Dynamic carbon-13 MAS NMR: application to benzene ring flips in polyaryl ethers.
Carbon-13 dynamic MAS NMR is used to determine the pi-flip rates of the phenyl rings in the low-molecular-weight members of the polyaryl ethers series (phenyl-O(-phenylene-O)n-phenyl). The first member in the series (diphenyl ether, n = 0) does not undergo measurable dynamic processes up to its melting point (28 degrees C). The second and third members (n = 1 and 2) exhibit, above room temperature, line broadening effects due to fast pi-flips of the terminal rings, while the spectra of the n = 1 homologue also exhibit line broadening for the inner phenylene ring. Kinetic parameters for the various pi-flip processes were derived by a detailed lineshape analysis of the MAS spectra. The measurements were extended to lower temperatures by time-reverse ODESSA experiments. The kinetic parameters derived from these experiments are, k(t)(300 K) = 31 s(-1), E(t) = 84 kJ/mol, and k(i)(300 K) = 1.3 s(-1), Ei = 77 kJ/mol for the n = 1 homologue and k(t)(300 K) = 3.2 s(-1), E(i) = 78 kJ/mol, for the n = 2 homologue, where the subscripts t and i refer to the terminal and inner benzene rings, respectively. For the simulation of the dynamic MAS spectra the Floquet expansion method was used. In an introductory chapter the Floquet method is reviewed with emphasis on the practical aspects of the computation procedure, on the sensitivity of the results to the isotropic and anisotropic chemical shift parameters, and on the form of the results in the limiting fast and slow exchange regimes.