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IR Spectroscopy CHM 411 Suroviec. I. Theory of IR Spectroscopy Frequency of absorbed radiation is the molecular vibration frequency responsible for absorption.

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Presentation on theme: "IR Spectroscopy CHM 411 Suroviec. I. Theory of IR Spectroscopy Frequency of absorbed radiation is the molecular vibration frequency responsible for absorption."— Presentation transcript:

1 IR Spectroscopy CHM 411 Suroviec

2 I. Theory of IR Spectroscopy Frequency of absorbed radiation is the molecular vibration frequency responsible for absorption process – which is directly proportional to wavenumber

3 A. Dipole Moment HCl Charge distribution around molecule is not symmetrical and therefore has a dipole moment

4 B. Rotation and Vibrational Transitions Rotational Vibrational

5 C. Types of Molecular Vibrations The relative positions of atoms in a molecule are not fixed but fluctuate continuously as a consequence of a multitude of different types of vibrational and rotational allowed states around the bonds in the molecule

6 II. Vibrational Modes It is possible to deduce the number and kinds of vibrations in simple diatomic and triatomic molecules

7 A. Motion of a molecule Need to consider: 1.Motion of a molecule through space (translational) 2.Rotation around center of gravity 3.Individual vibrations

8 B. Vibrational Coupling Energy of vibrations and wavelength of absorption is influenced by several factors: 1.Strong coupling between stretching vibrations only occurs when there is an atom in common to the 2 vibrations 2.Interaction between bending vibrations requires a common bond between vibrational groups 3.Coupling between stretching and bending vibrations can occur if stretching bond forms one side of an angle that varies 4.No interaction between groups separated by 2 or more bonds

9 Example: CO 2

10 Example: H 2 O

11 III. Instrumentation A. ATR

12 B. FTIR


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