Formation of hydrogen bonds ⑴ Between molecules of the same kind Now take HF as an example to illustrate the formation of hydrogen bonds.

In the HF molecule, due to the large electronegativity (4.0) of F, the electron pair is strongly biased towards the F atom, while there is only one electron outside the nucleus of the H atom, and its electron cloud shifts towards the F atom, making it

Almost in a proton state.

This partially positively charged hydrogen atom with a small radius and no inner electrons makes it possible for the F atom with a lone electron pair and a partially negatively charged F atom in another nearby HF molecule to get close enough to it, thereby generating electrostatic attraction.

This electrostatic attraction force is the so-called hydrogen bond.

For example, between HF and HF: ⑵ Between different kinds of molecules Not only can hydrogen bonds exist between the same kind of molecules, but also between some different kinds of molecules.

For example, between NH3 and H2O: Conditions for hydrogen bond formation ⑴ A hydrogen atom that forms a strong polar bond with a highly electronegative atom A ⑵ Small radius, large electronegativity, lone electron pair, and partial negative

The nature of the hydrogen bond between the charged atom B (F, O, N): the hydrogen nucleus on the strong polar bond (A-H), and the highly electronegative atom B containing a lone electron pair and carrying a partial negative charge

electrostatic attraction between.

} ⑶ General formula representing hydrogen bonding. If hydrogen bonding is written as a general formula, it can be represented by X-H...Y①.

In the formula, X and Y represent non-metal atoms with large electronegativity and small atomic radius such as F, O, and N.

X and Y can be two identical elements or two different elements.

⑷ Understanding of hydrogen bonding Although hydrogen bonding is very common and research on it is gradually deepening, people still have two different understandings of the definition of hydrogen bonding.

The first one calls the entire structure of X-H...Y a hydrogen bond, so the bond length of the hydrogen bond refers to the distance between X and Y. For example, the bond length of F-H...F is 255pm.

The second type calls H...Y a hydrogen bond, so the distance between H...F is 163pm as the bond length of the hydrogen bond.

We should pay attention to this difference when selecting hydrogen bond length data.

However, the understanding of hydrogen bond energy is consistent, and they both refer to the energy required to decompose X-H...Y-H into HX and HY.

2.

Strength of hydrogen bonds The strength of hydrogen bonds - bond strength can also be expressed in terms of bond energy.

Roughly speaking, hydrogen bonding energy refers to the energy required to break apart the H...Y bonds per unit amount of substance.

The bond energy of a hydrogen bond is generally below 42kJ·mol-1, which is much smaller than the bond energy of a hydrogen bond, and closer to the intermolecular force.

For example, the bond energies of hydrogen bonds and hydrogen bonds in water molecules are different.

Moreover, the activation energy required for the formation and destruction of hydrogen bonds is also small, and the spatial conditions for their formation are relatively easy to occur. Therefore, hydrogen bonds can be continuously formed and broken when substances are constantly moving.

3.

Intramolecular hydrogen bonds Certain molecules, such as HNO3 and o-nitrophenol molecules, can form intramolecular hydrogen bonds.

Due to the restriction of the ring structure, intramolecular hydrogen bonds often cannot be in the same straight line.

As shown in the figure 4.

The impact of hydrogen bond formation on material properties Hydrogen bonds are usually formed when a material is in a liquid state, but sometimes they can continue to exist in some crystalline or even gaseous materials after formation.

For example, hydrogen bonds exist in gaseous, liquid and solid HF.

There are many substances that can form hydrogen bonds, such as water, hydrates, ammonia compounds, inorganic acids and certain organic compounds.

The existence of hydrogen bonds affects certain properties of matter.

(1) When a substance with hydrogen bonds between molecules at melting point and boiling point melts or vaporizes, in addition to overcoming pure intermolecular forces, the temperature must also be raised and an additional amount of energy must be supplied to destroy the hydrogen bonds between molecules, so these

The melting point and boiling point of the substance are higher than the melting point and boiling point of the same series of hydrides.

Hydrogen bonds are formed within the molecule, and the melting and boiling points are often lowered.

For example, the melting point of ortho-nitrophenol (45°C) with intramolecular hydrogen bonds is lower than the meta-melting point (96°C) and para-position melting point (114°C) with intermolecular hydrogen bonds.

(2) Solubility In polar solvents, if hydrogen bonds can be formed between solute molecules and solvent molecules, the solubility of the solute increases.

The solubility of HF and HN3 in water is relatively large for this reason.

(3) Viscosity Liquids with hydrogen bonds between molecules generally have higher viscosity.

For example, polyhydroxy compounds such as glycerol, phosphoric acid, and concentrated sulfuric acid can form numerous hydrogen bonds between molecules, and these substances are usually viscous liquids.

(4) If hydrogen bonds are formed between density liquid molecules, association phenomena may occur, such as liquid HF. Under normal conditions, in addition to normal simple HF molecules, there are also complex molecules (HF) linked together by hydrogen bonds.

n.

nHF(HF)n where n can be 2, 3, 4….