December 19, 2024, 10:09:08 AM
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Topic: help identifying functional groups/identifying compound present in IR spectrum  (Read 8188 times)

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Offline ForensicInTraining

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I've attached a picture of the spectrum I have to identify. The only thing I think I have figured out is that there's a benzene ring from the peaks around 1582 and 1474, and an alkyl C-H stretch at 2897 which doesn't really help in regards to identifying the compound
 :( if anyone else can point out anything I've missed it would be greatly appreciated, this thing is driving me crazy  ???

Offline TheUnassuming

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I know this isn't what you want to hear, but you need to run the spectra again. 
The contrast the image is horrible to the point of not being able to see the peaks which with the excessive peak picking in the 1600-1400 region makes it nearly impossible to decipher what is going on. 
The massive CO2 peak also makes it hard to tell if the 2400-2600 ish peaks are anything either. 
Re-run the IR to clean it up a bit more.  Then (especially in the carbonyl/benzyl region) pick only the peaks that are big/key.  It looks like you have picked multiple points on fuzzy peaks.  Guessing salt plate or KBr pellet?
Typically when I TA'ed courses with problems like this we gave a list of compounds that were possible.  Were you given a list like that?  What other information have you learned about the compound? 
When in doubt, avoid the Stille coupling.

Offline ForensicInTraining

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Yea I was afraid that it wasn't good enough. I have two other spectra that look pretty much the same and no more sample left to run. These were KBr pellets. I attempted to make another one to hopefully get a better result but the pellet broke with the last of my sample. We're not given a list of any kind, we have to search and match it manually. I wasn't having much luck with that so I was hoping a different set of eyes might pick up something that I overlooked. The large peaks in the 2400-2300 range is present in the background spectrum and the spectrum of a KBr pellet with no sample so I've been ignoring those peaks while trying to identify the compound.

Offline TheUnassuming

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Ah, tough spot indeed.  So what is the full assignment?  You were given a mystery compound and told to run IR to see what you could find out?  An IR will only tell you a general yes/no to given functionalities but without any other knowledge isn't sufficient to tell you exactly what compound you have. 
 
I can't see the actual spectra well in the pic you have so please correct me if this is wrong.  The only real peaks are about 2900 and about 1500 (not in the fingerprint region), with the rest being noise/background?  If thats the case it seems like you have a pretty good answer for the general functionalities present on the molecule.   Knowing what the major peaks in the fingerprint region might also help narrow some specifics down.   
When in doubt, avoid the Stille coupling.

Offline Arkcon

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And to build on what TheUnassuming: said, if you can shut of peak picking, and use some other evidence you may have for what the sample possibly is, you can just highlight and pick peaks of interest.  That will help cleanup the spectrum for you, and anyone else, as well.  At least to show an instructor that you know what's background and what's not.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline ForensicInTraining

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The 2360 and the 2342 is background. We can't run any other tests just IR. It was a white powder and I know its a drug that's all the information I have besides the IR. I think it's aromatic but that's about it. I'm supposed to compare the finger print region to spectra of other drugs. We're then allowed to request that drug from the CLT and do IR on that to see if it matches up.

I've attached a picture of the spectrum without the peak pick.

Offline TheUnassuming

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Great, that picture is much clearer!  So it looks like your real peaks are ~ 2897, 1559, 1488, 1474, and 1225. Too bad those peaks at 24/2600 aren't real, those would very telling. 
So if you know its a drug that makes things a little easier(ish).  This is for an undergrad forensics chemistry course I assume?  What compound classes have you covered so far in the course? 
Its most likely an OTC drug like aspirin/caffine ect as I doubt they would spend a lot of money for a lab assignment and they won't give you anything dangerous in case of accidental exposure. 
The majority of commercial drugs have aromatic groups so you can make a good bet that one or both peaks in that region are from benzyl or similar.  The majority also have at least one heteroatom.  There are no OH/NH2 peaks which tells you something important as well.   
With these facts in mind, look over a good IR table again and see if you can come up with some theories about what all is present on your molecule.  Post what you come up with and we can go from there.
When in doubt, avoid the Stille coupling.

Offline ForensicInTraining

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Yea as far as drugs go the only thing they tell us is that they don't have controlled substances. They do have prescription drugs however. It's an instrumental class so the only thing we go over in lecture is how the instruments work and all the different parts, things like that. We don't cover drug classes or anything like that. The 2897 is a C-H stretch which doesn't help much, the 1559 is the aromatic, I believe it may have a nitrogen from either an N-O stretch (1488) or a C-N stretch in an aromatic amine (1225).

There were so many peaks that got picked up in the fingerprint region that it was hard to tell which was important so thank you for singling some out for me as I was mostly focusing on other peaks. If I'm correct with my interpretation of what these peaks represent I'll have a better idea what to look at when I'm in class again. 

Offline TheUnassuming

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haha... your chem dept must have rather good funding if they are going to give you prescription compounds for IR.  So the class is instrumental analysis (probably 300 level)?
Those seem like reasonable assumptions.  There are a good deal of peaks down in the finger print region which shouldn't be ignored by any means.  The strongest of the peaks could be real and should be considered in your evaluation in context of the already discussed peaks.  Together they could elucidate exactly what you have in your molecule.  The problem is that even in the higher frequency region you have a fair amount of noise, which makes it hard to trust the peaks down in the fingerprint region which aren't any stronger than the background. 
Given the overlap of ranges in the fingerprint region, even with a clean spectra of a known compound it can be hard to attribute a given peak to a specific functionality if the compound has very much complexity at all.   
When in doubt, avoid the Stille coupling.

Offline ForensicInTraining

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Yes it's a 300 level course. I actually figured out today what my drug was, and there was an important peak down there because there was a Cl in the compound. It was clomipramine if you're curious. Thank you so much for helping me, don't think I would've figured it out if I didn't go through my list again after you pointed out different peaks to me.  :)

Offline TheUnassuming

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Of course, glad I could help and its a great refresher for me as well. 
Oohh... good molecule for the assignment. 
Good luck in the course!
When in doubt, avoid the Stille coupling.

Offline ForensicInTraining

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Thank you!

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