APT spectra

With T set at V2J, the quaternary carbons generally appear with greater intensity than the other carbons, which will be of near-zero intensities, thereby allowing them to be distinguished, particularly from the CH2 carbons, as compared to the normal APT spectrum, in which both CH2 and quaternary carbons appear with positive amplitudes. A difference APT spectrum, in which an APT spectrum recorded with t set at %/is subtracted from another APT spectrum recorded with t set at /sj, can provide useful information. The methyl carbons will then appear with reduced intensities in the difference spectrum as compared to the methine carbons, allowing us to distinguish between them. 

The GASPE (or APT) spectrum of ethyl acrylate is shown here. Assign the signals to the various carbons. 

Fig. 19a-c Carbon-13 spectra of compound 1. a Standard spectrum (broad band decoupling) b APT spectrum c DEPT-135 spectrum... 

Figure 19 shows the normal (broad-band decoupled), APT and DEPT-135 spectra of model compound 1. Note that in the APT spectrum the solvent (CDC13) is visible, but not in the DEPT spectrum, where the two low-field quaternary aromatic carbons are also absent. 

The carbon spectrum, both in the broad-band decoupled form and as an APT spectrum. 

Fig. 3.17 The C JMOD (APT) spectrum of peracetylated glucose with D2 set to Note that the CDCl, triplet is visible.

If necessary, measure the chemical shifts from the ID C spectrum/lD "C DEPT ( C APT) spectrum. Draw up a table containing a column for chemical shifts, multiplicities and assignments. Leave enough space in the table to include the T, value for each carbon and a section for H/ C correlations. 

To illustrate the concept, Figure 6.16 shows the expected results of a normal 13C spectrum and an APT spectrum of 4-hydroxy-3-methyl-2-butanone. The APT spectrum shows all carbons including the quaternary C=0 and solvent carbons, and sorts the carbons into categories of CH and CH3 ( up peaks) and quaternary and CH2 ( down peaks). Note that sometimes APT spectra are presented upside down with CH and CH3 peaks down and quaternary and CH2 peaks up , but the deuterated solvent peak (no attached protons) tells us how to interpret it. 

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. 

Fig. 13. Left-, (a) ID CP spectrum (b) SS-APT spectrum with r = 4.5 ms and (c) SS-APT spectrum with r = 6 ms of cholesteryl acetate. Right Expansion of the spectra shown on the left between 12 and 45 ppm. The assignments are indicated above the peaks CH and CH3 groups can be distinguished by the fact that the CH resonances give intense negative peaks for r = 4.5 ms, that diminish in intensity for r = 6 ms, whereas the opposite effect is observed for CH3 resonances the CH2 groups give peaks that are weakly positive (like those around 27 ppm) or even null (like those around 30 ppm), whereas the quarternary carbons give intense positive resonances. (Taken from Lesage et al.201 with permission.)...

Now we are prepared to combine the APT (Section 12.10) and INEPT (Section 12.11.2) experiments into one of the most useful experiments in modem NMR. Like an APT spectrum, a DEPT (distortionless enhancement by population transfer) 13C spectrum is designed to display separate subspectra for CH, CH2, and CH3 carbon signals. And like an INEPT spectrum, signal intensity (i.e., sensitivity) arises by polarization transfer. 

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. 

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