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Magnitude of Molar Absorptivities (ε)• Process:
M + hν M* M + hνʹ + heat
• How probable?
ε ranges from 0 to ∼ 100,000 L/mol·cm
“forbidden” “allowed”
• In UV-vis absorption, photon provides enough energy to promote valence bonding electrons.
• Absorption wavelengths correlate with types of bonds in species under study.
excitation relaxation
(1) Species Containing π, σ, and n Electrons• Excitation energies for electrons in single σ
bonds restricted to vacuum ultraviolet region (λ < 185 nm)
• At longer λ’s, chromophores (functional groups) absorb UV-vis
• Complex spectra due to superposition of vibrational transitions on electronic transitions (vibronic)
• In chromophores, consider:• electrons participating in bonding (π)• nonbonding electrons (n)
Formation of a covalent bond
• Two atomic orbitals (AO’s) overlap to form two molecular orbitals (MO’s)
σ → σ* Transitions
ΔE large (λ < 150 nm)
ε = 10 - 10,000 L/mol·cm
n → σ* Transitions (halogens, N, O, S)
ΔE smaller (λ = 150-250 nm)
ε = 200 - 2000 L/mol·cm
n → π* and π → π* Transitions (org. chromophore)
ΔE small (λ = 200 - 700 nm)
ε = 10 - 10,000 L/mol·cm
• Red shift of λmax with increasing conjugation:
CH2=CHCH2CH2CH=CH2 λmax = 185 nm
CH2=CHCH=CH2 λmax = 217 nm
• Red shift of λmax with # of rings
Benzene λmax =204 nm
Naphthalene λmax =286 nm
• Blurred with solvent (Fig. 14-5)
(2) Absorption Involving d and f Electrons
• Most transition metal ions are colored (absorb in UV-vis) due to d → d electronic transitions
Fig 14-3
Why are transition metal ions colored?
Rationalized by Crystal-Field Theory:
• Normally, d-orbitals are degenerate
• When ligands bond to the metal ion, theycause different interactions with d electrons
• Result is splitting of the d-orbitals:
• ligand field splitting