Infra-red Spectroscopy | 红外光谱

📚 Infra-red Spectroscopy | 红外光谱

Infra-red (IR) spectroscopy is an analytical technique used to identify functional groups and covalent bonds in organic and inorganic molecules. When a molecule absorbs infra-red radiation, its bonds vibrate with larger amplitude, and the absorbed wavenumbers are displayed as peaks in an IR spectrum.

红外光谱是一种分析技术，用于鉴定有机和无机分子中的官能团和共价键。当分子吸收红外辐射时，其化学键以更大的振幅振动，被吸收的波数以峰的形式显示在红外光谱中。

1. What Is IR Spectroscopy? | 什么是红外光谱？

Infra-red spectroscopy uses the infrared region of the electromagnetic spectrum, typically from about 400 cm⁻¹ to 4000 cm⁻¹. It provides information about covalent bonds and functional groups rather than the complete structure of a molecule.

红外光谱利用电磁波谱中的红外区，通常在约400 cm⁻¹至4000 cm⁻¹范围内。它提供的是关于共价键和官能团的信息，而不是分子的完整结构。

In an IR spectrum, the horizontal axis is wavenumber in cm⁻¹ and the vertical axis is percentage transmittance. Absorption of IR radiation appears as downward-pointing peaks.

在红外光谱中，横轴是波数（cm⁻¹），纵轴是透光百分率。红外辐射的吸收表现为向下的峰。

---

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

Covalent bonds are not rigid; they stretch, bend and twist. When IR radiation matches the frequency of a molecular vibration, the molecule absorbs energy and the vibration becomes more excited.

共价键不是刚性的；它们会伸缩、弯曲和扭转。当红外辐射的频率与分子振动频率匹配时，分子吸收能量，振动变得更加剧烈。

For a vibration to be IR active, it must change the dipole moment of the molecule. For example, CO₂ has a symmetric stretch that produces no dipole change and therefore does not absorb IR radiation, while its bending and asymmetric stretching vibrations are IR active.

只有改变分子偶极矩的振动才具有红外活性。例如，CO₂ 的对称伸缩不产生偶极变化，因此不吸收红外辐射，而其弯曲振动和反对称伸缩振动具有红外活性。

---

3. Wavenumber and Energy | 波数与能量

Wavenumber is the reciprocal of wavelength and is expressed in cm⁻¹. A higher wavenumber means higher frequency and higher photon energy, because photons in the IR region have energy that matches vibrational energy gaps.

波数是波长的倒数，用 cm⁻¹ 表示。波数越高，频率和光子能量越高，因为红外区的光子能量与振动能级差相匹配。

wavenumber = 1 / λ (unit: cm⁻¹)

ΔE = hν = hc / λ

Stronger bonds generally absorb at higher wavenumbers, while heavier atoms absorb at lower wavenumbers.

较强的键通常在较高波数处吸收，而较重的原子在较低波数处吸收。

---

4. How an IR Spectrum Is Recorded | 红外光谱的测定方式

A sample is placed in the IR beam. Different bonds absorb at characteristic wavenumbers, and the detector measures how much radiation passes through the sample at each wavenumber.

将样品置于红外光束中。不同的化学键在特征波数处吸收，检测器测量每个波数下透过样品的辐射量。

The spectrum is usually plotted as percentage transmittance against wavenumber, so peaks point downwards. Modern instruments use Fourier transform methods for speed and sensitivity.

光谱通常以透光百分率对波数作图，因此峰向下。现代仪器采用傅里叶变换方法以提高速度和灵敏度。

---

5. Key Functional Group Absorptions | 官能团特征吸收

Particular bonds absorb in predictable regions of the IR spectrum. The table below summarises the most important absorptions expected at A-Level.

特定化学键在红外光谱的可预测区域吸收。下表总结了 A-Level 阶段最重要的吸收。

Bond	Functional group	Wavenumber range / cm⁻¹	Comment
O-H	Alcohols, phenols	3200-3600	Broad due to hydrogen bonding
O-H	Carboxylic acids	2500-3300	Very broad, overlaps C-H
N-H	Amines, amides	3300-3500	Sharp; primary amines often show two peaks
C-H	Alkanes	2850-2960	Strong
C-H	Alkenes, arenes	3000-3100	Often just above 3000
C≡N	Nitriles	2200-2260	Sharp, often medium intensity
C=O	Carbonyl compounds	1630-1750	Very strong
C=C	Alkenes	1620-1680	Variable, often weak
C-C, C-O, C-X	Various	Below 1500	Fingerprint region

These ranges help identify functional groups quickly, but exact positions can shift because of the surrounding structure.

这些范围有助于快速鉴定官能团，但具体位置可能因周围结构而移动。

---

6. The Carbonyl Group in IR | 红外光谱中的羰基

The C=O stretching absorption is one of the strongest and most useful signals in an IR spectrum. It appears around 1700 cm⁻¹, but the exact value depends on the carbonyl compound.

