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Understanding IR Spectroscopy: Theory, Vibrational Modes, and Instrumentation

Learn the theory behind IR spectroscopy, including dipole moments, rotational and vibrational transitions, and types of molecular vibrations. Discover how molecules move and vibrate, and explore the influence of vibrational coupling on absorption. Dive into the instrumentation used, including ATR and FTIR techniques. Enhance your knowledge of infrared spectroscopy with practical examples like CO2 and H2O.

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Understanding IR Spectroscopy: Theory, Vibrational Modes, and Instrumentation

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  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: • Motion of a molecule through space (translational) • Rotation around center of gravity • Individual vibrations

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

  9. Example: CO2

  10. Example: H2O

  11. III. Instrumentation A. ATR

  12. B. FTIR

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