Types of Electronic Transition
Electronic Transition
Electronic transition refers to the movement of an electron from one energy level to another within an atom or molecule. This process is a fundamental aspect of the interactions between light and matter and plays a crucial role in various fields such as chemistry, physics, and biology. The excitation of molecule by absorption of radiation in UV visible region involves promotion of electrons from bonding and nonbonding to antibonding orbital. There are σ and 𝜋 bonding orbitals associated with σ* and 𝜋* antibonding orbitals respectively. Non bonding orbitals are not associated with antibonding orbitals because nonbonding or lone pair of electrons does not form bond but attached with them.
Following electronic transitions are occurs in UV visible region-
σ → σ*, n → σ*, 𝜋 → 𝜋* and n → 𝜋*
Types of Electronic Transition
According to molecular orbital theory, when a molecule is excited by the absorption of energy (UV or visible light), its electrons are promoted from a bonding orbital to antibonding orbital.
The antibonding orbital which is associated with the excitation of σ-bonding orbital electron is called σ* antibonding orbital. So, σ to σ* transition takes place when σ electron is promoted to σ* orbital. This transition is denoted as σ → σ* transition.
When nonbonding (n) electron promoted to an antibonding (σ*) orbital, then it is denoted as n → σ* transition.
Similarly, When 𝜋 electron promoted to an antibonding (𝜋*) orbital, then it is denoted as 𝜋 → 𝜋* transition.
When nonbonding (n) electron promoted to an antibonding (v*) orbital, then it is denoted as n → 𝜋* transition.
The energy required for this transition is in the following order-
σ → σ* > n → σ* > 𝜋 → 𝜋* > n → 𝜋*
σ → σ* Transition
This type of transition takes place in saturated compounds having no hetero atoms. It is a high energy process because σ bond is generally very strong. The molecule in which all the bonds are sigma bonds do not show absorption in the normal UV- region 200-400nm. Usually this transition requires radiation of very short wavelength below 200nm. The usual spectroscopic technique cannot be used below 200nm, since oxygen present in air begins to absorb strongly. To study such high energy transitions below 200nm, the entire path length must be evacuated. Thus, the region below 200nm is commonly called vacuum ultraviolet region. The excitation of σ bond electron to σ* antibonding orbital occurs with net retention of electronic spin. It is called excited singlet state which may, in turn, gets converted to excited triplet state.
n→ σ* transition
This type of transition takes place in saturated compounds containing one hetero atom with unshared pair of electrons (n electrons). Some compounds undergoing this type of transitions are saturated halides, alcohols, ethers, aldehydes, ketones, amines etc. Such transitions require comparatively less energy than that required for σ → σ* transitions.
In saturated alkyl halides, the energy required for such a transition decreases with the increase in the size of the halogen atom (or decrease in the electronegativity of the atom). Hydrogen bonding shifts the UV absorptions to shorter wave lengths.
𝜋 → 𝜋* Transition (K-Band)
This type of transition occurs in the unsaturated centres of the molecule; i.e., in compounds containing double or triple bonds and also in aromatics. The excitation of 𝜋 electron requires smaller energy and hence, transition of this type occurs at longer wavelength. An electron of a double bond is excited to 𝜋* orbital. For example, alkenes, alkynes, carbonyl compounds, cyanides, azo compounds etc. show 𝜋 → v* transition.
n → 𝜋* Transition (R-Band)
In this type of transition, an electron of unshared electron pair on hetero atom gets excited to 𝜋* antibonding orbital. This type of transition requires least amount of energy out of all the transitions above and hence occurs at longer wavelengths. Saturated aldehyde show both types of transitions, i.e. low energy n → 𝜋* and high energy 𝜋 → 𝜋*. Absorption occuring at lower wavelength is usually intense. In saturated carbonyl compounds, two types of transitions take place-
High energy transitions
n → σ* (intense)
𝜋 → 𝜋* (intense)
Low energy transition
n → 𝜋* (weak).
In carbonyl compounds, the shift in the absorption depends upon the polarity of the solvent.
Which of the following electronic transitions do you expect to observe in the UV-Vis spectrum of 1,3-butadiene?
A. σ → σ*B. n → σ*
C. 𝜋 → 𝜋* ✔
D. n → 𝜋*
Correct arrangement of the various electronic transitions in the order of increasing energy.
A. n → σ* < 𝜋 → 𝜋* < n → 𝜋* < σ → σ*B. n → 𝜋* < 𝜋 → 𝜋* < n → σ* < σ → σ* ✔
C. n → σ* < n → 𝜋* < 𝜋 → 𝜋* < σ → σ*
D. σ → σ* < 𝜋 → 𝜋* < n → 𝜋* < n → σ*
UV-Vis spectroscopy of organic compounds is usually concerned with which electronic transition(s)?
A. σ → σ*B. n → σ*
C. n → π* and π → π* ✔
D. All of these
