Infra-red Spectroscopy | 红外光谱

📚 Infra-red Spectroscopy | 红外光谱

Infra-red (IR) spectroscopy is an essential analytical tool in organic chemistry, allowing chemists to identify functional groups present in a molecule by measuring the absorption of infrared radiation. It is a rapid, non-destructive technique that provides a unique 'fingerprint' for many compounds, making it indispensable in both research and quality control laboratories.

红外光谱（IR）是有机化学中一种重要的分析工具，通过测量分子对红外辐射的吸收，化学家可以确定分子中存在的官能团。它快速、无损，能为许多化合物提供独特的"指纹"，因此成为科研和质量控制实验室中不可或缺的手段。

1. The Electromagnetic Spectrum and the IR Region | 电磁波谱与红外区

Infra-red radiation lies between the visible and microwave regions of the electromagnetic spectrum, with wavelengths ranging from approximately 700 nm to 1 mm. In IR spectroscopy, we describe the radiation by its wavenumber (ν̂), measured in cm⁻¹, which is the reciprocal of the wavelength in centimetres and is directly proportional to frequency. The useful mid-IR region for organic analysis typically spans from 4000 cm⁻¹ to 400 cm⁻¹.

红外辐射位于电磁波谱的可见光区和微波区之间，波长范围大约为700 nm到1 mm。在红外光谱中，我们常用波数（ν̂，单位cm⁻¹）来描述辐射，波数是波长（以厘米为单位）的倒数，与频率成正比。用于有机分析的实用中红外区通常覆盖4000 cm⁻¹至400 cm⁻¹。

The energy of IR photons corresponds to the energy differences between vibrational energy levels of molecules. When a molecule is exposed to IR radiation, it can absorb photons of specific wavenumbers, promoting it from its ground vibrational state to an excited vibrational state.

红外光子的能量与分子振动能级之间的能量差相对应。当分子暴露于红外辐射时，可以吸收特定波数的光子，从振动基态跃迁到振动激发态。

2. Molecular Vibrations: Stretching and Bending | 分子振动：伸缩与弯曲

Molecules are not rigid; their bonds undergo constant vibrations. The two fundamental types of vibrational modes relevant to IR spectroscopy are stretching and bending. Stretching vibrations involve a change in bond length along the bond axis and can be symmetric or asymmetric. Bending vibrations change the bond angle and include scissoring, rocking, wagging and twisting motions.

分子并非刚性结构，它们的化学键在不断地振动。与红外光谱相关的两种基本振动模式是伸缩振动和弯曲振动。伸缩振动涉及沿键轴方向的键长变化，有对称和不对称之分。弯曲振动改变键角，包括剪式、摇摆、面外摇摆和扭曲等运动。

A non-linear molecule with N atoms has 3N – 6 fundamental vibrational modes. For example, water (H₂O) has three modes: symmetric stretching, asymmetric stretching and scissoring bending. Linear molecules such as CO₂ have 3N – 5 modes. Not all of these vibrations are necessarily IR active.

含有N个原子的非线型分子有3N – 6个基本振动模式。例如，水（H₂O）有三种模式：对称伸缩、不对称伸缩和剪式弯曲。线型分子如CO₂有3N – 5个模式。并非所有这些振动都会产生红外吸收。

3. Conditions for IR Absorption: Dipole Moment Change | 红外吸收的条件：偶极矩变化

ic molecule such as N₂ or O₂ does not alter the dipole moment, so these molecules are IR inactive. Conversely, the stretching of polar bonds like C=O or O–H causes a significant dipole change and gives rise to strong absorptions.

一个振动要吸收红外辐射，必须引起分子偶极矩的变化。像N₂或O₂这种同核双原子分子的对称伸缩不会改变偶极矩，因此它们是红外非活性的。相反，极性键如C=O或O–H的伸缩会引起显著的偶极矩变化，从而产生强吸收。

This selectivity means IR spectroscopy is particularly sensitive to functional groups with polar bonds, such as carbonyls, hydroxyls and amines. Even symmetrical molecules can show IR-active bending modes if they produce a temporary dipole, as in the bending of CO₂ at about 667 cm⁻¹.

