Hydrogen decoupling

Figure Bl.11.4. Hydrogen-decoupled 100.6 MHz C NMR spectrum of paracetamol. Both graphical and numerical peak integrals are shown.

The possible presence in the 4-chloro-4-hexenyl trifluoroacetate of small amounts of two cis-trans pairs of products of addition of trifluoroacetic to the triple bond without concomitant halogen shift remains speculative. In any event these compounds would be removed as ketones upon hydrolysis of the trifluoroacetate. Both the 4-chloro-4-hexenyl trifluoroacetate and the alcohol resulting from its hydrolysis have been shown to contain 9% of the (E) isomer. In the present study the hydrogen decoupled magnetic resonance spectra of the ester and alcohol were shown to contain peaks attributable to approximately 9% of E) isomer. 

Answers include the number of signals and (for (he spectrum without hydrogen decoupling) the splitting by directly attached hydrogens (N + 1 rule). Chemical shifts (based on Table 10-6) are very approximate. 

In the hydrogen-decoupled mode, the sample is irradiated with two different radio frequencies. The first radio frequency is used to excite the C nuclei. The second is a broad spectrum of frequencies that causes all hydrogens in the molecule to undergo rapid transitions among their nuclear spin states. On the time scale of a C-NMR spectrum, each hydrogen is in a time average of the two states, with the... 

C-NMR spectra are commonly recorded in a hydrogen-decoupled instrumental mode. In this mode, all C signals appear as singlets. 

Figure 10-32 NMR spectrum of methylcyclohexane with hydrogen decoupling. Each of the five magnetically different types of carbon in this compound gives rise to a distinct peak S = 23.1, 26.7, 26.8, 33.1, and 35.8 ppm.

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