THEORY

Electromagnetic Radiation

Electromagnetic (EM) radiation is a periodically changing or oscillating electric field propagating in a certain direction with a magnetic field oscillating at the same frequency but perpendicular to the electric field.

Figure 1. A schematic representation of EM radiation. The wavelength is represented by λ.

EM radiation may be considered as a traveling wave or as a stream of massless elementary particles, often called photons. As a wave, it can be characterized by its wavelength λ (the length of one wave), its frequency ν (the number of vibrations per unit time) and its wavenumber k (the number of waves per unit length

Interaction of Light and Molecules

Roughly, there are 3 possible effects of interaction between radiation and molecules. These are scattering, absorption, and emission. Absorption is the process by which the energy of a photon is taken up by the matter, and this process plays a key role in IR spectroscopy. There are several types of physical processes that could lie behind absorption, depending on the quantum energy of the particular frequency of EM radiation. For example, high energetic ultraviolet (UV) radiation can cause ionization and visible light usually causes electron transitions. As told before, the energy levels for all physical processes at the atomic and molecular levels are quantized, and if there are no available quantized energy levels with spacings which match the quantum energy of the incident radiation, then the material will be transparent to that radiation. In view of IR spectroscopy, I will now focus on the absorption of IR radiation by matter.

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Not: Çiğdem Hocamız yukarıdaki 14 Şubat 2015 tarihinde yayımladığımız notu güncellediği için yeni hali de ilk ulaşıldığı tarih olan 27 Ocak 2024 tarihinde konuya eklenmiştir.

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Fourier Transform Infrared (FT-IR) Spectroscopy

Electromagnetic Spectrum

There are three infrared regions; each has potential to provide different information:

1. Far-Infrared (400-33 cm-1): molecules containing heavy atoms, molecular skeleton and crystal lattice vibrations

2. Mid-Infrared (4000-400 cm-1): useful for organic analysis

3. Near Infrared (12820-4000 cm-1): very useful for quantitative analysis

Wavelength and Wavenumber

In IR regionwavenumber is usedgenerallywhich can be defined as thenumber of wavelengths per unit distance:

Wavenumber=1/ Wavelength (in cm)

* For the IR, wavelength is in microns.

* Wavenumber is typically in 1/cm, or cm-1

* 5 microns corresponds to 2000 cm-1

* 20 microns corresponds to 500 cm-1

• The primary source of infrared radiation is thermal radiation (heat).

• For a molecule to absorb IR, the vibrations within a molecule must cause a net change in the dipole moment of the molecule.

Infrared radiation can obtain :

1. The type of atoms within the molecule.

2. The type of bonds between atoms.

3. The molecular structure (by additional techniques such as NMR, mass spectroscopy, etc.)

4. From a quantitative point of view, infrared spectroscopy has a very well gained reputation for its power, flexibility, and reliability.

Molecular Vibrations

• In order to understand molecular vibrations, a bond can be treated as a simple harmonic oscillator composed of two masses (atoms) joined by a spring.

• Representation of a diatomic molecule with two generic atoms (of masses m1 and m2) connected by a spring.

• If masses m1 and m2 are equal, no change in the dipole moment will occur as the molecule vibrates.

• The HCl molecule possesses a permanent dipole moment, so it is infrared active.

• Molecules with a permanent dipole moment, such as water, HCl, and NO, are infrared active.

• The O2 molecule does not possess a permanent dipole moment, so it is infrared inactive.

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