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Chapter 13: Spectroscopy |
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Complex Coupling Patterns
- In reality, coupling patterns are often more complex than the simple n+1
rule since the neighbouring protons are often not equivalent to each other
(i.e. there are different types of neighbours).
- Different types of neighbours interact with different coupling
constants.
- In these cases, the "n+1" rule has to be refined so that each type
of neighbour causes n+1 lines.
- For example for a proton with two types of neighbour, then the number of
lines, L = (n1 + 1)(n2 + 1).
- However, in many cases the lines overlap with each other and the result
is further distortion from the "ideal" pattern.
Coupling patterns involving aromatic or alkene protons are often complex.
In the following example, 3-bromopropene (also
known as allyl bromide), we have coupling patterns that look more complex.
In particular, notice how the H on C2 is
coupled to all of the other H in the system, via three different coupling constants
since the neighbouring H are of different types.
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© Dr. Ian Hunt, Department of Chemistry, University
of Calgary
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