|
The chemical shift given by a particular proton is the frequency difference between its absorbtion and the absorbtion from tetramethylsilane, TMS, Si(CH3)4. The actual chemical shift depends on the radio frequency used, which in turn depends on the magnetic field strength. The shift can be made field-independent by defining it thus:
d = (n – no) x 106
= Dn x 106
no no The value of d is quoted in parts per million – this means that for a machine operating around 200MHz the shifts correspond to frequency differences of a few hundred Hz.
The chemical shift depends on the environment of the hydrogen atom concerned. The local external magnetic field, the one with which the nucleus interacts, is not the same as the applied external field. This is because the nucleus is shielded from this field to a greater or lesser extent by the other atoms in the vicinity and their electrons. The effect can be seen in the low-resolution spectrum of ethanal, CH3CHO, given below.
The less the shielding experienced, the higher the chemical shift. The relatively exposed hydrogen in the aldehyde group has d = 9.8, whereas the more heavily shielded hydrogen atoms in the methyl group are found at d = 2.2. TMS hydrogen atoms are by definition found at d = 0.
Typical 1H chemical shifts relative to TMS = 0.
RCOOH carboxylic acids +9 to +13 Can be broad and solvent-dependent RCONH2 amide +5 to +12 Broad and solvent-dependent RCHO aldehyde +8 to +10 Sharp C6H6 etc aromatics +6 to +10 R2C=CHR alkene +4 to +8 RNH2 amines +1 to +6 Broad and solvent-dependent ROH alcohols +0.5 to +8 Broad and solvent-dependent RCH2R methylene +1.5 to +4.5 RCH3 methyl 0 to +4
Chemistry contents NMR introduction Basis of NMR Proton environment and NMR spectra
