Unexpected Lines in an NMR Spectrum

things are not as simple as they may have appeared. From the discussions in Chapters 6 and 7 you might be under the impression that typical H NMR spectra exhibit just one sharp signal line for each H nucleus (or each set of equivalent H nuclei) and that the same thing is true for C spectra as well as for spectra of any other isotope. Actually, this is not usually the case. Instead, the individual signals expected on the basis of the molecule s symmetry are themselves often split into symmetrical patterns (multiplets) consisting of two or more lines. While these extra lines do make a spectrum appear more complex, they also offer valuable structural information that complements the chemical shift data. This chapter explains the source of these extra lines and shows how useful they can be for confirming the structures of molecules. 

The four-line pattern for the methylene hydrogens is a quartet and has multiplicity 4. Its chemical shift is midway between the second and third lines. Again, the three spacings between neighboring lines are all equal to the spacings in the triplet (6.9 Hz). However, in a quartet the relative intensity of the lines is 1 3 3 1. These intensity ratios, and the fact that the spacings in both multiplets are equal, are no accident. 

When reporting NMR data in condensed format, you list the chemical shift in 5 (ppm) of each multiplet, followed in parentheses by the type of multiplet (often abbreviated), coupling constant (in hertz), and, in the case of H spectra, relative signal area (intensity). By the way, the magnitude of J must 

I hope that by now your interest has been piqued. Why do equivalent hydrogens give rise to singlets in some cases and multiplets in others To understand the phenomenon of spin coupling, recall how the magnitude of the effective magnetic field experienced by a nucleus determines its processional frequency and thereby its chemical shift. If that is not second nature to you by now, perhaps you should review Section 6.1 before proceeding further. 

The number of different spin states M values) that n equivalent nuclei can adopt is 2nl + 1. Notice that there is only one way to have both up (M = j j = 1) and one way for both to be down (A/= - j - j = -1) but two combinations where one is up and one is down (M = j - j = 0). Thus, two equivalent hydrogens together give rise to three spin states (Af = 1,0, -1) with population ratio of 1 2 1. Does this ratio ring a bell  

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