Attached proton test pulse sequence

Figure 11 (A) The APT (attached proton test) pulse sequence. (B) Behaviour of the magnetization due to a nc- H...

Figure 2.4 (A) Pulse sequence for the gated spin-echo (GASPE) or attached proton test (APT) experiment. (B) Effect of the pulse sequence on the C magnetization vectors of a CH group.

The J-MODulated (JMOD) C experiment, also known as Attached Proton Test (APT) was the first and simplest way to determine "C multiplicities. In contrast to DEPT no polarization transfer is included in the pulse sequence (Fig. 3.16) and as a consequence the signals of quaternary carbons are visible in the spectrum, but the sequence is far less sensitive than DEPT or INEPT. The value of D2 is used to differentiate between the different carbon multiplicities. The signal intensities of quaternary carbons are not influenced by the value of D2 for D2 equal to 1 CH and CH, groups have maximum negative intensity and CHj has maximum positive intensity. For D2 equal to 1 /C2 J, ) only the signals of quaternary carbons are visible. 

The other experiment worth mentioning, which, by the way, is also obsolete, is the attached proton test or APT. This experiment is based on the different magnitudes of Tl—13C coupling for methine, methylene, and methyl groups. By adjusting certain delays in the pulse sequence (not given), quaternary and methylene carbons could be phased up, and methine and methyl carbons could be phased down. Since phase is arbitrary, this order could be reversed. This ability of distinguishing... 

FIGURE 12.1 Pulse sequence for attached proton test (APT).The initial pulse width can be set to conform to the Ernst angle in order to optimize sensitivity. 

Previously, it was reported that the chemical shift of the proton atoms of the C-12 of the fatty acid methyl ester can be found at 3.503 ppm (50). In the HMBC-NMR spectrum, there is a cross peak between this proton of C-12 and the carbon atom with a chemical shift of 95.23 ppm. This is a tertiary carbon atom. The discrimination between quaternary and tertiary C signals has been performed by using an attached proton test (APT) pulse sequence, one each second. The cross peak refers to the three-bond coupling of the H-12 protons with the anomeric center C-1 of lactose. The H-data of C-1 were found with the one-bond coupling constant, which is 150 MHz. With a COSY-NMR spectrum and its corresponding HMBC spectrum, the H and C NMR values of C-2, C-3, could be determined. 

Determination of the number of directly bound hydrogen atoms, called multiplicity assignment, is easy through application of editing pulse sequences, e.g., distortionless enhancement by polarization transfer (DEPT) or attached proton test (APT). 

MHz frequency as shown. A proton spectrum occurs over a chemical shift range of 10 ppm, which corresponds to 2.5 kHz at 500 MHz. As seen in Figure 3.22, all of the protons in the sample would see 98%-100% of the power of the 500 MHz radiation delivered and all would be excited simultaneously. A pulse programmer is used to control the timing and shape of the RF pulses used to excite the sample. Square wave pulses are commonly used, but multipulse experiments and 2D NMR experiments with other pulse shapes are performed. There are hundreds of pulse sequences and 2D experiments that have been developed, with curious names like attached proton test (APT), DEPT, INEPT, INADEQUATE, COSY, and many more, some of which will be discussed later in the chapter. Each pulse sequence provides specific and unique NMR responses that enable the analyst to sort out the NMR spectrum and deduce the chemical structure of a molecule. 

Figure 5.7. (A) The gated spin-echo (GASPE, or Attached Proton Test APT) pulse sequence. (B) Effects of a gated spin-echo sequence on a CH doublet, (a) At equilibrium (b) after the 90° pulse (c) after first period t, angle a = (Q — coo)t radians (d) after the 180° pulse and broad band decoupling (e) after final period t.

As an example, we discuss the so-called attached-proton test (APT). It involves the use of a spin echo, which is produced by a sequence of two pulses, as shown in Figure 3. 

Fig. 12. (a) The pulse sequence suitable for the solid-state attached proton test (SS-APT). The theoretical evolution of carbon signal intensity is shown in (b) and (c) as a... 

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