Proton decoupling field, temperature

Lyerla et al. measured Tic over a wide temperature range from room temperature down to 105 K [94], and concluded that Tic s of not only CH3 but also CH resonances depend on CH3 rotational motion, and that the broadening of the CH3 resonance below -100 °C is also due to modulation of CH3 rotational motion at the frequency of proton nutation in the presence of the decoupling field. Gomez et al. have also reported solid-state high-resolution 13C NMR spectra of isotactic polypropylenes [95]. They used samples characterized by X-ray crystallography and reconfirmed the results obtained by Bunn et al. 

N.J., USA) as a lyophylized powder and used without further purification. 1% NMR spectra were recorded on a Bruker HFX-90 hlgh-resolution spectrometer ( % resonance frequency 9.12 MHz). (The Instrument was equipped with a Bruker Fast Fourier transform unit, a Nlcolet 1o8o data hemdling system, a Bruker B-SV 2 proton decoupler, and a.Bruker deuterium lock system.) The probe was kept at 43 - 2°c with a flow of temperature controlled air (Bruker temperature control unit B-ST 1oo/7oo). Field frequency lock was obtained from D2O In a 2 mm capillary Inserted into the I0- or 15-mm sample tube. 27.36 MHz I NMR spectra of the rlbonuclease-nucleotlde complexes were recorded on a Bruker WH-27o spectrometer In the Fourier transform mode. The chemical shift data cure given In ppm relative to the external stemdard (4 M solution of 95%-enriched I5NH4NO3 In 2 M HNO3). 

Fig. 16 CP/MAS NMR spectra of Cd(mtn)Ni(CN)4.1/2QHi2 as a function of temperature, showing fade-out of the cyclohexanol resonances when the molecule reorients at rates comparable to the strength of the decoupling field [53]. Normal spectra are shown on the left, and dipolar dephased spectra, where resonances of carbons strongly coupled to protons are much weaker, on the right.

Boum describe their use of the line shape method in the temperature range —24° to — 82°C (deuterium decoupling was used so that only the unperturbed single proton resonance was observed) and of a double-resonance method in the temperature range —97° to — 116°C. This involved the observation of recovery of magnetization of one of the two lines in the spectrum after a saturating r.f. field applied to the other line was removed. Consistent rates of inversion were found from both methods as evidenced by linearity of the Arrhenius plot. The results do not agree with the spin-echo results of Allerhand et al In this type of work, while fairly consistent results of rate constants may be obtained, there is dispute as to how the thermodynamic parameters should be derived, even in the relatively simple case of cyclohexane. ... 

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