CNDO calculations for the rotation of aromatic rings in model analogues of PEEK

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.

Étude par CNDO/2 des mouvements de rotation des unités moléculaires constituant le squelette des polymères du type: PEK, PEEK et PEEKK

Diphenyl ether (DPE) and diphenyl ketone (DPK) are the molecular constituents of polymers such as PEK, poly(ether–ketone); PEEK, poly(ether–ether–ketone), and PEEKK, poly(ether–ether–ketone–ketone). A conformational analysis of DPE and DPK is examined by means of semiempirical calculations using CNDO/2 (Complete Neglect of Differential Overlap). The modelization is first run disregarding electronic interactions and steric effects, then introducing them in the calculations. We found that the absolute minimum energy conformer has torsional angles (θ1,θ2) = (30°,150°) for the DPE (with and without interactions) and (θ1,θ2) = (90°,90°) for the DPK (without interactions). Considering the polymer chains of PEK, PEEK, and PEEKK, the motion of DPE is expected to be easier than that of DPK in regions of local motions and for equivalent steric effects. Keywords: conformational analysis, diphenyl ether, diphenyl ketone, CNDO/2, rotational barriers.