Ultra Violet Spectroscopy

Ultraviolet spectroscopy is primarily used to measure the multiple bond or aromatic conjugation by measurement of energy absorbed when electrons are promoted to higher energy level. On passing electromagnetic radiation in the UV and Vis resign through a compound with multiple bonds; a portion of the radiation is normally absorbed by the compound. The amount of absorption depends on the wavelength of the radiation and the structure of the compound. The absorption of radiation is due to the subtraction of energy from the radiation beam when electrons in orbitals of lower energy are excited into orbitals of higher energy. The ultraviolet spectrum is simply a plot of wavelength of light absorbed verses the absorption intensity (absorbance or transmittance) and is conveniently recorded by plotting molar absorptivity (e) against wavelength (nm). Since (e) values range, in practice from as low as 10 to as high as 10,000, it is convenient to use log e  as the abscissa of UV spectrum.

A molecule contains electronic, vibrational and rotational energy levels. Each electronic level, within a molecule, is associated with a number of vibrational levels with less energy separation and each vibrational level in turn is associated with a set of rotational levels with even less energy separation. Due to the relatively larger amounts of energy associated with the ultraviolet radiation, they are capable of electronic excitations and induce transition in the electronic, vibrational and rotational energy levels of a molecule. Thus, the ultraviolet spectrum of a molecule results from transition between electronic energy levels accompanied by changes both in vibrational and rotational states.

The strength of absorption i.e. molar absorptivity or e_max is define by combined Beer’s- Lamberts law-

Log (I₀/I) = e.C. l = A

The principal characteristics of an absorption band are its position and intensity. The position of absorption corresponds to the wavelength of radiation whose energy is equal to that required for an electronic transition. The intensity of absorption is largely dependent on two factors:

1. The probability if interaction between the radiation energy and the electronic system
2. the difference between the ground state an excited state

 The molar absorptivity e is a constant for an organic compound at a given wavelength, and is reported as e_max (molar absorptivity at an absorption maximum). It may be mentioned that e is not dimensionless, but is correctly expressed in unit of 10^-2 m2 mol2 but the unit are by convention never expressed. The intensity of absorption is directly proportion to the transition probability. A fully allowed transition will have  e value greater than 10000 and it is called high intensity absorption. The transition with low transition probability and the  e value less than 100 are called low intensity transitions. Transitions with low intensity are due to forbidden transitions.