The Attached Proton Test APT

APT is a technique for -decoupled 13C spectra, which uses the phase (normal or upside down) of the 13C peaks as a way to encode information about the number of protons attached to a carbon Cq (quaternary carbon, no protons), CH (methine, one proton), CH2 (methylene, two), or CH3 (methyl, three). These spectra are called edited because the phase (positive absorptive or negative absorptive) is modified relative to a normal 13C spectrum in order to encode additional information. APT gives all of the information of a normal carbon spectrum with somewhat reduced sensitivity, and it tells you whether the number of attached protons is odd (CH3 or CH) or even (CH2 or quaternary). 

With this simple process, we have encoded information about the number of attached protons into the phase of the 13C peak. Proton decoupling during the acquisition period will give a spectrum with single peaks for each carbon resonance, pointing either up or down according to the number of attached protons. 

Group Pattern Angular Frequency in hertz Rotation in cycles after r = l/J Axis Phase 

If we choose the - -x axis as our reference, we will have positive peaks for the Cq and CH2 groups, and negative peaks for the CH and CH3 groups. Usually, the APT spectrum is phased the other way, using the —xr axis for the reference axis, so that the CH and CH3 peaks are positive and the Cq and CH2 peaks are negative. 

There is, however, one very important assumption we have made that is not practical The carbon resonance was assumed to be on-resonance. Obviously, we cannot guarantee this for all carbon resonances in a spectrum because different13 C peaks have different resonant frequencies. What happens if the carbon chemical shift is not at the center of the spectral window The magnetization, which starts on the / axis, will precess in the x-y plane at an angular frequency An, where Av is the position of the 13C resonance relative to the center of the spectral window on a hertz scale. Components of the multiplet will rotate a little faster or a little slower relative to the central component for example, the three components of a triplet will rotate at angular frequencies Av +./, Av, and Av — J. This additional rotation due to the chemical shift (Av) will affect the phase of the magnetization when acquisition 

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Figure 5-17 illustrates the result of the complete editing experiment for cholesteryl acetate, which gives negative peaks for CH and CH3 resonances and positive peaks for C and CH2. Proton irradiation during acquisition provides decoupling. This experiment affords a visual identification of the substitution pattern of all carbons, and has been called 7 modulation or the Attached Proton Test (APT). It exists in many variants. 

Figure 5-17 Lower The normal proton-decoupled spectrum of cholesteryl acetate. Upper The attached proton test (APT), phased so that CHt and quaternary carbons are positive and CH and CH3 carbons are negative. (Reproduced with permission from A. E. Derome, Modern NMR Techniques for Chemical Research Pergamon Press, Oxford, UK, 1987,...

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. 

In practice, however, we rarely collect a H-coupled spectrum because of the poor sensitivity associated with this experiment. More elegant experiments that require less instrument time are available to provide similar information. These experiments include the attached proton test (APT) [1] and the distortionless enhancement through polarization transfer (DEPT) [2] experiments. Careful examination of data generated by the heteronuclear multiple quantum... 

The NMR spectrum of the pure hydroxypropylated polystyrene showed peaks in the regions of d 21.5-26 ppm and d 64-67.5 ppm. Analysis by the attached proton test (APT) in conjunction with model compounds indicated that the region between 21.5 and 26 ppm corresponds to the methyl carbon resulting from attack of the polymeric organo-lithium at the least hindered carbon to form a secondary alcohol chain-end functional group (see (a) in eqn [7]). The area between 3 64 and 67.5 ppm was assigned to the carbon bonded to oxygen for two diastereomerically different products as shown by structure 1, where the chiral carbon atoms are labeled with asterisks. 

The spin-echo experiments described above have been developed into a powerful method for differentiating between CH3, CH2, CH, and quaternary carbon atoms. The Attached Proton Test (APT) utilizes a ated spin- cho (GASPE) pulse presented in Figure 5.7. Since the proton decoupler is switched... 

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... 

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