Covalent bonds are not rigid. They behave like springs, constantly vibrating by stretching (the bond length changes) and bending (the bond angle changes).
Each bond has a natural frequency of vibration. When infrared radiation of exactly that frequency passes through the sample, the bond absorbs the energy and vibrates more. Radiation of other frequencies passes straight through. This is why each type of bond absorbs at a characteristic wavenumber.The frequency at which a bond vibrates depends on:
Bond strength: stronger bonds vibrate at higher frequencies, so they absorb at higher wavenumbers. C=O (1680 to 1750 cm−1) absorbs at a higher wavenumber than C-O (1000 to 1300 cm−1).
Mass of the atoms: bonds to light atoms vibrate at higher frequencies. Bonds to hydrogen (O-H, N-H, C-H) absorb at high wavenumbers, from about 2500 to 3550 cm−1.A vibration only absorbs IR radiation if it changes the dipole of the molecule. Bonds between different atoms are polar, so they absorb. Molecules made of two identical atoms, such as N2 and O2, have no dipole to change, so they do not absorb IR radiation.Worked example: place C-O, C=O and C≡N in order of increasing absorption wavenumber.
All three bonds join atoms of similar mass, so bond strength decides. Triple bonds are stronger than double bonds, which are stronger than single bonds. Order: C-O (1000 to 1300 cm−1) < C=O (1680 to 1750 cm−1) < C≡N (2220 to 2260 cm−1).The O-H absorption of alcohols and acids is broad because hydrogen bonding gives a range of slightly different O-H bond strengths.
Each bond has a natural frequency of vibration. When infrared radiation of exactly that frequency passes through the sample, the bond absorbs the energy and vibrates more. Radiation of other frequencies passes straight through. This is why each type of bond absorbs at a characteristic wavenumber.The frequency at which a bond vibrates depends on:Bond strength: stronger bonds vibrate at higher frequencies, so they absorb at higher wavenumbers. C=O (1680 to 1750 cm−1) absorbs at a higher wavenumber than C-O (1000 to 1300 cm−1).
Mass of the atoms: bonds to light atoms vibrate at higher frequencies. Bonds to hydrogen (O-H, N-H, C-H) absorb at high wavenumbers, from about 2500 to 3550 cm−1.A vibration only absorbs IR radiation if it changes the dipole of the molecule. Bonds between different atoms are polar, so they absorb. Molecules made of two identical atoms, such as N2 and O2, have no dipole to change, so they do not absorb IR radiation.Worked example: place C-O, C=O and C≡N in order of increasing absorption wavenumber.
All three bonds join atoms of similar mass, so bond strength decides. Triple bonds are stronger than double bonds, which are stronger than single bonds. Order: C-O (1000 to 1300 cm−1) < C=O (1680 to 1750 cm−1) < C≡N (2220 to 2260 cm−1).The O-H absorption of alcohols and acids is broad because hydrogen bonding gives a range of slightly different O-H bond strengths.