26

The Simple Spin-Spin Coupling Rules

Consider only 3JH-H. This is called vicinal coupling.
•Observed: H’-C-C-H”
•typically not observed: H’-C-O-H” and H’-C-N-H” (fast proton exchange)

Splitting Pattern

The chemically equivalent H-atoms do not cause a splitting.
The splitting pattern is a related to the number of equivalent H-atoms at the
neigbouring atom.

•
•
•
•
•
•
•
•
•

0 H atoms as neighbor singlet
1 H atom as neighbor dublet (1:1) (e.g. -CH-CH-)
2 H atoms as neighbors triplet (1:2:1) (e.g. -CH-CH2-)
3 H atoms as neigbors quartet (1:3:3:1) (e.g. -CH –CH3)
4 H atoms as neigbors quintet (1:4:6:4:1) (e.g. -CH2-CH-CH2-)
5 H atoms as neigbors sextet
6 H atoms as neigbors septet (e.g. CH3-CH-CH3)
.... ...
n H atoms as neighbors n+1 lines

Intensity of the Multiplet Lines: Pascal Triangle

1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1

The intensity of all lines combined is proportional to the number of H-atoms responsible
for the resonance.Rules for coupling

Karplus Curve

For 3J couplings the magnitude of the coupling constant depends
angle:

on the dihedral

J

Theta

0o

90o

180o

10

10

H

H

R

H

H

R

R

H

R

H

9-12

14-16