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Differentiating Tautomers using 15N chemical shift information

As with many nitrogen-containing compounds, 15N chemical shift information can be critical in elucidating or confirming a candidate structure. Many elucidators extract 15N chemical shifts from such experiments as 15N NMR, 1H-15N HSQC or 1H-15N HMBC.

Below are two tautomers, from a previous blog, that differ in the hybridization state of the nitrogen. The 15N chemical shift for the structure on the left is expected somewhere between 260-300 ppm (referenced to NH3 (liq.)) whereas the 15N chemical shift for the structure on the right is expected around 160-190 ppm.

15n_nhoh_aug52008