A Comparison of NMR Spectra at Low-and High-Field Strengths

for example, the preceding spectrum, which shows three triplets and one quartet. Which triplet is associated with the quartet It is, of course, the one that has the same 7 values as are found in the quartet. The protons in each group interact to the same extent. In this example, with the 7 values given, clearly quartet A (7 = 7 Hz) is associated with triplet C (7 = 7 Hz) and not with triplet B or D (7 = 5 Hz). It is also clear that triplets B and D are related to each other in the interaction scheme. 

FIGURE 3.36 The NMR spectrum of 1-nitropropane. (a) Spectrum determined at 60 MHz (b) spectrum determined at 300 MHz (with expansions). 

With 300-MHz spectra, therefore, it is frequently necessary to show expansions if one wishes to see the details of the multiplets. In some of the examples in this chapter, we have used 60-MHz spectra—not because we are old-fashioned but because these spectra show the multiplets more clearly without the need for expansions. 

In most cases, the expanded multiplets from a high-field instrument are identical to those observed with a low-field instrument. However, there are also cases in which complex multiplets become simplified when higher field is used to determine the spectrum. This simplification occurs because the multiplets are moved farther apart, and a type of interaction called second-order interaction is reduced or even completely removed. Chapter 5 will discuss second-order interactions. 

In this section we wiU review the typical NMR absorptions that may be expected for compounds in each of the most common classes of organic compounds. These guidelines can be consulted whenever you are trying to establish the class of an unknown compound. Coupling behaviors commonly observed in these compounds are also included in the tables. This coupling information was not covered in this chapter, but it is discussed in Chapters 5 and 6. It is included here so that it wiU be useful if you wish to use this survey later on. 

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