📚 Analytical Techniques | 分析技术
In A-Level Chemistry, analytical techniques are the methods used to identify and quantify chemical compounds. They provide critical structural information, including functional groups, molecular mass, and atomic connectivity. Modern instrumental methods such as infrared spectroscopy, mass spectrometry, nuclear magnetic resonance spectroscopy, and chromatography are essential tools for chemists.
在A-Level化学中，分析技术是用于鉴定和定量化合物的方法。它们提供了关键的结构信息，包括官能团、分子质量和原子连接方式。现代仪器方法，如红外光谱、质谱、核磁共振波谱和色谱法，是化学家必不可少的工具。
1. Infrared Spectroscopy – Basic Principles | 红外光谱——基本原理
Infrared (IR) spectroscopy exploits the absorption of infrared radiation by covalent bonds within a molecule. When IR radiation is passed through a sample, bonds absorb specific frequencies that match their natural vibrational frequencies, causing stretching or bending. The resulting spectrum plots transmittance against wavenumber (cm⁻¹), revealing which functional groups are present.
红外光谱利用分子内共价键对红外辐射的吸收。当红外辐射通过样品时，键会吸收与其自然振动频率相匹配的特定频率，从而引起伸缩或弯曲振动。所得光谱是透过率对波数（cm⁻¹）的曲线，显示出存在哪些官能团。
The region below 1500 cm⁻¹ is known as the fingerprint region. It is highly characteristic of the whole molecule and is used to confirm identity by comparison with reference spectra. Above 1500 cm⁻¹, absorptions are assigned to specific functional groups.
低于1500 cm⁻¹的区域被称为指纹区。该区域对整个分子具有高度特征性，可通过与参考光谱比对来确认物质身份。1500 cm⁻¹以上的吸收峰则可归属于特定官能团。
| Bond / Functional Group | Wavenumber Range (cm⁻¹) | Intensity / Shape |
|---|---|---|
| O–H (alcohols, phenols) | 3200 – 3600 | Strong, broad |
| N–H (amines, amides) | 3300 – 3500 | Medium, often sharp |
| C–H (alkanes, alkenes, arenes) | 2850 – 3100 | Medium to weak |
| C≡N (nitriles) | 2200 – 2260 | Sharp |
| C=O (carbonyls) | 1680 – 1750 | Very strong, sharp |
| C=C (alkenes, aromatics) | 1620 – 1680 | Variable, often medium |
| C–O (alcohols, ethers, esters) | 1000 – 1300 | Strong |
上表列出了A-Level阶段需要识别的关键红外吸收。例如，宽而强的O–H吸收表明存在醇或酚，而1700 cm⁻¹附近的尖锐强峰表明存在羰基化合物。指纹区为分子提供了独特“签名”。
2. Interpreting IR Spectra – Diagnostic Clues | 解读红外光谱——特征线索
near 1700 cm⁻¹. If present, narrow the possibilities: carboxylic acids show a very broad O–H absorption overlapping the C–H region, while esters exhibit a strong C–O peak near 1200 cm⁻¹. Aldehydes give two weak C–H stretches around 2720 and 2820 cm⁻¹ alongside the C=O.
要鉴定未知化合物，首先寻找约1700 cm⁻¹附近的主要羰基峰。若存在，可进一步缩小范围：羧酸在C–H区域重叠处显示非常宽的O–H吸收，而酯在1200 cm⁻¹附近显示强C–O峰。醛类除了C=O外，还在约2720和2820 cm⁻¹处出现两个弱C–H伸缩峰。
Absence of a strong peak above 1600 cm⁻¹ and presence of a broad O–H band suggests an alcohol. The C–O stretch between 1000 and 1300 cm⁻¹ confirms this. Primary, secondary and tertiary alcohols may be distinguished in the fingerprint region with experience, but exam questions usually rely on combined data from other techniques.
