Vibrational Analysis (IR Spectroscopy from DFT)

 

Vibrational Analysis (IR Spectroscopy from DFT)

 “After optimizing a molecule using DFT, how do we know the structure is correct and stable?”

Then say:

“We verify it using vibrational analysis and compare with IR spectra.”

What is Vibrational Analysis?

Definition:

Vibrational analysis studies how atoms in a molecule move (vibrate) around their equilibrium positions.

Simple Explanation:

Atoms are not fixed — they are always vibrating (stretching and bending).

Daily Life Analogy

·         Atoms = balls

·         Bonds = springs

Like a spring system, atoms move back and forth.

Types of Vibrations

1. Stretching Vibrations

·         Change in bond length

✔ Types:

·         Symmetric stretching (In symmetric stretching, two or more bonds increase or decrease in length at the same time and in the same direction. In CO₂, both C=O bonds expand or contract simultaneously)

·         Asymmetric stretching (In asymmetric stretching, one bond increases in length while the other decreases at the same time. In CO₂, one C=O bond stretches while the other shrinks)

2. Bending Vibrations

·         Change in bond angle

 Types:

·         Scissoring (A bending vibration where two atoms move toward and away from each other, like the opening and closing of scissors.)

·         Rocking (bending motion where atoms move in the same direction, maintaining the bond angle but shifting position side-to-side.)

·         Wagging (bending vibration where atoms move up and down together out of the molecular plane. Both atoms move in the same direction (above and below plane))

·         Twisting (bending motion where one atom moves up while the other moves down, out of the molecular plane.)

Stretching = change in length
Bending = change in angle