Applications of UV-Vis Molecular Absorption Spectrometry

Chap 14

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)

Electron distribution in sigma and pi MO’s

Types of MO’s in formaldehyde

Electronic molecular energy levels

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)

UV absorptionspectra for

1,2,4,5-tetrazine

(Fig. 14-1)

(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

Colors of Visible Light

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

The d-orbitals

Effect of ligand field on d-orbital energies