Infrared Spectroscopy Key Points | 红外光谱考点精讲

📚 Infrared Spectroscopy Key Points | 红外光谱考点精讲

Infrared (IR) spectroscopy is a fast and non-destructive analytical technique that reveals the functional groups present in an organic molecule. It does this by measuring the absorption of infrared radiation as a function of wavenumber.

红外光谱是一种快速、无损的分析技术，能够揭示有机分子中存在的官能团。它通过测量红外辐射吸收随波数变化的情况来实现这一点。

1. Introduction to IR Spectroscopy | 红外光谱简介

Every covalent bond in a molecule vibrates at a characteristic frequency when it absorbs infrared light. These vibrations correspond to specific wavenumber ranges, and the pattern of absorbed energy forms an IR spectrum that acts as a molecular signature for functional groups.

分子中的每一个共价键在吸收红外光时都会以其特征频率振动。这些振动对应着特定的波数范围，被吸收能量的图谱就形成了红外光谱，它就像是官能团的分子签名。

The IR region of the electromagnetic spectrum used for organic analysis typically covers 4000–400 cm⁻¹. The spectrum is usually displayed with wavenumber on the x-axis and percentage transmittance on the y-axis, meaning that peaks point downwards.

用于有机分析的红外电磁波谱区域通常覆盖 4000–400 cm⁻¹。光谱通常以波数为 x 轴，以透过率百分数为 y 轴，这意味着吸收峰指向下方。

2. Molecular Vibrations and IR Activity | 分子振动与红外活性

For a molecular vibration to absorb infrared radiation, it must be accompanied by a change in the dipole moment of the molecule. This requirement makes the vibration IR active.

要使分子振动吸收红外辐射，它必须伴随着分子偶极矩的变化。这一要求使振动具有红外活性。

Diatomic molecules such as H₂ or N₂ have no permanent dipole and stretching them does not change the dipole, so they are IR inactive and do not absorb in the IR region. In contrast, polar bonds like C=O, O-H and C-Cl exhibit strong IR absorptions because their vibrations oscillate the dipole.

像 H₂ 或 N₂ 这样的双原子分子没有永久偶极，拉伸它们也不会改变偶极，因此它们是红外非活性的，在红外区域没有吸收。相反，C=O、O-H 和 C-Cl 等极性键会显示出强红外吸收，因为它们的振动会使偶极振荡。

The main types of molecular vibrations are stretching (symmetric and asymmetric) and bending (scissoring, rocking, wagging, twisting). Stretching vibrations generally appear at higher wavenumbers than bending vibrations.

分子振动的主要类型包括伸缩振动（对称与不对称）和弯曲振动（剪式、摇摆、面外摇摆、扭曲）。伸缩振动通常出现在比弯曲振动更高的波数处。

3. How an IR Spectrometer Works | 红外光谱仪工作原理

A modern FTIR (Fourier Transform Infrared) spectrometer uses an interferometer to split an infrared beam into two paths, creating an interference pattern. When a sample is placed in the beam, certain frequencies are absorbed by the bonds, and a computer transforms the resulting interferogram into a conventional spectrum.

现代傅里叶变换红外光谱仪使用干涉仪将红外光束分成两路，从而产生干涉图谱。当样品放入光路中时，某些频率的辐射被化学键吸收，计算机再将得到的干涉图转换成常规的红外光谱。

Samples can be analyzed as thin liquid films between salt plates, as a solid mixed with KBr and pressed into a disc, or as a gas in a cell with IR-transparent windows. The spectrometer scans the range of wavenumbers and compares the transmitted intensity to the incident intensity at each wavenumber.

样品可以作为盐片之间的薄层液体、与 KBr 混合并压成片状的固体，或者作为红外透明窗片气体池中的气体进行分析。光谱仪扫描整个波数范围，并比较每个波数处的透射光强与入射光强。

4. Reading an IR Spectrum: Transmittance vs Wavenumber | 解读红外光谱：透过率与波数

An IR spectrum plots % transmittance against wavenumber (cm⁻¹). A 100% transmittance means no absorption, while a downward peak indicates absorption of IR radiation at that wavenumber by a specific bond vibration.

红外光谱以透过率百分数对波数 (cm⁻¹) 作图。100% 透过率表示没有吸收，而一个向下的峰表示某一特定化学键振动在该波数处吸收了红外辐射。

The baseline at the top of the spectrum is usually close to 100% T. Key functional group peaks are identified by their position, shape, and intensity. The region above 1500 cm⁻¹ is most useful for determining functional groups, while the region below 1500 cm⁻¹ is known as the fingerprint region.

光谱顶部的基线通常接近 100% T。关键的官能团峰根据其位置、形状和强度进行识别。波数高于 1500 cm⁻¹ 的区域对于确定官能团最有用，而低于 1500 cm⁻¹ 的区域则被称为指纹区。

5. Characteristic Absorption Regions | 特征吸收区域

The table below summarises the most common distinctive IR absorptions that are examined at IB and AQA levels.

