ULTRAVIOLET-VISIBLE SPECTROSCOPY (UV-VIS)

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ULTRAVIOLET-VISIBLESPECTROSCOPY(UV-VIS)
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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 s or a p bonding orbital. Unshared
electrons are nonbonding (n) es. Excited
electrons reside in antibonding orbitals. Common
electronic transitions are n ? p p ?
p n ? s Difficult electronic transitions
are s ? p p ? s s ? s
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Relative Transition Energies
s
p
n
p
s
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UV-VIS
Peaks from electronic transitions are broad
because they encompass a large number of
vibrational and rotational states. These
absorbances follow Beers Law A ecl A
Absorbance, A.U. e molar
extinction coefficient, cm2/mmol c
concentration, M l pathlength,
cm Spectra are usually characterized by ?max,
the wavelength of maximum absorbance, and e at
this wavelength.
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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 ?max.
The intensity of the absorbance (e) is related
to the probability of the transition. The n ? p
transition requires less energy and occurs at
longer wavelength. The p ? p transition is
more generally more probable so has a greater
extinction coefficient.
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UV-VIS Spectrum
F
F
F
F
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UV-VIS Spectrum
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UV-VIS Spectrum
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ENDULTRAVIOLET-VISIBLESPECTROSCOPY(UV-VIS)