Because the 13C isotope is present at only 1.1% natural abundance, the probability of finding two adjacent 13C carbons in the same molecule of a compound is very low. As a result spin-spin splitting between adjacent non-equivalent carbons is not observed. However, splitting of the carbon signal by directly bonded protons is observed, and the coupling constants are large, ranging from 125 to 250 Hz. Methyl groups appear as quartets, methylenes as triplets, methines as doublets, and unprotonated carbons as singlets. Commonly, splitting of the signal by protons is eliminated by a decoupling technique which involves simultaneous irradiation of the proton resonances at 300 MHz while observing the carbon resonances at 75 MHz. The decoupling is accomplished with a second broad band, continuous, oscillating magnetic field B2 (as opposed to the pulsed B1 field), and the decoupling is continued during data collection. The B2 field causes rapid proton spin transitions such that the 13C nuclei lose track of the spin states of the protons. Figure 26 shows a proton decoupled 13C spectrum of ethyl acetate. The purpose of proton decoupling is to eliminate overlapping signal patterns and to increase the signal to noise ratio. Decoupling of the protons increases the signal to noise ratio by causing the collapse of quartets, triplets, and doublets to singlets and by causing a favorable increase in the number of carbons in the -spin state relative to the -spin state. The latter effect is called the Nuclear Overhauser Effect (NOE); how it causes this change in spin state populations will not be discussed here.
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