Charge-transfer (CT) absorptions: what is the typical magnitude of molar absorptivity for compounds exhibiting CT bands in UV–Vis spectra?

Introduction / Context:
Charge-transfer (CT) transitions involve electron movement between donor and acceptor moieties, producing intense absorption bands. Recognizing their large molar absorptivity helps assign spectral features and design sensitive assays.

Given Data / Assumptions:

• CT complexes form between electron-rich donors and electron-poor acceptors.
• Electronic transitions are allowed and often strongly absorbing.
• Solvent polarity can shift CT band positions.

Concept / Approach:
Empirically, CT bands exhibit ε values orders of magnitude larger than many d–d transitions. Typical ranges are 10^4–10^5 L mol^-1 cm^-1, sometimes higher, enabling detection at very low concentrations.

Step-by-Step Solution:
Compare ε scales: weak (d–d) ε ~ 10–100; medium (π→π*) ε ~ 10^3–10^4; CT often ≥ 10^4.Identify CT band characteristics: broad, intense, solvent-sensitive.Select the option describing “large” molar absorptivity.

Verification / Alternative check:
Classic donor–acceptor complexes (e.g., tetracyanoethylene acceptors) show prominent CT absorptions with high ε values in literature tables.

Why Other Options Are Wrong:
Options a and b underestimate ε for CT bands; d and e are incorrect by definition.

Common Pitfalls:
Confusing CT bands with ligand-field (d–d) transitions or with vibrational overtones.

Final Answer:
Large (ε commonly 10^4–10^5 L mol^-1 cm^-1 or higher).