INTRO, IR Conti.....

You may have come to think of a molecule as having rigid bond lengths and bond angles, this is not the actual case, since bond lengths and angles represent the average positions about which atoms vibrate. A molecule is not a rigid assemblage of atoms. A molecule can be said to resemble a system of balls of varying masses corresponding to atoms and springs of varying strengths (force constants ) corresponding to the chemical bonds of a molecule.

There are two types of molecular vibrations, stretching and bending. A stretching vibration is a rhythmical movement along the bond axis such that the inter-atomic distance is increasing or decreasing. A bending vibration may consist of a change in bond angle between bonds with a common atom or the movement of a group of atoms with respect to the remainder of the molecule without movement of the atom in the group with respect to one another.

Only those vibrations that result in a rhythmical change in the dipole moment of the molecule are observed in the IR. Various stretching and bending vibration of a molecule occur at certain quantized frequencies. When IR light of that same frequency is incident on the molecule, energy is absorbed and the amplitude of that vibration is increased. The frequency of the vibration remains unchanged. When the molecule reverts from the excited state to ground stale the absorbed energy released as heat.

Each atom has three degrees of freedom, corresponding to motions along any of the three cartesian coordinate axes (x, y, z). A polyatomic molecule of n atoms has 3n total degrees of freedom. However, 3 degrees of freedom are required to describe translation, the motion of the entire molecule through space. Additionally, 3 degrees of freedom correspond to rotation of the entire molecule. Therefore, the remaining 3n – 6 degrees of freedom are true, fundamental vibrations for nonlinear molecules. Linear molecules possess 3n – 5 fundamental vibrational modes because only 2 degrees of freedom are sufficient to describe rotation.