Chemical shift

- Inductive effects in alkanes-Anisotropy-Inductive effects in alkenes-δ values for different groups-13C chemical shift in different groups-Solvents-Rotation

Inductive effects in alkanes

Inductive effect and chemical shift

δ-values: Hybridization effects

δ values: sp3 < sp2 < sp

Carbons with sp2 hybridization hold the electrons closer to the nucelus than do sp3 carbons. This leadsto less shielding of the attached protons. Thus vinyl hydrogens have higher chemical shifts than alifatic hydrogens

Acetylenic protons (sp-hybridized C) would be expected to have a higher chemical shift than vinyl protons.

This is not true due to magnetic anistropy

Magnetic anisotropy

A magnetic field with non-uniform density or non-sperical distribution

The presence of π-electrons in a magnetic field sets up a small, localcurrent and an anisotropic magnetic field in its close proximity

Applies only to atoms with sp2 and sp hybridization

Inductive effects i alkenes

Alkynes

Chemical shift overview

Alkaner: δ-values

Alkener: δ values

Benzene derivatives: δ values

13C-Carbonyl chemical shift

13C-cyclic compounds

13C-Arenes

Cyclic alkenes

N-H and O-H chemical shift

δ: very dependent on solvent and concentration

R-O-H: 0,5 < δ < 4,5

R-N-H: 1,0 < δ < 5,0

Carboxylic acids (usually dimeric): 10 < δ 12

Enols: 12 < 18

In deuterated protic solvents: N-H, O-H and S-H can rapidly exchangeD with solvent and the 1H-NMR signal disappears.

Solvents

Non polar solvents: Small effect on chemical shift

Polart + benzene : significant effect on chemical shift

Slow rotation