Protons on Nitrogen Quadrupole Broadening and Decoupling

Nitrogen has a moderate-size quadrupole moment, and its spin transitions do not occnr as rapidly as those in the heavier halogens. Furthermore, the transitional rates and lifetimes of its excited spin states (i.e., its quadrupole moments) vary slightly from one molecule to another. Solvent environment and temperature also seem to affect the quadrapole moment. As a resnlt, three distinct situations are possible with a nitrogen atom  

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FIGURE 8.13 H NMR spectrum of pyrrole. The inset shows expansions of the resonances of the ring C—H protons. 

The resonance delocalization requires that the molecule adopt a planar geometry, and it thus interferes with free rotation. If the free rotation is slowed to the point that it takes longer than an NMR transition, the NMR spectrometer sees two different methyl groups, one on the same side of the C=N bond as the carbonyl group and the other on the opposite side. Thus, the groups are in magnetically different enviromnents and have slightly different chemical shifts. 

Nitrogen splits the attached hydrogens into a triplet 

The 3 methyl protons split each peak into a quartet 

FIGURE 6.12 An analysis of the H NMR spectrum of methylammonium chloride protons on nitrogen. 

The ammonium protons split the methyl resonance into a quartet (n + 1 = 4) intensities 1 3 3 1 

Elements that have / 5 have approximately spherical distributions of charge within their nuclei. Those that have 1 have ellipsoidal distributions of charge within their nuclei, and as a result have a quadrupole moment. 

Small quadrupole moment for nitrogen. In this case coupling is seen. An attached hydrogen (as in N—H) is split into three absorption peaks because of the three possible spin states of nitrogen (+1, 0, -1). 

This second simation is seen frequently in primary aromatic amines, such as substituted anilines. 

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