Infrared vibrations

The vibrational modes of diatomic or triatomic molecules might seem easy to classify, but this is not necessarily true for bigger molecules that experience different kinds of vibrations on all their bonds. In general, the vibrational modes are distinguished between stretching, when the length of the bond variates along its axis, and bending, which occurs when the angle between two bonds variates. A further distinction occurs when determining if the vibration is symmetrical or asymmetrical. For example, in symmetrical stretchings, the two bonds elongate and contract at the same time, while in an asymmetrical one, one bond contracts while the other elongate. Bending vibrations can be divided into wagging, twisting, rocking and scissoring, depending on the way the angle varies between the two bonds. This doesn’t necessarily mean that every molecule will show all of these vibrations, as depending on the structure of the molecule only some of these may happen.