Physical Chemistry , 1st ed.

Higher-resolution NMR spectra provide even more information about the

structure of a molecule. This is because nuclei that have magnetic moments

can interact, or couple,with each other. (The mechanism of coupling is actu-

ally through the electrons surrounding the nuclei, but we will not go into that

here.) Since a magnetic moment is caused by the nonzero spin of a nucleus,

this effect is called spin-spin coupling,and its magnitude is given by a spin-spin

coupling constantbetween nucleus iand nucleus j, labeled Jij. The Jijvalues have

units of Hz. The net effect of spin-spin coupling is to split an absorption into

multiple individual absorptions, much like the Zeeman effect splits electronic

absorptions. Figure 16.18 shows an example of spin-spin coupling on an NMR

spectrum. Internuclear interactions fall off rapidly with distance. For organic

compounds, most C and O atoms have a zero nuclear spin and so do not con-

tribute to spin-spin coupling. Only hydrogen atoms will interact with other

16.5 Nuclear Magnetic Resonance 577

8

ppm

6 4 2 0

Figure 16.16 A typical low-resolution NMR spectrum ofp-acetophenetidide, a simple disub-

stituted benzene derivative that has several chemically different hydrogens.

Figure 16.17 NMR absorptions are typically
measured in units of parts per million, which ul-
timately refer to the chemical shift of the partic-
ular nucleus in gauss units.

10

ppm

123456789 0

8
ppm

0

TMS

ClCH 2 CH 3

1234567

Figure 16.18 A high-resolution spectrum shows the effects of spin-spin coupling, mostly be-

tween hydrogens on adjacent atoms in a molecule.Source:S. E. Sen and K. S. Aniker,J. Chem.

Ed.,1996, 74: 570.