Paint Binders
Though paint binder molecules may have a characteristic
spectrum with both FTIR and Raman methods, their spectra can be easily masked
by other molecules in the paint. FTIR has the advantage here, because
the pigment molecules that mask the binder peaks in the Raman spectra are
not well absorbed by the IR. (12) The FTIR spectra can confirm the presence
of binders such as polyurethanes because of the polar C=O group and the
polar amine bonds. The polyurethanes are Raman active, but are not
readily shown in a spectrum of a paint sample.
Melamine and styrene are commonly used to crosslink the primary binder, helping with the film properties. These molecules have characteristic spectra with both methods, but are also partially masked in the Raman spectra by pigment molecules.
Melamine and styrene are commonly used to crosslink the primary binder, helping with the film properties. These molecules have characteristic spectra with both methods, but are also partially masked in the Raman spectra by pigment molecules.
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| Web MO Output Vibrational Frequencies Molecule Symmetry = C3
The C3 point group has only 2 irreducible
representations, A and E.
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Web MO Output Vibrational Frequencies Molecule Symmetry = CS
The CS point group has only 2 irreducible representations, A' and A''. |
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The symmetry labels listed here are a reflection of
the irreducible representation for a given symmetry operation. This is a
part of Group Theory. If you are interested in learning more about this
concept, the following site gives a good mathematical basis for the theory
The simple version is that a vibration will be infrared active if it belongs
to an irreducible representation of the point group that corresponds to one
of the three axes (x, y, or z). In other words, if the vibration causes
a change in the location of the individual atoms of a polar bond along one
of the axes, it will change the dipole moment and be IR active. The
vibration will be Raman active if it belongs to an irreducible representation
of the point group that corresponds to the quadratic functions of x, y, and
z (basically, the squares of each or combinations of any - these are shown
at the right hand side of the character table). (14)http://mathworld.wolfram.com/IrreducibleRepresentation.html and the following sites give more information about symmetry, point groups and character tables: http://bilbo.chm.uri.edu/CHM501/lecture10.html http://www.chemistry.nmsu.edu/studntres/chem639/cgi-bin/group1.cgi This site explains how to determine the vibrational frequencies associated with each of the symmetry operations! Pretty Cool! http://vergil.chemistry.gatech.edu/notes/grpthy-vib/node3.html |
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Web MO IR Spectrum
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Web MO IR Spectrum
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| Many of the melanine peaks are in the fingerprint
region of the IR spectrum. This means that in a mixture, they will probably
blend in with other peaks in this region. Since forensic scientists
usually have a reference spectrum for comparison, the peaks from melanine
will still give a unique pattern that will be easily matched. |
Only three of the major peaks in styrene fall outside of the fingerprint region. These three peaks may be more observable in the spectrum of a paint mixture. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Raman Spectra http://www.aist.go.jp/RIODB/SDBS/cgi-bin/direct_frame_top.cgi?lang=eng 
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Raman Spectra http://www.aist.go.jp/RIODB/SDBS/cgi-bin/direct_frame_top.cgi?lang=eng 
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| Notice that most of the Raman peaks for melanine
are in the lower part of the spectrum. This is where many of the pigments
absorb, so these peaks tend to get masked. |
Notice that the Raman peaks for styrene are
spread over a wider range than those for melanine. They still tend to
get masked, however, by the pigment molecules in the Raman spectrum. |
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