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

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;

Diamagnetic anisotropy of triple and double bonds shielded shielded

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

Heq Hax

Empirical Correlations for methylene of the type Y-CH2-X by adding Di to the chemical shift of methane; d = 0.23 + DX + DY

substituent constants Zi are added to the chemical shift of ethene; d = 5.28 + Zgem + Zcis + Ztrans

substituent constants Si are added to the chemical shift of benzene; d = 7.27 + ∑Si