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