First-Order Spin-Spin Coupling As closer the coupling nuclei as better they can interact and as higher is the coupling constant J. The number of bonds between two coupling nuclei is given as a prefix (e.g. a 3-bond coupling constant is given as 3 J).
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First-Order Spin-Spin Coupling
As closer the coupling nuclei as better they can interact and as higher is the coupling constant J. The number of bonds between two coupling nuclei is given as a prefix (e.g. a 3-bond coupling constant is given as 3J).
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-1/2
+1/2
+1/2
-1/2OR
RO
H H
Simple example for Spin-Spin Coupling
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OR
RO
H H
Each proton has a unique absorption (chemical shift δ) but this absorption is altered by magnetic nuclei nearby. In 1H-NMR other protons but also any other magnetic nuclei such as deuterium, 13C, 19F, and 31P.
First-order coupling is found for absorptions that satisfy ∆ν/J > 10 and any pair of chemically equivalent nuclei must also be magnetically equivalent.
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Pascal’s Triangle
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Spin-Spin Splitting Patterns
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H
HH
RO
RO
CH3
CH3
CH3
C-H sees CH2 protons (+1, 0, -1)
CH2 sees C-H proton (+1/2, -1/2)
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1H-NMR of 1,1,2-Trichloroethane
CH
CH2
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Typical A2X3 ethyl pattern
quartet
triplet
H-coupling in ethylbenzene
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H-coupling and Integration
isopropylbenzene
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1
6
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1H-NMR of Ethanol in CDCl3The proton is not on the O-atom of an individual molecule long enough for it to be affected by the methylene group!
CH3OHCH2
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1H-NMR of Ethanol in DMSO
No exchange!
CH3OH CH2
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Doublet of Quartets
CH3CH2OH
Methylene group couples to OH and CH3; 3J(CH2-OH) and3J(CH3-CH2)
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Integration in 1H-NMR
Integration 5:2:3
multiplet at high resolution
Benzyl Acetate
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Important variables of 1st-order 1H-NMR spectra
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