📚 NMR Spectroscopy: A Step-by-Step Guide to Experiment Analysis | 核磁共振光谱学实验解析
Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful analytical techniques for determining the structure of organic molecules. This guide walks you through the entire experimental process, from sample preparation to spectrum interpretation, with clear attention to the details that examiners expect.
核磁共振（NMR）光谱是测定有机分子结构最有力的分析技术之一。本指南将带你走完从样品制备到图谱解析的完整实验流程，并重点关注考官期望你掌握的细节。
1. Principles of NMR | NMR基本原理
Nuclei such as ¹H and ¹³C possess an intrinsic quantum property called spin. When placed in a strong external magnetic field, the spin energy levels split into two states: one aligned with the field (lower energy) and one opposed to it (higher energy).
像 ¹H 和 ¹³C 这样的原子核具有一种内在的量子属性——自旋。当置于强外部磁场中时，自旋能级会分裂为两个状态：与磁场同向（能量较低）和反向（能量较高）。
Applying radiofrequency radiation that matches the energy gap causes transitions between these levels. This absorption of energy is resonance. The resonance condition is:
当施加的射频辐射能量与能级差相匹配时，就会引起能级之间发生跃迁。这种能量吸收现象即共振。共振条件为：
ν = γB₀ / 2π
Here ν is the frequency of the absorbed radiation, γ is the magnetogyric ratio of the nucleus, and B₀ is the external magnetic field strength. Because different proton environments are shielded to different extents by surrounding electrons, they resonate at slightly different frequencies under the same magnetic field.
其中 ν 是被吸收辐射的频率，γ 是原子核的磁旋比，B₀ 是外部磁场强度。由于不同的质子环境受到周围电子的屏蔽程度不同，在同一磁场下它们发生共振的频率也会有微小差异。
2. The Spectrometer and Sample Preparation | 光谱仪与样品制备
A typical NMR spectrometer consists of a powerful superconducting magnet, a radiofrequency transmitter, a detector coil, and a computer system for signal averaging and data processing. The sample is placed in a narrow glass tube that spins steadily inside the magnetic field to ensure a uniform signal.
典型的NMR光谱仪由强超导磁体、射频发射器、检测线圈以及用于信号累加和数据处理的计算机构成。样品置于细玻璃管中，在磁场内匀速旋转以保证信号均匀。
Most organic samples are dissolved in a deuterated solvent such as CDCl₃, D₂O, or (CD₃)₂CO. Deuterated solvents are used because deuterium nuclei do not produce signals in a normal ¹H NMR spectrum, leaving the solute signals unobstructed.
大多数有机样品需要溶解在氘代溶剂中，例如 CDCl₃、D₂O 或 (CD₃)₂CO。之所以使用氘代溶剂，是因为氘核在普通 ¹H NMR 谱中不会产生信号，从而避免溶剂干扰溶质信号。
A small amount of tetramethylsilane (TMS) is added as an internal reference. TMS gives a single sharp peak at δ = 0 and is chemically inert, volatile, and chemically equivalent in all twelve protons.
通常还会加入少量四甲基硅烷（TMS）作为内标。TMS 在 δ = 0 处给出一个尖锐单峰，且化学性质稳定、易挥发，其十二个质子完全等价。
3. Chemical Shift and TMS | 化学位移与TMS
Chemical shift is the position of a resonance signal relative to the TMS reference. It is measured in parts per million (ppm) using the equation:
化学位移是某个共振信号相对于 TMS 参照峰的位置，单位是百万分之一（ppm），用以下公式表示：
δ = (ν_sample – ν_TMS) / ν_TMS × 10⁶ ppm
Protons attached to electronegative atoms, multiple bonds, or carbonyl groups are deshielded and appear at higher δ values (downfield). Protons surrounded by high electron density are shielded and appear at lower δ values (upfield).
与电负性原子、多重键或羰基相连的质子去屏蔽程度较高，出现在较大的 δ 值（低场）；处于高电子密度环境中的质子被屏蔽，出现在较小的 δ 值（高场）。
Common ¹H chemical shift ranges include alkyl CH₃ (0.9-1.5), CH₂ (1.2-1.6), CH (1.5-2.0), CH₃CO or CH₂CO (2.1-2.6), OCH₃ (3.7-4.0), vinyl (4.6-6.0), aromatic (6.5-8.5), and aldehyde (9.5-10.5).
