The APT Experiment

In order to pulse rapidly without a DT, it is necessary to locate any residual magnetization along the +z axis, where it can relax rapidly to its equilibrium value. This goal can be achieved by setting 0 = a + 90°(i.e., 90° 0 180°). A value of a is selected as it would be in an ordinary NMR experiment, and 90° is added to that number. Residual magnetization, therefore, starts along the -z axis and is driven back to the +z axis by the 180° pulse. 

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Figure 2.6 Signal intensities in the APT experiment depend on delay time x. The signal intensities of CH3, CHa, and CH carbons are shown as various curves as a function of x.

One disadvantage of the APT experiment is that it does not readily allow us to disdnguish between carbon signals with the same phases, i.e., between CH3 and CH carbons or between CH2 and quaternary carbons, although the chemical shifts may provide some discriminatory information. The signal strengths also provide some useful information, since CH3 carbons tend to be more intense than CH carbons, and the CH2 carbons are usually more intense than quaternary carbons due to the greater nuclear Overhauser enhancements on account of the attached protons. 

After the 90° pulse, the transverse magnetization vectors of C nuclei of C, CH, CH2, and CHj do not rotate synchronously with one another but rotate with characteristically different angular velocities during the same delay interval. This results in their appearing with differing (positive or negative) amplitudes. This forms the basis of the APT experiment. 

Chapter 6 has been almost completely rewritten. There is more emphasis on pulse sequences and on the use of inverse detection (e.g., HMQC and HMBC experiments). Some experiments from the Fifth Edition have been eliminated (e.g., /-Resolved), and others have been added. The chapter has been renamed Correlation NMR Spectrometry to better reflect the emphasis of the chapter. Because of this name change, the DEPT experiment has been moved to Chapter 5 the APT experiment has been eliminated. Gradient field NMR is presented as a recent development. Problems are assigned. 

These equations are plotted graphically in Figure 12.16. Note that with 0 set to 90° [a DEPT (90) subspectrum] the intensity of the CH signal is at a maximum, while CH2 and CH3 signals are zero. Compare these equations with those for the APT experiment [Eq. (12.3) and Figure 12.8],... 

The behavior of the four types of CH units in the APT experiment is illustrated for n = 1 and 3 in Figure 5-15 and forn = 0 and 2 in Figure 5-16. There are several versions of the APT pulse sequence, and one of the more popular is shown in Figure 7-2. Similar to the standard NMR experiment, the APT experiment is commonly performed in either of two ways. In one way, the initial pulse (0) can be set to 90°, and a relaxation delay (DT, Section 2-4f) of approximately 1 s employed. We saw in Section 2-4f, however, that the most efficient way to collect NMR data is to pulse continuously, without a DT and with a 90°. 

In the full editing APT spectrum [t = (7) ] shown in Figure 5-17, carbons with an odd number of attached protons (CH and CH3) are easily distinguished from those with an even number (CH2 and quaternary, as zero is considered to be an even number). If it is necessary to differentiate carbons within either of these two groups, the APT experiment can be rerun with different values of t. For t = (27)methylene and quaternary carbons can be distinguished, because the former are nulled while the latter have full intensity. Differentiating between methyl and methine carbons is less definitive. For t = 2/37, methyl signals have approximately one-third of the intensity of the methine resonances. 

In the DEPT experiment, results similar to those described here for the APT experiment are obtained. A variety of pulse angles and delay times are incorporated into the pulse sequence. The result of the DEPT experiment is that methyl, methylene, methine, and quaternary carbons can be distinguished from one another. 

The dependance of signal phases and intensities on delay time t in the APT experiment is shown in Fig. 2.6. If Jch is assumed to be 125 Hz and the delay time is accordingly set at l/J= V125 = 8 ms, then, as is apparent from Fig. 2.6, the quaternary and CHj carbons appear with maximum positive amplitudes while the CH3 and CH carbons afford maximum negative amplitudes [see vertical line at (a)]. If the delay time is adjusted to 6 ms [see vertical line at (b)], then quaternary carbons still appear with similar positive amplitudes, the CH2 carbons have weaker positive amplitudes, and the CH, and CH carbons have weak negative amplitudes. The... 

The APT experiment can be explained by using spin gymnastics to track fbe motion of fbe net magnetization vectors in fbe rotating frame at various points in fbe pulse sequence. Figure 6.9 shows the APT pulse sequence. The APT pulse sequence works by exploiting the nearly constant Iqh (this assumption is a poor one only when the molecule has a terminal H on a triple-bonded C). 

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