1. Induction, resonance, and hybridization modulate electron density at the proton, as the result of local electron currents around the nucleus;
precession of electrons attached to substituent
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;
Non-spherical electron density distributions generate 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 3cos2q-1, the magic angle in NMR;

3. Diamagnetic anisotropy of triple and double bonds
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;
less shielded
more shielded

5. Heq
Hax

7. Empirical Correlations
for methylene of the type Y-CH2-X by adding Di to the chemical shift of methane;
d = DX + DY

8. substituent constants Zi are added to the chemical shift of ethene;
d = Zgem + Zcis + Ztrans

9. substituent constants Si are added to the chemical shift of benzene;
d = ∑Si