Q.1.(b)Give principle of UV-visible spectroscopy, Explain why cellulose is white in colour, picric acid is yellow and aromatic diazotization and coupling produces different shades.
Ans.1. (b)The wavelength range of UV radiation starts at blue end of the visible light (above 4000 A°) and ends at 2000 A°. However, the wavelength of visible radiation starts at 8000 A° and ends at 4000 A°. Spectroscopically, visible light acts in the same way as UV light.
Ultraviolet absorption spectra arise from transition of electron or electrons within a molecule or an ion from a lower to a higher electronic energy level and the ultraviolet emission spectra arise from the reverse type of transition. For radiation to cause electronic excitation, it must be in the UV region of the electromagnetic spectrum.
When a molecule absorbs ultraviolet radiation of frequency ʋ sec-1, the electron in that molecule undergoes transition from a lower to a higher energy level or molecular orbital, the energy difference is given by
E = hʋ erg
The actual amount of energy requireddepends on the difference in enrgy between the ground state E0 and excited state E1 of the electron, Equation (1) becomes as
E1-E0 = hʋ
We know that the total energy of a molecule is equal to the sum of electronic, vibrational and rotational energy. The magnitude of these energies decreases in the following order Eelec, Evib, Erot.
As ultraviolet energy is quantized, the absorption spectrum arising from a single electronic transition must consist of a single discrete line. But a discrete line is not obtained because electronic absorption is superimposed upon rotational and vibrational sub-levels. For this reason the spectra of simple molecules in the gaseous state contain narrow absorption peaks where each peak is representing a transition from a particular combination of vibrational and rotational levels in the electronic ground state to a corresponding combination in the excited state.
In the case of complex molecules having more than two atoms, discrete bands coalesce to produce broad absorption bands or “band envelopes”.
Energy absorbed in the ultraviolet region produces changes in the electronic energy of the molecule resulting from transitions of valence electrons in the molecule. Three distinct types of electrons are involved in organic molecules i.e. σ, π &n electrons.
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