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Heyes, Stephen J.; Dobson, Christopher M.
doi: 10.1002/mrc.1260281309pmid: N/A
The channel‐type inclusion complex, (deoxycholic acid)2–ferrocene [(DCA)2–;ferrocene], was studied by 13C cross‐polarization/magic angle spinning NMR spectroscopy. The spectra indicate that two different DCA molecules are present in the crystalline structure. Relaxation data indicate a relatively restricted degree of reorientational freedom of the included ferrocene molecules. 2H NMR spectra of the inclusion complex with perdeuterated ferrocene reveal rapid, anisotropic librations of the principal axes of the ferrocene molecules about their mean positions, varying from <ca. ± 1° at 160 K to ca. ± 28° at 350 K. The anisotropic, librational nature of this molecular reorientation may be rationalized in terms of the packing features of the sheet‐like nature of the structures of such deoxycholic acid compounds.
Findlay, Anita; Harris, Robin K.
doi: 10.1002/mrc.1260281317pmid: N/A
A pulse sequence is presented which allows measurement of short T1 values (< 1 s) in solids, avoiding probe damage by the use of saturating sequences of 90° pulses. The use of the sequence is illustrated for a cross‐linked polymeric system which contains composite peaks having both short‐T1 and long‐T1 components.
Espidel, Joussef E.; Harris, Robin K.
doi: 10.1002/mrc.1260281306pmid: N/A
The construction of equipment capable of studying the 1H NMR of samples down to 26 K is described. The system includes a facility for obtaining matrix‐isolated compounds at cryogenic temperatures. Samples can be changed without removing the cryogenic equipment from the probe area. An NMR method of obtaining the true temperature at the sample is described. Illustrative spectra of both pure compounds and a matrix‐isolated system are provided.
Edwards, John C.; Ellis, Paul D.
doi: 10.1002/mrc.1260281312pmid: N/A
High‐field, 95Mo, solid‐state nuclear magnetic resonance spectra of (NH4)6Mo7O24 · 4H2O and (Bu4N)2Mo2O7 were obtained using solid‐echo techniques together with 1H cross‐polarization and dipolar decoupling. The results show that reliable second‐order powder line shapes can be obtained with excellent cross‐polarization enhancements of 66–86% of maximum theoretical enhancement. The lower than maximum enhancements are due to the relatively long TIS values, which are similar in magnitude to the TH1ρ values. Experiments were performed which reveal the dynamics involved in the cross‐polarization process, including measurement of the cross‐relaxation time constant and the 1H and 95Mo rotating‐frame spin‐lattice relaxation times. It was found that the principal relaxation process causing low gain in the observed signal was the 1H T1ρ, which was found to be on the order of 23–24 ms. The experimental results obtained indicate that selective cross‐polarization dynamics occur in molybdenum cluster species with inequivalent molybdenum sites. The 95Mo rotating‐frame spin‐lattice relaxation rate is long in comparison with the other relaxation processes, making molybdenum a good cross‐polarization candidate but precluding the use of multiple contact cross‐polarization.
Klinowski, Jacek; Anderson, Michael W.
doi: 10.1002/mrc.1260281313pmid: N/A
Insights into the structure of molecular sieves are provided by solid‐state NMR in ways which are herein reviewed and extended. The uses of 29Si, 27Al and 1H NMR are briefly discussed. Attention is then paid to recent work which examines, using 1H and 13C NMR, the chemical status of guest organic molecules and questions of shape selectivity, giving novel information relevant to catalytic reactions.
Tekely, Piotr; Canet, Daniel; Delpuech, Jean‐Jacques; Virlet, Joseph
doi: 10.1002/mrc.1260281305pmid: N/A
In crystalline polyoxymethylene, a system with tightly coupled protons, it is shown experimentally that the averaging of the dipolar local field of protons by multipulse proton homonuclear decoupling leads to a decrease in the spin–spin contribution to the 13C rotating frame relaxation rate.
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