这种选择性意味着红外光谱对带有极性键的官能团特别灵敏，例如羰基、羟基和氨基。即使是完全对称的分子，如果其弯曲振动能产生瞬态偶极，也可能表现出红外活性，例如CO₂在约667 cm⁻¹处的弯曲吸收。

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

In a typical Fourier-transform infrared (FT-IR) spectrometer, a beam of IR radiation is split into two paths. One passes through the sample, and the other serves as a reference. The transmitted beams are recombined, and the resulting interferogram is mathematically transformed into a spectrum. The spectrum displays the percentage transmittance (%T) against wavenumber.

在典型的傅里叶变换红外光谱仪（FT-IR）中，一束红外辐射被分成两路。一路通过样品，另一路作为参考。透射光束重新合束，生成的干涉图经数学变换后得到光谱图。光谱图显示透过率百分比（%T）对波数的关系。

Solid samples are often ground with an IR-transparent agent like KBr and pressed into a disc, while liquids can be placed between salt plates. The spectrum shows downward-pointing peaks, each corresponding to a specific bond vibration that absorbed IR energy at that wavenumber.

固体样品通常与KBr等红外透明介质研磨后压片，液体则可夹在盐片之间进行测量。谱图中的向下峰，每个峰对应于特定化学键在该波数处吸收了红外能量。

5. Interpreting an IR Spectrum: Axes and Conventions | 解读红外光谱图：坐标轴与标度约定

The horizontal axis of an IR spectrum is wavenumber (cm⁻¹), conventionally displayed from high wavenumber (4000 cm⁻¹) on the left to low wavenumber (400 cm⁻¹) on the right. The vertical axis is usually percent transmittance (%T), so the baseline is near 100% and absorption peaks point downwards. Stronger absorptions produce deeper peaks.

红外光谱的横轴是波数（cm⁻¹），通常以高波数（4000 cm⁻¹）在左、低波数（400 cm⁻¹）在右的方式展示。纵轴一般为透过率百分比（%T），因此基线靠近100%，吸收峰方向向下。吸收越强，峰谷越深。

Key regions are often discussed: the functional group region (4000–1500 cm⁻¹) contains stretching vibrations of common functional groups, while the fingerprint region (below 1500 cm⁻¹) is rich in complex bending and skeletal vibrations. Both regions must be analysed for a full structural interpretation.

通常会讨论两个关键区域：官能团区（4000–1500 cm⁻¹）包含了常见官能团的伸缩振动，而指纹区（低于1500 cm⁻¹）则富含复杂的弯曲振动和骨架振动。全面解析结构需要分析这两个区域。

6. Characteristic Absorption Bands for Organic Functional Groups | 有机官能团的特征吸收带

O–H stretch (alcohols, phenols, carboxylic acids): A broad, strong absorption in the range 3200–3550 cm⁻¹ is typical of hydrogen-bonded O–H groups. The breadth arises from the range of hydrogen-bond strengths in the sample. In carboxylic acids, the O–H absorption is so broad that it often overlaps with C–H stretches.

O–H伸缩（醇、酚、羧酸）：3200–3550 cm⁻¹范围内的宽而强的吸收是氢键合O–H基团的典型特征。峰的宽度来源于样品中氢键强度的分布范围。在羧酸中，O–H吸收非常宽，常与C–H伸缩振动重叠。

N–H stretch (amines, amides): Primary amines display two peaks around 3300–3500 cm⁻¹ (asymmetric and symmetric stretches), while secondary amines show a single peak in this region. Amide N–H stretches appear slightly lower, often near 3300 cm⁻¹.

