first-order spin-spin coupling - university of windsormutuslab.cs.uwindsor.ca/eichhorn/59-330...

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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