The position of an NMR peak is given by its chemical shift, symbol δ, measured in parts per million (ppm). The chemical shift is measured relative to a reference compound, which is given δ = 0 ppm.Chemical shift depends on the chemical environment of the nucleus. The electrons around a nucleus shield it from the external magnetic field.• A nucleus near an electronegative atom (O, N, Cl, Br) or a C=O group has electron density pulled away from it. It is deshielded and has a larger chemical shift.
• A nucleus surrounded only by C and H is more shielded and has a smaller chemical shift.On an NMR spectrum the δ axis runs from right to left: 0 ppm is on the right, and values increase to the left.Typical ranges are listed on the OCR Data Sheet, and you should use it in the exam rather than memorising values. The ranges are wider for 13C (about 0 to 220 ppm) than for 1H (about 0 to 12 ppm).Worked example: chloroethane, CH3CH2Cl, has two carbon environments. The CH2 carbon is bonded to the electronegative chlorine atom, so it is more deshielded and has the larger chemical shift. The CH3 carbon, further from Cl, has the smaller shift.Because δ is a ratio relative to a reference, the same compound gives the same chemical shifts on any NMR machine.
• A nucleus surrounded only by C and H is more shielded and has a smaller chemical shift.On an NMR spectrum the δ axis runs from right to left: 0 ppm is on the right, and values increase to the left.Typical ranges are listed on the OCR Data Sheet, and you should use it in the exam rather than memorising values. The ranges are wider for 13C (about 0 to 220 ppm) than for 1H (about 0 to 12 ppm).Worked example: chloroethane, CH3CH2Cl, has two carbon environments. The CH2 carbon is bonded to the electronegative chlorine atom, so it is more deshielded and has the larger chemical shift. The CH3 carbon, further from Cl, has the smaller shift.Because δ is a ratio relative to a reference, the same compound gives the same chemical shifts on any NMR machine.