1. Applications of UV-Vis Molecular Absorption Spectrometry
Chap 14

2. 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.

3. (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)

4. Formation of a covalent bond
Two atomic orbitals (AO’s) overlap to form two molecular orbitals (MO’s)

5. Electron distribution in sigma and pi MO’s

6. Types of MO’s in formaldehyde

7. Electronic molecular energy levels

8. σ → σ* Transitions
ΔE large (λ < 150 nm)
ε = ,000 L/mol·cm
n → σ* Transitions (halogens, N, O, S)
ΔE smaller (λ = nm)
ε = L/mol·cm
n → π* and π → π* Transitions (org. chromophore)
ΔE small (λ = nm)

9. 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)

10. UV absorption spectra for 1,2,4,5-tetrazine
(Fig. 14-1)
UV absorption
spectra for
1,2,4,5-tetrazine

11. (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

12. Colors of Visible Light

13. Why are transition metal ions colored?
Rationalized by Crystal-Field Theory:
Normally, d-orbitals are degenerate
When ligands bond to the metal ion, they cause different interactions with d electrons
Result is splitting of the d-orbitals:
ligand field splitting

14. The d-orbitals

15. Effect of ligand field on d-orbital energies