Unveiling the Spectrum: An Introduction to Spectroscopy

Introduction of Spectroscopy

·       Spectroscopy:

Spectroscopy is the technique where we studied about the interaction between the matter and electromagnetic radiation.

Matter is made up of atoms, molecules and ions. Electromagnetic radiation is the form of energy that is all around us and it has both electrical and magnetic characteristics. Light is the form of electromagnetic radiation. According to quantum mechanics, electromagnetic radiation has properties of both wave and particle like discrete packet of energy, called quanta and photons. The scattering of sun rays through raindrop give us the rainbow of different colors just like that when light is passed through the prism it is scattered into visible spectrum of primary colors. This visible light is merely a part of whole spectrum of electromagnetic radiation extending from the radio waves to cosmic rays. All of these electromagnetic radiations travel with same velocity but differ in terms of frequency and wavelength. An electromagnetic radiation can be characterized in different ways using different parameters as described below:

Wavelength (λ) is the distance between two successive crests or troughs of a wave in a beam of electromagnetic radiation.

Wave number is the number of waves passing per centimeter.
Velocity (c) of a wave is the distance through which a particular wave travels in one second.

Frequency (ν) is the number of waves passing through a point on the path of a beam of radiation per second.

Table 1: The Electromagnetic Spectrum

Radiation Type	Wave Length	Frequency	Applications
	λ, (A)	v=c/ λ (Hz)
Radio	1014	3 x 104
Nuclear Magnetic Resonance     1012		3 x 106
Television	1010	3 x 108	Spin Orientation
Radar	108	3 x 1010
Microwave	107	3 x 1011	Rotational
Far Infrared	106	3 x 1012	Vibrational
Near Infrared	104	3 x 1014
Visible	8 x 103    4 x 103	3.7 x 1014
		7.3 x 1014
Ultraviolet	3 x 103	1 x 1015	Electronic
X-Rays	1	3 x 1018
Gama Rays	10-2	3 x 1020	Nuclear Transitions
Cosmic Rays	10-4	3 x 1022

 

·       Nature of Electromagnetic radiations:

Electromagnetic radiation can be characterized in terms of energy possessed by each photon of radiation. Each photon has an energy which is proportional to the frequency of light. The energy required for the transition from a state of lower energy (E1) to state of higher energy (E2) is exactly equivalent to the energy of electromagnetic radiation that causes transition.

E1 – E2= E = hν = h c / λ

Where E is energy of electromagnetic radiation being absorbed, h is the universal Planck’s Constant, 6.624 x 10-27 Erg sec and ν is the frequency of incident light in cycles per second (cps Or hertz, Hz), c is velocity of light 2.998 x 1010 cm s-1 and λ = wavelength (cm)

Therefore, higher is the frequency, higher would be the energy and wavelength would be shorter. If wavelength is longer then energy and frequency would be lower. As we move from cosmic rays to ultraviolet and to radio waves, we are gradually moving towards the region of lower energies and wavelength of this region will be longer.

When organic molecules interact with the radiation of higher energy then it cause the promotion of electrons to higher energy levels or bond cleavage. When a molecule absorbs radiation of lower energy it undergoes molecular rotation or a bond vibration.

In organic chemistry, the important wavelength regions are the ultraviolet, visible and infrared.

ULTRAVIOLET (UV) Spectroscopy involves studying the interaction between ultraviolet light and matter. It's commonly used to analyze compounds based on their absorption or emission of UV radiation. UV spectroscopy is particularly useful for identifying functional groups in organic molecules and studying electronic transitions. UV-visible spectra display absorbance peaks corresponding to electronic transitions, providing insights into the molecular structure and concentration of a sample.

Infrared (IR) spectroscopy analyzes the interaction between infrared radiation and matter. It’s valuable for identifying functional groups in organic and inorganic compounds. Infrared spectra display absorption bands corresponding to vibrational transitions within a molecule. Different functional groups absorb at characteristic frequencies, enabling the identification of specific bonds or groups present in a sample. IR spectroscopy is widely used in chemistry, biochemistry, and various industries for structural analysis and compound identification.

·       UV-Visible Spectroscopy:

Introduction:

UV-VIS spectroscopy is considered as the most important spectrophotometric technique that is most widely used for the analysis of variety of compounds. This absorption spectroscopy use electromagnetic radiation between 190nm and 800nm and is divided into the ultraviolet (UV, 190-400 nm) and visible (VIS, 400-800 nm) regions. Since the absorption of ultraviolet or visible radiation by a molecule leads transition among electronic energy levels of the molecule, it is also often called as electronic spectroscopy. Absorption of visible and ultraviolet (UV) radiation is associated with excitation of electrons, in both atoms and molecules, from lower to higher energy levels. (3) Such electron transfer processes may take place in transition metal ions (d-d transitions and ligand-to-metal or metal-to-Ligand charge transfer transitions), and inorganic and organic molecules.

·       Principle:

UV-Visible spectroscopy is a technique that involves the measurement of the absorption of ultraviolet and visible light by molecules. The underlying principle revolves around the interaction of light with the electronic transitions within a molecule. When molecules absorb light, the energy can excite electrons from their ground state to higher energy states.

The energy absorbed is specific to the electronic structure of the molecule, and this is represented by characteristic absorption bands in the UV-Visible spectrum. The spectrum is obtained by plotting the absorbance of the sample against the wavelength of the incident light. Each molecule exhibits a unique UV-Visible spectrum, making this technique instrumental in identifying compounds and studying their electronic properties.

Beer-Lambert Law equation   is the principle behind absorbance spectroscopy.

The concentration of the sample can be determined directly from the absorption of spectra produced by these samples at specific wavelengths using the Beer-Lambert law.

·       Beer-Lambert law:

Lambert and Beer establishes a linear relationship between the absorbance of a substance, the concentration of the absorbing species, and the path length of the sample.

Mathematically, it is expressed as A = εlc, where A is the absorbance, ε is the molar absorptivity (or molar extinction coefficient), l is the path length of the sample, and c is the concentration of the absorbing species.

The molar absorptivity is a constant for a particular substance at a given wavelength, characterizing its ability to absorb light. The path length represents the distance the light travels through the sample, and concentration reflects the amount of the absorbing species present. This law is applicable under certain conditions, such as when the solution is dilute and the light source is monochromatic.

·       Electronic transitions:

In UV spectroscopy, electronic transitions involve the movement of electrons between different energy levels within an atom or molecule. When a molecule absorbs ultraviolet (UV) light, it promotes an electron from a lower energy level (ground state) to a higher energy level (excited state). The nature of these transitions is associated with the electronic structure of the molecule. Depending on the functional groups the organic molecules may undergo several possible transitions which can be placed in the increasing order of their energies viz. 

n to π* < n to σ* < π to π*  < σ to σ*

•   σ to σ* transition: Involving electrons in sigma orbitals, often seen in saturated compounds. Since it involves a large energy change, the resulting band is observed in the ultraviolet region 125nm-135nm. 
• π to π* transition: Common in conjugated systems, such as double bonds or aromatic compounds, where electrons move within pi orbitals. This transition corresponds to the weakest energy.
• n to σ* transition: The saturated compounds having lone pair (non-bonding) electrons undergo this type of transition. This type of transition involves smaller energy change as compared to σ to σ* transition and therefore, corresponding band is observed in the range 180nm-200nm (U.V region). Example: alcohol, water, ammonia, ether etc.  
• n to π* transition: Involving movement of electrons from non-bonding (n) orbitals to pi* anti-bonding orbitals, typical in carbonyl compounds.