📚 Hydrogen Bonding: Formation and Effects | 氢键的形成与影响
Hydrogen bonding is a unique type of intermolecular interaction that plays a crucial role in chemistry, physics, and biology. It is responsible for the high boiling point of water, the double-helix structure of DNA, and the folding of proteins. This article explores how hydrogen bonds form, their key characteristics, and their far-reaching effects on the properties of matter.
氢键是一种独特的分子间相互作用，在化学、物理和生物学中起着至关重要的作用。它解释了水的高沸点、DNA的双螺旋结构以及蛋白质的折叠。本文将探讨氢键如何形成、其主要特征，以及它对物质性质的深远影响。
1. What Is a Hydrogen Bond? | 什么是氢键？
A hydrogen bond is an electrostatic attraction between a hydrogen atom attached to a highly electronegative atom (such as N, O, or F) and another electronegative atom with a lone pair of electrons. It is denoted as X–H···Y, where X and Y are electronegative atoms.
氢键是连接在高电负性原子（如N、O或F）上的氢原子与另一个具有孤对电子的电负性原子之间的静电吸引作用，通常表示为X–H···Y，其中X和Y为电负性原子。
The hydrogen atom must be covalently bonded to a strongly electronegative atom, which polarises the bond and leaves the hydrogen with a significant partial positive charge (δ+).
氢原子必须与强电负性原子形成共价键，该原子使键极化，使氢原子带有显著的部分正电荷（δ+）。
The acceptor atom must have a lone pair of electrons to attract the hydrogen atom. Typical donor groups are –OH, –NH, and –HF; typical acceptors are O, N, and F.
受体原子必须具有孤对电子以吸引氢原子。常见的给体基团有–OH、–NH和–HF；常见受体有O、N和F。
2. Mechanism of Hydrogen Bond Formation | 氢键的形成机制
When hydrogen is bonded to a highly electronegative atom, the bonding electrons are unevenly shared. The electronegative atom gains a partial negative charge (δ−), and hydrogen gains a partial positive charge (δ+).
当氢与高电负性原子成键时，键合电子不均等共享。电负性原子获得部分负电荷（δ−），氢原子获得部分正电荷（δ+）。
The δ+ hydrogen is then attracted to the lone pair of electrons on a nearby electronegative atom (δ−). This Coulombic attraction is the essence of a hydrogen bond.
随后，带δ+的氢被附近电负性原子（δ−）上的孤对电子吸引。这种库仑吸引力就是氢键的本质。
The strength of the hydrogen bond depends on the electronegativity difference between H and X, and also on the distance and angle between the donor and acceptor.
氢键强度取决于H与X之间的电负性差，也取决于给体和受体之间的距离和角度。
3. Strength and Properties of Hydrogen Bonds | 氢键的强度与性质
Hydrogen bonds are stronger than typical van der Waals forces (2–20 kJ mol⁻¹) but much weaker than covalent or ionic bonds (150–400 kJ mol⁻¹). Their bond energies typically range from 10 to 40 kJ mol⁻¹.
氢键比典型范德华力（2–20 kJ mol⁻¹）强，但比共价键或离子键（150–400 kJ mol⁻¹）弱得多。其键能通常在10–40 kJ mol⁻¹之间。
They are directional: the strongest hydrogen bonds are linear (X–H···Y angle ≈ 180°). This directionality is crucial in dictating molecular structures.
氢键具有方向性：最强的氢键是线性的（X–H···Y角度≈180°）。这种方向性对决定分子结构至关重要。
They are also saturable: each hydrogen atom generally forms only one hydrogen bond, because it has only one positive region. The acceptor atom can form multiple hydrogen bonds depending on its lone pairs.
氢键具有饱和性：每个氢原子通常只能形成一个氢键，因为它只有一个正电区域。受体原子根据其孤对电子数量可以形成多个氢键。
| Type | Bond Energy (kJ mol⁻¹) | Relative Strength |
| Covalent bond | 150–400 | Strong |
| Hydrogen bond | 10–40 | Moderate |
| van der Waals force | 2–20 | Weak |
4. Types of Hydrogen Bonds: Intermolecular and Intramolecular | 氢键的类型：分子间和分子内
Intermolecular hydrogen bonds occur between different molecules, e.g. between water molecules. These are common in liquids and solids.
分子间氢键发生在不同分子之间，例如水分子之间。这类氢键常见于液体和固体。
Intramolecular hydrogen bonds occur within the same molecule, e.g. in 2-nitrophenol or o-hydroxybenzaldehyde. They can affect molecular geometry and stability.
分子内氢键发生在同一分子内部，例如在2-硝基苯酚或邻羟基苯甲醛中。它们会影响分子的几何构型和稳定性。
The presence of intramolecular hydrogen bonds often reduces the ability of the molecule to form intermolecular hydrogen bonds, which can change physical properties like boiling point.
分子内氢键的存在通常会降低分子形成分子间氢键的能力，从而改变沸点等物理性质。
5. Effect on Boiling Points | 对沸点的影响
Hydrogen bonding increases boiling points because extra energy is required to break the hydrogen bonds between molecules during vaporisation.
