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precession of electrons attached to substituent - PowerPoint PPT Presentation


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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.

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Presentation Transcript
slide1

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 generatediamagnetic anisotropy

slide2

Diamagnetic anisotropy of aromatic rings

change of sign at an angle of 54º44from the plane of the ring;

it is the zero value of the expression3cos2q-1, the magic angle in NMR;

slide4

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

slide5

Heq

Hax

slide7

Empirical Correlations

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

d = 0.23 + DX + DY