| Popis: |
The 1-D magic-angle spinning (MAS) nuclear magnetic resonance polarization transfer method, time-reverse one-dimensional exchange spectroscopy by side-band alternation (tr-ODESSA), was applied to the carbon-13 signals (at natural abundance) of a powder sample of 4,4′-diphenoxydiphenyl ether. The T1’s of the various carbons in the sample are of the order of several thousands of seconds, allowing dynamic measurements to be carried out over long time periods. The carbon-13 MAS spectrum of 4,4′-diphenoxydiphenyl ether is only partially resolved, but there are enough well-defined peaks to allow a complete analysis of the dynamic processes in this compound. In the tr-ODESSA experiments, all of the carbon peaks showed a dispersion (sigmoidal reduction in the peak intensity) at a characteristic time of about 20 s, which is identified with a spin diffusion process. This sets an upper limit to the time range over which chemical (physical) processes can be measured by the tr-ODESSA method. The peaks associated with the ortho/meta positions of the outer rings indeed exhibit, in addition, a dispersion at 10 to 100 ms, depending on temperature, which we associate with the π-flips of the outer rings. The rate constant for this process at 25°C is 6.5 s−1 and its activation energy is estimated at 62 kJ/mol. No dispersion that could be related to π-flips of the inner rings was observed in the tr-ODESSA results. An upper limit to the rate of this process can be set on the basis of the measured rate for spin diffusion. At room temperature this is more than two orders of magnitude slower than the π-flip rate of the outer rings. We also briefly report on some deuterium T1 and 2-D-exchange experiments on the lower homologues of the polyphenyl ether series. |
