱是研究复杂有机分子结构的最有价值的技术。然而，由于质子化学位移对化学和立体化学环境变化的敏感性相对较低，核磁共振波谱的应用受到了严重的限制。在与甾体和萜类的质子核磁共振谱相联系时经常使用的“亚甲基、亚甲基壳”等术语说明了不等价质子的共振的频繁重叠。位移试剂用于核磁共振谱中，通过改变原子核的磁性环境来降低原子核的当量，有两种类型：芳香族溶剂，如苯或吡啶，以及顺磁性金属络合物。后者的作用是与所研究化合物中合适的给体原子配位，从而扩大它们的配位壳并在溶液中形成新的络合物。除了键合电子的屏蔽效应外，顺磁金属离子还通过电子自旋密度的转移、通过共价键的形成、从金属离子到相关原子核的转移(接触位移)或未配对的电子磁矩的磁效应(假接触位移)来改变化学位移。第一排过渡金属络合物可用作位移试剂，可通过接触和假接触两种机制进行操作，尽管由于这些化合物的共价性质，前者占主导地位。不幸的是，这些位移试剂会引起严重的谱线展宽，从而对核磁共振谱的分辨率产生不利影响。1969年，Hinckleyl在这一领域取得了重大进展，引入了稀土-金属络合物作为位移试剂，从那时起，人们已经确定，稀土络合物产生的线宽加宽和位移几乎完全是由伪接触机制引起的。发现的最有用的配合物是稀土乙酰丙酮衍生物，其中一些是氟化的，表现出更大的位移能力。最常见的做法是在被研究的化合物(底物)中连续加入已知量的稀土移动剂(LSR)，并在每次添加后记录核磁共振谱。底物中每个质子的化学位移随着位移的增加而或多或少地发生变化。
Nuclear magnetic resonance (nmr) spectroscopy is a most valuable technique for structural investigations of complex organic molecules. However, owing to the relatively low sensitivity of proton chemical shifts to changes in the chemical and stereochemical environment, the application of nmr spectroscopy has been severely restricted. Such terms as the ‘methylene, methine envelope’used frequently in connection with the proton nmr spectra of steroids and terpenes illustrate this frequent overlapping of resonance of non-equivalent protons. Shift reagents are used in nmr spectroscopy to reduce the equivalence of nuclei by altering their magnetic environment, and are of two types: aromatic solvents such as benzene or pyridine, and paramagnetic metal complexes. The latter function by co-ordinating to suitable donor atoms in the compound under study, thereby expanding their co-ordination shell and forming a new complex in solution. Apart from effects due to shielding by bonding electrons, the chemical shifts are altered by the paramagnetic metal ion by a transfer of electron spin density, via covalent bond formation, from the metal ion to the associated nuclei (contact shift), or by magnetic effects of the unpaired electron magnetic moment (pseudocontact shift). First-row transition-metal complexes can be used as shift reagents and operate by both contact and pseudocontact mechanisms, although the former predominates owing to the covalent character of these compounds. Unfortunately, these shift reagents exhibit an adverse effect on the resolution of the nmr spectra by causing severe line-broadening. In 1969 Hinckleyl initiated a major advance in this field by introducing the use of a lanthanide-metal complex as a shift reagent and since then it has become established that lanthanide complexes produce far less linewidth broadening and give shifts which are caused virtually exclusively by the pseudocontact mechanism. The complexes found most useful are lanthanide acetylacetonate derivatives, some of which are fluorinated and exhibit greater shifting power. The most common practice is to successively add known amounts of the lanthanide shift reagent (LSR) to the compound under study (substrate) and record the nmr spectrum after each addition. The chemical shift of each proton in the substrate alters, to a greater or lesser degree, with each addition of shift