I.Absorbing Species

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I. Absorbing Species
Applications of UV/Vi Molecular Absorption
Spectrometry
Absorption of light is a two step
process Absorption M hn ? M Relaxation M
? M heat
The heat evolved (very minute) does not affect
the system temperature.
There are other modes of relaxation that cause
deviations in Beers Law. A) Photodecomposition
B) Fluorescence C) Phosphorescence
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Absorbing species containing s, p, and n
electrons (organic compounds).
Antibonding
Bonding
Formaldehyde
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Observed Electronic Transitions
s ? s and n ? s are high energy, short
wavelength transitions.
- s ? s l lt 185 nm (Vacuum UV)
- n ? s l 150 250 nm (mainly vac. UV)
- Very difficult to measure
n ? p and p ? p 200 700 nm

• The most important and useful
• transitions in molecular UV
• spectroscopy.

- Molar absorptivities (e) n ? p 10 100
L cm-1 mol-1 p ? p 1000 10,000
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ethylene absorbs at longer wavelengths ?max
165 nm, ? 10,000
The n to p transition is at even lower
wavelengths but is not as strong as p to p
transitions. It is said to be forbidden.
Example Acetone n-??? ? ?max 188 nm
? 1860 n??? ? ?max 279 nm ? 15
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Factors that change transition energies

• 1. Solvent Effect
• As polarity increases, ? ? for n ? p
• (shift to shorter ?, ? Blue shift)
• As polarity increases, ? ? for p ? p
• (Shift to longer ?, Red shift)
• As polarity increases, fine structure ?
• (Fine structure due to vibrational modes)

Absorption spectra for tetrazine
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UV-VIS spectra of 4-methyl-3-penten-2-one in
methanol (left) and heptane (right). The 320 nm
absorption is the n ? p transition, the 240 nm
is mainly p ? p
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2. Organic Chromophores Molecules having
unsaturated bonds or free nonbonding electrons
that can absorb radiation of relatively low
energy are called chromophores. Examples include
alkenes, alkynes, ketones, aldehydes, phenyl and
other aromatic species, etc.
2.a. Effect of Conjugation of Chromophores As
conjugation is increased in a molecule, more
delocalization (stability) of the p electrons
results. The effect of this delocalization is to
decrease the p molecular orbital. The result is
a decrease in transition energy from p - p and
thus a red or bathochromic shift. The molar
absorptivity will increase in this case and
better quantitative analysis will be achieved.
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Conjugation causes delocalization of p
electrons stabilizing p, therefore shifting
absorbance to longer wavelength (lower energy).
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Highly conjugated molecules are colored
Lycopene
ß-Carotene
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Absorption characteristics of some common
chromophores