📚 Infrared Spectroscopy: Principles and Applications | 红外光谱法：原理与应用
Infrared (IR) spectroscopy is a fundamental analytical technique in organic chemistry used to identify functional groups by measuring the absorption of infrared radiation by molecular bonds. It is fast, non-destructive, and requires only a small sample, making it an essential tool in both academic and industrial laboratories.
红外光谱法是有机化学中一种基础的分析技术，通过测量分子键对红外辐射的吸收来鉴定官能团。该方法快速、无损且所需样品量少，是学术和工业实验室中不可或缺的工具。
1. The Electromagnetic Spectrum and Molecular Vibrations | 电磁波谱与分子振动
Infrared radiation lies between the visible and microwave regions of the electromagnetic spectrum, with wavenumbers typically ranging from 4000 to 400 cm⁻¹. When IR radiation is absorbed, it excites vibrational energy levels within a molecule, causing bonds to stretch, bend, or wag.
红外辐射位于电磁波谱中可见光与微波区域之间，其波数范围通常在4000至400 cm⁻¹。当红外辐射被吸收时，它激发分子内的振动能级，使化学键发生伸缩、弯曲或摇摆。
Only bonds with a changing dipole moment can absorb IR radiation. This selection rule explains why symmetric molecules like O₂ or N₂ are IR-inactive, while bonds such as C=O, O–H, and N–H absorb strongly.
只有偶极矩发生变化的键才能吸收红外辐射。这一选择定则解释了为什么 O₂ 或 N₂ 等对称分子没有红外活性，而 C=O、O–H 和 N–H 等键则吸收强烈。
2. Wavenumber and Energy Relationship | 波数与能量的关系
Spectroscopists commonly use wavenumber (ṽ) rather than wavelength or frequency. The energy of a vibrational transition is directly proportional to the wavenumber, as shown in the equation below.
光谱学家通常使用波数（ṽ）而非波长或频率。振动跃迁的能量与波数成正比，如下式所示。
E = h × c × ṽ
Here, E is energy, h is Planck’s constant, c is the speed of light, and ṽ is the wavenumber in cm⁻¹. Because ṽ is directly proportional to E, higher wavenumbers correspond to stronger bonds or lighter atoms.
其中 E 是能量，h 是普朗克常数，c 是光速，ṽ 是以 cm⁻¹ 为单位的波数。由于 ṽ 与 E 成正比，较高的波数对应更强的化学键或较轻的原子。
3. Types of Molecular Vibrations | 分子振动的类型
The main vibrational modes are stretching and bending. Stretching can be symmetric (both bonds lengthen together) or asymmetric (one bond lengthens while the other shortens). Bending includes scissoring, rocking, wagging, and twisting.
主要的振动模式包括伸缩振动和弯曲振动。伸缩振动可以是对称的（两个键同时伸长）或不对称的（一个键伸长而另一个缩短）。弯曲振动包括剪式、摇摆、摆动和扭曲等。
- Stretching vibrations | 伸缩振动: occur at higher wavenumbers and generally appear in the 4000–1000 cm⁻¹ region.
- Bending vibrations | 弯曲振动: occur at lower wavenumbers, usually below 1500 cm⁻¹, in the “fingerprint” region.
4. The IR Spectrometer and Sample Preparation | 红外光谱仪与样品制备
A typical IR spectrometer passes a beam of IR radiation through a sample, then measures which wavenumbers are absorbed. Modern instruments use Fourier-transform (FT-IR) technology, which rapidly collects all frequencies simultaneously, improving speed and signal-to-noise ratio.
典型的红外光谱仪让一束红外辐射穿过样品，然后测量哪些波数被吸收。现代仪器采用傅里叶变换（FT-IR）技术，可同时快速采集所有频率，从而提高速度和信噪比。
Solid samples are often ground with KBr and pressed into a transparent disc, while liquids are placed in cells with NaCl or KBr windows. Thin films and mulls are also common methods.
固体样品通常与 KBr 研磨后压成透明薄片，液体则置于配有 NaCl 或 KBr 窗片的液池中。薄膜法和糊状法也常被使用。
5. Absorption Bands of Key Functional Groups | 主要官能团的特征吸收峰
Each functional group has a characteristic absorption range. Recognising these ranges is critical for interpreting IR spectra. The table below summarises the most important groups for A-level chemistry.
每个官能团都有其特征吸收范围。识别这些范围对于解释红外光谱至关重要。下表总结了A-level化学中最重要的官能团。
| Functional Group | 官能团 | Wavenumber (cm⁻¹) | 波数 | Notes | 备注 |
|---|---|---|
| O–H (alcohol) | 醇 | 3200–3600 (broad) | 宽峰 | Hydrogen bonding broadens the peak. |
| O–H (carboxylic acid) | 羧酸 | 2500–3300 (very broad) | 很宽 | Often overlaps with C–H peaks. |
| C=O (carbonyl) | 羰基 | 1630–1780 (strong) | 强峰 | Aldehydes, ketones, esters, acids all show C=O. |
| N–H (amine/amide) | 胺/酰胺 | 3300–3500 (medium) | 中等 | Primary amines show two peaks. |
| C≡N (nitrile) | 腈 | 2200–2260 (sharp) | 尖峰 | Distinct from C≡C (2100–2260 but weak). |
| C–H (alkane) | 烷烃 | 2850–2960 (medium) | 中等 | Also C–H bending at 1400–1500. |
6. The Fingerprint Region | 指纹区
The region below 1500 cm⁻¹ is called the fingerprint region. It contains many complex bending and skeletal vibrations that are unique to each molecule. Two pure samples of the same compound produce identical fingerprint patterns, allowing direct comparison with reference spectra.
