Chemical Shifts
Chemical shift values, symbolized by δ, are not precise, but typical - they are to be therefore regarded mainly as a reference. Deviations are in ±0.2 ppm range, sometimes more. The exact value of chemical shift depends on molecular structure and the solvent, temperature, magnetic field in which the spectrum is being record and other neighboring functional groups. Hydrogen nuclei are sensitive to the hybridization of the atom to which the hydrogen atom is attached and to electronic effects. Nuclei tend to be deshielded by groups which withdraw electron density. Deshielded nuclei resonate at higher δ values, whereas shielded nuclei resonate at lower δ values.
Examples of electron withdrawing substituents are -OH, -OCOR, -OR, -NO2 and halogens. These cause a downfield shift of approximately 2-4 ppm for H atoms on Cα and of less than 1-2 ppm for H atoms on Cβ. Cα is an aliphatic C atom directly bonded to the substituent in question, and Cβ is an aliphatic C atom bonded to Cα. Carbonyl groups, olefinic fragments and aromatic rings contribute sp2 hybridized carbon atoms to an aliphatic chain. This causes a downfield shift of 1-2 ppm at Cα.
Note that labile protons (-OH, -NH2, -SH) have no characteristic chemical shift. However such resonances can be identified by the disappearance of a peak when reacted with D2O, as deuterium will replace a protium atom. This method is called a D2O shake. Acidic protons may also be suppressed when a solvent containing acidic deuterium ions (e.g. methanol-d4) is used.
| Functional group | CH3 | CH2 | CH |
|---|---|---|---|
| CH2R | 0.8 | 1.3 | 1.6 |
| C=C | 1.6 | 2.0 | 2.6 |
| C≡C | 1.7 | 2.2 | 2.8 |
| C6H5 | 2.3 | 2.6 | 2.9 |
| F | 4.3 | 4.4 | 4.8 |
| Cl | 3.0 | 3.4 | 4.0 |
| Br | 2.7 | 3.4 | 4.1 |
| I | 2.2 | 3.2 | 4.2 |
| OH | 3.3 | 3.5 | 3.8 |
| OR | 3.3 | 3.4 | 3.7 |
| OC6H5 | 3.8 | 4.0 | 4.3 |
| OCOR | 3.6 | 4.1 | 5.0 |
| OCOC6H5 | 3.9 | 4.2 | 5.1 |
| OCOCF3 | 4.0 | 4.4 | / |
| CHO | 2.2 | 2.4 | 2.5 |
| COR | 2.1 | 2.2 | 2.6 |
| COOH | 2.1 | 2.3 | 2.6 |
| COOR | 2.0 | 2.3 | 2.5 |
| CONR2 | 2.0 | 2.1 | 2.4 |
| CN | 2.1 | 2.5 | 3.0 |
| NH2 | 2.5 | 2.7 | 3.0 |
| NR2 | 2.2 | 2.4 | 2.8 |
| NRC6H5 | 2.6 | 3.0 | 3.6 |
| NR3+ | 3.0 | 3.1 | 3.6 |
| NHCOR | 2.9 | 3.3 | 3.7 |
| NO2 | 4.1 | 4.2 | 4.4 |
| SR | 2.1 | 2.5 | 3.1 |
| SOR | 2.6 | 3.1 | / |
| =O (aliphatic aldehyde) | / | / | 9.5 |
| =O (aromatic aldehyde) | / | / | 10 |
| M-H (metal hydride) | / | / |
Read more about this topic: Proton NMR
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