system, however, there are electronic transtitions with lower energies that correspond to wavelengths longer than 200 nm. Let us compare the energy levels of ethylene and 1,3-butadiene.
wavelengths, the radiations having wavelength shorter than 200 nm are also absorbed by atmospheric gases. Therefore, in order to study the transitions brought about by these radiations in a molecule, the study should be conducted in vacuum.
The light source is usually a hydrogen lamp, and the optical and cells are made up of quartz because most other clear material absorb UV radiation.
Two empirical law has been formulated about absorption intensity.
2) Beer’s law: Beer’s law states that the absorption of a monochromatic radiation by a homogeneous medium is proportional to the number of absorbing molecules.
A ∝ C
|Fig.1 .2 Typical UV spectrum|
E = energy of radiation in ergs
h = Plank’s constant ( 6.62 x 10-27 ergs sec) and, λ= wavelength of electromagnetic radiation ( in hertz or cps )
Thus, when the frequency or wavelength of electromagnetic radiation is known, its energy ia also known. The arrangement of all types of electromagnetic radiation in the order of increasing wavelength or decreasing frequencies is called the electromagnetic spectrum.
The types of radiation within the electromagnetic spectrum are shown in Fig. 1.1. All electromagnetic radiation are fundamentally the same, the various forms differ only in energy.
|Fig. 1.1 Electromagnetic Spectrum|
If a molecule is placed in an electromagnetic radiation, a transfer of energy from the electromagnetic radiation to the molecule occurs when,
where, h = Plank’s constant
|Fig. 1 Shifts in absorption position and intensity|
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.
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 .
It is an effect in which the intensity of the absorption is an increases while wavelength is constant.
It is an effect in which the absorption is decreases while wavelength remain constant.
Nitro compounds are generally yellow in color, so it is clear
For example:- ethylene, acetylene, etc.
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
For Example:- −OH− , −OR−, −NH2−, −NHR−, −NR2−, −SH−, etc.
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