常见 ¹H 化学位移范围包括：烷基 CH₃（0.9-1.5）、CH₂（1.2-1.6）、CH（1.5-2.0）、CH₃CO 或 CH₂CO（2.1-2.6）、OCH₃（3.7-4.0）、烯氢（4.6-6.0）、芳氢（6.5-8.5）以及醛氢（9.5-10.5）。
4. Integration and Proton Counting | 积分与质子计数
Each ¹H NMR signal is accompanied by an integration curve. The height of the integration step is proportional to the number of protons giving that signal. Integration gives relative ratios, not absolute numbers.
每一个 ¹H NMR 信号都伴随一条积分曲线。积分台阶的高度与产生该信号的质子数成正比。积分给出的是相对比例，而不是绝对数目。
For example, a compound with three signals in a 3:2:1 ratio contains six protons total if its molecular formula has six hydrogens. Always compare integration ratios to the actual number of hydrogens from the molecular formula.
例如，某化合物的三个信号积分比为 3:2:1，如果其分子式中总共有六个氢，则分别对应 3、2、1 个氢。务必结合分子式中的氢总数来判断积分比所对应的实际质子数。
Integration is one of the first checks in exam-style problems. Missing integration is often the reason students cannot assign signals correctly.
积分是解答考试型题目时首先需要检查的信息之一。漏看积分比例往往是学生无法正确归属信号的常见原因。
5. Spin-Spin Splitting | 自旋-自旋裂分
Adjacent non-equivalent protons interact magnetically through chemical bonds, causing a signal to split into multiple peaks. The most common prediction rule is the n + 1 rule: a proton, or a group of equivalent protons, coupled to n equivalent protons on an adjacent atom appears as n + 1 peaks.
相邻的非等价质子通过化学键发生磁相互作用，导致信号裂分为多个峰。最常用的预测规则是 n + 1 规则：一个质子或一组等价质子，若与相邻原子上的 n 个等价质子耦合，则信号裂分为 n + 1 个峰。
Common multiplicity patterns follow Pascal’s triangle for relative intensities:
常见的多重峰相对强度遵循帕斯卡三角形：
| Multiplicity | Number of Peaks | Relative Intensity |
| Singlet | 1 | 1 |
| Doublet | 2 | 1:1 |
| Triplet | 3 | 1:2:1 |
| Quartet | 4 | 1:3:3:1 |
| Quintet | 5 | 1:4:6:4:1 |
For an ethyl group, the CH₃ protons are adjacent to a CH₂ group, so the CH₃ signal is a triplet (n = 2). The CH₂ protons are adjacent to three equivalent CH₃ protons, so the CH₂ signal is a quartet (n = 3).
例如乙基中，CH₃ 质子与 CH₂ 相邻，因此 CH₃ 信号为三重峰（n = 2）；CH₂ 质子与三个等价 CH₃ 质子相邻，因此 CH₂ 信号为四重峰（n = 3）。
The n + 1 rule works only when the coupled protons are equivalent and when the molecule does not contain many overlapping couplings. In more complex systems, second-order effects complicate the splitting pattern.
n + 1 规则仅在被耦合质子等价且分子中没有大量重叠耦合时适用。在更复杂的体系中，二级效应会使裂分模式变得复杂。
6. Step-by-Step Interpretation of a ¹H NMR Spectrum | ¹H NMR谱解析步骤
To interpret a ¹H NMR spectrum systematically, follow these steps. Each step should build a clear picture of the molecule’s structure.
要系统解析一张 ¹H NMR 谱，请按以下步骤进行。每一步都会帮助你逐步构建分子的结构图像。
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Count the number of signals. Each signal corresponds to one set of chemically equivalent protons. Symmetry reduces the number of signals.
数信号的个数。每个信号对应一组化学等价质子。分子对称性会减少信号数。
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Measure the integration ratio of each signal. This gives the relative number of protons in each environment.
测量每个信号的积分比例，获得各组环境的相对质子数。
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Record the chemical shift of each signal. This indicates the electronic environment, such as alkyl, alkene, aromatic, carbonyl-adjacent, or alcohol.