氢键使沸点升高，因为蒸发时需要额外能量来破坏分子间的氢键。
In the hydrides of groups 15–17, the first members (NH₃, H₂O, HF) have unexpectedly high boiling points compared to heavier analogues due to hydrogen bonding.
在15–17族氢化物中，第一个成员（NH₃、H₂O、HF）由于氢键而比更重的同系物具有异常高的沸点。
For example, H₂O boils at 100 °C, whereas H₂S boils at −60 °C. The difference is mainly due to hydrogen bonding in water.
例如，H₂O在100°C沸腾，而H₂S在−60°C沸腾。该差异主要归因于水中的氢键。
| Compound | Boiling point (°C) | Hydrogen bonding? |
| H₂O | 100 | Yes |
| H₂S | −60 | No |
| NH₃ | −33 | Yes |
| HCl | −85 | No (weak) |
6. Effect on Other Physical Properties | 对其他物理性质的影响
Hydrogen bonds also increase enthalpies of vaporisation and fusion, viscosity, and surface tension. Stronger intermolecular attraction means molecules stick together more firmly.
氢键还会增大汽化焓、熔化焓、黏度及表面张力。分子间吸引力越强，分子结合越紧密。
Viscosity of liquids such as water and glycerol is high due to hydrogen bonding. Similarly, surface tension is high, allowing insects like water striders to walk on water.
水、甘油等液体的高黏度归因于氢键。同样，表面张力很大，使水黾等昆虫能在水面行走。
Melting points of small molecules with hydrogen bonds (e.g., HF, NH₃) are also elevated relative to comparable molecules without H-bonds.
与没有氢键的类似分子相比，具有氢键的小分子（如HF、NH₃）的熔点也会升高。
7. Unusual Properties of Water | 水的反常性质
Water is a unique substance due to extensive hydrogen bonding. Its density maximum occurs at 4 °C, not at its freezing point.
水是独特的物质，因为存在广泛的氢键。其最大密度出现在4°C，而不是冰点。
In ice, water molecules form an open tetrahedral network held by hydrogen bonds; this makes ice less dense than liquid water, so ice floats.
在冰中，水分子通过氢键形成开放的四面体网络，这使冰的密度低于液态水，因此冰能漂浮。
Water has a very high specific heat capacity (4.18 J g⁻¹ °C⁻¹) because heat energy is used to break hydrogen bonds rather than raise kinetic energy.
水具有很高的比热容（4.18 J g⁻¹ °C⁻¹），因为热量用于破坏氢键，而不是增加分子动能。
8. Hydrogen Bonding in Biological Molecules | 生物分子中的氢键
In DNA, the two strands are held together by hydrogen bonds between complementary nitrogenous bases: adenine–thymine (A–T) forms two hydrogen bonds, and guanine–cytosine (G–C) forms three.
在DNA中，两条链通过互补氮碱基之间的氢键连接：腺嘌呤–胸腺嘧啶（A–T）形成两个氢键，鸟嘌呤–胞嘧啶（G–C）形成三个氢键。
These specific base-pairing rules ensure accurate copying of genetic information during replication.
这种特异性碱基配对规则确保复制过程中遗传信息的准确拷贝。
In proteins, hydrogen bonds between N–H and C=O groups in the peptide backbone stabilise secondary structures such as α-helices and β-pleated sheets.
在蛋白质中，肽骨架中N–H和C=O基团之间的氢键稳定了α-螺旋和β-折叠等二级结构。
Hydrogen bonding also occurs between side chains and with water, influencing protein folding and enzyme-substrate interactions.
侧链之间及其与水之间的氢键也影响蛋白质折叠和酶-底物相互作用。
9. Hydrogen Bonding and Solubility | 氢键与溶解性
A substance that can hydrogen bond with a solvent tends to be soluble in that solvent. For example, ethanol is completely miscible with water because both can form H-bonds.
能与溶剂形成氢键的物质易溶于该溶剂。例如，乙醇与水完全混溶，因为两者都能形成氢键。
The rule “like dissolves like” applies: polar solutes dissolve in polar solvents. Hydrogen bonding is a specific strong form of solvent-solute polarity interaction.
“相似相溶”规则适用：极性溶质溶解在极性溶剂中。氢键是溶剂-溶质极性相互作用的特定强形式。
As the hydrocarbon chain length in alcohols increases, the hydrogen-bonding effect diminishes relative to hydrophobic interactions, and solubility in water decreases.
随着醇中烃链长度增加，氢键效应相对于疏水相互作用减弱，水溶性随之降低。
10. Key Points for IB Exams | IB考试要点
Understand the conditions for hydrogen bonding: H bonded to N/O/F and a lone pair on another N/O/F.
理解氢键形成的条件：H与N/O/F成键，并且另一个N/O/F上有孤对电子。
Be able to draw hydrogen bonds using dotted lines (e.g., H–O···H) and explain the effects on boiling point, viscosity, and ice structure.
能够用虚线（如H–O···H）表示氢键，并解释其对沸点、黏度和冰结构的影响。
Recognise that hydrogen bonds are not true chemical bonds; they are intermolecular forces (though intramolecular forms exist).
认识到氢键不是真正的化学键；它们是分子间作用力（虽然也存在分子内形式）。
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