核磁共振Nuclear magnetic resonance spectroscopy

自旋量子数 I = 1/2 的原子核在外磁场 B₀ 中分裂为两个能级，能级差 ΔE = γħB₀，对应的共振频率在射频区（11.7 T 时 ¹H 约 500 MHz）。核外电子对外磁场产生屏蔽，不同化学环境的核共振频率略有差别，以相对四甲基硅烷（TMS）的化学位移 δ（ppm）表示。相邻核通过化学键的自旋-自旋耦合使信号裂分，裂分规律遵循 n+1 规则。

A nucleus with spin I = 1/2 splits into two energy levels in an external field B₀, separated by ΔE = γħB₀, which corresponds to a radio frequency (about 500 MHz for ¹H at 11.7 T). Surrounding electrons shield the nucleus, so nuclei in different environments resonate at slightly different frequencies, expressed as the chemical shift δ in ppm relative to tetramethylsilane (TMS). Spin–spin coupling through bonds splits signals according to the n+1 rule.

1938 年拉比用分子束首次观测到核磁共振，获 1944 年诺贝尔物理学奖。1946 年布洛赫和珀塞尔分别在凝聚态物质中观测到 NMR 信号，共获 1952 年诺贝尔物理学奖。1950 年前后普罗克特等发现化学位移，使 NMR 成为化学工具；1953 年瓦里安公司推出首台商用波谱仪。

Rabi first observed nuclear magnetic resonance in molecular beams in 1938 and received the 1944 Nobel Prize in Physics. In 1946 Bloch and Purcell independently detected NMR in bulk matter and shared the 1952 Nobel Prize in Physics. Around 1950 Proctor and others discovered the chemical shift, turning NMR into a chemical tool, and Varian built the first commercial spectrometer in 1953.

1966 年恩斯特发展脉冲傅里叶变换 NMR，大幅提高灵敏度，使 ¹³C 等低丰度核的测定成为常规；1970 年代他和耶纳等发展二维 NMR（COSY、NOESY 等），恩斯特获 1991 年诺贝尔化学奖。维特里希用多维 NMR 测定溶液中蛋白质结构，获 2002 年诺贝尔化学奖。同一原理还发展出医学磁共振成像（MRI）。

In 1966 Ernst developed pulsed Fourier-transform NMR, greatly raising sensitivity and making low-abundance nuclei such as ¹³C routine; in the 1970s he, Jeener and others developed two-dimensional NMR (COSY, NOESY and more), and Ernst received the 1991 Nobel Prize in Chemistry. Wüthrich used multidimensional NMR to solve protein structures in solution and shared the 2002 Nobel Prize in Chemistry. The same physics gave rise to magnetic resonance imaging (MRI) in medicine.

¹H NMR 提供四类信息：信号数目（化学不等价氢的种类）、化学位移（官能团环境，如醛氢 δ 9–10、芳氢 δ 6.5–8、烷基氢 δ 0.9–1.5）、积分面积（氢原子数之比）和裂分峰形与耦合常数 J（相邻氢的数目和空间关系）。测试通常在氘代溶剂（如 CDCl₃）中进行。

A ¹H spectrum gives four kinds of information: the number of signals (kinds of inequivalent hydrogen), chemical shifts (environments such as aldehyde H at δ 9–10, aromatic H at δ 6.5–8, alkyl H at δ 0.9–1.5), integrals (ratios of hydrogen counts), and splitting patterns with coupling constants J (the number and geometry of neighbouring hydrogens). Samples are usually run in deuterated solvents such as CDCl₃.