N–H伸缩（胺、酰胺）：伯胺在3300–3500 cm⁻¹附近显示两个峰（不对称和对称伸缩），而仲胺在此区域显示单峰。酰胺的N–H伸缩稍低，通常出现在3300 cm⁻¹附近。

C–H stretch (alkanes, alkenes, aromatics): Alkane C–H stretches appear just below 3000 cm⁻¹ (2850–2960 cm⁻¹). Alkene and aromatic C–H stretches occur above 3000 cm⁻¹ (3010–3100 cm⁻¹), providing a quick test for unsaturation. Aldehyde C–H stretches often give two weak peaks near 2720 cm⁻¹ and 2820 cm⁻¹.

C–H伸缩（烷烃、烯烃、芳烃）：烷烃C–H伸缩出现在3000 cm⁻¹以下（2850–2960 cm⁻¹）。烯烃和芳烃的C–H伸缩发生在3000 cm⁻¹以上（3010–3100 cm⁻¹），可快速判断不饱和性。醛基C–H伸缩常在2720 cm⁻¹和2820 cm⁻¹附近产生两个弱峰。

C=O stretch (carbonyl compounds): important IR absorptions, the C=O stretch, gives a sharp, intense peak in the region 1680–1750 cm⁻¹. The exact position depends on the compound class: saturated aldehydes ~1730 cm⁻¹, ketones ~1715 cm⁻¹, carboxylic acids ~1700 cm⁻¹, esters ~1735 cm⁻¹, and amides lower at ~1650–1690 cm⁻¹ due to resonance effects.

C=O伸缩（羰基化合物）：C=O伸缩是最重要的红外吸收之一，在1680–1750 cm⁻¹区域产生尖锐、强烈的峰。具体位置取决于化合物类型：饱和醛约1730 cm⁻¹，酮约1715 cm⁻¹，羧酸约1700 cm⁻¹，酯约1735 cm⁻¹，而酰胺因共振效应更低，约在1650–1690 cm⁻¹。

C=C stretch (alkenes, aromatics): Medium to weak absorptions around 1600–1680 cm⁻¹ indicate carbon–carbon double bonds. Aromatic rings often show two or three bands in this region, around 1600 cm⁻¹ and 1500 cm⁻¹. Conjugation lowers the stretching frequency and increases intensity.

C=C伸缩（烯烃、芳烃）：1600–1680 cm⁻¹附近的中等至弱吸收指示碳碳双键。芳环常在此区域显示两至三个谱带，约在1600 cm⁻¹和1500 cm⁻¹。共轭作用使伸缩频率降低，强度增大。

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C–O stretch (alcohols, ethers, esters): A strong absorption in the range 1000–1300 cm⁻¹ is typical of the C–O single bond. Alcohols show C–O near 1050–1150 cm⁻¹, while esters display two C–O stretches, one near 1200 cm⁻¹ and another near 1100 cm⁻¹.

C–O伸缩（醇、醚、酯）：1000–1300 cm⁻¹范围内的强吸收是C–O单键的典型特征。醇的C–O伸缩在1050–1150 cm⁻¹附近，而酯显示两个C–O伸缩，一个在约1200 cm⁻¹，另一个在约1100 cm⁻¹。

C–X stretch (halogenoalkanes): Carbon–halogen bonds absorb in the fingerprint region, with C–Cl near 600–800 cm⁻¹, C–Br lower, and C–I even lower. These peaks can help confirm the presence of halogens when combined with other spectral data.

C–X伸缩（卤代烷）：碳卤键吸收位于指纹区，C–Cl在600–800 cm⁻¹附近，C–Br更低，C–I更低。这些峰结合其他谱学数据可帮助确认卤素的存在。

7. The Fingerprint Region and Its Importance | 指纹区及其重要性

The region below 1500 cm⁻¹ is called the fingerprint region because the pattern of absorptions here is highly characteristic of the individual compound. It contains a complex mixture of bending vibrations, ring vibrations and skeletal modes that produce a unique profile, much like a human fingerprint.

