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.
I assume that carbon shifts also should allow a clear distinction between the two tautomers, since beta-carbons in the enamin part are normally found at higer field (ca. 5ppm ?). Are there carbon shift reported ?
ReplyDeleteHello Heinz,
ReplyDeleteI agree that these two tautomers can be differentiated by 13C chemical shifts. However, neither reference (Left structure: J.Heterocycl. Chem., 2002, v.39, p.131.) report any 13C NMR data.
In cases where the unknown contains multiple nitrogen atoms, many elucidators will collect 15N NMR data to assist in the elucidation process.