Spectroscopy 2.2

Elementary Idea of Electronic Spectroscopy, UV–Vis Spectroscopy with Related Rules and Its Applications

1. Introduction

Electronic spectroscopy studies transitions between electronic energy levels in atoms and molecules upon interaction with electromagnetic radiation, typically in the ultraviolet (UV) and visible (Vis) regions of the spectrum.

• UV region: ~190–400 nm
• Visible region: ~400–800 nm

When a molecule absorbs UV–Vis radiation, an electron is promoted from a lower-energy molecular orbital (usually bonding or non-bonding) to a higher-energy anti-bonding orbital.

2. Principle of Electronic Spectroscopy

1. Molecular Orbitals (MOs) — Electronic transitions occur between specific MOs:
   • σ → σ^* (high energy, deep UV)
   • n → σ^* (non-bonding to antibonding, medium energy)
   • π → π^* (common in conjugated systems, UV–Vis)
   • n → π^* (common in carbonyl compounds, UV)
2. The energy of the absorbed photon is related to the transition by:

   E = hν = ^hc⁄_λ

   where:
   • h = Planck's constant
   • ν = frequency
   • c = speed of light
   • λ = wavelength of absorbed light

3. Beer–Lambert Law

• A = absorbance
• ε = molar absorptivity (L·mol^-1·cm^-1)
• c = concentration (mol·L^-1)
• l = path length of the cell (cm)

4. Selection Rules

4.1 Spin Selection Rule

ΔS = 0 ? Spin multiplicity must remain unchanged; spin-forbidden transitions have weak intensity.

4.2 Laporte Selection Rule

In centrosymmetric molecules:

• g → u and u → g transitions are allowed
• g → g and u → u transitions are forbidden

5. Factors Affecting UV–Vis Absorption

• Conjugation increases wavelength (red shift).
• Solvent polarity affects n → π^* and π → π^* transitions.
• Substituents shift λ_max depending on electron-donating/withdrawing nature.

6. Applications of UV–Vis Spectroscopy

1. Qualitative: Functional group identification, impurity detection.
2. Quantitative: Concentration determination via Beer–Lambert law.
3. Structural: Degree of conjugation, cis/trans isomer differentiation.
4. Biological/Environmental: Protein/DNA quantification, pollution monitoring.

7. Summary Table