Download - 19 Molecular Spectroscopy
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UV/Vis absorption by organic compounds
requires that the energy absorbed correspondsto a jump from a populated orbital to an
unpopulated one.
!"
#"
n
#*
!*
bondingbonding
non-bonding
anti-bonding
anti-bonding
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N
N
N
N
Vapor phase spectrum
Here we have no
solvent and minimal
interaction betweenthe molecules
Vapor phase
In hexane
In water
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octahedral tetrahedral square planar
The type and degree of splitting depends on the
ligand and geometry of the complex that is formed.
This splitting of the d orbitals results in a d->d
transition that is in the UV/Vis range.
Chromium(III) examples
Ligand $maxCl- 736
H2O 573
NH3 462
CN- 380
N
N
3 + Fe2+ Fe2+
N
N
3
1,10-phenanthroline ferroin
IR radiation is of too low an energy to excite
electronic transitions.
Absorption is limited to vibrational and
rotational levels.
For liquids and solids, molecular rotation isoften limited so the major type of interaction is
vibrational.
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symmetricalstretching
asymmetricalstretching
scissoring
Due to the large number of vibrational states, IR
spectra can be very complex.
2 4 6 8 10 12 14
wavelength, mC-H
stretchC=O
stretch
C-Hbend
CH2rock
The real strength of IR is its ability to identify
functional groups.
Functional wavenumber wavelength
Group (cm-1) (m)
C-H, aliphatic 3000-2850 3.3-3.5
C-H, aromatic 3150-3000 3.2-3.3
O-H 3600-3000 2.8-3.3
C=O, aldehyde/ketone 1740-1660 5.7-6.0
-CH2Cl 1300-1200 7.6-8.2850-890 13.2-14
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R2NCS
S- Na+n + M
n+ R2NCS
S
M
n
dithiocarbamate
You want to make your measurement at a $max to
minimize errors and achieve maximum sensitivity.
Small error
Large error
identical variationsin wavelength.
The relationship between concentration and
absorption must be established.
Your methodshould only beused in thisrange.
Weve already covered the basics of absorbance
calculations.
Lets look at another type of absorbance
application - using variations in absorbance
during a titration.
This is an alternate to using an indicator and
especially useful for complexometric titrations.
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Experimental
setup
The absorbance of a solutioncan be monitored during atitration.
In this example, stirring will
cause the solution to circulatein the cuvette and theresponse can be measured.
You could also use a pumpor simply transfer a sample atknown intervals.
spectrophotometer
stirbar
The type of titration curve you obtain is
dependent on the reaction involved.
In this example
- the sample does not absorbat the measured wavelength
- the titrant will absorb
equivalence point
Determine what would cause these types oftitration curves.
Normal relaxation process
absorption relaxation
Large jumps are
called internal
conversion.
The process in
well understoodbut is temperature
dependenth%"
h%"
Fluorescence process
absorption fluorescence
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Just because a molecule demonstrates
fluorescence does not mean that there is nointernal conversion. Both processes occur.
The % of molecules that fluoresce is expressed
as the quantum yield.
Q =
We can consider all species can fluoresce but
for most, Q is very small.
rate of fluorescent relaxationtotal relaxation rate
Fluorescence spectra tend to look like
overlapping, mirror images of theoriginal absorption spectra.
The energy range is always lowerbecause some energy is always lost
to vibrational modes.
emission
excitation
300 350 400
Remember that the top
spectra is absorption
and the bottom isemission.
Now we not only need to
deal with the $max for
absorption but theemission $max.
Not many compounds undergo significant
fluorescence.
Aromatic fused ring structures are best.
Highly conjugated double bonds will also show
the effect to a smaller degree.
Basically, we need a rigid structure with limitedvibrational modes of relaxation.
Being an emission technique, it can be very
sensitive and have low detection limits.
Unfortunately, not many species undergo
fluorescence on their own.
We can extend the method by forming complexes
using fluorescent ligands or adding fluorescent
groups to other organic molecules.
OH
N C|
8-hydroxyquinoline
fluorene
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These are emission methods.
Fluorescent intensity = K c
It best to produce a calibration curve.
Must consider both the excitation and emission
$max values.
Look for the most sensitive combination.
intensity
concentration
Calibration curves are often linear over a 106 range.
Self-quenching
region