下表总结了 IB 和 AQA 考试中最常见的特征红外吸收。

Functional Group / 官能团	Wavenumber / cm⁻¹	Appearance
O-H (alcohols, phenols)	3200–3600	Broad, strong
O-H (carboxylic acids)	2500–3000	Very broad, often overlaps C-H
N-H (amines, amides)	3300–3500	Medium, often with one or two peaks
C-H (alkanes)	2850–2950	Medium to strong
C-H (aldehydes) – two weak bands	~2720 and ~2820	Weak but highly diagnostic
C≡C (alkynes)	2100–2260	Weak, sharp
C≡N (nitriles)	2200–2250	Medium, sharp
C=O (carbonyl) – ketones, aldehydes, carboxylic acids, esters, amides	1650–1750	Very strong, sharp
C=C (alkenes)	1620–1680	Medium to weak, often sharp
C-O (ethers, esters, alcohols)	1000–1300	Strong

Always remember that exact positions can shift by 10–30 cm⁻¹ depending on the molecular environment.

请始终记住，根据分子环境的不同，精确位置可能会产生 10–30 cm⁻¹ 的偏移。

6. O-H and N-H Stretching Vibrations | O-H 和 N-H 伸缩振动

The O-H stretching vibration is one of the most recognisable features. In alcohols and phenols, hydrogen bonding causes the O-H peak to be broad and strong, typically centred around 3300–3400 cm⁻¹. In a very dilute solution or in the gas phase where hydrogen bonding is minimised, the O-H peak becomes sharper and appears near 3600 cm⁻¹.

O-H 伸缩振动是最容易辨认的特征之一。在醇和酚中，氢键导致 O-H 峰又宽又强，通常以 3300–3400 cm⁻¹ 为中心。在极稀的溶液中或者在氢键最小化的气相中，O-H 峰会变得更尖锐，并出现在约 3600 cm⁻¹ 附近。

Carboxylic acids show an extremely broad O-H stretching band that extends from about 3300 cm⁻¹ down to 2500 cm⁻¹, often obscuring the C-H stretch. This broadness is due to strong hydrogen bonding in dimers. N-H stretching in amines and amides gives medium absorptions in the 3300–3500 cm⁻¹ region; primary amines often show two peaks (asymmetric and symmetric stretch), while secondary amines show a single peak.

羧酸显示出一个极为宽大的 O-H 伸缩带，从大约 3300 cm⁻¹ 一直延伸到 2500 cm⁻¹，常常掩盖了 C-H 伸缩峰。这种宽泛是由于二聚体中的强氢键造成的。胺和酰胺中的 N-H 伸缩振动会在 3300–3500 cm⁻¹ 区域给出中等强度的吸收；伯胺通常显示出两个峰（不对称和对称伸缩），而仲胺只显示一个峰。

7. C-H Stretching Vibrations | C-H 伸缩振动

The C-H stretch of alkanes appears just below 3000 cm⁻¹, typically in the 2850–2950 cm⁻¹ range. Alkene and aromatic C-H stretches are found just above 3000 cm⁻¹, so the 3000 cm⁻¹ boundary can help distinguish sp³ hybridised from sp² hybridised carbons.

烷烃的 C-H 伸缩振动出现在 3000 cm⁻¹ 以下，通常在 2850–2950 cm⁻¹ 范围内。烯烃和芳香烃的 C-H 伸缩振动则出现在 3000 cm⁻¹ 以上，因此 3000 cm⁻¹ 这一界限有助于区分 sp³ 杂化碳与 sp² 杂化碳。

Aldehydes give a very distinctive pattern: two weak C-H stretch bands near 2720 cm⁻¹ and 2820 cm⁻¹. The lower one is particularly useful because few other functional groups absorb in that region. Alkynes with a terminal ≡C-H bond show a sharp, strong C-H stretch around 3300 cm⁻¹.

醛基会产生一个非常独特的模式：两个弱的 C-H 伸缩带分别出现在约 2720 cm⁻¹ 和 2820 cm⁻¹。较低波数的那一个特别有用，因为几乎没有其他官能团在那个区域吸收。末端具有 ≡C-H 键的炔烃会在约 3300 cm⁻¹ 处显示一个尖而强的 C-H 伸缩峰。

8. Carbonyl (C=O) and Alkene (C=C) Stretching | 羰基和烯烃双键伸缩振动

The carbonyl stretching band is one of the most intense and reliable IR absorptions, falling in the 1650–1750 cm⁻¹ range. The exact position reveals information about the carbonyl environment. Simple ketones and aldehydes absorb near 1710–1725 cm⁻¹. Esters have their C=O at around 1735 cm⁻¹, while amides are shifted to lower wavenumbers, near 1650 cm⁻¹, due to resonance donation from the nitrogen.

