Chemical Shifts in NMR Spectra

The signal frequency that is detected in nuclear magnetic resonance (NMR) spectroscopy is proportional to the magnetic field applied to the nucleus. This would be a precisely determined frequency if the only magnetic field acting on the nucleus was the externally applied field. But the response of the atomic electrons to that externally applied magnetic field is such that their motions produce a small magnetic field at the nucleus which usually acts in opposition to the externally applied field. This change in the effective field on the nuclear spin causes the NMR signal frequency to shift. The magnitude of the shift depends upon the type of nucleus and the details of the electron motion in the nearby atoms and molecules. It is called a "chemical shift". The precision of NMR spectroscopy allows this chemical shift to be measured, and the study of chemical shifts has produced a large store of information about the chemical bonds and the structure of molecules.

在核磁共振（NMR）光谱学中检测到的信号频率与作用在核上的磁场成正比。如果作用在核上的唯一磁场只是外加磁场，那么这个频率将被精确确定。但是，原子电子对外加磁场的响应使得它们的运动在核处产生一个较小的磁场，通常与外加磁场方向相反。这种有效磁场的变化导致核自旋的频率发生偏移。偏移的大小取决于核的类型以及附近原子和分子中电子运动的细节。这种偏移被称为“化学偏移”。NMR光谱学的精度使得可以测量这种化学偏移，而对化学偏移的研究已产生了关于化学键和分子结构的大量信息。

The effective magnetic field at the nucleus can be expressed in terms of the externally applied field B₀ by the expression

核处的有效磁场可以表示为外加磁场B₀的函数

where σ is called the shielding factor or screening factor. The factor σ is small - typically
10^-5 for protons and <10^-3 for other nuclei (Becker).

其中σ称为屏蔽因子或筛选因子。σ这个因子通常很小——对于质子来说通常为10-5，对于其他核子则小于10-3（Becker）。

In practice the chemical shift is usually indicated by a symbol δ which is defined in terms of a standard reference.

实际上，化学位移通常用符号δ表示，该符号是基于标准参考物质定义的。

The signal shift is very small, parts per million, but the great precision with which frequencies can be measured permits the determination of chemical shift to three or more significant figures. The reference material is often tetramethylsilane, Si(CH₃)₄, abbreviated TMS. Since the signal frequency is related to the shielding by

信号位移非常小，通常以百万分之一来表示，但频率测量的高精度使得可以确定化学位移到三位或更多有效数字。参考物质通常为四甲基硅烷，即Si(CH3)4，简称TMS。由于信号频率与屏蔽有关，

化学位移也可以表示为

A sample of a chemical shift spectrum is given below. It is a sketch of a proton nmr spectrum included in Becker, Chapter 1. The high-resolution peaks can be identified with the functional groups in the molecule: δ = 1.23, (CH₃)₂; 2.16, CH₃C=O; 2.62,CH₂; 4.12, OH.

下面给出了一种化学移位谱的示例。它来自Becker，第1章中的一张质子核磁共振谱草图。高分辨峰可以与分子中的官能团对应：δ = 1.23, (CH3)2；2.16, CH3C=O；2.62, CH2；4.12, OH。