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Author: Subject: Help with FT-IR spectra
Bender84
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[*] posted on 10-12-2021 at 09:30
Help with FT-IR spectra


Hello World,

Recently I got the opportunity to work with FT-IR, so I'm quite new to this topic. Nonetheless I try to interpret as much as possible by my own and not to rely solely on the automatic matching software. Usually I'm quite comfortable with the samples I'm receiving to check, but this time I'm really stuck :( (see the attachment).

Does anyone have an idea what substance this actually may be? Or at least in what group of compounds should I search to try to identify this sample?

I'm quite sure that this is some benzoate but I can't figure out the peak at 3467 cm^-1. I don't know if it's a N-H stretch or water. The sample is a solid, transparent crystal. Colourless to slightly yellow. It melts at approx. 140*C.

Cheers.

IMG_20211210_180402_edit_202972413132048.jpg - 772kB
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Bedlasky
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[*] posted on 10-12-2021 at 22:24


That peak at 3467 would be NH. OH peak is usually broad, round.

From what your sample came from? You should have some basic idea what can be in your sample before analysis.




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Bender84
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[*] posted on 11-12-2021 at 06:44


I was told it is some TEMPO derivative. I GUESS it may have something in common with 4-hydroxy-TEMPO benzoate, but the peak at 3467 cm-1 doesn't fit.

I know that OH usually gives a broad band at 3000-3500 cm-1, but a strong sharp peak at 3550-3450 cm-1 is also expected for dimers.
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Lionel Spanner
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[*] posted on 14-12-2021 at 07:12


+1 for 3467/cm being an N-H amine stretch. Water would appear as a huge hump obscuring all the other peaks in the range.

The big peak at 1695/cm is a carbonyl stretch.

The large amount of peaks at lower frequencies suggest there's an aromatic system of some kind.

Beyond that you'd need more data to identify it positively - reactivity, NMR, MS, that sort of thing.




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Bmoore55
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[*] posted on 14-12-2021 at 11:07


If you can send me the data file I could search my database and see if I could find a match. It looks like you have a great spectra and it shouldn't be too difficult to search.
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[*] posted on 17-12-2021 at 06:54


There's a peak near 1700 cm-1 this is a very strong hint that your comopound contains a carbonyl (an oxygen double bonded to a carbon). Also there is no broad stretching in the 3000-3500 region so you probably don't have any OH or carboxylic acids in your molecule.
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[*] posted on 17-12-2021 at 08:16


Have you run it through AIST. that can help you get a rough match to another spectrum



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Bender84
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[*] posted on 17-12-2021 at 13:46


Hello Everyone,

Thanks for the answers!

I'm aware how the OH peak should look like, but I know that it is not always the case. Sometimes, although not to common, the OH peak is sharp and looks similar to N-H stretch band. As I wrote I would expect the N-H stretch to be at a lower wavelength.

Compare the spectra of 4-hydroxy-TEMP, free radical and its derivative: 4-hydroxy-TEMPO benzoate, free radical. Both have a piperidine ring in their structure but only 4-hydroxy-TEMPO has a (relative) sharp peak at approx. 3500 cm-1. Therefore I concluded that the peak I'm interested in and present in the spectra I posted, is due to OH group.

I obtained some "raw" crystals. They were contaminated with some petroleum products/ oil and looked yellow (from the smell I'm sure that in the liquid covering the crystals definitely some amine was present). I washed the crystals with IPA few times and then few times with demi water. The crystals were white after washing and looked like sugar. They did not dissolve in IPA or water. They do dissolve in acetone (not to easily, though) and THF (v.easy).

I dried the product for 24hrs at 100 degC in a lab oven. The crystals were transparent and had a slight yellow tint. I checked the melting point and it started to melt at 134.9*C. Crystals were fully melted at 147.7*C. I put some washed product in the test tube (1) and melted them few times. The liquid was orange (2) and after the crystals froze once again they were orange as well (3-7; each time the liquid became darker). I checked the spectra of the orange crystals and it wasn't to different from the white crystals. So I melted them once again and finally brought to a boil. When the melt coolled down it was a very dark red and very viscous liquid. I checked the spectra of this substance and there was no noticeable peak at ~3500 cm-1. The rest of the spectrum looked more or less the same. The best match was 4-hydroxy-TEMPO benzoate free radical, although I'm quite sure it is not the case or at least not the whole story.

I tried to recrystallize the substance in acetone in THF and I got a light orange crystals. Few minutes after the dissolution the solvents turned pinkish/ orange-ish and took ages to evaporate (probably because I did it in a test tube, because I wanted to obtain larger crystals).

@Traflic Acid
Yes, I did. Thanks to your advice :D Unfortunately with no results (I set the allowance to +/- 30).

@Bmoore55
Thanks! I'll send you spectra (.spa) file.

Best!

Edit: I forgot to mention that the vivid orange crystals on a single picture is 4-hydroxy-TEMPO benzoate free radical (,for visual comparison).


melting.jpg - 995kB spectra comparison.jpg - 656kB

4-BzO-TEMPO_free_radical.jpg - 424kB

[Edited on 17-12-2021 by Bender84]

[Edited on 17-12-2021 by Bender84]
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