Molecular orbital theory and electronic transition

In this particular article, Molecular orbital theory and electronic transition we are going to discuss assumptions of the molecular orbital theory. We are also going to discuss the different electronic transitions in detail.

electronic transitions in different molecular orbitals

according to molecular orbital theory, two molecular orbitals are formed by the combination of two atomic orbitals. molecular orbital of lower energy is called bonding molecular orbital. whereas molecular orbital of higher energy levels is called antibonding molecular orbital.

In molecular orbitals electron are filled in the same manner as in the atomic orbitals. the electron which forms the σ bond is called σ-electrons. whereas those used in the formation of π-bond is called π-electrons. and there are also some electrons which do not takes part in any bonding, these are called non-bonding electron or n-bonding electrons.

when electrons go excited they go in antibonding orbitals. antibonding molecular orbital of σ and π bonds are represented by σ* and π* respectively.  when a molecule absorbs radiations in the visible or UV region, then these type of transition are possibles-

  1. σ→σ* transitions
  2. n→σ* transitions
  3. π→π* transitions
  4. n→π* transitions
  5. σ→π* transitions
  6. π→σ* transitions

Transitions are arranged in order of increasing energy:

n-π* < π-π* < n-σ* < π-σ* <σ-π*< σ-σ*

the decreasing order of energy for these transitions are-

σ→σ*>n→σ*>π→π*>n→π*

Molecular orbital theory and electronic transition

However, most widely encountered transitions in organic compounds are as follows:

  1. σ→σ* transitions: The transition of an electron from a bonding sigma orbital of a molecule to the higher energy antibonding sigma orbitalis designated as σ-σ* transitions .the  energy required for these transitions are highest. and occur in far  UV – region. These transitions are shown by saturated hydrocarbons containing only σ- bonds viz. alkanes or saturated hydrocarbons. It may be noted that the ultraviolet region below 200 nm is less informative and hence is not considered for practical purposes because oxygen absorbs in this region.
  2. n→σ* transitions: saturated compounds containing atoms with unshared electrons pairs such as oxygen, nitrogen, or non-bonding electrons. These transitions are of lower energy than σ→σ* transitions .such transitions can be brought about by radiation in the region of 150-250nm with most absorption peaks appearing below 200nm. for example tetramethyl amine, methyl alcohol shows absorption at 227 and 174nm respectively.
  3. π→π* transitions: The π→π* transitions are of lower energy than n – σ* and are given by compounds having unsaturated centers e.g. >C=C<,- C=C- and CO and occur usually well within the region of the ultraviolet spectrometer. these transitions generally occur in UV region.
  4. n→π* transitions: The n-π* transitions are of lowest energy and are given by compounds having both non-bonding and π-electrons. bands due to n→π* are called R-bands. this transition requires the least amount of energy. for example- saturated aldehydes and ketones exhibit an absorption of low intensity at 279nm because of n→π* transition and absorption of high intensity around 195nm which is due to π→π* transition. The high energy transitions occur at shorter wavelengths and the ls lower energy transitions occur at longer wavelengths. 

The compounds containing saturated alkyl groups, alcohols and ether groups do not show absorption in the region 200-780 nm and hence are used as solvents for spectral determination.

Let us now consider some examples. The π→π* transitions for simple alkenes take place in the vacuum ultraviolet region. For example, ethylene absorbs at 175 nm. Conjugation of double bonds decreases the energy required for π-π* transition, and therefore, absorption shifts to longer wavelengths. For example, butadiene shows absorption at 217 nm corresponding to a π-π* transitions. If a number of double bonds are present in a conjugation as in β-carotene, the absorption may even get shifted to visible region and the compound would be colored. For example,β-carotene having eleven carbon-carbon double bonds in conjugation shown λmax at 451 nm corresponding to a π→π* transitions and is yellow in color.

Ultraviolet spectrum of a compound is mainly used for detecting the presence of conjugation and also for determining the nature of the conjugated system. However, it is seldom used for detecting the presence of individual functional groups.

Conclusion

The molecular orbital theory is used to explain the various transitions of electrons from one energy level to another energy level. In this particular article Molecular orbital theory and electronic transition, we have discussed many electronic transitions in detail and simplest way possible.

 

 

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