Ultraviolet (UV) Spectroscopy: Basics

 

Ultraviolet (UV) Spectroscopy 

It refers to the study of absorption of the ultraviolet (uv) radiation by the sample compound under observation. The UV radiation is used for the colourless compounds as they are transparent to the visible light and absorbs the light ranging from 200nm to 400nm which is UV radiation.

 

Upon absorption of light, the atoms or molecules undergo excitation to the higher excited level. The important point to notice here is that in UV spectroscopy (Visible spectroscopy also), there are electronic level transitions i.e. the valence electrons only undergo excitation.

Must have some basic knowledge of spectroscopy and its types. Grab it from here.

Basically, the molecular orbital can be termed as one of the following listed below:

·        LUMO: Lowest Unoccupied Molecular Orbital

·        LOMO: Lowest Occupied Molecular Orbital

·        HUMO: Highest Unoccupied Molecular Orbital

·        HOMO: Highest Occupied Molecular Orbital

We’ll use these terms further.

 

Absorption Spectrum

During the course of passing the radiation from sample, some amount of light gets absorbed by the atoms or molecules for the excitation purpose. The transmitted radiation on passing through a prism shows light spectrum with some gaps in it. These gaps refer to that part of light which got absorbed by the sample atoms or molecules. The spectrum so obtained is known as Absorption spectrum. And the wavelength of the radiation absorbed has energy equal to the energy difference b/w ground state and excited state.

 

 

Understanding Electronic Transitions

As it is clear that sigma (σ) electrons are the electrons involved in single bonds and possess lowest energy. The pi (∏) electrons are involved in conjugated bonds or we can say unsaturated compounds and possesses higher energy than sigma bonds. The non bonding (n) electrons are not involved in any bonding and possess the highest energy.

The energy level wise ranking of electrons is depicted in figure below. 

Electronic transitions can be from one molecular orbital level to the other unoccupied molecular orbital. Types of electronic transitions are depicted below.

 

It is obvious that during the course of excitation, the electrons will move from the highest filled molecular orbital to the lowest unoccupied molecular orbital i.e. the electrons will shift from HOMO to LUMO.

Beer’s Law (Brief)

The law states that during the absorption of radiation by the sample, the intensity of the monochromatic light beam decreases exponentially with increase in the concentration of absorbing species (atoms, molecules) arithmetically.

 

Lambert’s Law (Brief)

During the absorption process, if the thickness of the medium (through which the radiation is passed) is increased, the intensity of the monochromatic light decreases. And the rate of this decrease in intensity (of light) is directly proportional to the intensity of the incident light.

 

In simpler words we can say, the more is the intensity of incident light, more will be the rate of decrease in intensity of transmitted light on increasing the thickness of medium.

 

The general equation for the Beer – Lambert's law is:

A= Є. C .t

Where,    A= Absorbance or extinction coefficient

               Є = Molar extinction coefficient / Molar absorptivity

               C= Concentration of drug

               t= Path length

The law generally may not be applicable when

• ·        different forms of the absorbing species are in equilibrium
• ·        thermal equilibrium exist b/w ground state and low excited state
• ·        there is an associative interaction b/w solute and solvent