ULTRAVIOLET-VISIBLE SPECTROSCOPY (UV-VIS)

1. ULTRAVIOLET-VISIBLESPECTROSCOPY(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 σor 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* π* n π σ

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, cm2/mmol c = concentration, M l = pathlength, cm Spectra are usually characterized by λmax, 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 λmax. 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 Φ Φ Φ Φ

7. UV-VIS Spectrum

8. UV-VIS Spectrum

9. Extended Conjugation Beta-carotene (orange) Λmax = 450 nm

10. ENDULTRAVIOLET-VISIBLESPECTROSCOPY(UV-VIS)