若在1600 cm⁻¹以上缺乏强峰且存在宽O–H带，则可能为醇。1000–1300 cm⁻¹之间的C–O伸缩振动可确认。有经验时可在指纹区区分伯、仲、叔醇，但考试通常结合其他技术的数据。
Typical exam task: y and NMR data to deduce the structure. Always check for characteristic absorptions like O–H, C=O, C–O.
典型考题：可能给出光谱与实验式。需要结合红外、质谱与NMR数据推断结构。务必检查O–H、C=O、C–O等特征吸收。
3. Mass Spectrometry – Principles and Molecular Ion | 质谱——原理与分子离子峰
ng ionisation: M + e⁻ → M⁺ + 2e⁻. The molecular ion M⁺ (or parent ion) gives the relative molecular mass (Mᵣ) of the compound. The highest m/z peak in the spectrum usually corresponds to the molecular ion, unless fragmentation is extensive.
质谱法测量离子的质荷比（m/z）。样品气化后用高能电子轰击，使之电离：M + e⁻ → M⁺ + 2e⁻。分子离子M⁺（或称母离子）给出化合物的相对分子质量（Mᵣ）。谱图中最高的m/z峰通常对应分子离子，除非碎片化很严重。
Isotopic peaks provide additional information. Elements like chlorine and bromine exhibit characteristic M : M+2 patterns. Chlorine has two isotopes ³⁵Cl (75%) and ³⁷Cl (25%), so compounds containing one Cl atom show a 3:1 ratio of M to M+2 peaks. Bromine isotopes ⁷⁹Br and ⁸¹Br are nearly equal (1:1), so an organic bromide gives M and M+2 peaks of similar intensity.
同位素峰提供额外信息。氯和溴等元素呈现特征M : M+2模式。氯有两种同位素³⁵Cl（75%）和³⁷Cl（25%），因此含一个氯原子的化合物在M与M+2峰处呈现3:1的强度比。溴同位素⁷⁹Br和⁸¹Br丰度几乎相等（1:1），因此有机溴化物的M与M+2峰强度近似相等。
For one Br atom: M : (M+2) ≈ 1 : 1; For one Cl atom: M : (M+2) ≈ 3 : 1
一个溴原子：M : (M+2) ≈ 1 : 1；一个氯原子：M : (M+2) ≈ 3 : 1
4. Mass Spectrometry – Fragmentation Patterns | 质谱——碎片模式
After ionisation, excess energy causes the molecular ion to break apart into fragment ions. The mass spectrum displays a series of peaks at lower m/z values corresponding to these fragments. The fragmentation pattern acts like a fingerprint of the carbon skeleton and functional groups. The most stable carbocation fragments give the tallest peaks (base peak).
电离后，多余能量使分子离子断裂成碎片离子。质谱在较低m/z值处显示一系列碎片峰。碎片模式就像碳骨架和官能团的指纹。最稳定的碳正离子碎片产生最高峰（基峰）。
Common losses include methyl (•CH₃, 15 mass units), ethyl (•C₂H₅, 29), water (18), and acyl groups. For example, an aliphatic ketone often cleaves at the bond adjacent to the carbonyl, generating an acylium ion R–C≡O⁺. An alcohol may lose water to give a peak at M-18.
常见的中性片段丢失包括甲基（•CH₃，15个质量单位）、乙基（•C₂H₅，29）、水（18）和酰基。例如，脂肪酮通常在羰基相邻键处断裂，生成酰基离子R–C≡O⁺。醇可能失去水分子，在M-18处出峰。
To deduce the structure, identify the molecular ion peak, check for isotopic signatures, and then examine major fragment ions. Connect fragments to likely building blocks of the molecule. This information is always used together with IR and NMR.
推断结构时，先确定分子离子峰，检查同位素特征，然后分析主要碎片离子。将碎片与分子的可能结构块联系起来。这些信息始终与红外和核磁共振结合使用。
5. Introduction to Nuclear Magnetic Resonance (NMR) | 核磁共振简介
Nuclear magnetic resonance spectroscopy exploits the magnetic properties of certain nuclei. In A-Level chemistry, the focus is on ¹H (proton) NMR and ¹³C NMR. When placed in a strong magnetic field, nuclei align either with or against the field. Radiofrequency pulses flip their alignment, and the energy absorbed is detected. The chemical environment of a nucleus influences the precise frequency at which it resonates.
