What Causes Hydrogen Bonding?

Hydrogen bonding occurs between a hydrogen atom and an electronegative atom (e.g., oxygen, fluorine, chlorine). The bond is weaker than an ionic bond or a covalent bond, but stronger than van der Waals forces (5 to 30 kJ/mol). A hydrogen bond is classified as a type of weak chemical bond.

Why Hydrogen Bonds Form

The reason hydrogen bonding occurs is because the electron is not shared evenly between a hydrogen atom and a negatively charged atom. Hydrogen in a bond still only has one electron, while it takes two electrons for a stable electron pair. The result is that the hydrogen atom carries a weak positive charge, so it remains attracted to atoms that still carry a negative charge. For this reason, hydrogen bonding does not occur in molecules with nonpolar covalent bonds. Any compound with polar covalent bonds has the potential to form hydrogen bonds.

Examples of Hydrogen Bonds

Hydrogen bonds can form within a molecule or between atoms in different molecules. Although an organic molecule is not required for hydrogen bonding, the phenomenon is extremely important in biological systems. Examples of hydrogen bonding include:

• between two water molecules
• holding two strands of DNA together to form a double helix
• strengthening polymers (e.g., repeating unit that helps crystallize nylon)
• forming secondary structures in proteins, such as alpha helix and beta pleated sheet
• between fibers in fabric, which can result in wrinkle formation
• between an antigen and an antibody
• between an enzyme and a substrate
• binding of transcription factors to DNA

Hydrogen Bonding and Water

Hydrogen bonds account for some important qualities of water. Even though a hydrogen bond is only 5% as strong as a covalent bond, it's enough to stabilize water molecules.

• Hydrogen bonding causes water to remain liquid over a wide temperature range.
• Because it takes extra energy to break hydrogen bonds, water has an unusually high heat of vaporization. Water has a much higher boiling point than other hydrides.

There are many important consequences of the effects of hydrogen bonding between water molecules:

• Hydrogen bonding makes ice less dense than liquid water, so ice floats on water.
• The effect of hydrogen bonding on heat of vaporization helps make perspiration an effective means of lowering temperature for animals.
• The effect on heat capacity means water protects against extreme temperature shifts near large bodies of water or humid environments. Water helps regulate temperature on a global scale.

Strength of Hydrogen Bonds

Hydrogen bonding is most significant between hydrogen and highly electronegative atoms. The length of the chemical bond depends upon its strength, pressure, and temperature. The bond angle depends on the specific chemical species involved in the bond. The strength of hydrogen bonds ranges from very weak (1–2 kJ mol−1) to very strong (161.5 kJ mol−1). Some example enthalpies in ​vapor are:

F−H…:F (161.5 kJ/mol or 38.6 kcal/mol)
O−H…:N (29 kJ/mol or 6.9 kcal/mol)
O−H…:O (21 kJ/mol or 5.0 kcal/mol)
N−H…:N (13 kJ/mol or 3.1 kcal/mol)
N−H…:O (8 kJ/mol or 1.9 kcal/mol)
HO−H…:OH₃⁺ (18 kJ/mol or 4.3 kcal/mol)

_References

_{Larson, J. W.; McMahon, T. B. (1984). "Gas-phase bihalide and pseudobihalide ions. An ion cyclotron resonance determination of hydrogen bond energies in XHY- species (X, Y = F, Cl, Br, CN)". Inorganic Chemistry 23 (14): 2029–2033.}

_{Emsley, J. (1980). "Very Strong Hydrogen Bonds". Chemical Society Reviews 9 (1): 91–124.
Omer Markovitch and Noam Agmon (2007). "Structure and energetics of the hydronium hydration shells". J. Phys. Chem. A 111 (12): 2253–2256.}