Neighbouring group contributions

The neighbouring group contribution arises from the currents induced in nearby groups of atoms. Consider the influence of the neighbouring group X on the proton H in a molecule such as H-X. The applied field generates currents in the electron distribution of X and gives rise to an induced magnetic moment proportional to the applied field; the constant of proportionality is the magnetic susceptibility, χ (chi), of the group X. The proton H is affected by this induced magnetic moment in two ways. First, the strength of the additional magnetic field the proton experiences is inversely proportional to the cube of the distance r between H and X. Second, the field at H depends on the anisotropy of the magnetic susceptibility of X, the variation of χ with the angle that X makes to the applied field. We assume that the magnetic susceptibility of X has two components, χ|| and χ⊥, which are parallel and perpendicular to the axis of symmetry of X, respectively. The axis of symmetry of X makes an angle θ to the vector connecting X to H (1, where X is represented by the ellipse and H is represented by the circle). To examine the effect of anisotropy of the magnetic susceptibility of X on the shielding constant, consider the case θ = 0 for a molecule H-X that is free to tumble (2 and 3). Some of the time the H-X axis will be perpendicular to the applied field and then only χ_⊥ will contribute to the induced magnetic moment that shields X from the applied field. The result is deshielding of the proton H, or σ(neighbour) < 0 (2). When the applied field is parallel to the H-X axis, only χ_|| contributes to the induced magnetic moment at X. The result is shielding of the proton H (3). We conclude that, as the molecule tumbles and the H-X axis takes all possible angles with respect to the applied field, the effects of anisotropic magnetic susceptibility do not average to zero because χ_|| ≠ χ_⊥.

To a good approximation, the shielding constant σ(neighbour) depends on the distance r, the difference χ|| − χ⊥, as (see Further reading for a derivation)

where χ|| and χ⊥ are both negative for a diamagnetic group X. Equation 15.23 shows that the neighbouring group contribution may be positive or negative according to the relative magnitudes of the two magnetic susceptibilities and the relative orientation of the nucleus with respect to X. The latter effect is easy to anticipate: if 54.7° < θ ability of the field to induce a ring current, a circulation of electrons around the ring, when it is applied perpendicular to the molecular plane. Protons in the plane are deshielded (Fig. 15.9), but any that happen to lie above or below the plane (as members of substituents of the ring) are shielded.

Fig. 15.8 The variation of the function 1 – 3 cos2θ with the angle θ.

Fig. 15.9 The shielding and deshielding effects of the ring current induced in the benzene ring by the applied field. Protons attached to the ring are deshielded but a proton attached to a substituent that projects above the ring is shielded.

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