1. ULTRAVIOLET-VISIBLE SPECTROSCOPY (UV-VIS)

2. UV-VIS Absorbance of energy in the UV-Vis region results in the movement of an electron from the ground state to an excited state. Bonded electrons are either in a σ or a π bonding orbital. Unshared electrons are nonbonding (n) es. Excited electrons reside in antibonding orbitals. Common electronic transitions are: n  π* ; π  π* ;n  σ* Difficult electronic transitions are: σ  π* ; π  σ * ; σ  σ*

3. Relative Transition Energies σ σ*σ* π π*π* n

4. UV-VIS Peaks from electronic transitions are broad because they encompass a large number of vibrational and rotational states. These absorbances follow Beer’s Law: A = εcl A = Absorbance, A.U. ε = molar extinction coefficient, cm 2 /mmol c = concentration, M l = pathlength, cm Spectra are usually characterized by λ max, the wavelength of maximum absorbance, and ε at this wavelength.

5. UV-VIS UV absorbances normally occur in the 200-350 nm region of the spectrum (mμ is also used). The visible region is typically from 350-700 nm. The energy of the transition is indicated by λ max. The intensity of the absorbance ( ε) is related to the probability of the transition. The n  π* transition requires less energy and occurs at longer wavelength. The π  π* transition is more generally more probable so has a greater extinction coefficient.

6. UV-VIS Spectrum ΦΦ ΦΦ

9. END ULTRAVIOLET-VISIBLE SPECTROSCOPY (UV-VIS)