ULTRAVIOLET-VISIBLE SPECTROSCOPY (UV-VIS)

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*

Relative Transition Energies σ σ*σ* π π*π* n

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.

UV-VIS UV absorbances normally occur in the nm region of the spectrum (mμ is also used). The visible region is typically from 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.

UV-VIS Spectrum ΦΦ ΦΦ

END ULTRAVIOLET-VISIBLE SPECTROSCOPY (UV-VIS)