KEY CONCEPT
Wavenumbers (cm–1) are an analog of frequency. whereas More complex vibration patterns, caused by the motion of the molecule as a whole, can be seen in
the 1500 to 400 cm–1 range. This is called the fingerprint region because the specific absorbance
pattern is characteristic of each individual molecule. Spectroscopy experts can use this region to
identify a substance, but you won’t ever need to use it on the MCAT.
For an absorption to be recorded, the vibration must result in a change in the bond dipole moment.
This means that molecules that do not experience a change in dipole moment, such as those
composed of atoms with the same electronegativity or molecules that are symmetrical, do not
exhibit absorption. For example, we cannot get an absorption from O 2 or Br 2 , but we can from HCl or
CO. Symmetric bonds, such as the triple bond in acetylene (C 2 H 2 ), will also be silent.
KEY CONCEPT
Symmetric stretches do not show up in IR spectra because they involve no net change in
dipole movement.CHARACTERISTIC ABSORPTIONS
For the MCAT, you only need to memorize a few absorptions. The first is the hydroxyl group, O–H,
which absorbs with a broad (wide) peak at around one of two frequencies: 3300 cm–1 for alcohols,
and 3000 cm–1 for carboxylic acids. The carbonyl of a carboxylic acid pulls some of the electron
density out of the O–H bond, shifting the absorption to a lower wavenumber. The second is the
carbonyl, which absorbs around 1700 cm–1 with a sharp (deep) peak. In Table 11.1, notice how the
bond between any atom and hydrogen always has a relatively high absorption frequency and how,
as we add more bonds between carbon atoms, the absorption frequency increases. N–H bonds are
in the same region as O–H bonds (around 3300 cm–1), but have a sharp peak instead of a broad one.