📚 Infrared Spectroscopy for IB CCEA Chemistry | IB CCEA 化学：红外光谱考点精讲
Infrared (IR) spectroscopy is an essential analytical tool for identifying functional groups in organic compounds. In both the IB Diploma and CCEA A-Level chemistry specifications, the ability to interpret IR spectra and link absorption bands to bond vibrations is a core skill required for examinations.
红外光谱是鉴别有机化合物官能团的重要分析工具。在IB文凭课程和CCEA A-Level化学大纲中，解释红外谱图并将吸收带与键振动联系起来是考试要求掌握的核心技能。
1. Introduction to Infrared Spectroscopy | 红外光谱简介
Infrared spectroscopy exploits the interaction between infrared radiation and molecular vibrations. Organic molecules absorb IR energy at specific frequencies, resulting in a spectrum that acts as a ‘molecular fingerprint’. It is widely used to confirm the presence of functional groups such as alcohols, carbonyls and amines.
红外光谱利用红外辐射与分子振动的相互作用。有机分子在特定频率吸收红外能量，得到的谱图如同“分子指纹”。它广泛用于确认醇、羰基化合物和胺等官能团的存在。
A major advantage of IR spectroscopy is that it requires very small samples and can be performed on solids, liquids or gases. For IB and CCEA exams, students must be able to both interpret given spectra and predict characteristic absorption ranges.
红外光谱的一大优势是所需样品量极少，且可对固体、液体或气体进行测试。在IB和CCEA考试中，学生既要能解释给出的谱图，也要能预测特征吸收范围。
2. Principle of IR Absorption | 红外吸收原理
When a molecule is exposed to infrared radiation, its bonds absorb energy that matches the natural vibrational frequency of the bond. This occurs only if the vibration causes a change in the dipole moment of the molecule; symmetrical stretches in non‑polar bonds (e.g. N≡N) are IR inactive.
当分子暴露于红外辐射时，如果辐射频率与某个键的固有振动频率匹配，该键便会吸收能量。只有引起分子偶极矩变化的振动才能发生吸收；非极性键的对称伸缩（如N≡N）无红外活性。
The absorbed energy promotes bonds from a ground vibrational state to an excited state. The frequencies involved fall in the wavenumber range 4000–400 cm⁻¹, which corresponds to the mid‑infrared region of the electromagnetic spectrum.
吸收的能量将化学键从基态振动激发到激发态。涉及的频率落在波数范围4000–400 cm⁻¹，对应电磁波谱的中红外区域。
For a diatomic molecule, the vibrational frequency can be approximated using Hooke’s law: ν̃ ∝ √(k/μ), where k is the force constant of the bond and μ the reduced mass. Heavier atoms and weaker bonds absorb at lower wavenumbers.
对双原子分子，振动频率可用胡克定律近似：ν̃ ∝ √(k/μ)，其中k为键力常数，μ为约化质量。较重的原子和较弱的键在较低波数处吸收。
3. Types of Molecular Vibrations | 分子振动类型
IR‑active vibrations are categorised as stretching and bending. Stretching vibrations involve a rhythmic change in bond length and can be symmetric or asymmetric. Bending vibrations alter bond angles and include scissoring, rocking, wagging and twisting.
红外活性振动分为伸缩振动和弯曲振动。伸缩振动是键长的周期性变化，可分为对称和不对称伸缩。弯曲振动改变键角，包括剪式、面内摇摆、面外摇摆和扭曲。
In organic molecules, a CH₂ group can undergo six vibrational modes: symmetric stretch, asymmetric stretch, scissoring, rocking, wagging and twisting. Not all of these may be resolved in a typical spectrum, but they contribute to the complexity of the fingerprint region.
在有机分子中，一个CH₂基团可以有六种振动模式：对称伸缩、不对称伸缩、剪式、面内摇摆、面外摇摆和扭曲。并非所有模式都能在常规谱图中分辨，但它们共同构成了指纹区的复杂性。
For carbonyl (C=O) groups, the strong dipole change during stretching gives an intense absorption peak around 1700 cm⁻¹. This peak is one of the most reliable indicators of the presence of a carbonyl functional group.
对于羰基(C=O)，伸缩过程中偶极矩的明显变化在约1700 cm⁻¹处产生强吸收峰。该峰是判断羰基官能团存在的最可靠指标之一。
4. The IR Spectrum: Transmittance and Wavenumber | 红外谱图：透过率与波数
An IR spectrum plots percentage transmittance (%) on the vertical axis against wavenumber (cm⁻¹) on the horizontal axis. Peaks point downwards because absorption reduces transmittance. The wavenumber scale decreases from left (4000 cm⁻¹) to right (400 cm⁻¹).
红外谱图以透过率(%)为纵坐标，波数(cm⁻¹)为横坐标。由于吸收降低透过率，峰方向朝下。波数从左（4000 cm⁻¹）向右（400 cm⁻¹）递减。
Wavenumber is the number of waves per centimetre and is directly proportional to energy and frequency. A high wavenumber (e.g. 3000 cm⁻¹) corresponds to higher energy vibrations such as C–H stretches; low wavenumbers (e.g. 700 cm⁻¹) correspond to bending or heavier atom vibrations.
