Integration of 1H NMR Absorptions Proton Counting

Problem 13.18 How many peaks would you expect in the H NMR spectrum of 1,4-dimethyl-benzene (pom-xylene, or p-xylene) What ratio of peak areas would you expect on integration of the spectrum Refer to Table 13.3 for approximate chemical shifts, and sketch what the spectrum would look like. (Remember from Section 2.4 that aromatic rings have two resonance forms.) 

460 CHAPTER 13 Structure Determination Nuclear Magnetic Resonance Spectroscopy 

Any given molecule has only one of the three possible alignments of -CH2Br spins, but in a large collection of molecules, all three spin states are represented in a 1 2 1 statistical ratio. We therefore find that the neighboring -CFIj protons come into resonance at three slightly different values of the applied field, and we see a 1 2 1 triplet in the NMR spectrum. One resonance is a little above where it 

In the same way that the — CH3 absorption of bromoethane is split into a triplet, the -CH Br absorption is split into a quartet. The three spins of the neighboring -CH3 protons can align in four possible combinations all three with the applied field, two with and one against (three ways), one with and two against (three ways), or all three against. Thus, four peaks are produced for the -CH2Bf protons in a 1 3 3 1 ratio. 

Because coupling is a reciprocal interaction between two adjacent groups of protons, it s sometimes possible to tell which multiplets in a complex NMR spectrum are related to each other. If two multiplets have the same coupling constant, they are probably related, and the protons causing those multiplets are therefore adjacent in the molecule. 

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