The spins of neighboring groups of nuclei in a molecule are said to be coupledif
their spin states mutuallyinteract. The interactions, which involve electrons in
the intervening bonds, result in small variations in the effective magnetic fields
experienced by one group of nuclei due to the different orientations of the spin
angular momenta and magnetic moments of those in the neighboring group or
groups, and vice versa. These lead to the splitting of the resonance signal into
two or more components that are shifted slightly upfield and downfield respec-
tively from the position in the absence of coupling, the probabilities of each
being roughly the same because the permitted nuclear spin energy levels are
almost equally populated. Thus, the resonance signals for two single adjacent
nuclei with substantially different chemical shifts are each split into two compo-
nent peaks of equal intensity. Figure 7shows the splitting of the two proton
resonances in dichloroethanal, CHCl 2 - CHO, into a doublet, each with the same
separation between the component peaks, known as the coupling constant, J,
which is measured in Hz. The chemical shift of each doublet is taken to be the
mean value of those of its component peaks.Spin–spin
coupling
256 Section E – Spectrometric techniques
J (^) Hz
CHO
J (^) Hz
CHCl 2
Fig. 7. First order spin-spin coupling of the two adjacent protons in dichloroethanal.
Where there is more than one nucleus in a group, all possible combinations of
spin orientations must be considered, and this leads to further small upfield and
downfield shifts with increased multiplicity of the observed resonances (Fig. 8).
Statistical considerations also lead to variations in the relative intensities of the
components of each multiplet. The following general rules are applicable to
spin-spin coupling between nuclei with the same spin quantum number.
● The number of components in a multiplet signal is given by 2nI+1, where nis
the number of identical neighbouring nuclei in an adjacent coupled group,
and Iis the spin quantum number of the nuclei involved. For proton and
carbon-13 nuclei, whose spin quantum number is^1 ⁄ 2 , the number of compo-
nents is n+ 1 , and this is known as the n+ 1 rule.
● The relative intensities of the components in a multiplet signal are given by
Pascal’s triangle, which is based on the coefficients of the expansion of
(a+b)n. For proton spectra, the n+ 1 rule and Pascal’s triangle lead to the multi-
plicities and relative intensities for an observed resonance signal when there
are nadjacent identical nucleias shown in Table 5.
● In saturated structures, the effect is generally transmitted through only three
bonds, but in unsaturated structures it is transmitted further, e.g. around an
aromatic ring.
Commonly encountered spin-spin splitting patterns for two coupled groups
in the proton spectra of saturated molecules are illustrated in Figure 8. These are