波数是每厘米的波数，与能量和频率成正比。高波数（如3000 cm⁻¹）对应较高能量的振动，如C–H伸缩；低波数（如700 cm⁻¹）对应弯曲或较重原子的振动。
In exam questions, you are often provided with an IR table and a spectrum. You must locate the principal absorption bands, read off their wavenumbers and identify the functional groups responsible.
考试题目中通常会提供红外数据表和一张谱图。你必须定位主要吸收带，读出其波数并鉴定对应的官能团。
5. Fingerprint Region and Functional Group Region | 指纹区与官能团区
The IR spectrum is divided into two regions: the functional group region (4000–1500 cm⁻¹) and the fingerprint region (below 1500 cm⁻¹). The functional group region contains characteristic peaks that can be assigned to specific bonds, while the fingerprint region shows a complex pattern unique to the whole molecule.
红外谱图分为两个区域：官能团区（4000–1500 cm⁻¹）和指纹区（1500 cm⁻¹以下）。官能团区含有可归属为特定化学键的特征峰，而指纹区展示整个分子特有的复杂谱图。
The fingerprint region is used to confirm identity by comparison with a reference spectrum of the pure compound. No two different compounds (except enantiomers) show exactly the same pattern in this region. For exam purposes, focus your identification on the functional group region.
指纹区通过与纯化合物的参考谱图比对来确认物质身份。除对映体外，任意两种不同化合物在该区域不会呈现完全相同的谱图。为应对考试，请将鉴定重点放在官能团区。
A small peak in the fingerprint region is often difficult to assign, but broad, intense absorptions in the high‑wavenumber region are usually the most diagnostic.
指纹区的小峰通常很难归属，而高波数区域的宽强吸收最具诊断价值。
6. Characteristic Absorption Bands of Common Functional Groups | 常见官能团的特征吸收带
The table below summarises the most important IR absorption bands that IB and CCEA students are expected to know. All values are given in cm⁻¹; intensity descriptors: s = strong, m = medium, w = weak, br = broad, sh = sharp.
下表总结了IB和CCEA学生需要掌握的最重要红外吸收带。所有数值的单位为cm⁻¹；强度描述：s = 强，m = 中等，w = 弱，br = 宽，sh = 尖锐。
| Functional Group | Bond | Wavenumber / cm⁻¹ | Intensity & Notes |
|---|---|---|---|
| Alkane | C–H stretch | 2850–2960 | m to s |
| Alkene | =C–H stretch C=C stretch |
3020–3100 1620–1680 |
m w to m (absent if symmetric) |
| Alkyne | ≡C–H stretch C≡C stretch |
3300 2100–2260 |
s, sharp w (absent if symmetric) |
| Alcohol / Phenol | O–H (free) O–H (H‑bonded) C–O stretch |
3650–3590 3400–3200 1300–1000 |
sh, w br, s s |
| Carboxylic acid | O–H (broad) C=O stretch |
3300–2500 1725–1700 |
br, s (often overlaps C–H) s |
| Aldehyde | C=O stretch C–H (aldehyde) |
1740–1720 ~2720 and ~2820 |
s w, two peaks (useful diagnostic) |
| Ketone | C=O stretch | 1725–1705 | s |
| Ester | C=O stretch C–O stretch |
1750–1730 1300–1000 |
s s (often two bands) |
| Amide | N–H stretch C=O stretch |
3500–3140 1690–1630 |
m, often two peaks for –NH₂ s |
| Amine | N–H stretch | 3500–3300 | m, two peaks for –NH₂, one for –NH– |
| Nitrile | C≡N stretch | 2260–2220 | m, sharp |
In exams, you are not required to memorise the entire table, but you must know the most common ranges: O–H (broad ~3300), C=O (strong ~1700), C–O (~1000–1300) and C=C (~1650).
考试中无需记住整个表格，但必须掌握最常见范围：O–H（宽峰约3300）、C=O（强峰约1700）、C–O（约1000–1300）和C=C（约1650）。
7. Interpreting IR Spectra: Example of an Alcohol | 解析红外谱图：以醇为例
Consider a spectrum showing a broad, strong absorption centred near 3350 cm⁻¹, a sharp band at 2930 cm⁻¹ and an intense peak at 1050 cm⁻¹. The broad peak at 3350 cm⁻¹ indicates an O–H group involved in hydrogen bonding. The absence of a strong peak around 1700 cm⁻¹ rules out a carbonyl group. The peak at 1050 cm⁻¹ is typical of a C–O single bond stretch.
考虑一张谱图：在3350 cm⁻¹附近显示宽强吸收，2930 cm⁻¹处有尖峰，1050 cm⁻¹处有强峰。3350 cm⁻¹的宽峰表明存在形成氢键的O–H基团。1700 cm⁻¹附近无强峰排除了羰基。1050 cm⁻¹的峰是典型的C–O单键伸缩。
This small set of absorptions is conclusive for an alcohol. The C–H stretch appears around 2930 cm⁻¹, common in all organic compounds. By checking the fingerprint region for an exact match with reference data, one could identify the specific alcohol.
