📚 Hydrogen Bonding | 氢键
Hydrogen bonding is a special type of permanent dipole–dipole interaction that occurs between molecules containing a hydrogen atom covalently bonded to a highly electronegative element (nitrogen, oxygen, or fluorine) and a lone pair of electrons on a nearby electronegative atom. It is the strongest intermolecular force and plays a decisive role in determining the physical properties of many substances, as well as the structure and function of biological macromolecules.
氢键是一种特殊的永久偶极–偶极相互作用，发生在分子之间：一个氢原子与一个高电负性元素（氮、氧或氟）形成共价键，并与邻近电负性原子上的孤对电子相互吸引。它是最强的分子间作用力，对许多物质的物理性质以及生物大分子的结构和功能起着决定性作用。
1. What Is Hydrogen Bonding? | 什么是氢键？
A hydrogen bond is represented as X–H···Y, where X is a highly electronegative atom (N, O, or F) bonded to H, and Y is another electronegative atom possessing at least one lone pair. The dotted line indicates the intermolecular attraction, which is electrostatic in nature but partially covalent in very strong cases.
氢键表示为 X–H···Y，其中 X 是与 H 成键的高电负性原子（N、O 或 F），Y 是另一个带有孤对电子的电负性原子。虚线表示分子间吸引力，本质是静电作用，在极强的情况下也具有部分共价性。
This force is far stronger than ordinary dipole–dipole interactions or London dispersion forces but significantly weaker than covalent bonds. Typical hydrogen bond energies range from 10 to 40 kJ mol⁻¹, whereas a covalent O–H bond is about 460 kJ mol⁻¹.
这种力远强于普通的偶极–偶极力或伦敦色散力，但明显弱于共价键。氢键的典型能量范围为 10–40 kJ mol⁻¹，而共价 O–H 键约 460 kJ mol⁻¹。
2. Conditions for Hydrogen Bonding | 氢键的形成条件
For a hydrogen bond to form, two structural requirements must be met simultaneously:
要形成氢键，必须同时满足两个结构条件：
- A hydrogen atom must be covalently bonded to a highly electronegative atom (N, O, or F), making the bond highly polarised with a δ⁺ on H. 氢原子必须与高电负性原子（N、O 或 F）以共价键结合，使键高度极化，H 上带 δ⁺。
- The neighbouring molecule must contain an electronegative atom (N, O, or F) that possesses at least one available lone pair of electrons, which attracts the δ⁺ hydrogen. 邻近分子必须含有带至少一对孤对电子的电负性原子（N、O 或 F），吸引带 δ⁺ 的氢。
Hydrogen bonds do not form with chlorine, despite its high electronegativity, because the Cl atom is too large and its lone pairs are too diffuse to form a sufficiently strong attraction with the δ⁺ hydrogen.
尽管氯的电负性较高，但一般不形成经典氢键，因为 Cl 原子过大，孤对电子过于弥散，无法与 δ⁺ 氢形成足够强的吸引。
3. Electronegativity and Bond Polarity | 电负性与键的极性
The large difference in electronegativity between hydrogen (2.1) and N (3.0), O (3.5), or F (4.0) causes the shared electron pair in the X–H bond to be pulled strongly towards the electronegative atom. This creates a permanent dipole: the hydrogen carries a significant partial positive charge (δ⁺) while X carries a partial negative charge (δ⁻).
氢（2.1）与 N（3.0）、O（3.5）、F（4.0）之间较大的电负性差异，使得 X–H 键中的共用电子对被强烈拉向电负性原子。这产生了一个永久偶极：氢原子带有明显的部分正电荷（δ⁺），X 则带部分负电荷（δ⁻）。
The δ⁺ hydrogen, being a bare proton with a very small radius and no inner-shell electrons, can approach a lone pair on another electronegative atom very closely, giving the attraction its unusual strength.
带 δ⁺ 的氢实际上是一个半径极小且无内层电子的裸露质子，能够非常接近另一电负性原子的孤对电子，从而赋予这种吸引力异常大的强度。
4. Strength Compared with Other Intermolecular Forces | 与其他分子间力的强度比较
Hydrogen bonds are the strongest type of intermolecular force encountered in simple molecular substances. The table below compares approximate energy ranges:
氢键是简单分子物质中最强的分子间作用力。下表比较了近似的能量范围：
| Interaction type | 作用类型 | Typical energy / kJ mol⁻¹ | 典型能量 |
|---|---|
| London dispersion forces | 伦敦色散力 | 0.1 – 5 |
| Permanent dipole–dipole forces | 永久偶极–偶极力 | 1 – 10 |
| Hydrogen bonds | 氢键 | 10 – 40 |
Although hydrogen bonds are only about one-tenth the strength of covalent bonds, their collective effect in a condensed phase can dramatically raise melting points, boiling points, and enthalpies of vaporisation.