记录每个信号的化学位移，判断电子环境，例如烷基、烯基、芳基、羰基邻位或醇羟基。
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Analyze the multiplicity. Use n + 1 to infer how many equivalent protons are on adjacent atoms.
分析裂分模式，用 n + 1 规则推断相邻原子上有几个等价质子。
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Check for exchangeable protons such as OH and NH. These often appear as broad singlets and do not split neighboring protons under typical conditions.
检查可交换质子（如 OH、NH）。它们通常表现为宽单峰，且一般不会使相邻质子发生裂分。
7. ¹³C NMR Spectroscopy | ¹³C核磁共振谱
Carbon-13 has a natural abundance of only about 1.1%, and its magnetogyric ratio is different from that of the proton. In routine operation, broadband proton decoupling is used so that each distinct carbon environment appears as a single sharp line.
碳-13 的天然丰度只有约 1.1%，且其磁旋比与质子不同。常规测试中采用宽带质子去耦技术，使每种不同的碳环境表现为一条尖锐单线。
The number of ¹³C signals equals the number of unique carbon environments, provided the molecule is not symmetrical. Symmetrical pairs appear as one signal. This is particularly useful for confirming structural symmetry.
只要分子不具备对称性，¹³C 信号的数目等于不等价碳环境的数目。对称的成对碳只出现一个信号。这一点尤其有助于确认分子是否对称。
Typical ¹³C chemical shift ranges are: carbonyl carbons 160-220 ppm, aromatic carbons 110-175 ppm, alkene carbons 100-170 ppm, alkyne carbons 65-90 ppm, and saturated CH₃/CH₂/CH carbons 0-60 ppm.
常见 ¹³C 化学位移范围：羰基碳 160-220 ppm，芳碳 110-175 ppm，烯碳 100-170 ppm，炔碳 65-90 ppm，饱和 CH₃/CH₂/CH 碳 0-60 ppm。
The DEPT technique distinguishes CH₃, CH₂, and CH signals: CH and CH₃ appear positive, CH₂ appears negative in DEPT-135, while quaternary carbons do not appear. This helps identify carbon types without additional synthesis.
DEPT 技术可以区分 CH₃、CH₂ 与 CH 信号：在 DEPT-135 谱中，CH 与 CH₃ 为正峰，CH₂ 为负峰，季碳不出现。这有助于在不进行额外合成的情况下判断碳的类型。
8. Solvent Peaks and Impurities | 溶剂峰与杂质峰
Deuterated solvents always contain a small amount of non-deuterated impurities. These residual solvent protons appear as small peaks in the ¹H NMR spectrum and are often mistaken for sample signals.
氘代溶剂中总会残留少量非氘代杂质。这些残余溶剂质子会在 ¹H NMR 谱中出现小峰，容易被误认为样品信号。
Common residual peaks include CHCl₃ at δ 7.26, H₂O at δ 1.56, acetone at δ 2.05, and DMSO at δ 2.50. The TMS reference peak sits at δ 0.
常见残留峰包括：CHCl₃ 在 δ 7.26，H₂O 在 δ 1.56，丙酮在 δ 2.05，DMSO 在 δ 2.50。TMS 参照峰位于 δ 0。
In exam problems, you may be shown a spectrum containing a small impurity peak. Identify it by its chemical shift and integration, and do not assign it as part of the target molecule unless the molecular formula supports it.
考试题目中可能给出含有小杂质峰的谱图。请通过化学位移和积分辨认杂质峰，除非分子式支持，否则不要将其归属为目标分子的一部分。
9. Worked Example: Ethyl Acetate | 实例解析：乙酸乙酯
Consider a compound with molecular formula C₄H₈O₂. Its ¹H NMR spectrum shows three signals: a triplet at δ 1.2, a singlet at δ 2.0, and a quartet at δ 4.1, with integration ratio 3:3:2.
