Two of the spectra (2 and 3) show presence of a broad peak, which is normally suggestive of N-H protons. Spectrum 3 also shows the characteristic ethyl multiplets, with the chemical shift of the CH2 group indicating it to be next to a C=O.
Spectrum 2 shows a CH3 single also at about 2 ppm attached to C=O and a rather strange doublet at about 3ppm. This is normally characteristic of N-CH protons, but why a doublet? We will return to that later!
The spectrum of 1 shows a peak at 8 ppm indicating H-C=O, but again
The strange properties of 1 and 2 are due to strong double bond
character about the O=C-N bond (can you draw a resonance structure that
illustrates this?), which in the case of 1 results in separate resonances
fot
For spectrum of 2, the explanation may be:
a) because there are two isomers presents
b) because of coupling between the N-H and the methyl group.
How would you distiguish between these two explanations?