羰基伸缩带是最强和最可靠的红外吸收之一，位于 1650–1750 cm⁻¹ 范围。其精确位置揭示了有关羰基环境的信息。简单的酮和醛在约 1710–1725 cm⁻¹ 处吸收。酯的 C=O 约在 1735 cm⁻¹，而酰胺由于氮原子的共振供电效应，C=O 向低波数位移，接近 1650 cm⁻¹。

Carbonyl groups conjugated with a C=C double bond absorb at lower wavenumbers, typically 20–40 cm⁻¹ lower than the unconjugated analogue. For example, a conjugated ketone might appear at 1680 cm⁻¹. The C=C stretching bands of alkenes are generally weaker than C=O stretches and appear in the 1620–1680 cm⁻¹ region. Conjugation and symmetry affect the intensity; symmetrically substituted alkenes may show a very weak or absent C=C peak.

与 C=C 双键共轭的羰基会在更低的波数处吸收，通常比非共轭类似物低 20–40 cm⁻¹。例如，共轭酮可能出现在 1680 cm⁻¹。烯烃的 C=C 伸缩带通常比 C=O 伸缩带弱，出现在 1620–1680 cm⁻¹ 区域。共轭和对称性会影响强度；对称取代的烯烃可能会显示出非常弱甚至没有 C=C 峰。

9. The Fingerprint Region | 指纹区

The fingerprint region covers roughly 500–1500 cm⁻¹ and is rich in absorption bands arising from complex bending vibrations and skeletal modes that are unique to the whole molecule. Even if two molecules share the same functional groups, their fingerprint regions will differ, much like human fingerprints.

指纹区大约覆盖 500–1500 cm⁻¹，其中富含来源于复杂弯曲振动和骨架模式的吸收带，这些对整体分子来说是独一无二的。即使两个分子拥有相同的官能团，它们的指纹区也会有所不同，这非常像人类的指纹。

In an exam, you are usually expected to use the region above 1500 cm⁻¹ to identify functional groups, but you may also be asked to confirm that an unknown sample matches a reference by comparing the entire fingerprint pattern.

在考试中，通常要求你使用 1500 cm⁻¹ 以上的区域来鉴定官能团，但你也可能被要求通过比较整个指纹区域的图谱来确认未知样品与参考图谱是否匹配。

10. Identifying Functional Groups Using IR | 利用红外光谱鉴定官能团

To identify functional groups, focus on a few diagnostic zones: first check for an O-H or N-H peak around 3300 cm⁻¹; then look for a sharp, intense C=O peak near 1700 cm⁻¹; next examine the C-H stretch region around 3000 cm⁻¹ for unsaturation clues; finally, check for a sharp C≡C or C≡N peak near 2200 cm⁻¹.

要鉴定官能团，请关注几个诊断区域：首先检查 3300 cm⁻¹ 附近有无 O-H 或 N-H 峰；然后寻找 1700 cm⁻¹ 附近尖而强的 C=O 峰；接着检查 3000 cm⁻¹ 附近的 C-H 伸缩区域，寻找不饱和度的线索；最后检查 2200 cm⁻¹ 附近有无尖锐的 C≡C 或 C≡N 峰。

For example, if a spectrum shows a broad O-H band at 3300 cm⁻¹ and a C=O band at 1710 cm⁻¹ with no very broad absorption down to 2500 cm⁻¹, the molecule is likely a carboxylic acid? Actually, a carboxylic acid would show a very broad band extending to 2500 cm⁻¹, so an alcohol plus carbonyl might suggest a keto-alcohol or an ester with an O-H impurity. Careful piecing together of all evidence is essential.

例如，如果一张光谱在 3300 cm⁻¹ 处显示宽 O-H 带，并在 1710 cm⁻¹ 处显示 C=O 带，但没有一直延伸到 2500 cm⁻¹ 的极宽吸收，那么这个分子会不会是羧酸？实际上，羧酸会显示一个延伸到 2500 cm⁻¹ 的极宽吸收带，因此醇加羰基可能暗示着酮醇或是带有 O-H 杂质的酯。仔细拼合所有证据至关重要。

11. Factors Affecting Absorption Bands | 影响吸收带的因素

The exact wavenumber of an absorption is not fixed; it is influenced by many factors. Hydrogen bonding broadens O-H and N-H peaks and shifts them to lower wavenumbers. Conjugation of a C=O or C=C with another π-system reduces bond order, lowering the stretching frequency. Ring strain in cyclic ketones moves the C=O stretch to higher wavenumbers as the angle strain increases the s-character of the bond.

一个吸收峰的精确波数并非固定不变，它受到许多因素的影响。氢键会使 O-H 和 N-H 峰变宽并将其位移到更低的波数。共轭会使 C=O 或 C=C 与另一个 π 体系相连，从而降低键级，进而降低伸缩频率。环酮中的环张力会将 C=O 伸缩振动移向高波数，因为角张力增加了该键的 s 成分。

The physical state of the sample (solid, liquid, gas) and solvent interactions also alter band shapes and positions. For

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Infrared Spectroscopy Key Points | 红外光谱 考点精讲