The first region, from 4000 to 2500 cm⁻¹ is the region for C–H, N–H, and O–H bond stretching. Most of the atoms in an organic molecule (C, N, O, for example) are about the same weight (12, 14, 16...). Hydrogen is an order of magnitude lighter than any of these and so it dominates the stretching frequency by the large effect it has on the reduced mass, so any bond to H comes right at the left-hand end of the spectrum. Even the strongest bonds between non^-H atoms—triple bonds such as C≡C or C≡N— absorb at slightly lower frequencies than bonds to hydrogen: these are in the next region, the triple bond region from about 2500 to 2000 cm⁻¹. This and the other two regions of the spectrum follow in logical order of bond strength as the reduced masses are all about the same: C=C and C=O double bonds appear about 2000–1500 cm⁻¹ and at the right-hand end of the spectrum come single bonds, below 1500 cm⁻¹. These regions are summarized in this chart, which you should memorize.

Looking back at the spectrum of cyanoacetamide on p.65, we see peaks in the X–H region at about 3300 and 2950 cm⁻¹, which are the N–H and C–H stretches of the NH₂ and CH₂ groups. The one rather weak peak in the triple bond region (2270 cm⁻¹) is the C≡N group and the strong peak at about 1670 cm⁻¹ belongs to the C=O group. We shall explain soon why some IR peaks are stronger than others. The rest of the spectrum is in the single bond region. This region is not normally interpreted in detail but is characteristic of the compound as a whole rather in the way that a fingerprint is characteristic of an individual human being—similarly, it cannot be ‘interpreted’. It is indeed called the fingerprint region. The useful information from this spectrum is the presence of the C≡N and C=O groups and the exact position of the C=O absorption.