📚 Carbon-13 NMR Spectroscopy | 碳-13 核磁共振谱
Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful analytical tools for determining the structure of organic compounds. While 1H NMR reveals the hydrogen skeleton, carbon-13 NMR spectroscopy provides direct information about the carbon framework of a molecule. Since carbon atoms form the backbone of all organic molecules, ¹³C NMR is an indispensable technique in structural elucidation. The method relies on the magnetic properties of the ¹³C isotope, which has a natural abundance of about 1.1%, making it NMR-active despite its low occurrence.
核磁共振波谱是确定有机化合物结构最强大的分析工具之一。¹H 核磁共振显示氢骨架，而碳-13 核磁共振谱则提供关于分子碳骨架的直接信息。由于碳原子构成了所有有机分子的主干，¹³C NMR 成为结构解析中不可或缺的技术。该方法依赖于 ¹³C 同位素的磁性，其天然丰度约为 1.1%，虽然含量低，但仍具有 NMR 活性。
1. Introduction to ¹³C NMR Spectroscopy | ¹³C 核磁共振谱简介
³C isotope possesses a nuclear spin (I = ½), it can be observed by NMR, whereas the more abundant ¹²C isotope has no spin and is NMR-inactive. A ¹³C NMR spectrum displays a set of signals, each corresponding to a chemically distinct carbon environment within the molecule. The number of signals therefore indicates how many different types of carbon are present.
碳-13 核磁共振谱检测置于强磁场中的 ¹³C 核对射频辐射的吸收。因为只有 ¹³C 同位素具有核自旋（I = ½），它能被 NMR 观测到，而丰度更高的 ¹²C 同位素则无自旋，不具备 NMR 活性。一张 ¹³C NMR 谱图呈现一组信号，每个信号对应于分子内一个化学上不同的碳环境。因此，信号的数量就表明了存在多少种不同类型的碳。
2. The Basis of NMR: Nuclear Spin | 核磁共振的基础：核自旋
The ¹³C nucleus behaves like a tiny bar magnet due to its spin. When placed in an external magnetic field (B₀), its magnetic moment can align either with the field (lower energy α-state) or against it (higher energy β-state). The energy difference ΔE between these two states is proportional to the strength of the applied field. Irradiation with radio waves of exactly the right frequency (the Larmor frequency) causes transitions from the α to the β state, giving rise to the NMR signal.
¹³C 核由于其自旋而表现得像一个微小的条形磁体。当置于外磁场（B₀）中时，其磁矩可以顺着磁场方向排列（能量较低的 α 态），也可以逆着磁场方向排列（能量较高的 β 态）。这两种状态之间的能量差 ΔE 与外磁场强度成正比。用恰好具有正确频率（拉莫尔频率）的射频波辐照，能够引发从 α 态到 β 态的跃迁，从而产生 NMR 信号。
3. Chemical Shift and the δ Scale | 化学位移与 δ 标度
Not all ¹³C nuclei in a molecule resonate at exactly the same frequency. Electrons surrounding the nucleus shield it from the full effect of the external magnetic field. The actual field experienced by a nucleus is slightly reduced by this local diamagnetic shielding, so the resonance frequency is shifted. The chemical shift δ is defined as the difference in resonance frequency between the sample nucleus and a reference, divided by the spectrometer frequency, and is expressed in parts per million (ppm):
分子中并非所有 ¹³C 核都在完全相同的频率下共振。核周围的电子会屏蔽外磁场的全部影响。由于这种局部抗磁屏蔽，核实际感受到的磁场略微减弱，因此共振频率发生位移。化学位移 δ 定义为样品核与参考物的共振频率之差再除以光谱仪频率，并以百万分之一（ppm）表示：
δ = (νsample − νreference) / νspectrometer × 10⁶
Chemical shifts are independent of the spectrometer frequency, making it possible to compare spectra recorded on different instruments. The more deshielded a carbon nucleus is, the larger its δ value and the further downfield its signal appears.
