Some terms concerning UV
Some terms concerning UV:
Chromophore: A moiety of a molecule which is responsible for selective absorption of radiation in a given range of specially UV or visible region.
Auxochrome: A chemical group which does not give rise to an absorption band by itself, but upon being attached to a chromophore alters both the position and intensity of the absorption peak.
Bathochromic shift: It is a shift of the peak position (lmJ to a higher wavelength due to the effect of a substituent group or solvent, it is also known as x red shift.
Hypsochromic shift: It is a shift of Xmax 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 They involve n-¶* transition. The emax value is less than 100. The band at 279 mn observed in the UV spectrum of acetone x is an example of an R-band.
(ii) K-bands: These arise from ¶-¶* transition in¶-¶ conjugated systems and show emax 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 X values are between 230-270 mji and emax 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 emax values of these bands vary from 2000-14000. The bands at 210 mji with emax of 6200 for phenol is an example of E-band.
Instrumentation for UV spectrum:
Fig. 1 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.