Electronic transitions in UV–Vis spectroscopy of organic compounds: which transitions are most commonly analyzed for qualitative and quantitative studies?

Introduction / Context:
UV–Vis spectroscopy probes electronic transitions. For organic molecules, the most diagnostically useful transitions involve nonbonding (n) and pi (π) electrons, which produce bands sensitive to conjugation and substituents.

Given Data / Assumptions:

• Organic chromophores include carbonyls, alkenes, aromatics, and heteroatoms with lone pairs.
• Solvent and substituent effects shift band positions and intensities.
• Beer–Lambert law enables quantitation when bands are well defined.

Concept / Approach:
π → π* transitions are strong and occur in conjugated double-bond systems. n → π* transitions are typically weaker but are highly informative in carbonyls and other heteroatom-containing groups. σ → σ* requires high energy (far UV) and is less used for typical solution measurements.

Step-by-Step Solution:
Identify accessible transitions in 190–800 nm: π → π* and n → π* dominate.Recognize σ → σ* bands are usually below 200 nm (far UV) and less routinely measured.Therefore, select option c.

Verification / Alternative check:
Carbonyls show n → π* bands near 280–300 nm; aromatic rings show strong π → π* near 200–260 nm and characteristic longer-wavelength bands with conjugation.

Why Other Options Are Wrong:
Options a and b are incomplete; d is incorrect; e focuses on inorganic ligand-field transitions, not typical organic UV–Vis.

Common Pitfalls:
Overlooking solvent cutoff and instrument limitations in the far UV for σ → σ* bands.

Final Answer:
n → π* and π → π* transitions in conjugated systems and heteroatom lone-pair chromophores.