化学位移与光谱仪的频率无关，这使得在不同仪器上记录的谱图能够相互比较。碳核的去屏蔽程度越高，其 δ 值就越大，信号越出现在低场区域。
4. Reference Standard: Tetramethylsilane (TMS) | 参考标准：四甲基硅烷 (TMS)
The universally accepted reference for both ¹H and ¹³C NMR is tetramethylsilane, Si(CH₃)₄ (TMS). TMS is chosen because it is chemically inert, volatile (easily removed from the sample), and gives a single sharp signal from its twelve equivalent protons and four equivalent carbon atoms. In ¹³C NMR, the carbon atoms of TMS are highly shielded due to the electron-donating effect of silicon, so their signal appears at a very low frequency and is assigned a chemical shift of exactly 0 ppm.
¹H 和 ¹³C NMR 普遍接受的参考物是四甲基硅烷，Si(CH₃)₄ (TMS)。选择 TMS 是因为它化学惰性、易挥发（容易从样品中去除），并且其十二个等价的质子和四个等价的碳原子给出单一尖峰。在 ¹³C NMR 中，TMS 的碳原子由于硅的给电子效应而受到高度屏蔽，因此它们的信号出现在极低频率，并被指定为化学位移恰好 0 ppm。
In practice, a small amount of TMS is added to the sample, and all other signals are measured relative to it. The chemical shift scale therefore runs from 0 ppm (upfield, shielded) to about 220 ppm (downfield, deshielded) for most organic compounds.
实际使用中，将少量 TMS 加入样品，所有其他信号均相对于它进行测定。因此，对于大多数有机化合物，化学位移标度从 0 ppm（高场，屏蔽）一直延伸到约 220 ppm（低场，去屏蔽）。
5. Factors Affecting Chemical Shift | 影响化学位移的因素
The chemical shift of a ¹³C nucleus is primarily influenced by the electronegativity of nearby atoms, the hybridisation state of the carbon, and magnetic anisotropy effects. Electronegative substituents such as oxygen or halogens withdraw electron density, deshielding the carbon and shifting its signal to higher δ values. For example, the carbon atom in CH₃Cl appears at about 25 ppm, whereas CH₃OH has a shift near 50 ppm.
¹³C 核的化学位移主要受邻近原子的电负性、碳的杂化状态以及磁各向异性效应的影响。电负性取代基如氧或卤素会拉走电子密度，使碳去屏蔽，并使其信号移向更高的 δ 值。例如，CH₃Cl 中的碳约在 25 ppm 处出峰，而 CH₃OH 的碳化学位移接近 50 ppm。
Hybridisation also plays a key role: sp³-hybridised carbons appear in the 0– alkenes, aromatics) between 100 and 150 ppm, and sp-hybridised carbons (alkynes) around 70– roups (C=O), which are strongly deshielded, are found at the highest shifts, typically 160–220 ppm.
杂化状态也起着关键作用：sp³ 杂化的碳出现在 0–50 ppm 范围内，sp² 碳（烯烃、芳烃）在 100–150 ppm 之间，sp 杂化碳（炔烃）约在 70–90 ppm 之间。强烈去屏蔽的羰基（C=O）则出现在最高位移处，通常在 160–220 ppm。
6. Interpreting ¹³C NMR Spectra: Number of Signals | 解读 ¹³C NMR 谱：信号数目
The most important piece of information from a proton-decoupled ¹³C NMR spectrum is the number of signals. Each signal represents a set of chemically equivalent carbon atoms—carbons that are in identical environments, related by symmetry. For instance, ethanol (CH₃CH₂OH) gives two signals because the two methyl carbons of the CH₃ group are equivalent and the CH₂ carbon is in a different environment. Benzene, with its six identical sp² carbon atoms, shows only one signal.
从质子去耦 ¹³C NMR 谱中获得的最重要信息是信号的数目。每个信号代表一组化学等价的碳原子——即那些处于相同环境、通过对称性相关联的碳。例如，乙醇 (CH₃CH₂OH) 给出两个信号，因为 CH₃ 基团的两个碳是等价的，而 CH₂ 碳处于不同的环境。苯分子中有六个相同的 sp² 碳原子，因此只显示一个信号。
When deducing structure, it is essential to examine the symmetry of the molecule. A plane of symmetry or an axis of rotation can make seemingly different carbons equivalent. 1,4-dimethylbenzene, for example, has three types of carbon: the methyl carbons, the aromatic C–H carbons, and the substituted ring carbons.
