done using a partial decoupling technique known as off-resonance decoupling which allows only the
coupling between^13 C nuclei and protons directly bonded to them to be observed.
Examples of coupled and noise decoupled spectra for ethyl phenylacetate are shown in Figure 9.37 and
the approximate chemical shift positions for^13 C nuclei in different chemical environments in Figure
9.38. The shift positions of^13 C resonances are determined by electronegativity and anisotropy effects as
are proton resonances but in a more complex manner. In figure 9.37(a) the methyl quartet (δc14) is due
to coupling to three equivalent protons and the triplets at δc41 and δc60 are due to the two equivalent
protons in each methylene group. The^13 C attached to the oxygen (δc60) is deshielded more than that
attached to the carbonyl (δc41) whilst the^13 C of the carbonyl group itself appears at δc171. Longer
range^13 C-proton coupling (^13 C—C—H) is just discernible by broadening of the multiplets and the
carbonyl singlet. Coupling constants for protons directly bonded to^13 C nuclei are of the order of 100–
200 Hz whilst longer range coupling constants are typically between 0 and 2 Hz. For more complex
molecules, the noise decoupled spectrum is the more readily interpreted. Further examples are shown in
Figures 9.39 and 9.40.
Figure 9.37(^13) C NMR spectra of ethyl phenylacetate (from J. A. Moore and D. L. Dalrymple, Experimental
Methods in Organic Chemistry, W. B. Saunders Co., 1976).