Infrared Spectroscopy | 红外光谱考点精讲

📚 Infrared Spectroscopy | 红外光谱考点精讲

Infrared (IR) spectroscopy is a powerful analytical technique used to identify functional groups in organic molecules. By measuring the absorption of infrared radiation by covalent bonds, we can determine the types of bonds present in a compound. This topic is essential for GCSE CCEA Chemistry and focuses on interpreting IR spectra to identify key functional groups such as –OH, C=O, and –COOH.

红外光谱是一种用于鉴定有机分子中官能团的强大分析技术。通过测量共价键对红外辐射的吸收，我们可以确定化合物中存在的键类型。本专题是GCSE CCEA化学的重要组成部分，重点在于解读红外光谱图，识别–OH、C=O和–COOH等关键官能团。

1. What is Infrared Spectroscopy? | 什么是红外光谱？

Infrared spectroscopy exploits the fact that covalent bonds in molecules vibrate at specific frequencies when they absorb infrared radiation. Each type of bond absorbs IR radiation at characteristic wavenumber ranges, producing a spectrum that acts like a molecular ‘fingerprint’. The wavenumber (unit: cm⁻¹) is used instead of wavelength because it is directly proportional to energy.

红外光谱利用的是分子中的共价键在吸收红外辐射后会以特定频率振动这一原理。每种类型的键会在特征波数范围内吸收红外辐射，产生的光谱就像分子的“指纹”。使用波数（单位：cm⁻¹）代替波长，因为它与能量成正比。

Wavenumber range of interest: 4000 cm⁻¹ to 400 cm⁻¹.
关注的波数范围：4000 cm⁻¹ 至 400 cm⁻¹。
Absorption peaks point downwards in a conventional IR spectrum.
传统红外光谱图中吸收峰向下。

2. Molecular Vibrations and IR Absorption | 分子振动与红外吸收

For a bond to absorb IR radiation, its dipole moment must change during vibration. This means symmetrical diatomic molecules like O₂ or N₂ do not absorb IR. Organic bonds like C=O, O–H, and C–Cl undergo stretching and bending vibrations that alter the dipole moment, causing absorption at specific wavenumbers.

一个键要吸收红外辐射，其偶极矩必须在振动过程中发生变化。这意味着像氧气或氮气这样的对称双原子分子不会吸收红外。像C=O、O–H和C–Cl等有机键会发生改变偶极矩的伸缩和弯曲振动，从而在特定波数产生吸收。

Stretching vibrations: higher energy, higher wavenumber.
伸缩振动：能量较高，波数较高。
Bending vibrations: lower energy, lower wavenumber.
弯曲振动：能量较低，波数较低。

3. The Infrared Spectrometer | 红外光谱仪

A beam of infrared radiation covering a range of frequencies is passed through the sample. The amount of radiation absorbed at each frequency is measured by a detector and plotted as a spectrum. Modern instruments use Fourier Transform (FT) methods to collect data quickly. The sample can be a thin film, a solution, or pressed into a KBr disc.

一束包含不同频率的红外辐射穿过样品。检测器测量每个频率下辐射被吸收的量，并绘制成光谱图。现代仪器采用傅里叶变换方法快速收集数据。样品可以是薄膜、溶液或压入溴化钾片中。

Percentage transmittance is plotted against wavenumber.
以透光率为纵坐标，波数为横坐标。
100% transmittance means no absorption; dips indicate absorption.
100%透光率表示无吸收；下凹处表示吸收。

4. Interpreting an IR Spectrum | 如何解读红外光谱图

The IR spectrum is divided into two main regions. The region above 1500 cm⁻¹ contains peaks due to individual functional group vibrations, such as O–H and C=O. The region below 1500 cm⁻¹ is called the fingerprint region. In GCSE exams, you are expected to focus on the diagnostic peaks above 1500 cm⁻¹.

红外光谱图分为两个主要区域。1500 cm⁻¹以上的区域包含因单个官能团振动（如O–H和C=O）产生的峰。1500 cm⁻¹以下的区域称为指纹区。在GCSE考试中，你需要重点关注1500 cm⁻¹以上的特征峰。

Look for strong, sharp peaks to identify functional groups.
寻找强而尖锐的峰来识别官能团。
Ignore weak, complex patterns in the fingerprint region for basic identification.
在基础鉴定中忽略指纹区中弱而复杂的图谱。

5. The Fingerprint Region | 指纹区

The area between 1500 cm⁻¹ and 400 cm⁻¹ is unique for every compound, just like a human fingerprint. It contains complex bending and skeletal vibrations. While it is not used to pick out individual bonds in GCSE, it can confirm the identity of a substance by matching the entire spectrum to a reference database.

1500 cm⁻¹至400 cm⁻¹之间的区域对每种化合物都是独一无二的，就像人的指纹。它包含复杂的弯曲和骨架振动。在GCSE中不用于识别单个键，但可以通过将整个光谱与参考数据库匹配来确认物质身份。

6. Characteristic Absorptions: O–H and N–H | 特征吸收峰：O–H 和 N–H

The O–H bond in alcohols and carboxylic acids produces a broad, strong absorption around 2500–3300 cm⁻¹. In carboxylic acids, this O–H peak is very broad and often overlaps with the C–H stretch. N–H bonds in amines and amides give a sharp or medium peak near 3300–3500 cm⁻¹. The O–H peak shape is a key diagnostic feature.

