ULTRAVIOLET-VISIBLESPECTROSCOPY

(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 Electronic Transitions in the UV-Vis

Common Difficult

σ

σ*

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, cm2/mmol

c = concentration, M l = pathlength, cm

Spectra are usually characterized by λmax, the wavelength of maximum absorbance, and ε at this wavelength.

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.

UV-VIS Spectrum

O

Φ Φ

Φ Φ

UV-VIS Spectrum

UV-VIS Spectrum

ENDULTRAVIOLET-VISIBLE

SPECTROSCOPY(UV-VIS)