In case of visible spectroscopy the spectrum are plotted b/w adsorption & general UV visible spectrum can be represented as:- When
Λmax is the wavelength corresponding to the maximum adsorption.
In case UV visible spectrocopy the peaks are founded to boarder because in this case it involve the electronic transaction as well as vibration, rotational & translational are involved. In case of UV spectroscopy when we polt a graph absorption or intensity vs
Λmax then their can occur four different type of shift which are known as absorption shift.
|Fig. 1 Shifts in absorption position and intensity|
1) Bathochromic shift (Red shift):-
It is an affected by which absorption maxima is shifted towards the longer wavelength.In this type of shift intensity remain constant & wavelength is increses due to which it is also known as red shift.
2) Hypsochromic shift (Blue shift):-
It is an effect in which the absorption maxima is shifted towards the shorter wavelength can be done by decresing the conjucation. Thus these type of the shift is known as Blue shift .
In other word in this type of the shift intensity remain constant & wavelength is decreses.
3) Hyperchromic shift:-
It is an effect in which the intensity of the absorption is an increases while wavelength is constant.
4) Hypochromic shift:-
It is an effect in which the absorption is decreases while wavelength remain constant.
All these compounds which absorbed length of wavelength b/w 4000 Å to 8000 Å appears coloured to the human eye.
The exert color depends upon the wavelength of light absorbed by compounds.
Generally a chromophrous was considered any system which is responsible for in hence the color of the compounds
Nitro compounds are generally yellow in color, so it is clear
that nitro group is a chromophrous which in hence the yellow color.
Now chromophore may be defined as:-
“A covalent unsaturated group responsible for absorption in the UV or visible region is known as chromophore”.
Types of Chromophore
On the basis of the absorption of UV visible radiation chromophores are 2 types:-
i) Chromophrous in which the group contain Π e and they undergo π- π* transitions.
For example:- ethylene, acetylene, etc.
ii) Chromophrous which contain both Π e and non-bonding e.Such chromophrous under go two types of the transition.
i.e:- Π- Π* and n- Π*.
An auxochrome can be defined as any group which does not itself act as chromophrous whose presence bring about a shift of absorption bent towards the red ends of the spectrum.
The absorption of longer wavelength is due to the combination of chromphrous and a auxochroms to which rise new chromophrous.
Auxochrome + Chromphrous = New chromophrous
Auxochromic group do not show characteristics absorption in the UV visible spectrum.
For Example:- −OH− , −OR−, −NH2−, −NHR−, −NR2−, −SH−, etc.
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