醇和羧酸中的O–H键在2500–3300 cm⁻¹附近产生宽而强的吸收。在羧酸中，这个O–H峰非常宽，常与C–H伸缩振动峰重叠。胺和酰胺中的N–H键在3300–3500 cm⁻¹附近给出尖锐或中等强度的峰。O–H峰的峰形是一个关键的判断特征。

Alcohol O–H: broad, 3200–3550 cm⁻¹ (hydrogen bonding).
醇O–H：宽峰，3200–3550 cm⁻¹（氢键作用）。
Carboxylic acid O–H: very broad, 2500–3300 cm⁻¹, often masking C–H.
羧酸O–H：非常宽，2500–3300 cm⁻¹，常掩盖C–H峰。

7. Characteristic Absorptions: C=O and C–O | 特征吸收峰：C=O 和 C–O

The carbonyl group C=O gives a strong, sharp absorption in the range 1640–1750 cm⁻¹. The exact position depends on the type of compound: aldehydes and ketones absorb around 1710 cm⁻¹; esters around 1735 cm⁻¹; carboxylic acids around 1700 cm⁻¹. The C–O single bond in alcohols and esters shows a strong peak between 1000–1300 cm⁻¹.

羰基C=O在1640–1750 cm⁻¹范围内产生强而尖锐的吸收峰，确切位置取决于化合物类型：醛和酮约在1710 cm⁻¹；酯约在1735 cm⁻¹；羧酸约在1700 cm⁻¹。醇和酯中的C–O单键在1000–1300 cm⁻¹之间呈现强峰。

Presence of both broad O–H and C=O peak suggests a carboxylic acid.
同时存在宽O–H峰和C=O峰提示羧酸。
C–O peak confirms alcohol or ester when combined with other data.
C–O峰结合其他数据可确认醇或酯。

8. Characteristic Absorptions: C–H and C=C | 特征吸收峰：C–H 和 C=C

C–H bonds in alkanes and alkenes show absorption just below 3000 cm⁻¹. Saturated C–H stretches occur at 2850–2960 cm⁻¹, while unsaturated =C–H stretches appear above 3000 cm⁻¹. The C=C double bond gives a weak to medium sharp peak around 1620–1680 cm⁻¹. Conjugation can shift this peak to lower wavenumbers.

烷烃和烯烃中的C–H键在3000 cm⁻¹略下方显示吸收。饱和C–H伸缩振动出现在2850–2960 cm⁻¹，不饱和=C–H伸缩振动在3000 cm⁻¹以上出现。C=C双键在1620–1680 cm⁻¹附近给出弱到中等强度的尖峰。共轭作用可使该峰移向较低波数。

Look for =C–H above 3000 cm⁻¹ to confirm an alkene.
通过3000 cm⁻¹以上的=C–H峰确认烯烃。
A sharp peak near 1650 cm⁻¹ supports presence of C=C.
1650 cm⁻¹附近的尖峰支持C=C的存在。

9. Effect of Hydrogen Bonding on O–H Peak | 氢键对 O–H 峰的影响

Hydrogen bonding broadens the O–H absorption peak and shifts it to lower wavenumbers. The more extensive the hydrogen bonding, the broader the peak. In pure liquid alcohols, the O–H peak is very wide; in dilute solutions or in the gas phase, it becomes much sharper and appears at higher wavenumbers.

氢键会使O–H吸收峰变宽并向低波数移动。氢键作用越强，峰越宽。在纯液态醇中，O–H峰非常宽；在稀溶液或气相中，峰变得尖锐得多，并出现在较高波数。

Broad O–H peak in liquid sample: evidence of intermolecular hydrogen bonds.
液体样品中的宽O–H峰：表明存在分子间氢键。
Carboxylic acids form strong dimers; hence the very broad O–H absorption.
羧酸形成强二聚体，因此O–H吸收峰极宽。

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

In GCSE CCEA Chemistry exams, you will be given IR spectra and asked to identify the compound or functional groups present. Start by checking for a broad O–H peak near 3300 cm⁻¹, then look for a C=O peak around 1700 cm⁻¹. If both are present, think of carboxylic acids. If only C=O is present, consider aldehydes, ketones, or esters. Absence of these peaks suggests an alkane, alkene, or haloalkane.

在GCSE CCEA化学考试中，你会看到红外光谱图并被要求鉴定化合物或所含官能团。首先检查3300 cm⁻¹附近的宽O–H峰，然后寻找1700 cm⁻¹附近的C=O峰。若两者均存在，考虑羧酸。若仅存在C=O峰，考虑醛、酮或酯。若这两类峰均不存在，则可能是烷烃、烯烃或卤代烷。

Systematic approach: O–H region → C=O region → C–O region → C–H region.
系统方法：O–H区 → C=O区 → C–O区 → C–H区。
Typical exam question: Match the spectrum to the correct structure from a list.
典型考题：将光谱图与列表中的正确结构进行匹配。

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Infrared Spectroscopy | 红外光谱考点精讲