Determining the Number of Attached Hydrogens

The DEPT experiment is a modification of a basic NMR experiment called the attached proton test (APT) experiment. Although a detailed explanation of the theory underlying the DEPT experiment is beyond the scope of this book, an examination of a much simpler experiment (APT) should give you sufficient insight into the DEPT experiment so that you will understand how its results are determined. 

This type of experiment uses two transmitters, one operating at the proton resonance frequency and the other at the resonance frequency. The proton transmitter serves as a proton decoupler it 

This type of experiment uses two transmitters, one operating at the proton resonance freqnency and the other at the resonance frequency. The proton transmitter serves as a proton deconpler it is switched on and off at appropriate intervals during the pulse sequence. The transmitter provides the usual 90° pulse along the X axis, but it can also be programmed to provide pulses along the Y axis. 

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On the first stage of hydrogenation reaction the C/H composition of fulleranes can be approximately determined by XRD from increase of unit cell parameter. The crystal structure of fulleranes powder remains to be fee like in pristine C60 but the cell parameter increases proportionally to the number of attached hydrogen atoms due to increase of van der Waals diameter of molecules. Using chemical analysis on carbon/hydrogen ratio performed on samples hydrogenated at various conditions we constructed the dependence which could be used for approximate evaluation of hydrogen content from known cell parameter determined from powder XRD, see Fig. 5.3... 

Techniques developed in recent years make it possible to obtain large amounts of information from l3C NMR spectra. For example, DEPT-NMR, for distortionless enhancement by polarization transfer, allows us to determine the number of hydrogens attached to each carbon in a molecule. 

Determine the symmetry elements present in the following boranes and hence assign their symmetry point groups. The numbered circles in the polyhedra represent the boron atoms with the corresponding number of attached hydrogen atoms. 

A technique called DEPT NMR has been developed to distinguish among CH3, CH2, and CH groups. (DEPT stands for distortionless enhancement by polarization transfer.) It is now much more widely used than proton coupling to determine the number of hydrogens attached to a carbon. 

A very useful pulse sequence in spectroscopy is employed in the experiment called Distortionless Enhancement by Polarization Transfer, better known as DEPT. The DEPT method has become one of the most important techniques available to the NMR spectroscopist for determining the number of hydrogens attached to a given carbon atom. The pulse sequence involves a complex program of pulses and delay times in both the and ehannels. The result of this pulse sequenee is that carbon atoms with one, two, and three attached hydrogens exhibit different phases as they are recorded. The phases of these hydrogens will also depend on the duration of the delays that are programmed into the pulse sequence. In one experiment, called a DEPT-45, only carbon atoms that... 

Section 15.7) A spectroscopic method of determining the number of hydrogens attached to a carbon. 

The attached proton test (APT) is an alternative method of determining the multiplicities of carbons this method is rarely used because it does not produce the sensitivity enhancement achieved through polarization transfer, and it requires longer relaxation delays based on T/s. Research published a number of years ago used this method to determine the number of hydrogen atoms attached to... 

DEPT-NMR (Section 13.6) An NMR method for distinguishing among signals due to CH3, CH2, CH, and quaternary carbons. That is, the number of hydrogens attached to each carbon can be determined. 

The practical use and the advantage of proton off-resonance decoupling - less multiplet overcrowding and more signal noise relative to coupled spectra - is illustrated in Fig. 2.47. for a triterpene derivative in comparison to modern and more accurate methods for determination of CH multiplicity. An unequivocal assignment of the number of directly attached hydrogens may be possible for all carbons. 

Including the stereoisomer shown, the number of possible stereoisomers is x = 27 = 128 since there are six chirality centres and one double bond which can be either E- or Z-configured. The absolute configuration of the carbon atom at position 4 cannot be determined until the third sphere. In the first sphere there are three carbon atoms, in the second all carbon atoms are attached to two carbon atoms and one hydrogen atom. In the third sphere the priority order is 0,C,C > C,C,(C) > C,C,H and therefore this chirality centre is S- configured. 

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... 

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