How To Read COSY Spectra

2-Nitropropane. To see what type of information a COSY spectrum may provide, we shall consider several examples of increasing complexity. The first is the COSY spectrum of 2-nitropropane. In this simple molecule, we expect to observe couphng between the protons on the two methyl groups and the proton at the methine position. 

Isopentyl Acetate. In practice, we would not require a COSY spectrum to fully interpret the NMR spectmm of 2-nitropropane. The preceding analysis illustrated how to interpret a COSY spectrum using a simple, easy-to-understand example. A more interesting example is the COSY spectrum of isopentyl acetate (Fig. 9.14). 

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You may have noticed that each of the COSY spectra shown in this section contains additional spots besides the ones examined in our discussion. Often these extra spots have much lower intensities than the principal spots on the plot. The COSY method can sometimes detect interactions between nuclei over ranges that extend beyond three bonds. Besides this long-range coupling, 

606 Nuclear Magnetic Resonance Spectroscopy Part Five Advanced NMR Techniques 

FIGURE 10.14 COSY spectrum of isopentyl acetate. Notice the paired symmetry (AA, BB, CC ) around die diagonal line. 

Citronellol. The COSY spectrum of citronellol (see the structural formula on p. 534) is a third example. The spectrum (Fig. 10.14) is rather complex in appearance. Nevertheless, we can identify certain important coupling interactions. Again, lines have been drawn to help you identify the correlations. The proton on C6 is clearly coupled to the protons on C5. Closer examination of the spectrum also reveals that the proton on C6 is coupled through allylic (four-bond) coupling to the two methyl groups at C8 and C9. The protons on Cl are coupled to two nonequivalent protons on C2 (at 1.4 and 1.6 ppm). They are nonequivalent, owing to the presence of a stereocenter in the molecule at C3. The splitting of the methyl protons at CIO by the methine proton at C3 can also be seen, al- 

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