Spin Splitting

A very important characteristic of spin-spin splitting is that protons that have the same chemical shift do not split each other s signal Ethane for example shows only a single sharp peak m its NMR spectrum Even though there is a vicinal relationship between the protons of one methyl group and those of the other they do not split each other s signal because they are equivalent... 

A second type of structural information can be deduced from the hyperfine splitting in EPR spectra. The origin of this splitting is closely related to the factors that cause spin-spin splitting in proton NMR spectra. Certain nuclei have a magnetic moment. Those which are of particular interest in organic chemistry include H, " N, F, and P. 

The more infonnation you can extract from an NMR spectrum, the better your chances at aiiiving at a unique structure. Like spin-spin splitting, which complicates the appearance of a H NMR spectrum but provides additional infonnation, 2D NMR looks more complicated than it is while making structure determination easier. 

Spin-spin splitting of NMR signals results from coupling of the nucleai spins that aie separated by two bonds (geminal coup ling) or three bonds (vicinal coupling). 

Spin-spin splitting (Section 13.7) The splitting of NMR signals caused by the coupling of nuclear spins. Only nonequivalent nuclei (such as protons with different chemical shifts) can split one another s signals. 

Vicinal coupling (Section 13.7) Coupling of the nuclear spins of atoms X and Y on adjacent atoms as in X—A—B—Y. Vicinal coupling is the most common cause of spin-spin splitting in FI NMR spectroscopy. 

Figure 13.14 The origin of spin-spin splitting in bromo-ethane. The nuclear spins of neighboring protons, indicated by horizontal arrows, align either with or against the applied field, causing the splitting of absorptions into multiplets.

Spin-spin splitting in 1H NMR can be summarized in three rules. 

Rule 1 Chemically equivalent protons do not show spin-spin splitting. The equivalent protons may be on the same carbon or on different carbons, but their signals don t split. 

More Complex Spin-Spin Splitting Patterns 465... 

When the H NMR spectrum of an alcohol is run in dimethyl sulfoxide fDMSO) solvent rather than in chloroform, exchange of the O—H proton is slow and spin-spin splitting is seen between the O-H proton and C-H protons on the adjacent carbon. What spin multiplicities would you expect for the hydroxyl protons in the following alcohols ... 

Doublet (Section 13.11) A two-line NMR absorption caused by spin-spin splitting when the spin of the nucleus under observation couples with the spin of a neighboring magnetic nucleus. 

Multiplet (Section 13.11) A pattern of peaks in an NMR spectrum that arises by spin-spin splitting of a single absorption because of coupling between neighboring magnetic nuclei. 

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