In an NMR spectrum, the position of a peak is given by its chemical shift, symbol δ, measured in parts per million (ppm). It compares the absorption frequency of a nucleus with that of a reference compound, which is set at δ = 0.Chemical shift depends on the molecular environment of the nucleus. Electrons around a nucleus shield it from the applied magnetic field. When an electronegative atom such as O, N or a halogen is nearby, it pulls electron density away. The nucleus is deshielded and absorbs at a higher chemical shift. So the closer a carbon or hydrogen is to an electronegative atom or a C=O group, the larger its δ value.The ranges of chemical shift for different environments are given in the data booklet, so use it rather than memorising values. The general pattern is:
• 1H shifts run from about 0 to 12 ppm. H in an alkyl chain appears around 1 ppm; H on a carbon bonded to oxygen appears around 3 to 4 ppm; the H of an aldehyde (CHO) appears around 9 to 10 ppm.
• 13C shifts run from about 0 to 220 ppm. Alkyl carbons appear below about 50 ppm; C bonded to O appears around 50 to 90 ppm; C=O carbons appear furthest along, around 160 to 220 ppm.Example: in the 1H NMR spectrum of methoxymethane, CH3OCH3, the single peak is at about 3.3 ppm, because all the H atoms are on carbons bonded to oxygen. In ethane, CH3CH3, with no electronegative atom, the peak is at about 0.9 ppm.
• 1H shifts run from about 0 to 12 ppm. H in an alkyl chain appears around 1 ppm; H on a carbon bonded to oxygen appears around 3 to 4 ppm; the H of an aldehyde (CHO) appears around 9 to 10 ppm.
• 13C shifts run from about 0 to 220 ppm. Alkyl carbons appear below about 50 ppm; C bonded to O appears around 50 to 90 ppm; C=O carbons appear furthest along, around 160 to 220 ppm.Example: in the 1H NMR spectrum of methoxymethane, CH3OCH3, the single peak is at about 3.3 ppm, because all the H atoms are on carbons bonded to oxygen. In ethane, CH3CH3, with no electronegative atom, the peak is at about 0.9 ppm.