Decoupler radiofrequency

Bax A (1983b) A simple method for the calibration of the decoupler radiofrequency field strength. J Magn Reson 52 76-80... 

Figure 2. Partial 100 MHz P.M.R. Spectrum of 3,4,6-tri-O-acetyl-v-glucal (1) measured for a chloroform -d solution (A normal spectrum of the Hi and H2 resonances respectively (B) frequency sweep spin-decoupled spectrum of the Hi and H2 resonances, with a strong decoupling field centred on the Hs resonance (C), as in (B) above, but with an additional weak radiofrequency field centred on the high field transition of the H2 resonance (D), as in (B) above, but with a weak radiofreauency field centred on the low field transition...

The basic components of the solid state spectrometer are the same as the solution-phase instrument data system, pulse programmer, observe and decoupler transmitters, magnetic system, and probes. In addition, high-power amplifiers are required for the two transmitters and a pneumatic spinning unit to achieve the necessary spin rates for MAS. Normally, the observe transmitter for 13C work requires broadband amplification of approximately 400 W of power for a 5.87-T, 250-MHz instrument. The amplifier should have triggering capabilities so that only the radiofrequency (rf) pulse is amplified. This will minimize noise contributions to the measured spectrum. So that the Hartmann-Hahn condition may be achieved, the decoupler amplifier must produce an rf signal at one-fourth the power level of the observe channel for carbon work. 

The other is double resonance or spin decoupling which effects great simplifications in the spectra. A second radiofrequency field... 

Non-selective ID experiments as well as selective ID experiments including either selective weak pulses (ID ROESY, ID TOCSY) or selective continuous radiofrequency irradiation (ID homonuclear decoupling, ID NOE). 

In practice, spin decoupling experiments are conducted in the following way. First, the spectrum is recorded under normal conditions. Then the spectrum is recorded while a second radiofrequency emitter irradiates at the resonance frequency of the nuclei that are to be decoupled (Fig. 9.22). This double resonance technique is used to identify nuclei which are coupled and which cause interpretation difficulties in the spectrum. 

Figure 3.4. H-Decoupled CNMR spectra of 35% enriched I-[ CJ- and 2-[ C]pyruvate, in the presence of (A) Mn -pyruvate carboxylase, and (B) Mg -pyruvate carboxylase. Each spectrum, except the normal Fourier transform (NFT) spectrum, is the result of applying two radiofrequency pulses to the sample the delay between them is indicated in sec. From measurements of peak heights the T, values can be calculated, and a resonance has zero intensity at about 0.69 T,. Note that Mn shortens T, and has a differential effect on the two...

Heteronuclear NMR experiments, which can be performed with the standard equipment of practically all modern spectrometers, require in general three separate radiofrequency (RF) channels for both spectrometer and probe head. The first two channels deliver the H (for decoupling) and "X frequencies to the sample, and the third channel is commonly tuned to D and operates the field frequency lock. In most standard probe heads, these three frequencies are delivered via two concentric coils. The inner coil with the higher Q factor is generally used for detection, the outer one only for the application of pulses and decouphng. TWo general designs are in use in normal or forward probe heads, which are optimized for direct detection of X nuclei, the inner coil is a tuneable X coil and the outer coil is normally double tuned to and the lock frequency, while in inverse probe heads which are optimized for indirect detection of "X resonances via H, this order is reversed. 

---