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General
Description of IR Spectroscopy
Infrared (IR) or more correctly Vibrational Absorption (VA) spectroscopy
is the measure of absorbance of a sample, when it is illuminated by mid-infrared
light. The mid-infrared frequency region (4000-600 cm-1) is very important
in molecular spectroscopy because the wavelength of such absorption is
characteristic of specific types of chemical bonds. Therefore, VA spectroscopy
is applied for identifying organic as well as organometallic molecules.
VA Theory
The transition moment for infrared absorption can be expressed as:
where Xi and Xf are the initial
and final states, respectively, and μ is
the electric dipole moment operator, which is in turns defined as:
μ = μ0 + (r-re)(dμ/dr)
+ ... higher terms
where r is the distance
between atoms and μ0 is a constant,
corresponding to the permanent dipole moment at the equilibrium
bond length, re. Since < Xi | Xf > = 0, R can be simplified
to:
R
= < Xi | (r-re)(dμ/dr) | Xf >
This implies that there must be a change in the dipole moment of
a molecule during the vibration in order to absorb infrared radiation.This
implies that there must be a change in the dipole moment of a molecule
during the vibration in order to absorb infrared radiation.
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