Frequency-sweep decoupling

A detailed n.m.r. examination of some -D-g/ycero-pent-2-enopyra-nosyl derivatives has been reported. Frequency-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. 

All P.M.R. spectra were measured with a Varian HA 100 spectrometer operating in the frequency-sweep mode with tetramethylsilane as the reference for the internal lock. The double and triple resonance experiments were performed using a Hewlett Packard 200 CD audio-oscillator and a modified Hewlett Packard 200 AB audio-oscillator (vide infra). Spectra were measured using whichever sweep width was required to ensure adequate resolution of the multiplets under investigation, generally 250 or 100 Hz, and sweep rates were selected as necessary. Extensive use was made of the Difference 1 and Difference 2 calibration modes of the instrument, both for the decoupling experiments and for the calibration of normal spectra. 

A preferable experimental arrangement, which eliminates both of the above disadvantages, is the so-called frequency-sweep N.M.D.R. experiment. Here H0 is kept constant at all times, o>2 is located on the center of the resonance to be irradiated and then the spectrum is observed by slowly sweeping o>i through the spectrum. Since a>2 remains at all times on the center of the resonance to be decoupled, this experiment enables one to remove simultaneously all of the couplings caused by that particular resonance. All of the experiments discussed below were performed under these conditions. 

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...

Adiabatic pulse A type of pulse employing a frequency sweep during the pulse. This type of pulse is particularly efficient for broadband decoupling over large sweep widths. 

Fig. 15. Adiabatic decoupling of 13CO from 13C with a compensating pulse applied on the other side of the peaks. The compensating and decoupling pulses have the same shape but opposite frequency sweep. Due to the Bloch-Siegert effects, both the left and the right peaks are pushed towards the center while the centre peak is balanced and remains in its position. Reprinted from Ref. 47 with permission from Elsevier.

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. 

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.]...

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. 

Fluorine-19 NMR data were acquired at a frequency of 188.22 MHz with a Varian XL-200 spectrometer. Typically, 100 transients were accumulated from a 5% polymer solution by volume in dimethylformamide-d7 placed in a 5 mm sample tube at 120 C with internal hexafluorobenzene as a reference ( = 163 ppm). A sweep width of 8000 Hz was used with 8 K computer locations (acquisition time 0.5s) and a 5.0 s delay between 90 pulses (9.0 s duration). Proton heteronuclear coupling was removed by broad-band irradiation centered at 200 MHz. A modified Bruker WH-90 spectrometer allowed carbon-13 NMR spectra to be obtained with simultaneous proton and fluorine-19 broadband decoupling (13). 

In this technique, the frequency of a second radiofrequency transmitter (the decoupler) is set either upheld or downheld from the usual sweep width of a normal proton spectrum (i.e., off resonance). In contrast, the frequency of the decoupler is set to coincide exactly with the range of proton resonances in a true decoupling experiment. Furthermore, in off-resonance decoupling, the power of the decoupling oscillator is held low to avoid complete decouphng. 

Spin decoupling is used to simplify the analysis of complex spectra and is performed by modulation of the radio frequency radiation. The sample is irradiated with the resonance frequency of one nucleus while sweeping through the entire spectral region. 

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