Chemical Shift

The position of an NMR signal is given as its chemical shift, δ, measured in parts per million (ppm) relative to a reference compound.The reference is tetramethylsilane (TMS), Si(CH₃)₄, defined as δ = 0. TMS is used because all 12 of its hydrogen atoms (and all 4 carbon atoms) are in the same environment, so it gives a single sharp peak; this peak is at a lower chemical shift than almost all organic compounds; and it is inert, non-toxic and volatile, so it is easily removed afterwards.Chemical shift depends on how much a nucleus is shielded by electrons. An electronegative atom such as O, N or Cl pulls electron density away from nearby atoms. Their nuclei are deshielded, so they have a larger chemical shift. The closer the nucleus is to the electronegative atom, the bigger the effect.Approximate ¹H shifts (always use the Data Booklet):
• alkyl C-H: 0.5 to 2.0
• H on a carbon next to C=O: 2.0 to 2.6
• H on a carbon bonded to O: 3.3 to 4.3
• H on a benzene ring: 6.5 to 8.0
• aldehyde -CHO: 9 to 10
• carboxylic acid -COOH: 10 to 12
¹³C shifts cover a wider range, 0 to about 220, with C=O carbons at the high end.Worked example: in chloroethane, CH₃CH₂Cl, the CH₂ protons are on the carbon bonded directly to chlorine, so they are more deshielded and appear at a higher chemical shift than the CH₃ protons.