UV Visible Spectroscopy & Electronic Spectroscopy

UV Visible Spectroscopy & Electronic Spectroscopy

UV visible spectroscopy may be defined as the absorption of UV visible radiation by a molecule lead to transition among the electronic energy level of the molecules so it is also called as electronics  spectroscopy as well as UV visible spectroscopy.

Electronics spectra involves the elevation of a electron from bonding orbital to the antibonding orbital as the electronic level are quantized. Electron will jump to a higher energy level only by observing light of the frequency which is exactly corresponding to particular quantum of energy.

A particular energy transition in electronics spectrum is associated with specific wavelength of light and should be responsible for appearance of sharp peak but in reality broads absorption band appears this is due to the factor that electronic transition is always associated with corresponding rotational and vibrational changes which are also quantised.

Beer-Lambert Law

According to this law,when a beam of monochromatic light is passed through dilute solutions,the ratio of intensities of incident and transmitted light are related to concentration and length of solution through which it passes

Molar extinction coefficent is constant for a particular compound at given wavelength.

Height of the peak in an electronic spectrum depends upon the number of molecules undergoing transitions and it is directly propotional to the log E.

Note- E mean epslon

Therefore "E" is the measure of the intensity of absorption at particular wavelength of any specific compound.

More the value of E more will be allowed transitions so E is a measure of forbidden and allowed transition.

Electronic Transitions

When a molecule absorbs UV-Visible light of transition at particular energy.It is assumed that onle one electron is promoted to higher energy levels and that all other electron are unaffected.

The most probable Delta E transition would appesr to the involve promotion of one electron from highest occupied molecular  orbitalHOMO to the lowest unoccupied molecular orbital LUMO.

The general type of electron transitions are allowed in the UV visible transitions are

Disadvantages:-

Broad peak of compound.

Only unsaturated compounds like alkene,alkyne,carbonyl conjugated systems show transitions in this region.

Advantages:-

Used to identify unsaturated compounds