Vibrational spectroscopy, especially as applied in Infrared (IR) Spectroscopy, is a vital tool in pharmaceutical analysis.
Understanding the vibrational modes of polyatomic molecules is essential to interpreting IR spectra.
For polyatomic molecules, there are various ways the atoms can vibrate, and these vibrational movements are termed as 'modes.'
The number of these vibrational modes (or fundamental modes) for a non-linear molecule is given by the formula 3N-6, where N is the number of atoms in the molecule.
For linear molecules, the number of vibrational modes is 3N-5.
The fundamental modes of vibrations in polyatomic molecules can be categorized into two major types:
1. Stretching Vibrations:
Symmetric Stretching: All atoms in the molecule move towards or away from the center of mass uniformly. This keeps the molecule's shape unchanged.
Asymmetric Stretching: Different atoms in the molecule move towards or away from the center of mass at different rates. This changes the molecule's shape.
2. Bending Vibrations (or Deformation Vibrations):
Scissoring (or Symmetric Bending): Two atoms move towards or away from each other while staying in the plane of the molecule.
Rocking: One part of the molecule rocks in the plane of the molecule.
Wagging: One part of the molecule wags in and out of the plane of the molecule.
Twisting (or Torsional Vibration): One part of the molecule moves in a way that twists it out of the plane of the molecule.