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Topic: Unexplained band in IR  (Read 2824 times)

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Offline snadders

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Unexplained band in IR
« on: December 10, 2023, 09:36:57 AM »
This is my first time posting here so please let me know if there is anything I could improve!

To get right into it, I have prepared the compound ethyl 2,2-dimethyl-5-phenyl-1,3-dioxolane-4-carboxylate and recorded an ATR-IR spectrum of it. Most of it looks as expected except for a weak but prominent band at 2255 cm -1.

The wavenumber makes me immediately think nitrile or alkyne since they have very characteristic bands in that exact region. The problem is, my compound is neither of those or even resembles anything else that absorbs there. The solvents and reagents used to prepare it should not absorb at that wavenumber either. No literature I can find for my compound includes that band. The compound is reasonably pure according to NMR.

I am at a loss as to what causes this. Are there any common laboratory impurities that could cause the absorption (like silicone grease sometimes showing up in NMR)? Could it really be my dioxolane and nobody else reports the band somehow? Has anyone ever had anything similar happen and can maybe steer me in the right direction of what im missing?

Thanks so much in advance for any help and I'm thrilled to be a part of this community now!

Offline Babcock_Hall

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Re: Unexplained band in IR
« Reply #1 on: December 11, 2023, 03:34:47 PM »
This is a long shot, but do you see a strong signal near 1128 cm-1?
« Last Edit: December 11, 2023, 04:09:38 PM by Babcock_Hall »

Offline snadders

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Re: Unexplained band in IR
« Reply #2 on: December 12, 2023, 09:27:43 AM »
Thanks for your answer. It's hard to tell, theres a lot going on in that region since I'm dealing with a molecule with a lot of C-O stretches. What is your suspicion? Maybe there are other signals to look for or I could see it in NMR.

Offline Babcock_Hall

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Re: Unexplained band in IR
« Reply #3 on: December 12, 2023, 10:27:50 AM »
https://www.youtube.com/watch?app=desktop&v=BHFIzSi9t_k

My hypothesis is that you are observing an overtone band; overtones appear at twice the frequency of the main band and with much less intensity.  I found some examples of carbonyl (C=O) overtones showing up around 3500 cm-1, but I was less successful searching for C-O overtone examples, which might appear in this general region.  I saw some signals in the IR spectrum of anisole at AIST which had some weak signals, but they were not assigned, nor there exact frequencies given.  However, my knowledge of IR is quite limited, and I am not sure what effect Fermi resonance, if present, might have on the position of the overtone.  Is there an IR spectrum available online of this compound?

Offline Corribus

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Re: Unexplained band in IR
« Reply #4 on: December 12, 2023, 11:31:34 AM »
Could also be a combination band.

https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Vibrational_Spectroscopy/Vibrational_Modes/Combination_Bands%2C_Overtones_and_Fermi_Resonances

As a general rule, it is difficult to assign every band in an FTIR spectrum of a complex molecule. If you see peaks you cannot explain, I would not lose sleep over it. They could be overtone or combination bands, impurities, etc, with little way to make the distinction. For basic characterization in organic chemistry, you should view FTIR as more of a supportive technique than as a primary identification tool like NMR.
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 snadders

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Re: Unexplained band in IR
« Reply #5 on: December 12, 2023, 12:51:06 PM »
Thanks to you both for the suggestions that it might be overtones. I've also seen them before but usually only for carbonyls as mentioned. Also, it would be weird that out of all the big C-O peaks only one would give an overtone band and also not even at precisely twice the frequency.. There are reported IR bands for the structure but no spectra I could find.

I'm not worried that I made the wrong compound since all other data fits, but I do need to find some kind of explanation for it in the exam report I'm writing, or my professor might ask questions. We've been taught that peaks around the 2000 cm-1 region basically only come from triple bonds and cumulated double bonds.

Offline Babcock_Hall

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Re: Unexplained band in IR
« Reply #6 on: December 12, 2023, 02:02:19 PM »
Methanol (AIST spectral database) has an intense signal at 1030 cm-1 and another signal at 2046 cm-1 that may be an overtone of it.  The IR spectrum of methyl tert-butyl ether (MTBE) has a signal at 2160 cm-1 that may be an overtone of a signal at 1087 cm-1.  Corribus would know better than I would, but overtone frequencies are slightly different from exactly twice the fundamental, owing to anharmonicity IIUC.
« Last Edit: December 12, 2023, 02:21:51 PM by Babcock_Hall »

Offline Corribus

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Re: Unexplained band in IR
« Reply #7 on: December 12, 2023, 02:48:56 PM »
True, ν0  :rarrow: v2 transition is usually slightly smaller in energy than twice ν0 :rarrow: v1 because energy levels get closer together as they approach the dissociation limit. The difference is usually small and may not be obvious depending on how broad the peaks in question are.
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 snadders

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Re: Unexplained band in IR
« Reply #8 on: December 12, 2023, 03:03:40 PM »
That is really interesting and good to know, thank you! Although from what I just read about how anharmonicity affects overtones, they are usually at a lower frequency than a multiple of the fundamental. But that was just from a quick google search, so take it with a grain of salt.

At this point I feel like I'm going way beyond the scope of this exam though, so the overtone explanation is good enough for now. I will look more into this though, it's pretty cool stuff. Thanks again for your help.


Offline snadders

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Re: Unexplained band in IR
« Reply #9 on: December 12, 2023, 03:12:14 PM »
My previous reply was to Babcock_Hall. I didn't see your reply Corribus, sorry. But it confirms what I read, if I understand right.

I should probably add that the tallest peak that could give rise to the overtone is at 1192 cm-1. I feel like that would be quite a big difference.

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