Heteronuclear Decoupler Modes

The standard 13 C experiment leaves the decoupler on continuously to take advantage of the NOE enhancement (1H decoupler on at very low power during the relaxation delay) and to get decoupling of JCH (1H decoupler on at low power during the acquisition of the FID). Even when decoupling is not used during acquisition (i.e., when you want to observe fully

Figure 4.14

coupled 13 C multiplets) you should keep the decoupler on during the relaxation delay to get the benefit of the heteronuclear NOE. Decoupling that is applied only during the relaxation delay is sometimes called gated decoupling because the decoupler signal is "gated" on and off during each transient. We saw the gated-decoupled 13 C spectrum of phenetole in Figure 4.2 (bottom). Occasionally you may want to measure quantitative 1H-decoupled 13C spectra that can be integrated just like proton spectra to determine the number of carbons represented by each line. To get accurate peak areas, you will have to increase the relaxation delay to be at least 5 x T1 for the most slowly relaxing 13C in the sample. You will also want to turn the decoupler off during the relaxation delay to eliminate the heteronuclear NOE, which would enhance some peaks more than others. This experiment is the reverse of gated decoupling so it is called inverse gated decoupling. Figure 4.14 shows the inverse-gated 13C spectrum of phenetole with a relaxation delay of 30 s. While it is difficult to integrate these very sharp peaks with a noisy baseline, the integrals and peak heights clearly follow the pattern 1:2:1:2:1:1 for the six resonances, showing that the ortho and meta positions of the aromatic ring represent two carbons each. Without any 1H decoupling at all the 13 C spectrum is very weak and split into complex and overlapping multiplets, so this mode is not used. In fact, the first sign that your 1H decoupler is not working is that your 13C spectra only show the solvent (e.g., CDCl3) peak.

Figure 4.15 shows the 13C spectrum of sucrose with four different decoupling modes: decoupler off, decoupler on during the relaxation delay (A: NOE only), decoupler on during the acquisition of the FID (B: decoupling only), and decoupler on continuously (decoupling and NOE). The effect of the NOE can be seen by comparing the first and second spectrum (none vs. A only), and the effect of decoupling can be seen by comparing the second and fourth spectra (A only vs. A and B). The increase in peak height seen with decoupling is due to the multiplet signals being combined into a single, tall peak. In the third spectrum (B only) we are already seeing some NOE enhancement due to NOE buildup during the acquisition of the FID that does not completely dissipate during the relaxation time.

A Only B Only

Figure 4.15

A Only B Only

Figure 4.15

4.8.1 Parameters for Decoupler Gating

Varian uses the decoupler mode (dm) parameter to determine when the decoupler should be turned on and off during the pulse sequence. Time periods A, B, C, ... are defined during any pulse sequence, and the definition of these time periods can be observed by entering dps (display pulse sequence). For the general 1D sequence called "s2pul" (simple 2-pulse), the periods A, B, and C are defined as follows:

Relaxation delay Pulse Delay Pulse Acquisition

The B period is irrelevant in a simple 13C experiment because pi and d2 are usually set to zero.

These four decoupling modes along with their Bruker equivalents can be defined as follows:

Varian Bruker Definition Result dm = "yyy" pulprog: zgdc Continuous decoupling. Singlets with enhancement.

dm = "yyn" pulprog: zggd Gated decoupling. Multiplets with enhancement.

dm = "nny" pulprog: zgig Inverse-gated decoupling. Singlets without enhancement.

dm = "nnn" pulprog: zg No decoupling. Multiplets without enhancement.

where enhancement refers to the nuclear Overhauser effect. Bruker uses different pulse programs (parameter pulprog) for each application, but Varian uses different settings of the parameter dm.

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