I have already attempted to interpret some of the peaks. Please tell me if they are correct/incorrect and anything that I have missed.

Unknown #49

Unknown #50

  • $\begingroup$ i think you have made those interpretations very cautiously and will full effort because the trend followed by them is clearly visible . i mean as the position increases the intensity decreases i.e. they are inversely proportional to each other i cant detect any wrong or miscalleneous point in your graph $\endgroup$ Jun 28, 2014 at 10:01
  • 1
    $\begingroup$ In your first spectrum, I would be skeptical about the alkynic $\ce{#C-H}$ bond you've identified at approx. $3400\mathrm{cm^{-1}}$, as there is no corresponding alkynic $\ce{C#C}$ signal (typically roughly in the $2100-2300\mathrm{cm^{-1}}$ region). $\endgroup$
    – Greg E.
    Jul 28, 2014 at 18:38

2 Answers 2


In your first spectrum, the band at 3517 $\mathrm{cm^{-1}}$ indeed fits best to the $\ce{N-H}$ stretch of a primary amine (source). However, I would not assign the shoulder of this band to a $\ce{#C-H}$ stretch. Such a band would be sharper and perhaps more intense, and as Greg E. already pointed out in his comment to your question, there is also no corresponding $\ce{C#C}$ stretch band present (reference). The absence of bands in the range between 2500-2000 $\mathrm{cm^{-1}}$ allows you to rule out some other functional groups with double or triple bonds, like nitriles, thiocyanates, isocyanates, carbodiimides or azides. The band at 1715 $\mathrm{cm^{-1}}$ is a typical $\ce{C=O}$ stretch, and while it could be a ketone, the presence of the broad band at 2932 $\mathrm{cm^{-1}}$ (which could actually be a double band) makes it likely to be an aldehyde, since the double band can then be assigned to the $\ce{=C-H}$ stretch of the CHO group (it could also include other $\ce{C-H}$ stretches due to its width).

Your second spectrum appears a bit more complex. The broad, intense band at 3298 $\mathrm{cm^{-1}}$ in combination with the band at approximately 1680 $\mathrm{cm^{-1}}$ could be assigned to an amide, but I would expect a stronger carbonyl band in this case. Nevertheless, the intensity and width of the 3298 $\mathrm{cm^{-1}}$ band suggest the presence of a functional group capable of H-bonding. The bands between 2900-2800 $\mathrm{cm^{-1}}$ could be assigned to alkyl $\ce{C-H}$ stretches (source). For the compound to be aromatic, there should be aromatic $\ce{C-H}$ stretch bands present, slightly above 3000 $\mathrm{cm^{-1}}$ (source).


My interpretation of the first spectrum:

  • I would say that the 3517 band belongs to OH group. This is a typical shape OH group. Amines look more like two bands with a shoulder. However secondary amine is also a possibility. Other bands of OH should be the broad band between 1400 and 1300 from which the sharp bands arise. Than probably 1166, which seems to be tertiary OH group.
  • ≡C−H stretch is not right. Is is an overtone of the 1715 vibration.
  • The four bands around 2932 are CH3 and CH2 stretches.
  • 1461 is an antisymetric deformation of CH3, 1411 is scisorring of CH2 and 1359 is an umbrella of CH3.
  • The rest is a fingerprint region. I assume this is an aliphatic ester molecule.

The second spectrum I am not sure, however:

  • An aromatic vibrations are unlikely, there should be vibrations just over 3000, which are missing and the 2000 region also looks different.
  • There are aliphatic vibration just like in the first spectrum.
  • 3298 is not typical for amine. However I cannot rule it out. It might be an OH-group as well.

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