Introduction 
     UV spectroscopy involves the measurement of absorption of 
    light in the visible and ultraviolet regions (visible region 400-
    800 nm ; uv region 200-400nm) by the substance under 
    investigation. Since the absorption of light involves the 
    transition from one electronic energy level to another within a 
    molecule, UV spectroscopy is also known as electronic 
    spectroscopy. This technique is complementary to fluorescence 
    spectroscopy, in that fluorescence deals with transitions from 
    the excited state to the ground state, while absorption 
    measures transitions from the ground state to the excited 
    state.
     Principle of absorption spectroscopy
      Molecules containing π-electrons or non-bonding electrons (n-
     electrons) can absorb the energy in the form of ultraviolet or 
     visible light to excite these electrons to higher anti-bonding 
     molecular orbitals. The more easily excited the electrons (i.e. 
     lower energy gap between the HOMO and the LUMO), the longer 
     the wavelength of light it can absorb.
      A very important condition for a molecule to absorb 
     electromagnetic radiation is that the energy of photon of radiation 
     must be equal to the energy difference between two vibrational or 
     rotational or electronic energy states of the molecule. A record of 
     the amount of radiation absorbed or transmitted by a given 
     sample as function of wavelength of radiation is called absorption 
     spectrum.
               Beer’s Lambert law 
      
   When a beam of monochromatic light is passed through a 
   substance dispersed in a non-absorbing solvent, 
   absorption of light is directly proportional to molar 
   concentration of the absorbing substance as well as path 
   length of the sample substance. 
   Using the Beer-Lambert law:
                                   
   where A is the measured absorbance, in Absorbance Units 
   (AU),      is the intensity of the incident light at a given 
   wavelength,     is the transmitted intensity, L the path length 
   through the sample, and c the concentration of the absorbing 
   species. For each species and wavelength, ε is a constant 
   known as the molar absorptivity or extinction coefficient.
         Electronic 
         excitations 
    
       sigma to sigma* transition : 
    very high energy required 
    consequently occur at short 
    wavelength. Eg: methane.
    
       n to sigma* transition: occur at 
    long wavelength than sigma to 
    sigma*. Eg: methyl chloride .
    
       n to pi*transition: require small 
    amount of energy and take place 
    within the range of ordinary uv 
    spectrophotometer. Eg : carbonyl 
    group of saturated aldehydes and 
    ketones. 
    
       pi to pi* transitions : relatively 
    high energy requirement than n 
    to pi * transitions and absorption 
    generally takes place outside the 
    ordinary uv region. Eg : aldehydes 
    and ketones.
    Effect of conjugation 
    
      conjugation of double 
    bonds lowers the energy 
    required for electronic 
    transition, molecules 
    containing conjugated 
    systems absorb radiations of 
    longer wavelength than in 
    case of non conjugated 
    systems. 
    
      Eg: 1,3-butadiene shows 
    max at 217 nm in contrast to 
    ethylene which shows at 
    175nm.
    
      energy gap between 
    HOMO and LUMO decreases.
    
      as the gap decreases 
    position of energy moves to 
    longer wavelength which falls 
    within the ordinary uv region.