Infrared Spectroscopy-Principles, Instrumentation and Application
- 1. IR SPECTROSCOPY SURIYA MARIMUTHU Ph.D. Scholar, C/O Prof. Chia-Hsiang Chen, KMU-Taiwan 19 NOV 2023 1 ADVANCED ANALYTICAL CHEMISTRY
- 2. CONTENT This is for Your’s time… IR Spectroscopy Dipole Moment Changes during Vibrations and Rotations Types of Molecular Vibrations Potential-energy diagram Selection Rules Vibrational Modes IR Instrumentation Summary 2
- 3. What is the purpose of using IR? What is Spectroscopy ? What is Electromagnetic radiation ? This is for Your’s time… 3 What is the principle of IR spectroscopy?
- 4. The infrared (IR) region of the spectrum encompasses radiation with wavenumbers ranging from about 12,800 to 10 cm21 or wavelengths from 0.78 to 1000 µm. The IR spectrum is usually subdivided into three regions, NEAR IR MIDDLE IR FAR IR IR SPECTROSCOPY 4
- 5. Dipole Moment Changes during Vibrations and Rotations Heteronuclear diatomic molecules are IR Active ? Homonuclear species are IR Inactive ? Types of Molecular Vibrations Two types of Vibrations Stretching Vibration Bending Vibration The relative positions of atoms in a molecule are not fixed but instead fluctuate continuously Why? 5
- 6. Stretching Vibrations Symmetric Vibrations Asymmetric Vibrations Bending Vibrations In plane rocking In plane scissoring Out of plane wagging Out of plane twisting 6
- 7. Potential-energy diagram F = - ky Hooke’s law 7
- 8. Potential Energy of a Harmonic Oscillator ⅆ𝐸 = 𝐾𝑦 ⅆ𝑦 0 𝐸 ⅆ𝐸 = 0 𝑦 𝑦 ⅆ𝑦 ⅆ𝐸 = −𝐹 ⅆ𝑦 𝐸 = 1 2 𝑘𝑦2 8
- 9. Vibrational Frequency 𝐹 = 𝑚𝑎 𝑎 = ⅆ2 𝑦 ⅆ𝑡2 𝑚 ⅆ2 𝑦 ⅆ𝑡2 = −𝑘𝑦 𝑦 = 𝐴 cos 2𝛱𝜈𝑚𝑡 ⅆ2 𝑦 ⅆ𝑡2 = −4𝜋2𝜈𝑚 2 𝐴 cos 2𝛱𝜈𝑚𝑡 1 2 3 4 5 𝐴 cos 2𝛱𝜈𝑚𝑡 = 4𝜋2𝜈𝑚 2 𝑚 𝑘 𝐴 cos 2𝜋𝜈𝑚𝑡 𝑣𝑚 = 1 2𝜋 𝑘 𝜇 = 1 2 𝜋 𝑘 𝑚1 + 𝑚2 𝑚1𝑚2 6 9
- 10. Selection Rules Δ𝜈 = ±1 Vibrational Modes For a linear molecule is given by 3N-5 For a Non-linear molecule is given by 3N-6 Four factors tend to produce fewer experimental bands than would be expected from the theoretical number of normal modes. The symmetry of the molecules . The energies of two or more vibrations are identical or nearly identical. The absorption intensity. The vibrational energy. 10
- 11. How many vibrational modes are possible for CO2 ? Why is it that although the CO2 molecule has four vibrational modes only two peaks are seen on its IR spectrum? How many vibrational modes of CO2 are IR active? 11
- 12. IR Instrumentation Three types of instruments for IR absorption measurements are commonly available: (1) Dispersive spectrophotometers with a grating monochromator (2) FT spectrometers employing an interferometer (3) Nondispersive photometers using a filter or an absorbing gas that are used for analysis FT spectrometer type of instrument has been largely displaced for mid- and far-IR measurements – Why? 12
- 13. 13 IR Sources The Nernst Glower The Globar Incandescent Wire Source The Mercury Arc The Tungsten Filament Lamp The Carbon Dioxide Laser Source Semiconductor IR Laser Sources
- 14. 14 IR Transducers Pyroelectric Transducers Photoconducting Transducers Thermal Transducers 1. Thermocouples 2. Bolometers
- 18. Very high-resolution work that is encountered with gaseous mixtures having complex spectra resulting from the superposition of vibrational and rotational bands. The study of samples with high absorbances. The study of substances with weak absorption bands. Investigations requiring fast scanning such as kinetic studies or detection of chromatographic effluents. Collecting IR data from very small samples. obtaining reflection spectra. IR emission studies. 18 Advantages of FT Spectrometers
- 19. SUMMARY The infrared (IR) region of the spectrum encompasses radiation with wavenumbers ranging from about 12,800 to 10 cm21. The IR spectrum is usually subdivided into three regions, the near-IR, the mid-IR, and the far-IR. IR inactive-No net change in dipole moment occurs during the vibration or rotation of homonuclear species such as O2, N2, or Cl2. The selection rule states that Δ𝜈 = ±1. Three types of instruments for IR absorption measurements are commonly available: Dispersive spectrophotometers with a grating monochromator, FT spectrometers employing an interferometer and Nondispersive photometers using a filter or an absorbing gas that are used for analysis of atmospheric gases at specific wavelengths. 19
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