Selection Rules and the Beer Lambert Law

The fact that there are many electronic transitions possible, however, does not mean that they can or will occur. There are complex selection rules based on the symmetry of the ground and excited states of the molecule under examination. Basically, electronic transitions are allowed if the orientation of the electron spin does not change during the transition and if the symmetry of the initial and final functions is different these are called the spin and symmetry selection rules, respectively. However, the so-called forbidden transitions can still occur, but give rise to weak absorptions. 

According to the Beer-Lambert law, the absorbance. A, of a solution is directly proportional to the path length (Z, length of the cell containing the solution, in cm) and the concentration of the absorbing molecule (c, in moles per litre), according to equation (2.3)  

10 from those values in units of 100 cm mol ). The molar absorptivity, e, is a measure of the intensity of the absorption and usually ranges from 0 to 10 (units of 100 cm moT ). The greater the probability of a particular absorption and its associated electronic transition, the greater the value for that transition. For most molecules, absorptions associated with 7t TI transitions have higher e values than the n ir transitions. 

In general, forbidden transitions give rise to low-intensity (low s) absorption bands ( 10,000), but two important forbidden absorptions are seen quite commonly the transition of ketones at 

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