这组吸收峰足以确证醇的存在。C–H伸缩出现在约2930 cm⁻¹，这在所有有机化合物中都常见。通过将指纹区与参考数据精确匹配，可以鉴定具体是哪种醇。
For a phenol, the O–H stretch also appears broad near 3350 cm⁻¹, and the aromatic C=C stretching gives additional peaks around 1600–1450 cm⁻¹. Aromatic compounds show C–H stretch just above 3000 cm⁻¹.
对于酚，O–H伸缩同样在约3350 cm⁻¹呈宽峰，芳环的C=C伸缩还会在1600–1450 cm⁻¹范围给出额外吸收。芳香族化合物的C–H伸缩出现在略高于3000 cm⁻¹处。
8. Interpreting IR Spectra: Carbonyl Compounds | 解析红外谱图：羰基化合物
The strong C=O stretch is the hallmark of carbonyl compounds. Its exact position helps distinguish aldehydes, ketones, carboxylic acids, esters and amides. Aldehydes also show a unique pair of weak C–H stretches around 2720 and 2820 cm⁻¹, often called the ‘aldehyde handle’.
强C=O伸缩峰是羰基化合物的标志。其确切位置有助于区分醛、酮、羧酸、酯和酰胺。醛还在约2720和2820 cm⁻¹处显示一对独特的弱C–H伸缩峰，常被称为“醛基把手”。
A carboxylic acid gives a very broad O–H stretch that starts around 3300 cm⁻¹ and often extends down to 2500 cm⁻¹, partially obscuring the C–H absorption region. The C=O of a carboxylic acid absorbs at 1725–1700 cm⁻¹, slightly lower than a ketone.
羧酸的O–H伸缩非常宽，从约3300 cm⁻¹开始，常下延至2500 cm⁻¹，部分掩盖C–H吸收区域。羧酸的C=O吸收在1725–1700 cm⁻¹，比酮略低。
Esters display a C=O band at a higher wavenumber (1750–1730 cm⁻¹) than ketones, and two strong C–O stretches between 1300 and 1000 cm⁻¹. These C–O bands are a strong clue for an ester when a carbonyl is present.
酯的C=O带出现在比酮更高的波数（1750–1730 cm⁻¹），且在1300–1000 cm⁻¹之间有两个强C–O伸缩峰。当谱图已有羰基信号时，这些C–O带是有力指向酯的证据。
9. Factors Affecting Absorption Position | 影响吸收位置的因素
Several structural factors shift the wavenumber of an absorption. For C=O, conjugation with a double bond or an aromatic ring lowers the wavenumber by 20–40 cm⁻¹ due to reduced double‑bond character. Ring strain in cyclic ketones increases the C=O stretching frequency; for example, cyclobutanone absorbs around 1775 cm⁻¹.
结构因素会使吸收波数发生偏移。对C=O而言，与双键或芳环共轭会降低双键成分，使波数降低20–40 cm⁻¹。环酮中的环张力升高C=O伸缩频率；例如环丁酮在约1775 cm⁻¹吸收。
Hydrogen bonding dramatically broadens O–H and N–H bands and shifts them to lower wavenumbers. In concentrated alcohols or carboxylic acids, intermolecular hydrogen bonds cause the broadness, while in dilute solutions or gas phase, a sharper ‘free’ O–H peak may be observed near 3600 cm⁻¹.
氢键会使O–H和N–H带显著变宽并移向低波数。在浓醇或羧酸中，分子间氢键导致宽峰，而在稀溶液或气相中，可能在约3600 cm⁻¹观察到尖锐的“游离”O–H峰。
Electronic effects from neighbouring electronegative atoms can also shift absorptions. For example, an acid chloride C=O absorbs near 1800 cm⁻¹ because of the inductive effect of chlorine.
邻近电负性原子的电子效应也会使吸收移动。例如，酰氯中C=O在约1800 cm⁻¹吸收，因为氯的吸电子诱导效应增强了羰基键力常数。
10. IR Spectroscopy in Structural Determination | 红外光谱在结构鉴定中的应用
In modern organic analysis, IR spectroscopy is used alongside mass spectrometry and NMR. An IR spectrum quickly confirms or excludes the presence of functional groups such as OH, C=O, NH₂ and C≡N, guiding further investigation. It does not provide the molecular formula or the complete skeleton.
在现代有机分析中，红外光谱与质谱和核磁共振结合使用。红外谱图可快速确认或排除OH、C=O、NH₂和C≡N等官能团的存在，指引进一步检测。它不能直接给出分子式或完整骨架。
Typical exam questions provide an IR spectrum of an unknown compound along with its molecular formula. Identifying functional groups from the spectrum allows you to narrow down the structural possibilities. Often you are then asked to draw and name the compound.
典型的考题会给出未知化合物的红外谱图和分子式。通过谱图识别官能团能缩小可能的结构范围，随后经常要求你画出并命名该化合物。
For instance, a compound C₃H₆O with a strong band at 1715 cm⁻¹
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