We get the singlets consistently seen in carbon spectra because of the way we record the spectra. The values of ¹JCH are so large that, if we recorded 13C spectra with all the coupling constants, we would get a mass of overlapping peaks. When run on the same spectrometer, the frequency at which ¹³C nuclei resonate turns out to be about a quarter of that of the protons. Thus a ‘400 MHz machine’ (remember that the magnet strength is usually described by the frequency at which the protons resonate) gives 13C spectra at 100 MHz. Coupling constants (¹JCH) of 100–250 Hz would cover 2–5 ppm and a CH₃ group with ¹JCH of about 125 Hz would give a quartet covering nearly 8 ppm (see the example on the previous page).

Since the proton-coupled ¹³C spectrum can so easily help us to distinguish CH₃, CH₂, CH, and quaternary carbons, you might wonder why they are not used more. The above example was chosen very carefully to illustrate proton-coupled spectra at their best. Unfortunately, this is not a typical example. More usually, the confusion from overlapping peaks makes this just not worthwhile. So ¹³C NMR spectra are recorded while the whole 10 ppm proton spectrum is being irradiated with a secondary radio frequency source. The proton energy levels are equalized by this process and all coupling disappears. Hence the singlets we are used to seeing. For the rest of this chapter, we shall not be introducing new theory or new concepts; we shall be applying what we have told you to a series of examples where spectroscopy enables chemists to identify compounds.

مواضيع ذات صلة

Enolization is catalysed by acids and bases

Evidence for the equilibration of carbonyl compounds with enols

Tautomerism: formation of enols by proton transfer

Hydration of alkynes

Breaking a double bond completely: periodate cleavage and ozonolysis

Adding two hydroxyl groups: dihydroxylation

Electrophilic addition to alkenes can produce stereoisomers

Hydration of alkynes

Breaking a double bond completely: periodate cleavage and ozonolysis

Adding two hydroxyl groups: dihydroxylation

Electrophilic addition to alkenes can produce stereoisomers

Electrophilic additions to alkenes can be stereospecific