Noncovalent Bonds

Hydrogen (H) bonds are part of a class of intermolecular interactions known as noncovalent bonds. A noncovalent bond is any relatively weak intermolecular attraction that does not involve the sharing of electrons. Noncovalent bonds include hydrogen bonds, van der Waals forces, ionic bonds, and hydrophobic bonds.

In a water molecule (H₂O), which contains a bond between two H atoms and one oxygen (O) atom, the van der Waals attraction, which is a weak, noncovalent attractive force between two distinct molecules resulting from instantaneous dipoles, causes the molecule to bend such that the O atom is on one side, the H atoms are on the other, and the two ${\rm {H{-}O}}$ bonds are at an angle of 104.5°. Since oxygen is highly electronegative, the oxygen atom draws the electrons toward its nucleus and away from the hydrogen nucleus, and ${\rm {H{-}O}}$ forms a polar covalent bond. With oxygen on one "side" of the molecule and hydrogen on the other, the H₂O molecule has a net dipole moment, which is a vector quantity that defines the extent of the charge on either side of a polar covalent bond, with the direction that points from the positive side of the bond toward the negative side, and the entire molecule is polar.

In water the partially positively charged hydrogen poles are attracted to the partially negatively charged oxygen poles of other water molecules, and as water molecules move closer together, they arrange themselves so as to maximize the intermolecular ${\rm {H{-}O}}$ interactions. This intermolecular attraction is an example of a hydrogen bond. A hydrogen bond is a weak bond that results from an attraction between a positively charged hydrogen in one molecule and a negatively charged atom in another. Hydrogen bonds can form with any molecule that contains a positively charged dipole. Since both nitrogen and oxygen are more electronegative than hydrogen, all ${\rm {O{-}H}}$ and ${\rm {N{-}H}}$ bonds are polar, and H atoms in these bonds can form hydrogen bonds. ${\rm {C{-}H}}$ bonds, in contrast, are nonpolar, and the H atoms do not form hydrogen bonds.

Hydrophobic bonds are another type of noncovalent bond. To be hydrophobic is having a weak or no affinity to water. A hydrophobic bond is a force that pushes nonpolar molecules together in an aqueous environment. When a nonpolar compound, such as an oil, is mixed with water, the oil molecules aggregate. This is not because the nonpolar molecules are attracted to each other; rather, it is because the nonpolar molecules are repelled by the polar water molecules. Although hydrogen bonds and the other noncovalent bonds are weaker than covalent bonds, they are responsible for many physical properties in water and other biological molecules. It is because of hydrogen bonds, for example, that water exists in liquid form on Earth. Nonpolar molecules of similar molecular weight have much lower freezing and boiling points, but the attractive force of hydrogen bonds holds water molecules together at higher temperatures. Hydrogen bonds also play a critical role in the structures of large biological molecules, such as DNA and proteins. Without the stabilizing force of hydrogen bonds, molecules of DNA, RNA, and proteins would fall apart and be unable to function in cells.