Infrared-Spectroscopyinstrumentation

Infrared (IR) Spectroscopy

  • Definition: IR spectroscopy studies the interaction of infrared radiation with matter and provides insights into chemical nature and molecular structure.

  • Spectrum Acquisition Methods:

    • Absorption of IR radiation (most common)

    • Emission and reflection techniques.

  • Applications: Used extensively for analyzing organic materials, polyatomic inorganic molecules, and organometallic compounds.

Overview of IR Spectroscopy Concepts

  1. Electromagnetic Radiation

  2. Vibrations

  3. Principle of IR Experiment

  4. IR Spectrum

  5. Types of Vibration

  6. CGF/Fingerprint Regions

  7. IR Activity of Vibrations

  8. Interpretation of IR Spectra

  9. Instrumentation

  10. Sample Preparation

Electromagnetic Radiation

  • Propagation: Constant velocity in a vacuum; defined by the equation: c = λ × ν.

  • Units:

    • 1 Å (10^-10 m), 1 nm (10^-9 m), 1 mm (10^-6 m).

  • Energy:

    • Described as particles or quanta, related to a stream of energy defined by the equations from Einstein, Planck, and Bohr.

The Electromagnetic Spectrum

  • Wavelength vs Frequency:

    • High-energy regions (γ-rays, X-rays)

    • Visible light

    • IR, microwave, and radio waves.

  • Basic Processes Involved: Molecular, atomic, nuclear processes can be observed.

Infrared Region Limits

  • Red Light:

    • 800 nm, 0.8 mm, 12500 cm^-1

  • Near IR:

    • 0.8 - 2.5 mm, 12500 - 4000 cm^-1

  • Mid IR:

    • 2.5 - 50 mm, 4000 - 200 cm^-1

  • Far IR:

    • 50 - 1000 mm, 200 - 10 cm^-1

Molecular Spectra Types

  1. Electronic/Vibronic Spectra: (UV-visible-near IR)

  2. Vibrational/Vibrational-Rotational Spectra: (IR region)

  3. Rotational Spectra: (Microwave region)

Vibrational Energies

  • Absorption Range: Infrared radiation (10,000 – 100 cm^-1) absorbed by organic molecules, converting it into vibrational energy.

Harmonic Oscillator Model

  • Concept: A mass connected to a spring with restoring force proportional to displacement.

  • Vibrational Frequency Influences:

    • Increased force constant (bond strength)

    • Decreased atomic mass.

Anharmonic Oscillator Model

  • Potential Energy: Deviates from the simple harmonic oscillator near equilibrium distances, approximated by Morse Potential.

Energy Transitions in Vibrational Spectra

  • Energy Level Differences: Transition from n to n+1 follows quantization; weaker transitions observed as overtones.

  • Room Temperature Statistics: 1% or fewer molecules are in excited states without radiation, affecting observed transitions.

Selection Rules

  • Absorption Condition: A molecule must experience a change in dipole moment to absorb IR radiation.

  • Electronegativity Values: Important for determining molecular behavior in IR spectra.

Group Frequencies and Fingerprint Region

  • Group Frequencies: Characteristic absorption frequencies for functional groups (e.g., Carbonyl 1650 to 1740 cm^-1).

  • Fingerprint Region: From 1300 to 400 cm^-1, characteristic of the whole molecule, used for identification via comparison.

Main Uses of IR Spectroscopy

  1. Molecular Structure Determination: Bond lengths and angles in gaseous molecules.

  2. Qualitative and Quantitative Analysis: Monitoring trace gases, and direct analysis using infrared absorption.

Instrumentation in IR Spectroscopy

  • Dispersive Instruments: Use monochromators for spectral scanning.

  • FTIR Systems: Preferable for far-IR and mid-IR analysis due to high sensitivity and speed.

Sample Preparation Techniques

  • Gas Samples: Glass or metal cell with NaCl/KBr windows, may involve multipass cells for efficiency.

  • Liquid Samples: Filmed or “sandwiched” between NaCl plates, with adjustable pathlengths.

  • Solid Samples: Obtained as KBr discs or mulls, often prepared under specific protocols for clarity in results.

Advantages of FTIR

  • High Resolution: Can resolve closely spaced lines (less than 0.1 cm^-1).

  • High Sensitivity: Suitable for small sample quantities, and results are reproducible.

  • Fast and Inexpensive: Ideal characteristics for routine use in laboratories.

References

  • J. Workman et al. , “Applied Spectroscopy”, Academic Press, 1998.

  • J.M. Hollas, “Modern Spectroscopy”, John Wiley & Sons, 1996.