1. Applications of UV/VIS
Yongsik Lee

2. 14B absorbing species Excitation Relaxation Formation of M*
Lifetime 1-10 nsec
Experience relaxation
Relaxation
Photochemical rxn
Luminescence
heat

3. Three types of electronic transition
Involving p, s, and n electrons
Involving d and f orbital electrons
Charge transfer electrons

4. Molecular orbitals (MO)
Sigma orbital
Rotaionally symmetric charge density around the axis of the bond
Pi orbital
Parallel overlap of atomic p orbitals
Nodal plane along the axis of the bond
Maximum density in regions above and below the plane

5. MO in formaldehyde Nonbonding electrons
Unshared electrons between atoms
Not participate in chemical bonding

6. Molecular energy levels

7. Sigma-sigma* transitions
Requires large energy
Usually in VUV
Not used much in UV/VIS
For C-H bond
Methane = abs max at 125 nm
Ethane = abs max at 135 nm
For C-C bond
Less bonding energy, longer abs wavelength

8. N-sigma* transitions Region 150-250 nm
Table 14-1 some examples of absorption
Bond itself dependent not chemical strucuture of the molecule
Solvent effect
Shift to shorter wavelength in the presence of polar solvents
Water or ethanol

9. n – pi*, pi-pi* transitionsnm
Unsaturated absorbing center required
Ideal for UV-Vis spectrometry of organic chromophore

10. Spectrum lmax shift

11. List of simple chromophores
only molecular moieties likely to absorb light in the 200 to 800 nm region
pi-electron functions
hetero atoms having non-bonding valence-shell electron pairs.
The oxygen non-bonding electrons in alcohols and ethers do not give rise to absorption above 160 nm. Consequently, pure alcohol and ether solvents may be used for spectroscopic studies.
The presence of chromophores in a molecule is best documented by UV-Visible spectroscopy
but the failure of most instruments to provide absorption data for wavelengths below 200 nm makes the detection of isolated chromophores problematic.

12. Natural organic pigments

13. Terminology for Absorption Shifts
                                                          
Terminology for Absorption Shifts
each additional double bond in the conjugated pi-electron system
shifts the absorption maximum about 30 nm in the same direction.
Also, the molar absorptivity (ε) roughly doubles with each new conjugated double bond.
extending conjugation generally results in bathochromic and hyperchromic shifts in absorption
                                                          

14. Conjugated dienes

15. Unsaturated ketone
The spectrum of the unsaturated ketone illustrates the advantage of a logarithmic display of molar absorptivity.
The π __> π* absorption located at 242 nm is very strong, with an ε = 18,000.
The weak n __> π* absorption near 300 nm has an ε = 100.

16. UV/VIS of Aromatoc compound
E2 band
Exhibits very strong light absorption near 180 nm (ε > 65,000)
weaker absorption at 200 nm (ε = 8,000)
B band
a group of much weaker bands at 254 nm (ε = 240)
Only this group of absorptions are completely displayed
because of the 200 nm cut-off characteristic of most spectrophotometers.

17. Added conjugation of benzene
The added conjugation in naphthalene, anthracene and tetracene -> bathochromic shifts of absorption bands.
All the absorptions do not shift by the same amount
for anthracene and tetracene the weak absorption is obscured by stronger bands that have experienced a greater red shift.
As might be expected from their spectra, naphthalene and anthracene are colorless, but tetracene is orange.

18. Woodward-Fieser Rules for Calculating the λmax of Conjugated Dienes and Polyenes

19. UV data sheet

20. Calculating the π -> π* λmax of Conjugated Carbonyl Compounds

21. Woodward-Fieser Rules

22. UV data sheet

23. Physical states & spectra

24. Inorganic ions
Most transition metal ions are colored (absorb in UV-vis) due to d -> d electronic transitions

25. Color of the sample Remember: Solution absorbs red appears blue-green
Solution absorbs blue-green appears red

26. Five d orbitals Electron density distribution of d orbitals
Xy, xz, yz are similar in space (between 3 axes)
X2-y2, z2 are along the axes

27. Effect of ligand field on d-orbital energies
Ligands cause different interactions with d electrons
ligand field “splitting” theory

28. Ligand field strength Ligand field increase -> D increase
The lmax decrease

29. Homework
14-1, 14-6, 14-7