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 ph enyl rings in the low-molecular-weight members of the polyaryl ethers serie s (phenyl-O(-phenylene-O)(n)- phenyl). The first member in the series (diph enyl ether, n = 0) does not undergo measurable dynamic processes up to its melting point (28 degreesC). The second and third members (n = 1 and 2) exh ibit, above room temperature, line broadening effects due to fast rr-flips of the terminal rings, while the spectra of the n = 1 homologue also exhibi t 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 t ime-reverse ODESSA experiments. The kinetic parameters derived from these e xperiments are, k(t)(300 K) = 31 s(-1), E-t = 84 kJ/mol, and k(i)(300 K) = 1.3 s(-1) E-i = 77 KJ/mol for the n = 1 homologue and k(t)(300 K)= 3.2 s(-1 ), E-i = 75 KJ/mol, for the n = 2 homologue, where the subscripts t and i r efer to the terminal and inner benzene rings, respectively. For the simulat ion of the dynamic MAS spectra the Floquet expansion method was used. In an introductory chapter the Floquet method is reviewed with emphasis on the p ractical aspects of the computation procedure, on the sensitivity of the re sults to the isotropic and anisotropic chemical shift parameters, and on th e form of the results in the limiting fast and slow exchange regimes. (C) 2 000 Academic Press.