尽管氢键强度只有共价键的十分之一左右，但在凝聚相中，其集体效应可以显著提高熔点、沸点和蒸发焓。
5. Hydrogen Bonding in Water | 水中的氢键
Each H₂O molecule can form up to four hydrogen bonds – two through its hydrogen atoms (donor) and two through the lone pairs on oxygen (acceptor). In liquid water, a dynamic three‑dimensional network of hydrogen bonds is constantly forming and breaking, accounting for water’s anomalously high boiling point (100 °C) compared to other Group 16 hydrides.
每个 H₂O 分子最多可形成四个氢键——两个通过自身的氢原子（供体），两个通过氧上的孤对电子（受体）。液态水中，氢键在不断形成和断裂的动态三维网络，这解释了水相对于其他第16族氢化物异常高的沸点（100 °C）。
This extensive hydrogen bonding also explains water’s very high specific heat capacity (4.18 J g⁻¹ K⁻¹), high enthalpy of vaporisation (40.7 kJ mol⁻¹), and high surface tension, all of which are essential for supporting life and moderating Earth’s climate.
广泛的氢键也解释了水极高的比热容（4.18 J g⁻¹ K⁻¹）、高蒸发焓（40.7 kJ mol⁻¹）和高表面张力，这些特性对维持生命和调节地球气候至关重要。
6. The Structure of Ice and the Density Anomaly | 冰的结构与密度异常
When water freezes, the hydrogen bonds lock the H₂O molecules into a fixed, open tetrahedral lattice. Each oxygen atom is surrounded by four hydrogen atoms, two covalently bonded and two hydrogen-bonded, leaving large hexagonal channels in the structure.
当水结冰时，氢键将 H₂O 分子锁定在一个固定的、开放的四面体晶格中。每个氧原子被四个氢原子围绕，两个共价键合，两个通过氢键连接，结构中出现六边形通道。
This open arrangement makes ice less dense (0.917 g cm⁻³) than liquid water (1.000 g cm⁻³ at 4 °C). Consequently, ice floats on water, insulating the liquid below and allowing aquatic life to survive in cold climates. The density maximum of water at 4 °C is also a direct result of the competition between thermal expansion and the formation of an ordered hydrogen‑bonded network.
这种开放排列使冰的密度（0.917 g cm⁻³）低于液态水（4 °C 时 1.000 g cm⁻³）。因此冰浮在水面上，隔绝了下方液态水，使水生生物能在寒冷气候中存活。水在 4 °C 时密度最大，也是热膨胀与有序氢键网络形成竞争的直接结果。
7. Boiling Point Trends of Hydrides (Groups 14–17) | 氢化物沸点趋势（第14–17族）
When the boiling points of the hydrides of Groups 14 to 17 are compared, H₂O, HF, and NH₃ show anomalously high values because of hydrogen bonding. For example:
当比较第14至17族氢化物的沸点时，H₂O、HF 和 NH₃ 因氢键而表现出异常高的数值。例如：
- H₂O (bp 100 °C) vs H₂S (bp –60 °C), H₂Se, H₂Te. 水 vs 硫化氢等。
- HF (bp +20 °C) vs HCl (bp –85 °C), HBr, HI. 氟化氢 vs 氯化氢等。
- NH₃ (bp –33 °C) vs PH₃ (bp –88 °C), AsH₃, SbH₃. 氨 vs 磷化氢等。
The graph of boiling points against period shows sharp peaks at NH₃, H₂O, and HF. Without hydrogen bonding, these compounds would exhibit much lower boiling points, consistent with their smaller molar masses.
沸点对周期的关系图在 NH₃、H₂O 和 HF 处呈现尖锐的峰值。若没有氢键，这些化合物的沸点会低得多，与它们较小的摩尔质量相符。
8. Viscosity and Surface Tension | 粘度和表面张力
Liquids with extensive hydrogen‑bonding networks, such as water, glycerol, and concentrated sulfuric acid, display high viscosity because the intermolecular forces resist flow. For example, glycerol (propan-1,2,3-triol) has three –OH groups per molecule, forming a dense three‑dimensional hydrogen‑bond network, giving it a syrup‑like consistency.
具有广泛氢键网络的液体，如水、甘油和浓硫酸，表现出高粘度，因为分子间作用力阻碍流动。例如，甘油（丙三醇）每个分子有三个 –OH 基团，形成致密的三维氢键网络，使其呈糖浆状稠度。
Surface tension is also enhanced by hydrogen bonding. Water molecules at the surface experience a net inward pull because they form fewer hydrogen bonds than those in the bulk liquid. This results in the ‘skin’ effect that allows insects such as water striders to walk on water.
氢键也会增强表面张力。表面的水分子由于形成的氢键比内部少，受到了净向内的拉力，产生了“膜”效应，使水黾等昆虫能在水面上行走。
9. Hydrogen Bonding in Alcohols and Carboxylic Acids | 醇和羧酸中的氢键
Alcohols contain the hydroxyl (–OH) group, which allows them to act as both hydrogen‑bond donors and acceptors. Consequently, ethanol (C₂H₅OH, bp 78 °C) has a much higher boiling point than propane (C₃H₈, bp –42 °C) despite their similar molar masses.
