Q.1 Which of the following electronic transitions is not possible in UV-visible region?
- n → π*
- σ → σ*
- π → π*
- None of these
σ → σ* electronic transition is not possible in the typical UV-visible region (200-800 nm) as it requires very high energy and occurs in the far UV (<150 nm).
Question Solution
The correct answer is σ → σ*. Standard UV-Vis spectrophotometers operate above 200 nm, where σ → σ* transitions are inaccessible due to air absorption and equipment limits.
Option Analysis
-
n → π*: Possible; low-energy transition (200-400 nm) in carbonyls like ketones.
-
σ → σ*: Not possible; high-energy, far UV region (<150 nm), beyond routine UV-Vis range.
-
π → π*: Possible; common in conjugated systems (200-700 nm).
-
None of these: Incorrect, as σ → σ* is excluded.
The electronic transition not possible in UV-visible region is σ → σ*, a high-energy shift inaccessible to standard spectrophotometers. This key concept in molecular spectroscopy helps analyze organic compounds via allowed transitions like n → π* and π → π*.
Types of Transitions
UV-Vis spectroscopy detects electron jumps from bonding/non-bonding to antibonding orbitals using 200-800 nm light.
Transition Energy Level Wavelength Range Observed in UV-Vis? n → π* Low 200-400 nm Yes σ → σ* Very High <150 nm No π → π* Moderate 200-700 nm Yes σ → σ* involves sigma electrons, demanding far UV energy not practical for routine analysis.
Why σ → σ* Excluded
These transitions occur below 200 nm, where oxygen absorbs light, requiring vacuum setups. π → π* and n → π* dominate due to relevance in π-systems and lone pairs.
Exam Relevance
For GATE Life Sciences or CSIR-NET, focus on allowed transitions for spectral interpretation in molecular biology. σ → σ* tests understanding of instrumental limits.
-
