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What is a hydrogen bridge bond?

Hydrogen bridge bonding is a special type of dipole-dipole interaction, belonging to the Van der Waals forces, in which a hydrogen atom joins two or more molecules without covalently bonding. We do not speak of an electron compartment, but of a mainly electrostatic phenomenon.

As its name suggests, hydrogen acts as a bridge, so it must be placed between two atoms. Hydrogen is linked (HX) to a very electronegative atom (such as N, O and F), and it approaches another electronegative atom but of a neighboring molecule. This results in the formation of the hydrogen bridge X ··· HX.

Note that in the representation X ··· HX, the dots symbolize the hydrogen bond, while the dash represents the covalent bond between H and X. That said, let’s look at the hydrogen bond between two water molecules, where X is the oxygen atom: H 2 O ··· H-OH (lower image).

In the image above we see that seven water molecules are held together thanks to their hydrogen bonds, represented by blue lines. A hydrogen bridge alone is not very strong, but when there are billions of them, they give matter abnormal and unexpected properties.

Characteristics of the hydrogen bridge

Compositional

The hydrogen bond should really be represented as X ‘··· HX, where X is what is known as the hydrogen bond donor, because it is covalently bonded to hydrogen (gives it).

Meanwhile, X ‘is the acceptor of the hydrogen bond, present in a neighboring molecule (receives it). Thus, we have a donor atom (X), a hydrogen (H) and an acceptor atom (X ‘) making up the hydrogen bridge (X’ ··· HX).

Geometric

When we think of a bridge, flat or arched surfaces come to mind. Since it is assumed that the bonds do not bend, we will then have two distances: X ··· H and HX, which make up the hydrogen bridge X ··· HX.

Between these two distances there is an angle, which is often 180º; that is, the three atoms of our bridge rest on the same horizontal (or vertical) line.

When the angle is different from 180º, the X ··· HX bridge is no longer straight or linear, but acquires other geometries.

On the other hand, the distances in the hydrogen bonds are not identical. The distance HX is shorter than X ··· H, which can be seen in the image of the water molecules. For example, the distance HX is usually 110 pm (1 · 10 -12 m), while the other distance X ··· H is 160 pm onwards.

Associative

A special feature of the hydrogen bond is that it allows molecules to associate much more closely with each other. They don’t go around ignoring each other. Therefore, it establishes a momentary order in the sinuses of the liquids; and in the case of solids, they contribute to the definition of their crystals.

Where we see a hydrogen bridge we can think in association and, therefore, in a certain order (although dynamic and changing) on ​​molecular scales.

Energetic

Breaking a hydrogen bridge is not very difficult. Water molecules, for example, are breaking and creating them all the time while they are mobilized. But breaking many of them at the same time would involve disassociating an endless number of molecules. We are talking about supplying an energy such that it breaks moles of said hydrogen bonds (6.02 · 10 23 X ··· HX).

Thus, the strength of the hydrogen bond varies depending on the identities and nature of the molecules. For example, the force of the hydrogen bond O ··· HO between water and alcohol is 5 kcal / mol: it takes 5 kcal of energy to break one mole of that hydrogen bridge in question.

Examples of hydrogen bonds

Water

Hydrogen bonds between water molecules were discussed at first, but the effect they have on their properties was not mentioned. Thanks to them, water boils at 100 ºC, leaving far behind the boiling points of related molecules such as H 2 S, which boils at -60 ºC; or H 2 Se, which boils at -41.25 ºC.

This abysmal difference is due to the hydrogen bonds of water, which also define other of its anomalous properties, such as its enormous specific heat, ice crystals, its dielectric constant, etc.

Ethanol

Hydrogen bridges in an ethanol crystal

Now let’s look at another hydrogen bridge: the one between ethanol molecules, CH 3 CH 2 OH (above). Note how the CH 3 CH 2 OH molecules are arranged in such a way that their hydrogen bonds are established CH 3 CH 2 HO ··· HOCH 2 CH 3 (dotted lines).

However, the molecules are too ordered to assume that we are talking about liquid ethanol, but instead make up a crystal (solid ethanol).

The hydrogen bridge described for ethanol is similar to that of other alcohols, with the difference that their carbon skeletons can hinder the efficiency of said bridges.

Acetic acid

Hydrogen bridges between two acetic acid molecules. Source: Jynto, CC0, via Wikimedia Commons

Acetic acid, CH 3 COOH, is capable of establishing two hydrogen bonds at the same time that unite two molecules at the same time. Because they are two molecules linked by hydrogen bonds, we speak of a dimer.

Note that one of these hydrogen bonds is C = O ··· HO and the other OH ··· O = C. Acetic acid has the peculiarity that it exists in the vapor phase as this dimer.

Cellulose

Hydrogen bridges between various cellulose chains. Source: Laghi.l, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons

Let us now look at more diverse and multiple hydrogen bonds. Cellulose, a natural polymer, consists of chains made up of various β-glucose units.

Each chain is kept attached to another thanks to many hydrogen bonds (upper image), which reinforce the cohesion between the chains.

DNA

Hydrogen bridges between the nitrogenous bases of DNA . Source: CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

So far we have seen the associative effect that hydrogen bonds have to impose order between molecules. But what about a macromolecule? In a macromolecule, such as DNA, we find internal or intramolecular hydrogen bonds between its nitrogenous bases thymine, adenine, guanine, and cytosine (above).

The intramolecular hydrogen bonds between these nitrogenous base pairs make the DNA molecule acquire a double helix structure, which is ideal for its replication. If these hydrogen bonds are broken by heating, the double helix will end up splitting into two individual segments or bands.

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