Hydrogen-decoupled spectra

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

Putting together the information from all three spectra makes it possible to tell the number of hydrogens attached to each carbon. The CH carbons are identified in the DEPT-90 spectrum, the CH2 carbons are identified as the negative peaks in the DEPT-135 spectrum, the CH3 carbons are identified by subtracting the CH peaks from the positive peaks in the DEPT-135 spectrum, and quaternary carbons are identified by subtracting all peaks in the DEPT-135 spectrum from the peaks in the broadband-decoupled spectrum. 

Deuteration of one end of the allyl moiety in these compounds removes the equivalence of the two positions and in place of the single line for the terminal position, two separate absorptions should appear (W). One, in the normal decoupled spectrum, is a singlet for the hydrogen substituted carbon, and the other a weak quintet for the deuterated end which would be difficult to observe. These two signals would bracket the normal singlet. If a mixture of deuterated and undeuterated allyl compound is used, therefore, two easily observable peaks should appear, one in the normal position, the other shifted. In the spectra of allyllithium and allylsodium the line from the deuterated compound appeared I and 11 Hz upfield respectively, at 0°C, of the normal lines. The potassium compound only showed a somewhat broadened line. At -80°C the separation for allyllithium was 22 Hz. 

Several techniques have been developed that enable the number of hydrogens attached to the carbon to be determined. An older technique, called off-resonance decoupling, allows hydrogens and carbons that are directly bonded to couple but removes any longer-range coupling. In an off-resonance decoupled spectrum, a CH3 appears as... 

The best procedure for obtaining a 13C NMR spectrum is to run the spectrum twice The singlets in the broadband-decoupled spectrum indicate the number of nonequivalent types of carbon atoms and their chemical shifts. The multiplicities of the signals in the off-resonance-decoupled spectrum indicate the number of hydrogen atoms bonded to each carbon atom. 13C spectra are often given with two traces, one broadband decoupled and the other off-resonance decoupled. If just one trace is given, it is usually broadband decoupled. Figure 13-45 shows both spectra for butan-2-one. 

Let s consider how we might solve this structure, given only the 13C NMR spectrum and the molecular formula. As we have seen in Figures 13-41 and 13-42, the signal at 173 ppm is appropriate for a carbonyl carbon. The off-resonance-decoupled spectrum shows a singlet at 173 ppm, implying that no hydrogens are bonded to the carbonyl carbon. 

Given the chemical shifts of 13C absorptions, suggest likely types of carbons. Use either the off-resonance-decoupled spectrum or the DEPT 13C spectra to determine the number of hydrogen atoms bonded to a given carbon atom. Problems 13-42, 43, 45, and 48... 

Most convincing evidence for the equatorial orientation of H-4 was provided by the hydrogen deuterium-decoupled spectrum of... 

In a proton-decoupled spectrum, the total NOE for a given carbon increases as the number of nearby hydrogens increases. Thus, we usually find that the intensities of the signals in a spectrum (assuming a single carbon of each type) assume the order... 

An example of a NMR spectrum is shown in Figure 5.1. It shows the NMR spectrum of butan-l-ol, CH3-CH2 CH2 CH2-OH. The figure shows two traces. The upper is known as the decoupled spectrum, and shows one line for each carbon atom in a different environment. In this case, there are four lines, since all the four carbon atoms are in different environments, and hence have different chemical shifts. The lower trace shows the undecoupled spectrum, in which each of these peaks is split into a multiplet by the attached hydrogen atoms. 

This method works well in a molecule which has well spaced peaks, but is less successful in larger molecules. An alternative method is to measure what it known as a DEPT spectrum (Distortionless Enhancement by Polarisation Transfer). The DEPT spectrum plots the CH3, CH2 and CH peaks separately. It does not record carbon atoms which do not have a hydrogen atom attached, so we need a decoupled spectrum as well as a DEPT spectrum. The spectra for butan-l-ol are shown in Figure 5.2. 

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. 

Compound Qhas the molecular formula CyHs- The broad-band proton decoupled spectrum of Qhas signals at 8 50 (CH), 85 (CHs). and 144 (CH). On catalytic hydrogenation Qis converted to R (CyHig)- Propose structures for Qand R... 

When we obtain a proton-decoupled spectrum, the intensities of many of the carbon resonances increase significantly above those observed in a proton-coupled experiment. Carbon atoms with hydrogen atoms directly attached are enhanced the most, and the enhancement increases (but not always linearly) as more hydrogens... 

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... 

Owing to their importance in biological systems, porphyrins have been the subject of several studies intended to determine the nature and rates of the exchange phenomena involved. Thus, for octaethylporphyrin vOEP) (K 14) and meso tetraphenylporphyrin (TPP) (G 8) (Y 17), the two resonances observed at low temperature on the decoupled spectrum coalesce as the temperature is raised, indicating a raprid exchange of the two central hydrogens between the four nitrogen sites. 

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