Spectroscopic selection rules (intro)
Quantum Chemistry (UG) · Chemistry
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The n → π* transition in formaldehyde is weak (ε ≈ 10) but π → π* is strong (ε ≈ 10,000). Why? Step 1: n → π* is symmetry-forbidden (poor orbital overlap, Laporte-like restriction). Step 2: It still appears weakly because molecular vibrations momentarily break the symmetry (vibronic coupling). Step 3: π → π* is fully allowed — good overlap, no restriction. Step 4: Result: a 1000× difference in intensity, exactly as selection rules predict.