醇分子含有羟基（–OH），可作为氢键供体和受体。因此，乙醇（C₂H₅OH，沸点 78 °C）的沸点远高于分子量相近的丙烷（C₃H₈，沸点 –42 °C）。
Carboxylic acids form particularly strong hydrogen‑bonded dimers in the liquid and vapour phases. Two carboxylic acid molecules pair up via two hydrogen bonds between their –COOH groups, creating an eight‑membered ring. This dimerisation effectively doubles the molecular mass, leading to exceptionally high boiling points; ethanoic acid (bp 118 °C) boils well above propan-1-ol (bp 97 °C) of comparable size.
羧酸在液相和气相中能形成特别强的氢键二聚体。两个羧酸分子通过它们的 –COOH 基团之间的两个氢键配对，形成八元环。这种二聚作用使有效分子量翻倍，导致沸点异常高；乙酸的沸点（118 °C）远高于分子大小相似的丙-1-醇（97 °C）。
10. Hydrogen Bonding in Biological Molecules | 生物分子中的氢键
Hydrogen bonding is fundamental to the structure and function of DNA. The two strands of the double helix are held together by specific hydrogen bonds between complementary base pairs: adenine (A) pairs with thymine (T) via two hydrogen bonds, while guanine (G) pairs with cytosine (C) via three hydrogen bonds. This complementarity allows precise replication and transcription.
氢键对 DNA 的结构和功能至关重要。双螺旋的两条链通过互补碱基对之间的专一氢键连接在一起：腺嘌呤（A）与胸腺嘧啶（T）通过两个氢键配对，鸟嘌呤（G）与胞嘧啶（C）通过三个氢键配对。这种互补性确保了精确的复制和转录。
In proteins, hydrogen bonds stabilise the secondary structures. In an α‑helix, C=O groups form hydrogen bonds with N–H groups four amino acids further along the chain. In β‑pleated sheets, hydrogen bonds link C=O and N–H groups on adjacent polypeptide strands running either parallel or antiparallel.
在蛋白质中，氢键稳定了二级结构。在 α-螺旋中，C=O 基团与沿链向前第四个氨基酸的 N–H 基团形成氢键。在 β-折叠片中，氢键连接相邻多肽链上平行或反平行的 C=O 和 N–H 基团。
11. Intermolecular vs. Intramolecular Hydrogen Bonding | 分子间与分子内氢键
Most hydrogen bonds are intermolecular, linking separate molecules and affecting bulk properties such as boiling point and solubility. However, certain molecules possess functional groups positioned so that a hydrogen bond can form within the same molecule – this is intramolecular hydrogen bonding.
大多数氢键是分子间的，连接不同的分子并影响如沸点和溶解度等宏观性质。然而，某些分子的官能团位置使之能在同一分子内形成氢键——这就是分子内氢键。
A classic example is 2‑nitrophenol, where the –OH group donates a hydrogen bond to the nearby –NO₂ group. This intramolecular bond reduces the molecule’s ability to form intermolecular hydrogen bonds, resulting in a lower boiling point compared to its 3‑ and 4‑nitrophenol isomers. In salicylic acid, intramolecular hydrogen bonding stabilises the conjugate base, enhancing the acid’s strength relative to benzoic acid.
典型例子是 2-硝基苯酚，其中 –OH 基团向邻近的 –NO₂ 基团提供氢键。这种分子内键降低了分子形成分子间氢键的能力，导致其沸点低于 3‑和 4‑硝基苯酚异构体。水杨酸中，分子内氢键稳定了共轭碱，使该酸的强度相对于苯甲酸增强。
12. Summary and Exam Tips | 总结与考试技巧
Hydrogen bonding arises from a highly polarised X– s of many compounds. In diagrams, always draw a dashed line from the lone pair to the δ⁺ hydrogen, label partial charges, and ensure the X–H···Y arrangement is roughly linear (≈180°).
氢键源于高度极化的 X–H 键和邻近 N、O 或 F 原子上的孤对电子。它解释了水的异常性质、DNA 的稳定性以及许多化合物的高沸点。在示意图中，一定要从孤对电子向 δ⁺ 氢画虚线，标出部分电荷，并确保 X–H···Y 排列大致呈直线（≈180°）。
When answering exam questions, precise language is crucial: refer to “hydrogen bonding” not just “dipole–dipole”, specify the role of lone pairs, and compare energies where appropriate. Remember that sulphur, chlorine, and carbon do not form hydrogen bonds in the same sense, despite electronegativity values.
回答考题时，准确的表述至关重要：要指明“氢键”，而不仅仅是“偶极–偶极”，说明孤对电子的作用，并在适当处比较能量。记住，尽管硫、氯和碳也有电负性，但它们并不形成通常意义上的氢键。
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