在推断结构时，检查分子的对称性至关重要。一个对称面或对称轴可以使看似不同的碳原子变得等价。例如，1,4-二甲苯有三种类型的碳：甲基碳、芳环上的 C–H 碳和取代的环碳。
7. Chemical Shift Ranges for Common Functional Groups | 常见官能团的化学位移范围
The table below summarises the typical ¹³C chemical shift ranges for carbon atoms in various structural environments. These ranges are extremely useful for identifying functional groups in an unknown compound.
下表总结了不同结构环境中碳原子的典型 ¹³C 化学位移范围。这些范围对于鉴定未知化合物中的官能团极其有用。
| 碳环境 Carbon environment |
化学位移 δ / ppm |
|---|---|
| 烷烃 C (sp³) / alkane C | 0 – 50 |
| 与氧 / 氮相连的 C (C–O, C–N) | 40 – 90 |
| 炔 C (sp) / alkyne C | 70 – 90 |
| 烯 / 芳烃 C (sp²) / alkene/aromatic C | 100 – 150 |
| 羰基 C=O (醛、酮) / carbonyl | 190 – 220 |
| 羰基 C=O (酸、酯、酰胺) / acid, ester, amide | 160 – 185 |
These ranges are not rigid but give a reliable guide. Overlap can occur, and the exact shift depends on additional substituents. Nevertheless, the appearance of a signal above 150 ppm almost always indicates a carbonyl group.
这些范围并非绝对固定，但提供了可靠的指引。信号之间有可能重叠，具体的位移还依赖于其他取代基。尽管如此，出现在 150 ppm 以上的信号几乎总是指示羰基的存在。
8. Proton Decoupling and Why ¹³C NMR Spectra Are Simple | 质子去耦及 ¹³C 谱为何简单
In natural samples, ¹³C nuclei are surrounded by ¹H nuclei, and coupling between them (¹JC–H typically 120–250 Hz) would produce complex multiplet patterns. If all ¹³C–¹H couplings were visible, the spectrum would be extremely crowded and difficult to interpret. To simplify the spectrum, a technique called broadband (or noise) proton decoupling is routinely used. During acquisition, a strong radiofrequency field irradiates all the ¹H nuclei, causing them to flip rapidly between spin states. This effectively removes the ¹³C–¹H coupling, so each chemically distinct carbon appears as a single sharp peak.
在天然样品中，¹³C 核被 ¹H 核包围，它们之间的耦合（¹JC–H 通常为 120–250 Hz）会产生复杂的多重分裂图案。如果能看见所有的 ¹³C–¹H 耦合，谱图将极度拥挤而难以解释。为了简化图谱，常规采用一种称为宽带（或噪声）质子去耦的技术。在采集信号期间，用强的射频场辐照所有 ¹H 核，使它们自旋态快速翻转。这实际上消除了 ¹³C–¹H 耦合，因此每种化学上不同的碳都表现为单一尖峰。
Proton decoupling also provides a sensitivity advantage through the nuclear Overhauser effect (NOE), which enhances the intensity of the ¹³C signal. This makes the standard ¹³C NMR spectrum much easier to acquire and to analyse.
质子去耦还能通过核 Overhauser 效应（NOE）提高 ¹³C 信号的强度，从而带来灵敏度上的优势。这使得标准 ¹³C NMR 谱极易采集和分析。
9. Solvents in ¹³C NMR Spectroscopy | ¹³C NMR 中的溶剂
Samples for ¹³C NMR are normally dissolved in deuterated solvents, most commonly CDCl₃ (deuterated chloroform). Ordinary solvents that contain hydrogen would give huge ¹H signals and would also produce their own intense carbon signals, overwhelming the spectrum of the sample. In CDCl₃, the hydrogen atoms are replaced by deuterium (²H), which has a different magnetic moment and does not interfere in the ¹H or ¹³C frequency ranges. The solvent serves a dual purpose: dissolving the sample and providing a deuterium lock signal that stabilises the magnetic field during the experiment.
