Simple coupling patterns (those that obey the n+1 rule) are observed in systems where a set of magnetically equivalent protons are only coupled to one other set of neighboring magnetically equivalent protons or where a set of magnetically equivalent protons are coupled to multiple sets of neighboring magnetically equivalent protons by identical coupling constants.
An example of the first instance can be observed in methyl ethyl ketone (2-butanone). Ha protons are only coupled to Hb protons and vice versa. The typical triplet-quartet splitting pattern for an ethyl group is observed.
Figure 28 contains the NMR spectrum for methyl ethyl ketone. In this instance the n+1 rule is observed.
Figure 28. Simple coupling in ethyl methyl ketone.
In propyl formate, Ha protons are only coupled to Hb protons and a triplet is observed for this signal. Additionally, Hc protons are only coupled to Hb protons and a triplet is observed for this signal also. However, Hb protons are coupled to both Ha and Hc protons, but the coupling constant to both types of protons is identical. Therefore, Hb protons are apparently split by 5 protons and a sextet is observed for this signal, obeying the n+1 rule.
Figure 29 contains the NMR spectrum for propylformate. In this instance the n+1 rule is observed.
Figure 29. Simple coupling in propylformate.
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