Notice the two protons, Ha and Hb, on 1,1-dibromo-2,2-dichloroethane. Because of their proximity,
the magnetic environment of Ha can be affected by Hb, and vice-versa. Thus, at any given time, Ha
can experience two different magnetic environments because Hb can be in either the α- or the β-
state. The different states of Hb influence the nucleus of Ha, causing slight upfield and downfield
shifts. There is approximately a 50% chance that Hb will be in either of the two states, so the
resulting absorption is a doublet: two peaks of identical intensity, equally spaced around the true
chemical shift of Ha. Ha and Hb will both appear as doublets because each one is coupled with one
other hydrogen. To determine the number of peaks present (as doublets, triplets, and so on), we use
the n + 1 rule: if a proton has n protons that are three bonds away, it will be split into n + 1 peaks.
(One caveat: do not include protons attached to oxygen or nitrogen.) The magnitude of this
splitting, measured in hertz, is called the coupling constant, J.
KEY CONCEPT
The splitting of the peak represents the number of adjacent hydrogens. A peak will be split
into n + 1 subpeaks, where n is the number of adjacent hydrogens.Let’s try a molecule that has even more coupled protons. In 1,1-dibromo-2-chloroethane, shown in
Figure 11.6, the Ha nucleus is affected by two nearby Hb nuclei, which together can be in one of four
different states: αα, αβ, βα, or ββ.
Figure 11.6. 1,1-Dibromo-2-chloroethaneAlthough there are technically four different states, αβ has the same effect as βα, so both of these
resonances occur at the same frequency. This means we will have three unique frequencies, αα, αβ
or βα, and ββ. Ha will thus appear as three peaks (a triplet) centered on the true chemical shift, with
an area ratio of 1:2:1.