Field-sweep decoupling

There are two basically different methods of spin decoupling field-sweep decoupling, in which the frequency difference between the decoupling field and the observing field is kept constant the spectrum is swept, and frequency-sweep decoupling, in which the value of H2, is kept constant as the value of Hi changes. The latter is more commonly used for routine studies and the results are easier to interpret. 

It should be noted that it would have been totally impossible to have effected these decoupling experiments by the field-sweep method a>i has to be scanned ca. 100 Hz to observe the H2 resonances which of course would require < >2 to be scanned by an equal amount. Since it is not possible to obtain a decoupling field of this magnitude, the above experiment would merely result in partial decoupling of the H2 resonances. 

All of the experiments described above were performed by the field-sweep method, in which the radiofrequencies i and a>2 are passed through the spectrum by changing the magnetic field. This method has serious disadvantages when removal of large couplings is required, and, under these conditions, recourse should be had to frequency-sweep experiments. It is also possible to introduce more than one decoupling field and, hence, to effect triple-resonance experiments. 

R.N. Emanuel) were recorded at 220 MHz on a Veurian HR220 field-sweep spectrometer (P.C.M.U., A.E.R.E., Harwell) 220 MHz proton-decoupling experiments were made by Dr. R.A. Spragg on a Perkin-Elmer R3U... 

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 1H NMR spectra of parbendazole was recorded with a JEOL-PS 100 NMR spectrometer operating at a frequency of 100 MHz and a magnetic field strength of 2.349 T. Spectra were determined over the region 10.8-0.0 parts per million (ppm), with a sweep time of 250 s. Chemical shifts were recorded as S (delta) ppm downfield from tetra-methylsilane (TMS). Proton noise and off-resonance decoupled 13C NMR spectra were measured on a JEOL FX 90Q Fourier Transform NMR spectrometer operating at 90 MHR and spectral width of 5000 Hz (220 ppm). All measurements were obtained with the compound being dissolved in deuterated dimethyl sulfoxide (DMSO-d6) for dT NMR and in deuterated trifluoroacetic acid (TFA-dx) for 13C NMR. 

Fig. 5.—Diagrammatic Representation of the Relationship of the Observe and Irradiate" Fields Used for Performing Various Double-resonance Experiments in the Frequency-sweep Mode. [A, For spin-decoupling, the irradiation field is positioned at the mid-point of the X-resonance B, for spin-tickling, the irradiation field is set on transition X-2 (for both of these experiments, the effect of the perturbation is monitored by scanning the observing field) and C, for an INDOR experiment, the observing field is held positioned on the transition X-2, and now it is the irradiating field that is scanned.]...

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