Fundamental Modes of Vibrations in Polyatomic Molecules

Fundamental Modes of Vibrations in Polyatomic Molecules

Fundamental Modes of Vibrations in Polyatomic Molecules explain bond stretching and bending in molecular analysis.

Molecular Vibrations

• In polyatomic molecules, atoms are not stationary but vibrate around their equilibrium positions.
• These vibrations can be described as periodic motions and are quantized, meaning they occur at specific energy levels.
• The two main types of molecular vibrations are:

1. Stretching Vibrations:

   • Involve changes in the bond length between atoms.
     1. Symmetric Stretching: Bonds expand and contract simultaneously in the same direction.
     2. Asymmetric Stretching: One bond shortens while another lengthens.

2. Bending Vibrations:

   • Involve changes in the bond angle between atoms.
     1. Scissoring: Two atoms move toward and away from each other.
     2. Rocking: Atoms move in the same direction, rocking back and forth.
     3. Wagging: Atoms move up and down out of the plane.
     4. Twisting: One atom moves up while another moves down, causing a twist.

Degrees of Freedom

• Translational Degrees: Movement of the entire molecule in space (three degrees).
• Rotational Degrees: Rotation of the molecule around its axes (three degrees for nonlinear, two for linear molecules).
• Vibrational Degrees: Calculated using the formula:
  • Nonlinear Molecules: 3N−63N – 63N−6
  • Linear Molecules: 3N−53N – 53N−5
  • Where NNN is the number of atoms in the molecule.

Click Here to Watch the Best Pharma Videos