NMR SpectroscopyChemical ShiftsChemical shifts are always reported in δunits:δ= (Downfield shift from Me4Si in Hz)(Spectrometer frequency, MHz)109876543210δppmHROHHXHX=O,Cl,BrXHX=N,SAlkanesHH1H Chemical ShiftsThe ranges above provide an estimate of the chemical shift for simple molecules, but don't help very much whenthere are multiple substituents. A simple scheme can be used to estimate chemical shifts of protons on sp3carbons. Use a base shift of δ0.9 for methyl groups, δ1.2 for CH2groups, and δ1.5 for CH groups, and add to these thefollowing values for each αsubstituent:OC(=O)R3.0OR2.3Br2.2Cl2.4Aryl1.4C(=O)R1.0C=CPhClOHBase shift CH:1.5αPh:1.4αOH:2.35.3Base shift CH2:1.2αCl:126.96.36.199Calculated:Calculated:Observed:4.8Observed:3.65BoincreasesνodecreasesUpfieldShieldedBodecreasesνoincreasesDownfieldDeshieldedHigh frequencyLow frequency•e-BeBoB = Bo- Beνo= γB/2πHA(magnetic field at nucleus)(Larmor precession frequency of HA)Chemical shifts have their origin in the circulation of electrons induced by the magnetic field, which reduces the actualfield at the nucleus. Thus a higher magnetic field has to be aplied to achieve resonance.ReichChem 345HOHHPLT j:\_345\345-nmr-handout.plt p. 1/19 Scale: 0.65
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