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We found an unknown of exact mass 217.082. Is there a tool that one can search by an exact mass get a list of possible compounds. We have considered desmethylnaproxen as a possibility, but found its mass as 217.086 for the proton adduct.

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    $\begingroup$ What's the error bar on either measurement? $\endgroup$
    – Zhe
    Feb 14 '19 at 15:25
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I guess that would be what you are looking for: http://www.chemcalc.org/mf_finder/mfFinder_em_new This one gives you theoretical combinations of atoms, that fit the mass.

You could also check the sdbs database for organic compounds. https://sdbs.db.aist.go.jp This one gives you existing compounds but it is rounding to 1 decimal so it gives you everything with 217.1 which is not perfect.

I just used the frist calc and then checking all close results (green) at SigmaAldrich to see, which formula matches a compound they sell. There are just two from the List: $\ce{C10H16ClNO2}$ and $\ce{C11H11N3O2}$

So I would focus on these.

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  • $\begingroup$ Thanks. I suppose though that without the proton adduct that 216.074 is the actual weight I'm looking for. I'll try that on the sites you provided. Our precision on a Thermo Q-Exactive is at least three decimal places. $\endgroup$ Feb 14 '19 at 18:21
  • $\begingroup$ @Michael Muehlbauer: Please see the Late addition in my answer. $\endgroup$ Feb 15 '19 at 18:28
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By your question, I assume you may be taking Spectroscopic Analysis of Organic Compounds course. I'm not sure which book you are using but I recommend Silverstein's book (Ref.1), Spectrometric Identification of Organic Compounds, which is better (I have only 7th edition, but current edition is available as well). That's said, this book lists formula masses of possible compounds using Formula masses of different combinations of $\ce{C, H, N}$, and $\ce{O}$ to four decimal points (You have given exact mass of your compound with only three decimals). These formula masses in the book are based on the exact mass of the most abundant isotope of each element. Well, following are the closest given in Appendix A of Ref.1:

Formula mass: 217.0852 ($\ce{C11H11N3O2}$); Formula mass: 217.0865 ($\ce{C13H13O3}$); and Formula mass: 217.0892 ($\ce{C16H11N}$).

Late Addition:

Formula mass: 216.0774 ($\ce{C11H10N3O2}$) (Ref.1, pp. 62).

Reference:

  1. R. M. Silverstein, F. X. Webster, D. J. Kiemble, Spectrometric Identification of Organic Compounds, 7th Edition; John Wiley & Sons, Inc.: Hoboken, NY, 2005.
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