某化合物分子式为 C₄H₈O₂。其 ¹H NMR 谱显示三个信号：δ 1.2 三重峰、δ 2.0 单峰、δ 4.1 四重峰，积分比为 3:3:2。
| Signal | δ / ppm | Multiplicity | Integration | Assignment |
| 1 | 1.2 | triplet | 3H | CH₃CH₂ |
| 2 | 2.0 | singlet | 3H | CH₃CO |
| 3 | 4.1 | quartet | 2H | OCH₂ |
The singlet at δ 2.0 corresponds to the CH₃ group attached to the carbonyl carbon. The quartet and triplet at δ 4.1 and δ 1.2 are the classic pattern of an OCH₂CH₃ fragment, where the two protons on OCH₂ couple with three CH₃ protons to give a quartet, and the CH₃ protons couple with two OCH₂ protons to give a triplet.
δ 2.0 的单峰对应与羰基相连的 CH₃ 基团。δ 4.1 的四重峰和 δ 1.2 的三重峰是 OCH₂CH₃ 片段的典型特征：OCH₂ 上两个质子与三个 CH₃ 质子耦合产生四重峰，而 CH₃ 质子与两个 OCH₂ 质子耦合产生三重峰。
The ¹³C NMR spectrum shows four signals at approximately δ 171 (C=O), δ 60 (OCH₂), δ 21 (CH₃CO), and δ 14 (CH₃CH₂), confirming the ethyl acetate structure. The degree of unsaturation from C₄H₈O₂ is one, which matches the carbonyl double bond.
¹³C NMR 谱显示四个信号，分别在约 δ 171（C=O）、δ 60（OCH₂）、δ 21（CH₃CO）和 δ 14（CH₃CH₂），进一步确证乙酸乙酯结构。由 C₄H₈O₂ 计算的不饱和度为 1，正好对应羰基双键。
10. Advanced Techniques: COSY, HSQC, and HMBC | 高级方法：COSY、HSQC 与 HMBC
Two-dimensional NMR methods are used when one-dimensional spectra are too crowded or ambiguous.
当一维谱过于拥挤或难以判断时，需要使用二维核磁共振方法。
COSY (Correlation Spectroscopy) shows which protons are coupled to one another. A cross-peak between two signals indicates that those protons are within a few bonds of each other, typically on adjacent carbons.
COSY（相关谱）可以显示哪些质子彼此耦合。两个信号之间的交叉峰表明这些质子相隔约三根化学键以内，通常是相邻碳上的质子。
HSQC (Heteronuclear Single Quantum Coherence) correlates each ¹H signal directly to the ¹³C signal of the carbon to which it is attached. This helps assign carbon shifts and detect missing hydrogens.
HSQC（异核单量子相关谱）将每个 ¹H 信号直接关联到其所连接碳的 ¹³C 信号，有助于归属碳位移并判断缺失的氢。
HMBC (Heteronuclear Multiple Bond Correlation) detects correlations between protons and carbons separated by two or three bonds. It is invaluable for connecting fragments that have no direct proton-proton coupling.
HMBC（异核多键相关谱）可以检测相隔两到三根键的质子与碳之间的相关信号。它对于连接没有直接质子-质子耦合的结构片段特别有价值。
These techniques are rarely tested at A-level but are often introduced in higher-level examinations or interview discussions.
这些技术虽然很少在 A-level 阶段直接考查，但在更高水平考试或面试讨论中经常出现。
11. Exam Tips and Common Mistakes | 考试技巧与常见错误
Many students lose marks in NMR questions because of small but repeated errors. The following checklist will help you avoid them.
许多学生在 NMR 题目中失分，往往是因为一些细小但反复出现的错误。以下清单可帮助你避免这些常见问题。
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Always state that TMS is the internal reference for δ = 0. Do not say the spectrum was referenced to water unless the question specifies it.
始终说明 TMS 是 δ = 0 的内标。除非题目特别指明，否则不要说以水为参照。
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Before assigning integration values, multiply the ratio so that the total number of hydrogens matches the molecular formula.
在归属积分值前，应将积分比放大到使氢总数与分子式一致。
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Check for molecular symmetry. For example, benzene has six equivalent protons but only one signal.
检查分子对称性。例如苯有六个等价质子，但只有一个信号。
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Do not apply the n + 1 rule to OH and NH protons when exchange is fast; they usually appear as broad singlets.
当交换速率快时，不要把 n + 1 规则应用于 OH 和 NH 质子；它们通常
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