低于1500 cm⁻¹的区域被称为指纹区，因为此处的吸收模式对特定化合物具有高度特征性。它包含了弯曲振动、环振动和骨架振动的复杂混合物，产生独特的图谱，就像人的指纹一样。

While functional group identification relies mainly on the region above 1500 cm⁻¹, the fingerprint region is critical for confirming the identity of a compound by comparison with a reference spectrum. Two compounds that have the same functional groups will display identical fingerprint patterns only if they are the same molecule (including stereoisomers in some cases).

虽然官能团鉴别主要依赖1500 cm⁻¹以上的区域，但指纹区对于通过与标准谱图对比确认化合物身份至关重要。具有相同官能团的两种化合物，只有当它们是完全相同的分子（有时包括立体异构体）时，才会显示完全相同的指纹图案。

8. Using IR Spectroscopy to Identify Unknowns | 利用红外光谱鉴别未知化合物

When approaching an unknown IR spectrum, start by examining the region above 1500 cm⁻¹. Look for characteristic peaks: a broad absorption around 3300 cm⁻¹ suggests O–H or N–H; a sharp intense peak near 1700 cm⁻¹ points to a carbonyl group; peaks above 3000 cm⁻¹ indicate unsaturated C–H bonds. Then move to the fingerprint region to match against a known database if possible.

在解析一个未知红外光谱时，首先检查1500 cm⁻¹以上的区域。寻找特征峰：3300 cm⁻¹附近的宽吸收提示O–H或N–H；1700 cm⁻¹附近的尖锐强峰指向羰基；3000 cm⁻¹以上的峰指示不饱和C–H键。然后进入指纹区，如果可能，与已知数据库比对。

For example, a spectrum showing a broad band at 2500–3300 cm⁻¹ overlapping with C–H and a strong C=O peak at 1700 cm⁻¹ strongly suggests a carboxylic acid. The absence of O–H or N– ation such as elemental analysis, mass spectrometry and NMR for definitive identification.

例如，一个谱图在2500–3300 cm⁻¹出现与C–H重叠的宽峰，且在1700 cm⁻¹有强C=O峰，强烈提示为羧酸。存在羰基但无O–H或N–H峰则可能指向酯或酮。红外数据通常结合元素分析、质谱和核磁共振等其他信息以实现最终鉴定。

9. Typical Exam Questions and Data Interpretation | 典型考题与数据解析

At A-Level, you will often be given an IR spectrum alongside a data table of characteristic absorptions. Questions typically ask you to identify the functional groups present, explain your reasoning with specific wavenumbers, and sometimes deduce a plausible structure consistent with the molecular formula and IR evidence.

在A-Level考试中，通常会给出一个红外光谱图，并附有特征吸收数据表。试题通常要求你识别存在的官能团、说明你的推理并引用具体波数，有时还需要根据分子式和红外证据推导出合理的结构。

For instance, an exam question might state: ' O₂. Its IR spectrum shows a broad peak at 3000 cm⁻¹ and a sharp, strong peak at 1720 cm⁻¹.' From this, you should recognise the broad O–H of a carboxylic acid (the O–H stretch from 2500–3300 cm⁻¹ overlapping the C–H region) and the C=O of a carboxylic acid around 1700–1725 cm⁻¹, suggesting propanoic acid.

例如，一道考题可能陈述："某有机化合物分子式为C₃H₆O₂，其红外光谱在3000 cm⁻¹显示宽峰，在1720 cm⁻¹显示尖锐强峰。" 由此，你应该识别出羧酸的宽O–H（O–H伸缩在2500–3300 cm⁻¹与C–H区域重叠）和羧酸C=O在1700–1725 cm⁻¹附近的吸收，从而推断该化合物为丙酸。

ercise where several spectra are provided and you must link each to a given set of compounds. Focus on the presence or absence of key peaks: a broad O– excludes carbonyls, and so forth. Always refer explicitly to wavenumber ranges from the data sheet.

