Glutamic Acid (2-aminopentadioic acid)
Spectrum and carbon skeletal structure obtained from Skeletal structure labeled with Adobe Photoshop.
Fig. 1. Carbon skeletal structure of Glutamic acid. The carbons have been numbered for reference.

Fig. 2. The H-NMR of Glutamic acid, which shows four signals of varying size and intensity. Although four signals are observed, only three correspond to H atoms of Glutamic acid. The largest signal seen is due to the solvent that Glutamic acid had been dissolved in. The signal suggests that D2O was utilized, supported further by the absence of a proton signal for –COOH groups, which would be expected between 10 and 12ppm, and a signal for the amine protons, which would be expected at ~5 ppm and broad in morphology.

Fig. 3. This signal at 4.8 ppm is consistent with D2O, a solvent frequently used for proton NMR.
Fig. 4. This multiplet is observed between 1.9 and 2.18 ppm. The location of this multiplet is consistent with the predicted location of the protons attached to the 3rd carbon because they are farthest from the –COOH group and therefore least hindered by electrons. 15 spikes are observed and are due to extensive coupling of the protons at the 3rd carbon with surrounding protons. Such extensive splitting is due to the chiral center at carbon 2, which renders the surrounding protons to be diastereotopic and non-equivalents. The expected number of spike due to spin-spin coupling would be 24 as each proton of the 3rd carbon would have a triplet of doublet of doublets, which would be 12. As there are two protons on the 3rd carbon, the number is doubled to 24.

Fig. 5. This triplet from ~2.31-2.37 is associated with the two equivalent protons of the 4th carbon, split by the protons on the 3rd carbon. The location of this signal is predicted by the proximity of the 4th carbon protons to the –COOH group, which is expected cause a chemical shift between 2 and ppm. Smaller peaks flanking the main signal may be due to impurities. The splitting of the peaks of the triplet may be due the lone proton on the second carbon, although this would not be expected as splitting rarely occurs when protons are separated by 4 bonds.

Fig. 6. This doublet of doublets, between ~3.73-3.765 is attributed to the proton on the 2nd carbon, split by the diastereotopic protons on the 3rd carbon. The location of these signals is as expected due to deshielding by the –COOH group and the –NH2 group. Both groups may cause deshielding in the 2-3ppm range, but as both groups are proximal to the proton, the deshielding is relatively additive.

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