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| IR Spectroscopy |
IR spectroscopy
History
William Weber Coblentz is widely regarded as
the pioneer of infrared (IR) spectroscopy, having published the results of a
large study of compounds in 1905. His work demonstrated that chemical
functional groups had specific and characteristic IR absorptions, and he was
able to accurately record the IR spectra of 135 compounds - a feat that still
stands today, some 60 years later.
The method of IR spectroscopy gained traction
during World War II, when a method for characterizing synthetic rubber
formulations was needed for the war effort. This led to the development of the
first commercial instruments from Beckman and Perkin Elmer, and in 1957, Perkin
Elmer introduced the first low-cost IR spectrophotometer, the Model 137, priced
at just $3800.
The Coblentz Society was formed shortly after
to educate early practitioners in the art, and the method was used widely.
However, it experienced a significant resurgence in the sciences with the
advent of Fourier transform IR (FT-IR) instruments in the late 1960s and early
1970s. These instruments could collect spectra in a matter of seconds, and, by
signal averaging, spectra of very high quality could be measured.
It is worth noting that the discovery of
infrared radiation predates Coblentz's work by almost a century. In 1800, the
German-born British-astronomer William Herschel conducted a simple experiment
in which he dispersed sunlight through a prism and placed a thermometer at the
location of each colour, thus discovering infrared radiation.
Introduction
Infrared
Spectroscopy is an analytical technique used to identify organic compounds by
determining the intensity and wavelength of light absorbed. This versatile tool
measures vibrations in molecules which help to distinguish different types of
molecules based on their physical and chemical properties. From identifying or
analyzing minor components in a mixture, confirming a compound's identity, and
monitoring reactions, to studying the behavior of a sample under various
conditions.
The
infrared section of the electromagnetic spectrum, or light with a longer
wavelength and a lower frequency than visible light, is the subject of IR
spectroscopy, sometimes known as infrared spectroscopy. The study of a
molecule's interaction with infrared light is known as infrared spectroscopy.
Three methods can typically be used to investigate the idea of IR spectroscopy:
reflection, emission, and absorption measurements. Finding the functional
groups of molecules—relevant to both organic and inorganic chemistry—is the
main use of infrared spectroscopy.
IR
Spectroscopy identifies infrared light frequencies that are absorbed by
molecules. Due to the fact that these particular light frequencies match the
frequency of the bonds in molecules, molecules have a tendency to absorb them.
Infrared radiation contains the energy needed to excite molecular bonds and
cause them to vibrate more violently. However, only polar bonds will interact
with electromagnetic infrared radiation. A molecule can be excited by the
electromagnetic wave's electric field component because it has distinct regions
of partial positive and negative charge.
The
dipole moment of the particular molecule changes in parallel with the change in
vibrational energy. The polarity of the bond affects how much energy is absorbed.
Symmetrical non-polar bonds in N=N and O=O are unable to interact with an
electric field; hence they cannot absorb IR radiation.
IR Region values
The
region between 4000 cm-1 and 1600 cm-1 has bands that identify the functional
group that is present. It is possible to recognize their bands and utilize them
to ascertain the functional group of an unidentified molecule.
The region between 1600 cm-1 and 667 cm-1, known as the fingerprint region, has bands that are specific to each molecule and resemble a fingerprint. These bands are used to compare the spectra of different compounds.
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| IR Value |
Principle
The
IR spectroscopy theory is based on the idea that molecules have a tendency to
absorb particular light frequencies that are unique to the corresponding
structure of the molecules. The energies depend on the atomic masses, the
related vibronic coupling, and the geometry of the molecular surfaces.
For
instance, the molecule may be able to absorb the energy present in the incident
light, which will cause it to rotate more quickly or vibrate more loudly.
Sample preparation
Infrared
spectroscopy can be used with samples that are solid, liquid, or gaseous.
Solid sample
By
crushing the sample with an oily-textured mulling agent, solid samples can be
created. This mull can now be spread thinly on a salt plate for measurement.
Liquid sample
Since
salt plates are transparent to IR light, liquid samples are often held between
two of them while being analyzed. Sodium chloride, calcium fluoride, or even
potassium bromide can be used to make salt plates.
Gaseous sample
Gaseous
samples can have concentrations measured in parts per million, the sample cell
must have a comparatively long path length, meaning light must travel a
relatively significant distance inside the sample cell.
Sensitivity
A
sample as little as 1 to 10 grams can now be identified using infrared
spectroscopy. Infrared spectroscopy may be used to analyze almost all organic
molecules as well as certain inorganic ones. It has numerous uses and can be
utilized in both qualitative and quantitative analysis.
Why water is not used as a solvent in IR
Water
cannot be used as a solvent for IR spectroscopy because it has two high
infrared absorption peaks. Additionally, alkali halide discs, which are widely
used in IR, can be dissolved in water because it is a polar solvent.
For
a molecule or sample to exhibit an infrared spectrum, a change in the electric
dipole moment of the functional group must occur during the vibration based on
the selection rule for IR transitions.
IR light Source
An
infrared spectroscopy thermal light source called a Globar is used. It is a
silicon carbide rod that has been electronically heated to a temperature of
1,000 to 1,650°C and has a diameter of 5 to 10 mm and a length of 20 to 50 mm
(1,830 to 3,000 degrees Fahrenheit).
Solvents used in IR spectroscopy
The
two most common solvents are carbon tetrachloride (CCl4) and carbon disulfide
(CD). Solvents for polar materials include chloroform, methylene chloride,
acetonitrile, and acetone. Solids reduced to fine particles can be analyzed as
a thin paste or mull.
IR Spectra
An IR spectrum is a graph plotted with the infrared light
transmitted on the Y-axis against frequency or wave number on the X-axis.
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