Types of Electronic Transition in UV-Visible Spectroscopy
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| Types of Electronic Transition in UV-Visible Spectroscopy |
“UV-Visible absorption spectroscopy is the measurement
of the attenuation or weakening of the strength of a light beam when it passes
through the sample or after reflection from a sample surface.”
Types of Transitions in UV-Visible Spectroscopy
UV-light and visible light cause the same transitions in
molecules so they are commonly combined and called as UV-Visible, when they
interact with the matter then it becomes UV-Visible spectroscopy. This kind of
spectroscopy is also called electronic spectroscopy because electronic
transition occurs in molecules when they absorb UV-Visible light promotion of
electrons from low to high energy level occurs, and only outermost shell electron transitions occurs in compounds.
By using UV-Visible spectroscopy or from the types of
transitions we can determine the number of double bonds conjugated molecules,
aromatic conjugated, simple conjugated, or non-conjugated molecules.
Regions in
UV-Visible Spectra
UV-Visible absorption region ranges between 10 nm to
800 nm where 10 to 400 nm is Ultra
Violet region and 400 to 800 nm is a visible
region. The region between 200 to 800 nm is the most prevalent region for the electronic transition so this region is named as excitation region. The region below 200 nm is called as vacuum region. Because oxygen and
nitrogen that are present in the atmosphere show absorption in spectra, so vacuum
instruments are required for this region's studies. The region between 200 to 400
nm is known as the ordinary or quartz
region.
Absorption laws
Beer’s Law
“This law states that when a beam of monochromatic
light passes through a homogenous absorbing medium. The rate of decrease of
intensity of radiation with a thickness of absorbing medium is proportional to
the intensity of the incident light as well as concentration.”
Mathematically,
-dI/ dx = K`IC
Lambert Law
“This law states that when a beam of monochromatic
light passes through a homogenous absorbing medium. The rate of decrease of
intensity of radiation with a thickness of absorbing medium is proportional to
the intensity of the incident light.”
Mathematically,
-dI/ dx = K`I
Beer`s Lambert law
“Beer-Lambert law states that the concentration of the
sample and path length is directly proportional to the absorbance of the light.”
A = ε Lc
Here,
A = Amount of light absorbed for a particular
wavelength by the sample
ε = Molar absorptivity coefficient
L = Distance covered by the light through the solution
c = Concentration of the absorbing species
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| Beer`s Lambert law |
Molar absorptivity
ε = A/ Lc
The sample's molar absorptivity, also called the
extinction coefficient, is represented by the symbol. The ability of the sample
to absorb light at a specific wavelength is related to a special physical
constant of the sample's chemistry.
By dividing both the concentration and the length of
the solution that the light travels through, molar absorptivity is found.
Following are
cases where Beer's law is destroyed:
- when many molecular kinds are in balance with one another.
- Solute and solvent combine to generate an association complex.
- when using fluorescent chemicals.
As the ratio of light entering a solution to light
leaving it determines how much light travels through it, absorbance has no units. Absorbance is sometimes expressed in
"absorbance units," which is abbreviated as AU and has no dimension.
It is necessary to use a diluted solution, avoid light
beam scattering, and employ monochromatic electromagnetic radiation.
Beer-Lambert law at high concentrations
The Beer-Lambert law fails at larger concentrations
because its linearity is restricted to chemical and instrumental parameters.
The molecules' closeness gets so close as the solution's concentration rises
that the absorptivity changes. Moreover, the refractive index varies when the
concentration is high. when the excited state and the ground state have reached
thermal equilibrium.
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| Beer-Lambert law at high concentrations |
Now we discuss different types of transitions that are
as follows,
ϭ to ϭ⃰ transitions
These type of transitions occurs in saturated
hydrocarbons that contain sigma bonds.
Absorption occurs near 150 nm that is of high
energy for saturated hydrocarbons because the sigma bond is a strong bond and those
organic compounds in which all the valence electrons are involved in the
formation of a sigma bond do not show absorption in normal UV-Visible region that
is in the range of 180-400nm. Electrons excite from one sigma orbital to
another.
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| ϭ to ϭ⃰ transitions |
n to ϭ⃰ transitions
For these types of transitions, the wavelength range is
between 150 to 250nm. These transitions occur when non-bonding electrons are promoted to anti-bonding sigma orbitals. Most commonly these transitions occur
in saturated compounds that contain one heteroatom along with unshared pair of
electrons. For example, ethers, alcohols, aldehydes, ketones, etc. These
transitions require less amount of energy as compared to the first one. Water and
methyl chloride absorbs at 167nm and 169nm in the UV-Visible region respectively.
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| n to ϭ⃰ transitions |
ᴫ to ᴫ⃰ transitions
The wavelength range for these types of transition is between 170nm to 190/200 nm. These occur when electrons move from one pi
bon to another pi bond. Compounds like alkenes, alkynes, carbonyl, cyanides, and azo
show these types of transitions. Normally if we increase one alkyl group in
olefins the wavelength increase by 3 to 5 nm. Carbonyl compounds show absorbance at 180nm.
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| ᴫ to ᴫ⃰ transitions |
n to ᴫ⃰ transitions
These electronic transitions occur when electrons of
an unshared pair of heteroatom get promoted to anti-bonding orbitals. The
wavelength range is high from all above and energy is lower (as we know energy
and wavelength have an inverse relationship with each other) as compared to the above
three. The Wavelength range is between 200 to 700 nm.
These
are of total six types of electronic transitions, the transitions that are
discussed above are allowed transitions and their energy order is as follows,
ϭ to ϭ⃰ transitions > n to ϭ⃰ transitions > ᴫ to
ᴫ⃰ transitions > n to ᴫ⃰ transitions
The other two types of transition are forbidden
transitions that are ϭ to ᴫ⃰
transitions and ᴫ to ϭ⃰ transitions.
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| Electronic Transitions |
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