另一种常见题型是匹配练习，提供多个谱图，让你将其与给定的一组化合物对应。聚焦关键峰的有无：宽O–H峰可排除酮，无C=O可排除羰基化合物等。务必明确引用数据表中的波数范围。

10. Factors Affecting Peak Positions and Intensities | 影响峰位和强度的因素

Several factors can shift the absorption wavenumber or alter the intensity of peaks. Hydrogen bonding lowers the O–H or N–H stretching frequency and dramatically broadens the peak, as seen when comparing a free O–H at ~3650 cm⁻¹ with the hydrogen-bonded O–H of an alcohol near 3350 cm⁻¹.

多种因素会改变吸收波数或峰强度。氢键作用使O–H或N–H伸缩频率降低并显著增宽，例如，自由的O–H在大约3650 cm⁻¹，而醇中氢键合的O–H则在3350 cm⁻¹附近。

ches to lower wavenumbers because delocalisation weakens the bond order. For example, a conjugated ketone might absorb at 1680 cm⁻¹, while a saturated ketone absorbs near 1715 cm⁻¹. Ring strain in cyclic carbonyls raises the stretching frequency; cyclobutanone absorbs at a higher wavenumber than cyclohexanone.

共轭效应使C=O和C=C伸缩移向低波数，因为电子离域降低了键级。例如，共轭酮可能在1680 cm⁻¹吸收，而饱和酮在1715 cm⁻¹附近。环张力使环状羰基化合物的伸缩频率升高；环丁酮比环己酮吸收在更高的波数。

Sample state also matters: spectra recorded in solution or in the solid state may differ from gas-phase spectra due to intermolecular interactions. In an exam context, you are typically expected to interpret the data given, but awareness of these effects helps explain deviations from typical values.

样品状态也有影响：由于分子间相互作用，溶液或固体中记录的谱图可能不同于气相谱图。在考试情景中，你通常需要解读给定数据，但了解这些效应有助于解释偏离典型值的情况。

11. Limitations of IR Spectroscopy | 红外光谱的局限性

IR spectroscopy is excellent for detecting functional groups but does not directly give the molecular mass or full structural connectivity. Complex molecules with many functional groups may produce crowded, overlapping spectra that are difficult to interpret without additional techniques. IR cannot easily distinguish between members of a homologous series because the functional group absorptions dominate, and the fingerprint region may not be unique enough for unambiguous identification without reference spectra.

红外光谱非常适于检测官能团，但不能直接给出分子量或完整的结构连接信息。具有许多官能团的复杂分子可能产生拥挤、重叠的谱图，没有其他技术辅助则难以解析。IR不容易区分同系物，因为官能团吸收占主导，而指纹区若无标准谱图可能不足以实现明确鉴别。

Moreover, some vibrations are IR inactive, such as the stretching of symmetric homonuclear diatomics. This means IR alone cannot detect certain bonds, and it is often used alongside Raman spectroscopy for a complete vibrational picture. At A-Level, the focus is on the positive identification of common organic functional groups, and limitations are acknowledged mainly to avoid over-interpretation.

此外，某些振动是红外非活性的，如同核双原子分子的伸缩。这意味着单独使用IR无法检测某些键，通常与拉曼光谱联用才能得到完整的振动图像。在A-Level阶段，重点是常见有机官能团的鉴别，承认其局限性主要是为了避免过度解读。

12. Summary and Key Points for Revision | 总结与复习要点

technique for functional group analysis. Memorise the characteristic wavenumber ranges: O–H (broad, 3200–3550 cm⁻¹), N–H (3300–3500 cm⁻¹), C–H (alkanes below 3000 cm⁻¹, alkenes/aromatics above 3000 cm⁻¹), C=O (sharp, 1680–1750 cm⁻¹), C=C (~1600–1680 cm⁻¹), and C–O (1000– he fingerprint region (below 1500 cm⁻¹) is used for compound identification by comparison.

红外光谱是官能团分析的核心技术。熟记特征波数范围：O–H（宽峰，3200–3550 cm⁻¹），N–H（3300–3500

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