用于 ¹³C NMR 的样品通常溶解在氘代溶剂中，最常用的是 CDCl₃（氘代氯仿）。含有氢的普通溶剂会产生巨大的 ¹H 信号，同时也会产生自身强烈的碳信号，淹没样品谱图。在 CDCl₃ 中，氢原子被氘（²H）取代，氘具有不同的磁矩，不会在 ¹H 或 ¹³C 频率范围内造成干扰。溶剂具有双重作用：溶解样品，并提供氘锁信号以在实验中稳定磁场。
Deuterated solvents themselves do give a ¹³C signal, for example CDCl₃ shows a characteristic triplet at about 77 ppm due to ¹³C–²H coupling. Fortunately, this solvent peak usually appears in a region that does not obscure most analyte signals.
氘代溶剂本身确实会给出 ¹³C 信号，例如 CDCl₃ 由于 ¹³C–²H 耦合而在约 77 ppm 处表现出特征的三重峰。幸运的是，这个溶剂峰通常出现的位置不会遮盖大多数待测物的信号。
10. Integration and Peak Intensities | 积分与峰强度
Unlike ¹H NMR spectroscopy, the area under a ¹³C peak is not reliably proportional to the number of equivalent carbon atoms contributing to that signal. This is because the efficiency of relaxation and the NOE enhancement vary greatly among different carbon types. Quaternary carbons (with no attached hydrogens) often give weak signals, while methyl carbons may appear strong. As a result, integration is not routinely used in ¹³C NMR for quantification. What matters most are the number of signals and their chemical shifts.
与 ¹H NMR 谱不同，¹³C 峰下面的面积并不可靠地与贡献该信号的等价碳原子数目成正比。这是因为不同碳类型的弛豫效率和 NOE 增强效应差异很大。不带质子的季碳常常给出弱信号，而甲基碳的信号可能很强。因此，在 ¹³C NMR 中通常不将积分用于定量。最重要的是信号的数目及其化学位移。
Nonetheless, peak heights or intensities can offer qualitative clues. Symmetry-related carbons still contribute to the same signal, so a signal from a set of two equivalent carbons may sometimes appear taller than one from a unique carbon, but the relationship is not strictly quantitative.
尽管如此，峰高或强度仍能提供定性的线索。由对称性相关联的碳原子会贡献到同一信号，因此一组两个等价碳的信号有时显得比一个独特碳的信号更高，但这种关系并非严格的定量关系。
11. Applications in Structure Determination | 在结构测定中的应用
¹³C NMR spectroscopy is a cornerstone of organic structure elucidation. By combining the number of signals with their chemical shifts, a chemist can deduce the carbon skeleton and identify functional groups. For instance, a compound with the molecular formula C₄H₈O that shows four distinct ¹³C signals between 0 and 60 ppm, with no signals above 160 ppm, is unlikely to contain a carbonyl group. If the same compound shows a signal near 205 ppm, it strongly suggests a ketone.
¹³C NMR 谱是有机结构解析的基石。通过将信号数目与化学位移相结合，化学家可以推导出碳骨架并鉴定官能团。例如，一个分子式为 C₄H₈O 的化合物如果在 0–60 ppm 之间显示四个不同的 ¹³C 信号，而没有高于 160 ppm 的信号，它就不大可能含有羰基。如果同一化合物在 205 ppm 附近有信号，则强烈提示其为酮。
When used alongside ¹H NMR, infrared spectroscopy, and mass spectrometry, ¹³C NMR enables the unambiguous identification of isomers and the confirmation of synthetic products. A classic example is distinguishing propanal (CH₃CH₂CHO) from propanone (CH₃COCH₃): propanal shows an aldehyde carbon at about 200 ppm and three signals total, while propanone gives a carbonyl signal near 205 ppm but only two signals in total due to symmetry.
当与 ¹H NMR、红外光谱和质谱联用时，¹³C NMR 能实现对异构体的明确区分，以及合成产物的确证。一个经典的例子是区分丙醛 (CH₃CH₂CHO) 和丙酮 (CH₃COCH₃)：丙醛在约 200 ppm 处显示醛基碳信号，共有三个信号；而丙酮由于对称性，在 205 ppm 附近给出一个羰基信号，但总共只有两个信号。
12. Summary | 总结
Carbon-13 NMR spectroscopy is an indispensable tool in modern organic chemistry. It directly reveals the number of chemically distinct carbon environments in a molecule and, through chemical shift values, provides a detailed fingerprint of the functional groups present. The routine use of proton decoupling gives rise to simple, single-line spectra that are easy to interpret. With TMS as
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