1

local electron currents around the nucleus for spherical e-density distribution;

Purely magnetic effects of substituents also can have major effects on proton shielding if the group has a non-spherical shape generating a non-local field;

precession of electrons

Non-sphericalelectron density

distributions generate

diamagnetic anisotropy

Diamagnetic anisotropy

2

Diamagnetic anisotropy of aromatic rings

- change of sign at an angle of 54º44 from the plane of the ring;

- it is the zero value of the expression 3cos2θ-1, the magic angle in NMR;

3

Diamagnetic anisotropy of triple and double bonds

5.25 5.25

shielded

shielded

shielded

4

Diamagnetic anisotropy of C-C single bonds

The larger non-local shielding effect is seen when the C-C bond is perpendicular to B0;

more shielded

less shielded

5

Complex examples for diamagnetic shielding

6

Typical chemical shift ranges of functional groups

7

Empirical Correlations for Chemical Shifts

for methylene of the type Y-CH2-X by adding ∆i to the chemical shift of methane;

δ = 0.23 + ∆X + ∆Y

8

substituent constants Zi are added to the chemical shift of ethene;

δ = 5.28 + Zgem + Zcis + Ztrans

9

substituent constants Si are added to the chemical shift of benzene;

δ = 7.27 + ∑Si