I. Absorbing Species
-
Upload
malachi-sanford -
Category
Documents
-
view
26 -
download
0
description
Transcript of I. Absorbing Species
I. Absorbing Species
• Absorption of light is a two step process:
Absorption M + h 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 Beer’s Law.
A) Photodecomposition
B) Fluorescence
C) Phosphorescence
Applications of UV/Vi Molecular Absorption Spectrometry
Absorbing species containing , , and n electrons
(organic compounds).
Antibonding FormaldehydeBonding
Observed Electronic Transitions• * and n * are high energy, short
wavelength transitions. - * < 185 nm (Vacuum UV)
- n * = 150 – 250 nm (mainly vac. UV) - Very difficult to measure
• n * and *
200 – 700 nm - The most important and useful
transitions in molecular UV
spectroscopy.
- Molar absorptivities ():
n * 10 – 100 L cm-1 mol-1
* 1000 – 10,000
ethylene absorbs at longer wavelengths: max = 165 nm, = 10,000
hv
C C
C O
n
hv
n
nExample: Acetone
n-max =188 nm ; = 1860
nmax = 279 nm ; = 15
The n to π* transition is at even lower wavelengths but is not as strong as π to π* transitions. It is said to be “forbidden.”
1. Solvent Effect
As polarity increases, λ ↓ for
n π*
(shift to shorter λ, Blue
shift)
As polarity increases, λ ↑ for
π π*
(Shift to longer λ, Red shift)
As polarity increases, fine
structure ↓
(Fine structure due to vibrational
modes)
Factors that change transition energies
Absorption spectra for tetrazine
UV-VIS spectra of 4-methyl-3-penten-2-one in methanol (left) and
heptane (right). The ~320 nm absorption is the n π* transition,
the ~240 nm is mainly π π*
*n *n
In methanol In heptane
*
2. Organic ChromophoresMolecules 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 ChromophoresAs conjugation is increased in a molecule, more delocalization
(stability) of the π electrons results. The effect of this
delocalization is to decrease the π * molecular orbital. The result
is a decrease in transition energy from π - π * and thus a red or
bathochromic shift. The molar absorptivity will increase in
this case and better quantitative analysis will be achieved.
• Conjugation causes delocalization of p electrons stabilizing p*, therefore shifting absorbance to longer wavelength (lower
energy).
Highly conjugated molecules are colored
Lycopene
β-Carotene
Absorption characteristics of some common chromophores