核磁共振波谱法利用某些原子核的磁性。A-Level化学重点关注¹H（质子）NMR和¹³C NMR。置于强磁场中时，原子核顺向或逆向排列。射频脉冲使其翻转，吸收的能量被检测。原子核的化学环境影响其共振的精确频率。
ectron-donating groups shield the nucleus, shifting the peak upfield (lower δ); electron-withdrawing groups deshield it, shifting the peak downfield (higher δ).
化学位移（δ）衡量信号与参比物四甲基硅烷（TMS，δ = 0 ppm）的距离。给电子基团屏蔽原子核，使峰移向高场（低δ）；吸电子基团去屏蔽，使峰移向低场（高δ）。
NMR spectra are recorded in deuterated solvents (e.g. CDCl₃) to avoid interference from the solvent's own protons. Only nuclei with odd mass numbers give NMR signals.
核磁共振谱在氘代溶剂（如CDCl₃）中记录，以避免溶剂本身质子的干扰。只有质量数为奇数的原子核才能产生NMR信号。
6. ¹H NMR – Chemical Shifts and Integration | 氢谱——化学位移与积分
A ¹H NMR spectrum provides four key pieces of information: the number of different proton environments (number of peaks), the chemical shift of each environment (δ value), the relative number of protons in each environment (integration trace), and the splitting pattern due to neighbouring protons. This is often remembered as 'N+1 rule' for splitting.
¹H NMR谱提供四条关键信息：不同质子环境的数目（峰数）、每种环境的化学位移（δ值）、每种环境中的相对质子数（积分曲线）以及邻近质子造成的裂分模式。裂分常用‘n+1规则’记忆。
Integration is given as a step-like trace or as a number beneath each peak. The ratio of the step heights corresponds to the ratio of protons in that environment. For example, a ratio of 3:2:1 might indicate a –CH₃ group, a –CH₂– group, and an –OH proton.
积分以阶梯状曲线或峰下数字给出。阶梯高度比对应于该环境中质子数之比。例如，3:2:1的比例可能表示一个–CH₃、一个–CH₂–和一个–OH质子。
| Proton Environment | Chemical Shift (δ / ppm) |
|---|---|
| R–CH₃ (alkyl methyl) | 0.7 – 1.2 |
| R–CH₂–R (alkyl methylene) | 1.2 – 1.6 |
| CH₃–C=O (methyl ketone) | 2.0 – 2.5 |
| –CH₂–Cl / –CH₂–Br | 3.0 – 4.0 |
| R–O–CH₂–/ R–O–CH₃ | 3.3 – 4.0 |
| R–OH (alcohol, variable) | 1.0 – 5.5 (broad, exchangeable) |
| Aromatic protons (Ar–H) | 6.5 – 8.5 |
| R–CHO (aldehyde) | 9.5 – 10.0 |
| R–COOH (carboxylic acid) | 10.0 – 13.0 |
上表概括了关键质子环境的典型化学位移。活泼质子（如OH、NH）的峰通常较宽，且加入D₂O重水交换后会消失。
7. ¹H NMR – Spin-Spin Splitting (n+1 Rule) | 氢谱——自旋-自旋裂分（n+1规则）
Protons on adjacent carbon atoms (vicinal, three bonds apart) couple to split each other's signals. The number of peaks in a multiplet is given by n+1, where n is the number of equivalent protons on the neighbouring atom(s). A proton with no neighbouring protons appears as a singlet (s).
相邻碳原子上的质子（邻位，相隔三个键）相互偶合，使彼此信号裂分。多重峰中的峰数遵循n+1规则，其中n为相邻原子上等价质子的数目。没有邻位质子的环境表现为单峰（s）。
Thus, a –CH₃ group next to a –CH₂– gives a triplet for the –CH₂– (n=3, led to quartet for the CH₃). A –CH– adjacent to a – (from CH₃) and a doublet (from CH). The relative intensities in a multiplet follow Pascal's triangle: doublet 1:1, triplet 1:2:1, quartet 1:3:3:1.
