Electronic transitions

         When continuous radiation passes through a transparent material, a portion of the radiation may be absorbed. If that occurs, the residual radiation, when it is passed through a prism, yields a spectrum with gaps in it, called an absorption spectrum. As a result of energy absorption, atoms or molecules pass from a state of low energy (the initial, or ground state) to a state of higher energy (the excited state). The electromagnetic radiation that is absorbed has energy exactly equal to the energy difference between the excited and ground states. 

       In the case of ultraviolet and visible spectroscopy, the transitions that result in the absorption of electromagnetic radiation in this region of the spectrum are transitions between electronic energy levels. In UV-spectroscopy the molecule undergoes electronic transitions involve sigma, pi, and n electrons.

As a molecule absorbs energy, an electron is promoted from an occupied orbital to an unoccupied orbital of greater potential energy

     The absorption of UV or visible radiation corresponds to the excitation of outer electrons. There are three types of electronic transition that can be considered.
  

1. Transitions involving p, s, and n electrons
2. Transitions involving charge-transfer electrons
3. Transitions involving d and f electrons 

When an atom or molecule absorbs energy, electrons are promoted from their ground state to an excited state. In a molecule, the atoms can rotate and vibrate with respect to each other. These vibrations and rotations also have discrete energy levels, which can be considered as being packed on top of each electronic level.

electronic transitions

The absorption of ultraviolet and visible radiation in organic molecules is restricted to certain functional groups (chromophores) that contain valence electrons of low excitation energy.


Possible electronic transitions of p, s, and n electrons are;
    

Generally, the most probable transition is from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).

electronic transitions

 s ® s^* Transition

• An electron in a bonding sigma orbital of a molecule is exited to the corresponding anti-bonding orbital by the absorption of radiation.
• Large energy is required to induce s ® s^* Transition because of the large distance between sigma and sigma star.

             Example

                                     Alkane

n ® s^* Transitions

• Saturated compounds containing atoms with lone pairs (non-bonding electrons) are capable of n ® s^* transitions.
• These transitions usually need less energy than s ® s^* transitions

Example

                  Oxygen, Halogens and Nitrogen compounds

n ® p^* and p ® p^* Transitions

• Most absorption spectroscopy of organic compounds is based on transitions of n or p electrons to the p^* excited state because the energy required for processes brings the absorption peaks into the spectral region.
• Both transitions require the presence of an unsaturated functional group to the pi orbital.

 Example

   p ® p^*          In alkenes, carbonyl compounds, alkynes, azo compounds, and so on 

   n ® p^*           In carbonyl compounds

Not all of the transitions that at first sight appear possible are observed. Certain restrictions, called selection rules, must be considered.

• One important selection rule states that transitions that involve a change in the spin quantum number of an electron during the transition are not allowed to take place; they are called “forbidden” transitions. The 

  ^{n ® p*}  

  transition is the most common type of forbidden transition.
• Other selection rules deal with the numbers of electrons that may be excited at one time, with symmetry properties of the molecule and of the electronic states.

Charge - Transfer Absorption

• Many inorganic species show charge-transfer absorption and are called charge-transfer complexes.
• For a complex to demonstrate charge-transfer behavior, one of its components must have electron-donating properties and another component must be able to accept electrons.

• Absorption of radiation then involves the transfer of an electron from the donor to an orbital associated with the acceptor.