If one of the bonded atoms (m 1 or m 2) is a hydrogen (atomic mass =1), the mass ratio in the equation is roughly unity, but for two heavier atoms it is much smaller. The infrared stretching frequencies of these groups vary in the same order, ranging from 1100 cm -1 for C-N, to 1660 cm -1 for C=N, to 2220 cm -1 for C≡N.Īpproximate Infrared Stretching Frequencies For example, a C=N double bond is about twice as strong as a C-N single bond, and the C≡N triple bond is similarly stronger than the double bond. In the analogy of a spring, it corresponds to the spring's stiffness. The force constant (f) is proportional to the strength of the covalent bond linking m 1 and m 2. The equation on the right describes the major factors that influence the stretching frequency of a covalent bond between two atoms of mass m 1 and m 2 respectively. It requires more energy to stretch (or compress) a bond than to bend it, and as might be expected, the energy or frequency that characterizes the stretching vibration of a given bond is proportional to the bond dissociation energy. Transitions between vibrational energy states may be induced by absorption of infrared radiation, having photons of the appropriate energy. At ordinary temperatures these bonds vibrate in a variety of ways, and the vibrational energies of molecules may be assigned to quantum levels in the same manner as are their electronic states.

We have noted that the covalent bonds of molecules are not rigid, but are more like stiff springs that can be stretched and bent. Infrared Spectrometry The Nature of Vibrational Spectroscopy

Aldehyde ir spectrum