Spin-spin Coupling
Absorption bands arising from adjacent protons are split into multiplet peaks by a mutual interaction of
the spins. The effect is due to small variations in the effective field experienced by a proton when
neighbouring nuclei can occupy two or more energy levels or spin states. It is transmitted through the
intervening bonds by a tendency for electron and nuclear spins to be paired.
Consider the case of two single (methine) protons HA and HX attached to adjacent carbon atoms and
with quite different chemical shifts (Figure 9.32(a)). The field experienced by HA is increased or
decreased slightly by the two allowed spin states of HX, designated and ↓, and which in the gross
sample are virtually equally populated. This results in the absorption band for HA splitting into a doublet
whose peak intensities are in the ratio 1:1. The effect is mutual in that the two almost equally populated
spin states of HA cause the HX absorption to split into an identical doublet. The spacing
Figure 9.32
Coupling between adjacent single protons. (a) First-order coupling, AX.
(b) Second order coupling, AB → A 2.