How would you interpret the IR spectrum of your product if it's peaks match the literature spectrum, but the signals are weak? Could the argument that the sample was very dilute and lots of water was in it work as a viable explanation?

What about in a scenario where the experimental IR spectrum has all necessary peaks but is missing a key peak in, for example, the 2000-3000 region?

I synthesized acetylglycine and am trying to see why I am missing a peak. For the purposes of this lab, it's not necessary to have absolute purity of compound but for my own curiousity, i'm trying to figure it out. I'm trying to consider all the physical reasons for why my IR may not match the literature, while my melting point ranges are sufficiently accurate, meaning my product shouldn't have too many impurities.

I believe a dimerization could have taken place where for example, a carboxylic acid terminal of one molecule hydrogen bonds with the carboxylic acid terminal of another. Carbonyl oxygen to hydroxyl group, hydroxyl to carbonyl oxygen. This could lead to a suppression of the hydroxyl (-OH) signal.

  • $\begingroup$ Could you show the data? Is the key peak strong or weak in the literature spectrum? Impurities would not explain a missing peak. What is the expected concentration in your sample (let's say you assume 100% yield), and how does it compare to the concentration in the literature spectrum? Are you using a solvent correction and do your solvents absorb in the 2000-3000 cm(-1) region? $\endgroup$ Sep 26 '19 at 15:29

Intensity is (in general) proportional to sample concentration, so yes, if you are performing IR in solution, then an overly dilute sample would have less intense peaks. But solution IR is somewhat uncommon; I suspect you are either doing IR on your sample in neat form (using an ATR stage) or in a pressed KBr plate? If the latter, you may not have used enough sample.

Regarding your missing peak: sometimes peak intensities can change, depending on your sample prep. For example, if the peak in question corresponds to proton capable of hydrogen bonding (e.g., -COO-H, -NR2-H, -O-H, etc.) then the sharpness and intensity of the peak will depend on the extent of H-bonding your sample, which, in turn, can depend on things like sample concentration, the presence of water, etc. Sometimes these peaks get broadened to the point of being invisible if your sample is not very concentrated.

If your missing peak corresponds to a C-H, C-C, C-O, C-N, or some other non-labile bond, then there really isn't an explanation other than your compound not being what you think it is...

  • $\begingroup$ i was just speculating but in actuality i was using a solid sample and i was trying to see if i placed too little of the 'dust' on the IR spot. but your answer is good and helps me think about the possibilities. Thank you! $\endgroup$
    – Ahmer Imam
    Sep 23 '19 at 18:15

I think all modern teaching labs use attenuated total reflectance (ATR)-FTIR systems... you place some crystals and then collect the spectrum right away. ATR is least sensitive technique because the IR penetrates only only a few nanometers of the sample. In good old school times, one would dry the sample-literally bone dry, then oven-dried KBr was ground with the sample and a pellet was made under intense pressure. The resulting spectrum was a high quality spectrum with reasonable transmission values. Water and remaining solvents can often ruin the IR spectrum. There is always a comprise in life in terms of quality vs. quantity. ATR is fast but spectrum quality is not that great.

  • $\begingroup$ Good answer. I just want to point out that ATR has many uses and pros, otherwise it seems that it only allows for quick analysis. $\endgroup$
    – Alchimista
    Sep 24 '19 at 8:35

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