Hydrogen bonds are weak bonds formed between hydrogen and electronegative atoms. They can be intermolecular or intramolecular and are responsible for the complex structure of proteins. The most common example is in water, where it is responsible for the high boiling point. The length and strength of the bond vary with temperature, pressure, and environment.
A hydrogen bond is a relatively weak bond that hydrogen atoms form with electronegative nitrogen, oxygen, or fluorine atoms. Hydrogen bonds are weaker than ionic, covalent, and metallic bonds, but are still slightly strong on their own, typically ranging in energy from 5 to 30 kJ/mole. In contrast, weak covalent bonds have an energy of about 155 kJ/mol. Hydrogen bonding can be an intermolecular (between molecules) or intramolecular (between different parts of a molecule) bond. This type of binding can occur in both organic molecules, such as DNA, and inorganic molecules, such as water. Hydrogen bonding is partially responsible for the complex secondary and tertiary structure of proteins.
The most ubiquitous and simplest example of hydrogen bonding is in water, where each water molecule is bonded to four adjacent water molecules through hydrogen bonding. The oxygen atom in each water molecule has two lone electrons to offer, which are readily bonded by the hydrogen atoms in other water molecules. Furthermore, the two hydrogen atoms attached to each oxygen bond to the oxygen molecules in the adjacent water molecules. This intermolecular bond is responsible for the relatively high boiling point of water. Water has an extremely high boiling point compared to materials made up of similarly sized molecules. If these bonds didn’t exist, water would boil at a temperature similar to carbon dioxide (which boils at -78°C or -108.4°F), and life as we know it would be impossible.
A hydrogen bond consists of a hydrogen bond acceptor, the target atom, and a hydrogen bond donor, the hydrogen atom itself. Sometimes, in molecules such as chloroform (CHCl3), carbon can be involved in hydrogen bonding, especially when surrounded by electronegative atoms such as chlorine. A hydrogen bond is unusual and often called an electrostatic dipole-dipole interaction (a weak intermolecular interaction), grouping it with more transient bonds caused by momentary quantum fluctuations in electric charge, but it also has many characteristics of much stronger covalent bonds, where clouds of electrons actually overlap directly. These features include directionality, strength, shorter-than-typical production interatomic distances in transient van der Waals interactions, and a small number of interaction partners, which is diagnostic of stronger bonds.
The length of the hydrogen bonds varies with the strength of the bond, temperature and pressure. Bond strength also depends on a number of factors, including temperature, angle of bond, pressure, and environment. In water, the typical length of a hydrogen bond is 1.97 (197 pm).
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