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Infra-Red (IR) Spectroscopy Applied Chemistry Course: CHY101 or Vibrational spectroscopy.

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Presentation on theme: "Infra-Red (IR) Spectroscopy Applied Chemistry Course: CHY101 or Vibrational spectroscopy."— Presentation transcript:

1 Infra-Red (IR) Spectroscopy Applied Chemistry Course: CHY101 or Vibrational spectroscopy

2 The interaction of radiation with matter

3 Fig Source: bend

4 Stretching and Bending Types of Molecular Vibrations: Vibrations fall into the basic categories of stretching and bending.  Stretching vibration involves a continuous change in the inter-atomic distance along the axis of the bond between two atoms.  Bending vibrations are characterized by a change in the angle between two bonds and are of four types: scissoring, rocking, wagging, and twisting.

5 Stretching Vibration

6 Bending Vibration

7 H 2 O: Stretching and Bending Vibrations  Bending is easier than stretching -- happens at lower energy (lower wavenumber)  Bond Order is reflected in ordering -- triple > double > single (energy) with single bonds easier than double easier than triple bonds  Heavier atoms move slower than lighter ones Fig Source: bend

8

9  Vibrations which do not change the dipole moment are Infrared Inactive (homonuclear diatomics). Selection Rules Selection rule:  A molecule will absorb infrared radiation if the change in vibrational states is associated with a change in the dipole moment (  ) of the molecule. Dipole moment is greater when electronegativity difference between the atoms in a bond is greater. Some electronegativity values are: H 2.2; C 2.55; N 3.04; O 3.44; F 3.98; P 2.19; S 2.58; Cl 3.16 Homonuclear diatomic molecules (O 2, N 2, H 2, Cl 2 ) – IR Inactive Heteronuclear diatomic molecules (HF, HCl) – IR Active

10 Fig. The asymmetric stretching vibration of the CO2 molecule, showing the fluctuation in the dipole moment. Asymmetric Stretching Vibration of the CO 2

11 Fig. The bending motion of the carbon dioxide molecule and its associated dipole fluctuation. Bending motion of the CO 2


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