Naphthalene impurities spectroscopy

[woodywood]

Dear all,

I was doing cleaning of naphthalene through zone refinement. After it is finished impurities from the naphthalene concentrate completely in one place of the ampule (lower part). I was interested to see which type of impurities are collected and did a spectroscopy measurement. Couple of samples were measured (NS1 - NS5) which you can see on figure I attached. There are clear absorption peaks on the NS3 absorption line.

My question is, is anyone familiar with elements, molecules or any other particles that would absorb light in this way?To me it seems that 622 nm, 595 nm, 572 nm, 549 nm and 526 nm are actually the same transition only different vibronic levels. Therefore I would say that these belong to the same particle. Maybe even 686 nm and 655 nm could belong there although they have a little bit higher spacing. For 512 nm, 463 nm and 445 nm I have no idea how to relate them.

I searched these wavelengths, but unfortunately I could not find any candidate that would fit.

Another constraint. Since these are spectra obtained from naphthalene I would suppose that impurities must be related to the process of naphthalene synthesis. That is only my assumption.

I would be very grateful if someone could help me about this.

Best.

 

[Bystander]

Naphthalene is one of a series of related polycyclic aromatics; the "chickenwire" family.

 

[ModestyKing]

Hi,
I'm not an expert, but looking at this (http://nopr.niscair.res.in/bitstream/123456789/2661/1/IJPAP 45(9) 714-720.pdf), it's possible you have some compound with atom weights similar to Carbon (like Boron or Nitrogen, both common enough interstitial defects in naphthalene). The peaks you are seeing may be due to B-B-B bending, B-C-B bending, B-C-C bending, same for nitrogen. The paper above that I just Googled has CCC bending showing up in Raman as a peak at 508. I figure the different closely related peaks (esp. in the 500's) are simply due to different configurations of the triple atom bending happening, whether it be B-C-C, C-N-C, etc. So it may be you just have a big enough density of interstitial defects all in one place that you're now getting a good signal.

(If I misinterpreted your experimental procedure or anything, sorry!)

 

[ModestyKing]

Sorry for the double post, I don't seem to see an edit button for my previous one.
-- I ran a DFT calculation of the Raman spectrum of Naphthalene, but with a Nitrogen interstitial in one of the sites (see attached picture). I got a spectrum relatively close to what you got, so it may be Nitrogen interstitials - the frequencies were off by around 5-10, but it could be just because I ran it with pretty basic methods/basis sets. Anyways, good luck!

 

[woodywood]

Hi,

thank you both for replying, specially to ModestyKing.

I would like to ask you if you could upload spectrum you obtained there. Also, I am interested if you could give me some more details on the program you are using to calculate it.

Anyway I really appreciate your help about this issue I have.

best!

 

[ModestyKing]

Hi there,
I've blown it up on the same range as your data so you can see the minor peaks as well. This isn't exactly your spectrum - for example, it's missing the mode at 655, and a couple others are too small to see even when blown up, but this wasn't exactly a professional-grade calculation (to make it a quick calculation, I used an easy functional, not too accurate integration grids, small basis set, etc.). Plus, other peaks could correspond to the Nitrogen being interstitial'd at different locations in your Naphthalene, I only did that one location. It's worth looking into, though I can't say this is for sure the right answer.

I used ORCA, which is a quantum chemistry code that has a lot of documentation and easy-start guides for all the calculations I did, just Google them to find the different websites (ORCA has its manual, plus a user wiki, plus a forum, all on different websites/servers, so it can be hard to find at first). I then used Gabedit to visualize the spectrum results and vibrational modes. I pulled basic Naphthalene coordinates off of a NIST database I think, but I optimized it quantum mechanically in ORCA first before running my calculations.

 

[woodywood]

Thank you very much the information about the software. I will do some simulations and see what comes out.