因此，与–CH₂–相邻的–CH₃会使–CH₂–裂分为四重峰（n=3），而–CH₃自身被–CH₂–裂分为三重峰。与–CH₃相邻的–CH–使–CH₃呈二重峰，–CH–呈四重峰。多重峰的相对强度遵循帕斯卡三角形：双重峰1:1，三重峰1:2:1，四重峰1:3:3:1。
Coupling constants (J) are measured in Hz and are independent of the spectrometer frequency. Equivalent protons (e.g. the three protons in a methyl group) do not couple with each other. Labile protons (OH, NH) often do not cause splitting and may appear as broad singlets.
偶合常数（J）以Hz为单位，与谱仪频率无关。等价质子（如甲基中的三个质子）彼此不偶合。活泼质子（OH、NH）通常不引起裂分，可能呈现宽单峰。
8. ¹³C NMR Spectroscopy | 碳-13核磁共振
¹³C NMR detects signals from the ¹³C isotope (about 1.1% natural abundance). Each chemically distinct carbon atom in a molecule gives one peak in the spectrum. The chemical shift range is much wider than in ¹H NMR, typically 0–220 ppm. No integration or splitting is observed because ¹³C–¹³C coupling is rare and proton decoupling is routinely applied.
¹³C NMR检测¹³C同位素（自然丰度约1.1%）的信号。分子中每种化学不等价的碳原子在谱图中给出一个峰。化学位移范围比¹H NMR宽得多，通常为0–220 ppm。因¹³C–¹³C偶合罕见且常规使用质子去耦，谱图中不观测积分或裂分。
The number of peaks therefore equals the number of non-equivalent carbon environments. Symmetrical elements reduce the number of peaks. For example, benzene gives one peak (all six carbons equivalent), while methylbenzene gives five peaks for the aromatic region plus one for the methyl carbon.
因此，峰数等于非等价碳环境的数目。对称元素会减少峰数。例如，苯给出一个峰（所有六个碳等价），而甲苯在芳环区给出五个峰加上甲基碳的一个峰。
| Carbon Environment | Chemical Shift (δ / ppm) |
|---|---|
| R–CH₃ (alkyl) | 5 – 35 |
| R–CH₂–R (alkyl) | 15 – 40 |
| C–O (alcohols, ethers) | 50 – 90 |
| Alkene / Arene C (C=C) | 100 – 150 |
| C=O (carbonyl: aldehydes, ketones) | 190 – 210 |
| C=O (esters, carboxylic acids) | 160 – 185 |
该表总结了常见碳环境的¹³C化学位移。碳谱特别适合检测羰基和季碳，这些在氢谱中不直接出现。碳谱与氢谱结合可完整确定碳骨架。
9. Thin-Layer Chromatography (TLC) | 薄层色谱法
Thin-layer chromatography is a simple, quick method for separating and identifying small amounts of substances. A stationary phase (usually silica gel or alumina coated on a glass or plastic plate) and a mobile phase (a solvent or solvent mixture) enable differential migration. Components separate based on their relative affinity for the stationary and mobile phases.
薄层色谱法是一种分离和鉴定少量物质的简单快速方法。固定相（通常为涂在玻璃或塑料板上的硅胶或氧化铝）和流动相（溶剂或混合溶剂）实现差异迁移。各组分根据其对固定相和流动相的相对亲和力而分离。
After development, spots are visualised under UV light or using a developing agent. The retention factor Rf is calculated: Rf = distance moved by substance / distance moved by solvent front. Rf values are compared with known standards under identical conditions.
展开后，斑点在紫外光下或使用显色剂观察。计算比移值Rf：Rf = 物质移动距离 / 溶剂前沿移动距离。在相同条件下与已知标准品比较Rf Published by TutorHao | A-Level Chemistry Revision Series | aleveler.com
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