C=O 伸缩吸收是红外光谱中最强、最有用的信号之一。它出现在约1700 cm⁻¹附近，但具体数值取决于羰基化合物。

Aldehydes usually absorb at about 1720-1740 cm⁻¹, ketones at about 1705-1725 cm⁻¹, carboxylic acids at about 1700-1725 cm⁻¹, esters at about 1735-1750 cm⁻¹, and amides at about 1630-1690 cm⁻¹.

醛通常在约1720-1740 cm⁻¹处吸收，酮在约1705-1725 cm⁻¹，羧酸在约1700-1725 cm⁻¹，酯在约1735-1750 cm⁻¹，酰胺在约1630-1690 cm⁻¹。

The strong C=O peak is often the first clear evidence that a carbonyl group is present, but additional peaks are needed to identify the exact functional group.

强 C=O 峰通常是羰基存在的第一个明确证据，但还需要其他峰来确定具体的官能团。

---

7. O-H and N-H Stretching Absorptions | O-H 与 N-H 伸缩吸收

Alcohols and phenols show a broad O-H stretching absorption at about 3200-3600 cm⁻¹. The broadness is caused by hydrogen bonding between molecules.

醇和酚在约3200-3600 cm⁻¹处显示宽的 O-H 伸缩吸收。宽峰由分子间氢键引起。

Carboxylic acids have an even broader O-H absorption from about 2500 to 3300 cm⁻¹, which often overlaps the C-H absorptions. Amines and amides give sharp N-H peaks around 3300-3500 cm⁻¹.

羧酸的 O-H 吸收更宽，从约2500至3300 cm⁻¹，常与 C-H 吸收重叠。胺和酰胺在约3300-3500 cm⁻¹处产生尖锐的 N-H 峰。

A primary amine often shows two N-H peaks, a secondary amine shows one, and a tertiary amine has no N-H peak because it has no N-H bond.

伯胺通常显示两个 N-H 峰，仲胺显示一个，而叔胺没有 N-H 峰，因为它没有 N-H 键。

---

8. The Fingerprint Region | 指纹区

The region below about 1500 cm⁻¹ is called the fingerprint region. It contains many complex bending and stretching vibrations that are unique to each molecule.

约1500 cm⁻¹以下的区域称为指纹区。它包含许多复杂的弯曲和伸缩振动，对每个分子都是独特的。

This region is used to confirm identity by comparing the unknown spectrum with reference spectra. It is difficult to assign every peak in this region to a specific bond.

该区域通过与参考光谱比对来确认物质身份。很难将这一区域的每个峰归属于特定化学键。

Two different compounds may have similar functional group peaks, but their fingerprint regions are almost always different.

两种不同的化合物可能具有相似的官能团峰，但它们的指纹区几乎总是不同。

---

9. Interpreting Spectra and Comparing Compounds | 谱图解析与化合物比较

When analysing an IR spectrum, look first for strong characteristic peaks: a broad O-H, a sharp N-H, a strong C=O, a C≡N, and C-H absorptions above or below 3000 cm⁻¹.

解析红外光谱时，首先寻找强特征峰：宽 O-H、尖锐 N-H、强 C=O、C≡N 以及高于或低于3000 cm⁻¹的 C-H 吸收。

An alcohol can be distinguished from an alkane by its broad O-H peak. An aldehyde can be distinguished from a ketone by a weak C-H absorption near 2720 cm⁻¹ in the aldehyde spectrum.

醇可通过其宽 O-H 峰与烷烃区分。醛可通过约2720 cm⁻¹附近的弱 C-H 吸收与酮区分。

A carboxylic acid shows both a very broad O-H and a C=O absorption, whereas an ester shows C=O without the very broad O-H. A nitrile is easily recognised by a sharp peak around 2200-2260 cm⁻¹.

羧酸同时显示非常宽的 O-H 和 C=O 吸收，而酯显示 C=O 但没有非常宽的 O-H。腈可通过约2200-2260 cm⁻¹处的尖锐峰轻松识别。

---

10. Factors Affecting Absorption Position | 影响吸收位置的因素

Stronger bonds vibrate at higher wavenumbers because more energy is required to stretch them. Heavier atoms vibrate at lower wavenumbers because they move more slowly.

较强的键在较高波数处振动，因为需要更多能量才能使其伸缩。较重的原子在较低波数处振动，因为它们运动较慢。

Hydrogen bonding broadens O-H and N-H absorptions and lowers their stretching frequency. Resonance and conjugation can weaken a double bond and shift C=O or C=C absorptions to lower wavenumbers.

氢键使 O-H 和 N-H 吸收变宽，并降低其伸缩频率。共振和共轭可使双键变弱，使 C=O 或 C=C 吸收向较低波数移动。

These shifts are useful because they help explain why the same functional group can absorb at slightly different positions in different molecules.

Published by TutorHao | A-Level Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）