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Instrumental Test 2c
Terms in this set (25)
deals with alteration of the vibrational
energy of chemical bond due to absorption of
Vibrational Spectroscopy can be classified as
o Infrared spectroscopy
o Raman spectroscopy
Vibrational Spectroscopy infrared measurements
can be made from
12,800-10 cm-1 ( 0.78-1000 μm ): It subdivided
o 12,800-4000 cm-1
It is known as the overtone region.
o 4000-200 cm-1 (Mid-infrared; MID-IR)
The region where vibrational and rotational
o 200-10 cm-1 (Far-infrared; FAR-IR)
o It is known as the rotation region.
Vibrational Spectroscopy formula
wavenumber, v = 1/wavelength(in cm) or
v = 10,000/wavelength (in um)
Energy of infrared radiation in vibrational spectroscopy
o insufficient to cause the excitation of
o can cause excitation of molecular vibrations,
and sometimes of rotation as well.
in vibrational spectroscopy Not all molecular vibrations results in the
absorption of IR radiation.
in vibrational spectroscopy to be IR active
the dipole moment of the
molecule must change as the vibration
Types of molecular vibrations in vibrational spectroscopy
o Stretching (along chemical bonds and involves
bond length changes).
o Bending (bond angles changes).
in vibrational spectroscopy bending vibrations occur at
lower frequencies than the corresponding stretching
Bending vibration formula
Ev = (v-1/2)hvo
v = the vibrational quantum number (0,1,2,3...,)
vo = the bond'sfundamental vibrational frequency.
in vibrational spectroscopy most molecules are
in their ground vibrational state (νo).
in vibrational spectroscopy fundamental absorption lines
are due to transitions in which Δν=±1.
in vibrational spectroscopy Overtones (weaker absorption lines) are
due to transitions in which Δν= ±2 or ±3.
Number of possible normal vibrational modes in vibrational spectroscopy
a) 3n-5 (linear molecules)
b) 3n-6 (nonlinear molecules)
types of Infrared Spectroscopic Measurements
o Transmission infrared
o Diffuse reflectance infrared
o Attenuated total reflectance infrared
Basic Components of an IR
radiation source, Monochromator or interferometer , detector
Some common IR detectors are:
a.Thermal ( Thermocouple, bolometer, and
b.Photon-sensitive or pyroelectric devices
(Deuterated triglycine sulfate or DTGS and
mercury cadmium telluride or MCT)
To produce IR radiation by electrical heating
some common radiation sources
a) A Nernst glower (common)
b) A Globar
c) Nichrome wire
Types of Spectrometers
Dispersive infrared, Fourier transform infrared (FTIR), Interferometer
Dispersive infrared spectrometers employ a
Fourier transform infrared (FTIR) spectrometers employ a
An interferometer has three active
moving mirror, fixed mirror, beam splitter
How an interferometer work?
It utilizes a beam splitter to transmit half of the
radiant beam from the source to a fixed mirror and
the remaining half to a moving mirror. The
transmitted beams are reflected from the respective
mirrors back to the beam splitter and then combined.
The combined beam produces an interference pattern
due to changes in the relative position of the moving
mirror to the fixed mirror. The resulting beam is then
passed through the sample and the signal generated
An infrared spectrum is made up of :
o The functional group region (4000-1300 cm-1 )
o The finger print region (1300-667 cm-1)
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