UV Spectra Of Benzene & its derivatives, polycyclic aromatic compounds and heterocyclic compounds

Lecture Notes:
UV Spectra Of Benzene & its derivatives, polycyclic aromatic compounds and heterocyclic compounds /M.Sc/NET/B.Pharma
🔍 Vibronic Coupling Enhances Forbidden Transitions: The fine structure of benzene’s B band is attributed to vibronic coupling, where vibrational transitions couple with weak electronic transitions, increasing their intensity. This phenomenon underscores the importance of considering vibrational-electronic interactions in interpreting UV spectra, especially for compounds with forbidden electronic transitions.

🧬 Substituent Effects on λ_max Explained by Hyperconjugation and Steric Hindrance: Para-substituted methyl groups on benzene increase conjugation via hyperconjugation, causing a bathochromic shift (increase in λ_max). Conversely, ortho substitution introduces steric hindrance reducing conjugation and causing a hypsochromic shift (decrease in λ_max). This highlights the delicate interplay between electronic effects and steric factors in determining spectral properties.

⚡ Lone Pair Delocalization Leads to n → π Transitions and Electron Transfer Bands:* In compounds like aniline and phenol, lone pairs on nitrogen or oxygen interact with benzene π electrons, creating n → π* transitions and electron transfer bands (ET bands). The intensity of these bands depends on the extent of delocalization, which can be hindered by steric effects from substituents, demonstrating how molecular geometry influences electronic transitions.

📈 Extension of Chromophore by Electronically Complementary Groups: When electron-donating and electron-withdrawing groups are present para to each other on benzene, their combined conjugation extends the chromophore, effectively lowering the energy gap between HOMO and LUMO and causing a bathochromic shift. This provides a molecular basis for designing compounds with tailored absorption properties by strategic substitution.

🌟 Conjugation Length Correlates with Visible Light Absorption in PAHs: Polycyclic aromatic hydrocarbons exhibit increasing λ_max with the number of fused benzene rings due to extended conjugation reducing the π → π* energy gap. When sufficiently extended, absorption enters the visible region, explaining why larger PAHs are colored. This principle is fundamental in organic electronics and dye chemistry.

🔄 Symmetry and Transition Probability in Heteroaromatic Compounds: The lecture emphasizes how molecular symmetry affects the allowance of electronic transitions. Benzene’s high symmetry results in forbidden transitions with weaker bands, while substitution by nitrogen in pyridine lowers symmetry, making π → π* transitions more allowed and intense. This insight is critical for understanding spectral differences between related compounds and the role of symmetry in spectroscopy.

💧 Environmental Effects on UV Spectra Reveal Molecular Interactions and Tautomerism: Solvent polarity, hydrogen bonding, and pH can alter UV absorption bands, especially in heteroaromatics like pyridine derivatives. These changes reflect shifts in electronic structure and tautomeric equilibria, enabling UV spectroscopy as a diagnostic tool for studying molecular environment, protonation states, and tautomer distributions.

These insights collectively deepen the understanding of UV spectral behavior in aromatic systems and their derivatives, linking molecular structure, electronic transitions, and environmental factors.
00:00 UV spectra of benzene & its derivatives
00:25:45 UV spectra of polycyclic aromatic compounds
00:29:55 UV spectra of Heteroaromatic compounds

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