CPMAS spectroscopy temperature

Apart from structural factors annular tautomerism is strongly influenced by the aggregate state of the compound. Gas-phase and solid state studies of tautomerism have received much attention recently because of the development of methods such as X-ray crystallography, CPMAS NMR, microwave spectroscopy, etc. Low temperature H NMR is the simplest and most straightforward method to determine Kx it only requires that proton transfer be slow enough to observe separate signals for both tautomers, and that the equilibrium is not shifted too much towards one of the tautomers (about 5% of the minor tautomers seems the limit). More detailed information on tautomerism of key azole systems is given below. 

The phase transformation behavior of Py-D3C was far simpler than that reported for tetrahydrofuran/N2- and tetrahydrofuran/Xe-D3C in reference 10. We attribute this difference in part to impurities in the samples employed, samples that contained methanol and ethylenediamine according to 13 C CPMAS NMR spectroscopy. We have observed that use of Si(OCH3)4 as a silica source or ethylenediamine as a catalyst in clathrasil synthesis introduces defects that can alter phase transition temperatures by as much as 30 °C and/or introduce new phase transformations. 

C CPMAS NMR studies of NH-pyrazoles and indazoles <1993CJC678> concluded that (1) the tautomer present in the solid is also the major tautomer in solution (2) certain pyrazole C-substituents (CF3, Br, Ar, Het) prefer position 3 (i.e., they are 3-substituted pyrazoles 239) whereas others (Bu1, Pr1, CH3) prefer position 5 (i.e., they are 5-substituted pyrazoles 240) (3) 3(5)-ferrocenylpyrazole should be a 50 50 mixture of both tautomers in the solid state. Low-temperature 13C NMR spectroscopy of 3(5)-phenylpyrazole shows that at -20 °C it exists as a mixture of 80% of the 3-phenyl tautomer 239 (R1 = H, R2 = Ph) and 20% of the 5-phenyl tautomer 240 (R1 = H, R2 = Ph) <1991G477>. 

The great power of NMR compared with other spectroscopic techniques is the ability to study both the frequency and geometry of molecular rearrangements in the solid state. It is true to say that there have been few studies of molecular motion in poly(imide)s using NMR spectroscopy. The main reason for this has been the previously limited temperature range of commercial CPMAS probes. Therefore, future work in poly(imide)s is expected to exploit the ground-breaking efforts of other workers in the field of multidimensional NMR spectroscopy in the solid state [3]. 

Interestingly, H(//-H)Os3(CO)io(NH3) is stable for a long time in the presence of NH3 but it decomposes when dissolved in chlorinated solvents yielding 1 and free NH3. The compound seemed, however, to be sufficiently stable in toluene solution to enable complete characterization in solution by H and C NMR spectroscopy. By comparing the NMR spectra obtained in solution (at —80 °C) (Fig. 3) and in the solid state (room temperature, CPMAS) and by measuring the nuclear Overhauser effect between the NH3 and the hydride resonances it was possible to establish that in solution and in the solid state H(,u-H)Os3(CO)io(NH3) adopts... 

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