Molecular Vibrations
The molecular vibrations help determine if a gas can absorb infrared radiation. A molecular vibration will only occur in a molecule if infrared radiation of a particular frequency or wavelength is absorbed. A vibration in a molecule is any change in shape of the molecule. A molecule can only absorb infrared radiation when it experiences a change in dipole moment during a vibration. This change can only occur in two type of vibrational modes: stretching and bending. There are two types of stretches, symmetric and asymmetric. A molecules vibrational motions must be known in order to determine if the gas is a greenhouse gas.

A symmetric stretch occurs when atoms move in opposite directions as the bonds shrink or stretch. This is symmetric with respect to the central atom. Because of this, there is no change in the dipole moment and this movement cannot result from the absorption of infrared radiation. The picture below shows the symmetric stretch of water. Notice how the hydrogens are moving in the same amount.

An asymmetric stretch occurs when some atoms move in the same direction while others move in the opposite direction. This is not symmetric with respect to the central atom so there isn't a change in dipole moment. An asymmetric stretch will result from the absorption of infrared radiation. The asymmetric stretch in the image below is showing how the hydrogens bonds are both increasing and decreasing, making it asymmetric.

A bend occurs when different atoms move downward and upward away from the central atom. This motion causes an imbalance in electronegativity and change in polarity. This motion can result from the absorption of infrared radiation. The bending motion is shown in the picture below alongside the other vibrational modes of water.

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