Decoupling power

The residual coupling JR of an individual carbon with the carbon-proton coupling constant JCH is a linear function of the decoupling frequency offset A f2 from the protons attached to that carbon, provided the constant decoupling power y BJ2 n is sufficiently... 

In single-frequency off-resonance decoupled (SFORD) spectra the magnitude of the coupling interaction between 13C and H is reduced so that normally only one bond C-H coupling patterns are observed and an A X situation is assumed. In such a decoupling the residual coupling JK is less than /(C-H) and depends upon the decoupling power (yB2) and the decoupler offset (Av). For most applications to structural work... 

There are more advanced experiments such as DEPT (Chapter 7) that observe 13 C and use the decoupler to supply high power, short duration ( hard ) pulses at the XH frequency. This requires full power from the decoupler, but the parameters dpwr andpll7 are avoided for these pulses. Setting decoupler power to the maximum might lead to disastrous mistakes because the decoupler can only deliver full power for short ( 10 pis) periods of time without burning up the decoupler, the probe, and the sample. Instead, the parameters pp (Varian) and p3 (Bruker) are used for the 90° pulse width for decoupler hard pulses and pplvl (Varian) and pl2 (Bruker) indicate the power level for short-duration high-power decoupler pulses. 

Quantitative analyses were performed on a 200 MHz spectrometer with 10 kHz sample spinning. A pulse length of 5 ps was used for the l3C 90° flip angle (r.f, field = 4,67 G). The proton decoupling power corresponds to a 90° flip angle duration of 7 pt (r.f, field — 0.84 G) and it is applied continuously. Tetramethylsilane was used as external reference for the chemical shifts (5) in ppm. 

Answer The decoupler power was left at the low power setting during the acquisition. The power was not high enough to decouple the protons My so we see the quartet splitting scaled to a smaller value by the low-power decoupling. 

In many cases one finds that considerably more off-resonance experiments are performed than are strictly necessary. This is clearly a considerable waste of spectrometer time when weak samples are involved. In fact two separate coherent experiments will always suffice (195) provided that the decoupler power is known with precision. It is then necessary to solve a quartic equation. Since this will probably be done by iteration it may be helpful to perform one or two additional off-resonance experiments so that spurious solutions can be rejected. 

In off-resonance decoupling experiments of this type difficulties may arise for certain levels of the decoupling power, since then the combined effects of spin-tickling and spin population transfer may lead to certain important lines having very low intensity. It has therefore been suggested (196) that the amplitude of the irradiating rf field used should... 

In some respects, the H-decoupling operation can be viewed as a simultaneous H NMR experiment. Just as there is a transmitter offset that positions the X-nucleus observation frequency, so is there a corresponding decoupler offset for the H-decoupling fiequency. Many spectrometers have both transmitter and decoupler power levels. Three parameters, however, are specific to decoupling and have no counterparts among the spectral observation parameters that have been discussed. 

As a general rule, the transmitter and decoupler power levels are set so that both the H and 90° /p s are in the range of 5-10 ps. Pulse widths less than 5 ps are inaccurate, because pulse rise and fall times are in the microsecond range. Probes having /p s appreciably greater than 10 ps may be unable to execute those pulse sequences which require several transmitters to be rapidly switched on and off. 

We saw in Section 2-4k that (i) NMR spectra are usually recorded with WALTZ proton decoupling for spectrometers whose field strengths are 9.4 T or above (or whose H frequencies are 400 MHz and above) and (ii) one of the components of WALTZ decoupling, the decoupler field strength yB2, has to be determined for each individual spectrometer. The quantity 762 is a function of both the decoupler power level and characteristics of the probe. In addition, 7 2 depend on the sample itself if highly ionic solutions are being ex-... 

Fig. 4 Sequence of 150 MHz Ca lines of glycine with progressively decreasing decoupling power, step -1 dB and -0.25 dB (insert). Rotational resonances occur at double, single and fractional (1/3) rotation frequency...

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