Figure 2. Effect of ¹⁵N labelling for distinguishing the amide and imidic acid tautomers.

Infrared spectra were computed of the amide and Z-Z imidic acid forms of acetamide in bulk water (panels (a,b)), and in combined molecular (two D₂O molecules) and bulk water (panels (c,d)). The chemical structures of the computed molecules are included in the figures. Results for unlabelled tautomers are shown in black, for ¹⁵N-labelled tautomers in red. Tautomerisation to the imidic acid is distinguishable by a prominent 14-cm⁻¹ downshift of the C=N stretch by ¹⁵N labelling in the spectral region of 1,650–1,700 cm⁻¹.