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Offline patata

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Raman spectra assignment
« on: October 09, 2014, 11:21:53 AM »
I would like some help with determining the symmetry group of an oligomer!
The oligomer is the one shown in this page: http://www.designermoleculesinc.com/files/BMI-1700_(R1191)_-_Imide_Extended_BMI.pdf and it would be really helpful if anybody had an answer about its symmetry group in order to help me assign correctly its raman spectrum.

Online mjc123

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Re: Raman spectra assignment
« Reply #1 on: October 09, 2014, 12:01:02 PM »
That, in a conformation it would adopt in reality, has no symmetry - it would be C1. You are never going to be able to do a point group analysis to predict the Raman spectrum. The best you can do is consider the approximate symmetry of the various portions of the molecule, e.g. you would expect the symmetric C=O stretch of the imide to be stronger than the asymmetric stretch. Also compare the spectra of model compounds, e.g. phthalimide, Bisphenol A and a long chain alkane.

Offline patata

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Re: Raman spectra assignment
« Reply #2 on: October 09, 2014, 01:56:51 PM »
Thank you very much for the answer mjc123. Could you please do me one more favour and propose a good book to me in order to gain basic knowledge on these things? I just begun to work with Raman, as you can see, and working with polymers peaks assignment isn't the most easy thing.I found much literature on IR but not so  much on Raman. Thank you very much in advance for your help.

Offline Corribus

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Re: Raman spectra assignment
« Reply #3 on: October 09, 2014, 02:14:40 PM »
Raman and IR are complimentary techniques. The functional group transitions show up at virtually the same place but the selection rules are different, so weak peaks in IR can be strong in Raman. As mjc123 mentioned, with the exception of the simplest molecules, these techniques are more useful for functional group identification. Big molecules are, to use a very scientific term, floppy, and do not often vibrate as a single coordinated whole. Vibrations are coupled to some degree, of course, but to an approximation, a carbonyl stretch on one end of a big molecule vibrates independently from an amino group on the other end of a big molecule. This is true for both Raman and IR.

The best book I know for a good intro to practical vibrational (including Raman) is Physical Methods for Chemists by Russell Drago, but I'm fairly certain it's out of print. This is a good book on physical methods anway. If you're just looking for where certain peaks show up in a spectrum, MOST companies sell spectral libraries that include entire maps for common polymers. They'll even try to identify your sample based on the spectrum. This is certainly true for IR spectra and may be available on Raman instruments as well.

http://www.amazon.com/Physical-Methods-Chemists-Russell-Drago/dp/0030751764

Spectrometric Identification of Organic Compounds by Silverstein and Webster has a lot of information about experimental IR transitions, and many of these will be identical (by absorption energy) for Raman.

There are also good textbooks purely on Raman spectroscopy, but whether you need one of them depends on the level of detail you plan on going into, and what your physical chemistry and spectroscopy background is.

May I ask, is there a particular reason you're using Raman instead of FTIR? The latter is sufficient in most cases for characterization and identification of organic compounds. Raman is a more specialized technique.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline patata

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Re: Raman spectra assignment
« Reply #4 on: October 10, 2014, 04:41:07 AM »
I have just started my phd and the bigger part of it is evaluating stress concentration in graphitic structures during tensile loading. At that time although, I have to study the polymerization of BMI 1700 with a trifuran catalyst via Raman spectroscopy in order to measure reaction kinetics, as a deliverable for the programm which is paying for my phd. Getting in touch for the first time with polymers spectra I thought that a good sequence of steps would be taking an extended spectrum of the material, finding out which peaks correspond to which bond vibrations and then selecting a peak which remains constant during polymerization and another which will be decreasing in order to plot it against time and evaluate the order of the reaction and its kinetic characteristics. But the literature seems to luck a little bit in Raman spectra of the material that I am using and thats why I am trying to do the assignment by myself. The main reason for using Raman instead of IR is that this is the available technique in my lab, considering the time needed for such a study I couldn't work in another lab.

Offline Corribus

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Re: Raman spectra assignment
« Reply #5 on: October 10, 2014, 09:41:25 AM »
Ok, makes sense. Be aware that it's unlikely you will be able to assign every peak in the vibrational spectrum. Especially in polymers, which have a lot of inhomogeneity, a lot of transition regions just turn out to be big blobs over overlapping peaks. (Though, smart libraries do a good job of identifying polymers from these blobs). What you want to do is look for specific tell-tale functional groups, often carbonyls and the like.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

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