2- chloropropionic acid :
The spectrum contains a doublet and a quartet (J=8Hz) corresponding to the coupling of one proton with three neighbours, this is an AX₃ spectrum.
Example (2)
Ethyl bromide : CH₃CH₂Br.
This is an A₂X₃ case. It containing a triplet and a quartet.
Example (3)
1-Nitropropane CH₃CH₂CH₂NO₂
In this compound there are three groups of protons, each group couplings with it's near neighbours, so that the central methylene group couples both with the methyl protons and with the terminal methylene protons. The methyl group appeared as triplet, and the terminal methylene group also appears as a triplet. Since both couple with the central methylene group : (n+1)= (2+1) =3. The methyl protons shown no coupling with the terminal methylene protons since coupling over four σ bonds is rarely observed.
The central methylene group can be deal with by considering the successive couplings , first with the methyl group and then with the terminal methylene protons. The methyl protons split the central methylene signal into a quartet : the two terminal methylene protons should now split each line of this quartet into a triplet, giving twelve lines in all. In the spectrum only six lines are observed, shows that considerable overlapping of the predicted twelve lines has taken place. In fact the two coupling constants involved (J CH₃,CH₂ and J CH₂ ,CH₂) are equal and an easier way to consider the central methylene group is to add the total number of neighbours with which it couples (CH₃ and CH₂) and then apply the (n+1) rule. There are five coupling neighbours, therefore the multiplicity is 6.
Example (4)
Furfural (AMX) system.
In unsaturated systems and aromatic systems, it's frequently possible to observe three groups of protons A, M and X, each of which couples with the other two. For that such a system to be first order, the chemical shift positions of the protons must be relatively well separated, just as A,M and X are separated in the alphabet. The nmr spectrum of furan-2-aldehyde furfural showing such as AMX systems : the coupling pattern involves interaction between protons separated by four bonds.
The three nuclear protons each giving rise to a four line signals, so that twelve lines in all are observable that Aldehyde protons appears as a singlet.
Protons A couples with x, which splits a signal into a doublet but A also couples with M. So that each lines of A is further split into two, giving four lines in all. The signals for protons A showing two splitting, JAM and JAX, and is therefore a double doublet rather than a quartet.
Similarly the M signals is split into two by coupling with X, and each lines is split into two by coupling with A. Two coupling constants are again seen, JAM, and JMX and M appears are a double doublet.
Finally the X signals is split into a double doublet by two successive couplings, JMX and JAX.
A full analysis of an AMX spectrum involves identifying all three J values, JAM, JAX, and JMX. Note that each J values appears in two different multiplets, and that each multiplet contains two different J values.
AMX coupling in schematic from :
Spectra of furfural (only HA, HM, HX are shown.
The nmr spectrum shows below many vinyl compounds. For example, vinyl acetate shows AMX coupling systems.
Super
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