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Spectoscopy 2017 3

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Why do we get splitting of signals in NMR spectrum • Splitting occur when different kinds of protons are close enough for their magnetic fields to influence each other – a situation called spin-spin coupling 1 Spin-spin splitting 2 1 Spin-spin splitting 3 The ways in which the magnetic fields of three protons can be aligned 4 2 Splitting patterns Singlet s 8/28/2017 Quartet q Doublet d Triplet 1:3:3:1 2 tCHEM320 Spectroscopy 1:1 1:2:1 Quintet 1:4:6:4:1 Pascal’s triangle 5 6 3 Splitting is observed if the protons are separated by two or three s bonds Long-range coupling occurs when the protons are separated by more than three bonds and one of the bonds is a double or a triple bond 7 More Examples of 1H NMR Spectra 8 4 More Examples of 1H NMR Spectra 9 Coupling Constants The coupling constant (J) is the distance between two adjacent peaks of a split NMR signal in Hertz Coupled protons have the same coupling constant 10 5 Coupling Constants • In compounds where there are free rotation around single bonds, all coupling constants are the same and N+1 rule can be use • However, if free rotation is no longer possible (steric hindrance, cyclic compounds), the N+1 rule can no longer be used. 8/28/2017 CHEM320 Spectroscopy 2 11 A Splitting Diagram for a Doublet of Doublets 12 6 1H-1H Coupling Constants Gem = geminal = on the same carbon 8/28/2017 CHEM320 Spectroscopy 2 13 The trans coupling constant is greater than the cis co ...
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