Engineering Chemistry S Datta

514 ENGINEERING CHEMISTRY
Bathochromic shift: It is a shift of the peak position (λ max
) to a higher wavelength due
to the effect of a substituent group or solvent, it is also known as red shift.
Hypsochromic shift: It is a shift of λ max
to lower wavelength. It is also known as blue
shift.
Hyperchromic and hypochromic effects: These terms refer to an increase and
decrease in absorptivity of the molecule respectively.
Types of absorption bands: There are four types of absorption bands. They occur due
to electronic transition of a molecule.
(i) R-bands: These are observed in compounds containing such groups as
C=O,
—NO 2
etc. They involve n – π* transition. The ε max
value is less than 100. The band
at 279 mµ observed in the UV spectrum of acetone is an example of an R-band.
(ii) K-bands: These arise from π–π * transition in π–π conjugated systems and show ε max
greater than 10,000. 1,3,5-hexatriene is an example of such a conjugated system.
(iii) B-bands: These are due to aromatic and heterochromatic systems. The λ max
values
are between 230-270 mµ and ε max
less than 2000. These bands are called benzenoid
bands. In the presence of K-bands the position of the B-band is shifted to larger
wavelengths. The UV spectrum of benzaldehyde contains K, R and B bands.
(iv) E-bands: These are also known as ethylenic bands and are characteristic of the
aromatic systems as are the B-bands. Only difference is: they occur at lower wavelengths.
The presence of an auxochromic group shifts an E-band to a higher wavelength. The
ε max
values of these bands vary from 2000-14000. The bands at 210 mµ with ε max
of 6200 for
phenol is an example of E-band.
Instrumentation for UV spectrum
Fig. 24.2 A simplified diagram.
Radiation source. The source for the UV range is usually high pressure hydrogen or
deuterium discharge lamp, which covers a range of 200-375 mµ. A xenon arc or, a mercury
lamp provides a more intense radiation. The source employed for the visible range is 6 or 12V
tungsten automobile head lamp bulb. UV is a plot of absorbance vs. wavelength.
Applications of UV in analytical chemistry
The working formula:
A = ε b c
is derived from Lambert-Beer’s law, where A is the absorbance, E is molar extinction coefficient,
b is the path length in cm and c is molar concentration (mol l –1 ). The absorbance values of the