Types of Vibrations in Molecules

For the CO2 molecule, we can draw a symmetric stretch, an asymmetric stretch, and a bending vibration, as shown  

The CO2 molecule on the left is undergoing a symmetric stretch, the one in the middle an asymmetric stretch and the one on the right an in-plane bend. The symmetric 

For simple molecules, this approach predicts the number of fundamental vibrations that exist. Use of the dipole moment rule indicates which vibrations are IR-active, but the IR spectrum of a molecule rarely shows the number of absorption bands calculated. Fewer peaks than expected are seen due to IR-inactive vibrations, degenerate vibrations, and very weak vibrations. More often, additional peaks are seen in the spectmm due to overtones and other bands. 

The requirements for the absorption of IR radiation by molecules can be summarized as follows  

The natural frequency of vibration of the molecule must equal the frequency of the incident radiation. 

The CO2 molecule on the left is undergoing a symmetric stretch, the one in the middle an asymmetric stretch, and the one on the right an in-plane bend. The symmetric stretching vibration does not change the dipole moment of the molecule. This vibrational mode does not absorb IR radiation—it is said to be IR inactive. However, the other two modes of vibration do change the dipole moment—they are IR active. The asymmetric stretching frequency occurs at 2350 cm and the bending vibration occurs at 666 cm .  

For diatomic and triatomic molecules, it is possible to work out the number and kind of vibrations that occur. To locate a point in 3D space requires three coordinates. To locate a molecule containing N atoms in three dimensions, 3N coordinates are required. The molecule is said to have 3N degrees of freedom. To describe the motion of such a molecule, translational, rotational, and vibrational motions must be considered. Three coordinates or degrees of freedom are required to describe translational motion and three degrees of freedom are required to describe rotational motion about the molecule s center of gravity. This leaves 3N - 6 degrees of freedom to describe vibrational motion. There are 37V - 6 possible normal modes of vibration in a molecule of N atoms. For example, the water molecule contains 3 atoms, so it has 3 x 3 = 9 degrees of freedom and (3 x 3) - 6 = 3 

The oop bending mode and the in-plane bending mode already shown both occur at 666 cm. They are said to be degenerate. Both are IR active, but only one absorption band is seen since they both occur at the same frequency. 

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Infrared spectroscopy measures the light absorbed by different types of vibrations in molecules. 

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