低于1500 cm⁻¹的区域称为指纹区。该区域包含许多复杂的弯曲振动和骨架振动，每种分子都有独特的图案。两个同一种纯化合物的样品会产生完全相同的指纹图谱，因此可与参考光谱直接比对。
Although this region is difficult to assign to specific bonds, it is extremely valuable for identifying unknown compounds by matching databases or known standards.
虽然该区域难以逐一归属于特定化学键，但它通过匹配数据库或已知标准物来鉴定未知化合物时极为宝贵。
7. Interpreting an IR Spectrum | 红外光谱的解析
A systematic approach begins by checking the presence or absence of a C=O peak near 1700 cm⁻¹. Next, look for O–H or N–H peaks above 3000 cm⁻¹, and then examine the C–H region near 2900 cm⁻¹. Finally, use specific combinations to distinguish between functional groups.
系统解析的第一步是检查1700 cm⁻¹附近是否存在 C=O 峰。其次，观察3000 cm⁻¹以上是否有 O–H 或 N–H 峰，然后检查2900 cm⁻¹附近的 C–H 区域。最后，通过特征组合区分不同官能团。
- Aldehyde | 醛: C=O plus a sharp C–H peak at ~2720 cm⁻¹ (doublet).
- Ketone | 酮: C=O, no O–H, no aldehyde C–H.
- Carboxylic acid | 羧酸: very broad O–H centred near 3000 cm⁻¹ plus C=O.
- Ester | 酯: C=O plus a strong C–O peak at 1000–1300 cm⁻¹.
- Alcohol | 醇: broad O–H, but no C=O.
8. Applications in Qualitative Analysis | 在定性分析中的应用
IR spectroscopy is used to verify the identity of a product after a reaction, monitor reaction progress, and detect impurities. For example, the appearance of a C=O peak in an oxidation experiment confirms that an alcohol has been successfully oxidised to a ketone or aldehyde.
红外光谱法可用于反应后确认产物身份、监测反应进程以及检测杂质。例如，在氧化实验中若出现 C=O 峰，即可确认醇已成功氧化为酮或醛。
In forensic science, IR spectroscopy identifies paint, plastic, and drug samples. In pharmaceutical quality control, it confirms that a raw material matches the expected structure without destroying the sample.
在法医学中，红外光谱法可鉴定油漆、塑料和药物样品。在药品质量控制中，它可以无损地确认原料是否符合预期结构。
9. Advantages and Limitations | 优点与局限性
The main advantages of IR spectroscopy are speed, versatility, and ease of use. It works with solids, liquids, gases, and thin films, and requires very little sample preparation when using ATR (attenuated total reflectance) accessories.
红外光谱法的主要优点是快速、通用且易于操作。它适用于固体、液体、气体和薄膜，且使用 ATR（衰减全反射）附件时几乎无需制样。
However, it is not the best technique for determining exact molecular mass or full structure. Raman spectroscopy and mass spectrometry complement IR by providing additional information, and NMR gives detailed connectivity of atoms.
然而，它并不是确定精确分子质量或完整结构的最佳方法。拉曼光谱和质谱可提供补充信息，而 NMR 则能给出原子的详细连接方式。
10. Worked Example | 实例解析
A compound with molecular formula C₄H₈O₂ shows a strong peak at 1740 cm⁻¹ and a very broad peak centred around 3000 cm⁻¹. The spectrum also shows a strong C–O peak at 1220 cm⁻¹. What is the most likely structure?
某化合物的分子式为 C₄H₈O₂，其红外光谱在1740 cm⁻¹处有强吸收峰，在3000 cm⁻¹附近有很宽的吸收峰，同时在1220 cm⁻¹处有强 C–O 峰。最可能的结构是什么？
The strong C=O peak suggests a carbonyl group. The very broad O–H peak indicates a carboxylic acid. Possible structure: butanoic acid, CH₃CH₂CH₂COOH.
强 C=O 峰表明存在羰基，极宽的 O–H 峰表明是羧酸。可能的结构：丁酸，CH₃CH₂CH₂COOH。
If the molecule were an ester, the O–H peak would be absent. Therefore, the combination of a very broad O–H and a strong C–O at 1220 cm⁻¹ strongly supports a carboxylic acid structure.
若该化合物为酯，则不应出现 O–H 峰。因此，极宽的 O–H 峰和1220 cm⁻¹处强 C–O 峰共同支持羧酸结构。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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