The dynamics of aromatic ring flipping in solid 4,4 '-diphenoxydiphenylether as studied by carbon-13 tr-ODESSA NMR

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 nat ural abundance) of a powder sample of 4,4'-diphenoxy diphenyl ether. The T- 1's of the various carbons in the sample are of the order of several thousa nds of seconds, allowing dynamic measurements to be carried out over long t ime 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-ODE SSA experiments, all of the carbon peaks showed a dispersion (sigmoidal red uction in the peak intensity) at a characteristic rime of about 20 s, which is identified with a spin diffusion process. This sets an upper limit to t he time range over which chemical (physical) processes can be measured by t he 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 pi-flips of the outer rings. The rate constant for this process at 25 degrees C is 6.5 s(-1) and its activation energy is estimated at 62 kJ/mol. No dispersion that could be related to K-flips of the inner rings was observed in the tr-ODESSA resu lts. An upper limit to the rate of this process can be set on the basis of the measured rate for spin diffusion. Ar room temperature this is more than two orders of magnitude slower than the pi-flip rate of the outer rings. W e also briefly report on some deuterium T-1 and 2-D-exchange experiments on the lower homologues of the polyphenyl ether series.