Two-dimensional 2D NMR analysis of a betaine

Note also that the spectral resolution is too poor to resolve three-bond couplings. For clarification the second methylene of 

The strong peak at 3.2 ppm can be clearly identified from its chemical shift as being the N-methyl peak. The only issue is the labelling of peaks b and d how do we know which is which We would expect a priori that b should be deshielded relative to d because it has a carbonyl carbon adjacent to it. But how can we be absolutely sure of this assignment This is where 2D NMR comes into its own. 

Let us now see how the cross-peaks give us information about connectivity in the molecule. Starting with the methyl peak ( z ) at 0.85 ppm on the diagonal, we find two cross-peaks correlating it to the (CH2) peak at 1.25 ppm. Of course, this tells us what we know anyway, namely that the methyl carbon must be directly bound to one of the —CH2— carbons in the alkyl chain the peak at 1.25 ppm in turn has two further cross-peaks to the CH2 at 1.7 ppm (peak e ). This again confirms what we already know, viz, the methylene carbon e must be directly 

These cross-peaks result from one-bonded couplings, i.e. each 

---