Ab Initio Calculations of Conformational Effects on 13C NMR Spectra of Amorphous Polymers

Abstract

A new approach for the simulation of inhomogeneouly broadened solid state NMR spectra is presented. The geometry dependence of the chemical shift is investigated and used to analyze the local molecular organization of amorphous glassy polymers by comparison of simulated lineshapes with experimental spectra. The method combines statistical conformational models, force-field optimization and the advanced quantum chemical ab initio IGLO scheme for the calculation of chemical shifts. An introduction to these topics is given together with a detailed derivation of our approach. The novel scheme is used to assess the validity of statistical models of polymer conformations and to discriminate between competing theories. Moreover, the molecular orbital contributions to the chemical shift, the configurational splitting in solution, and the role of the anisotropy of the chemical shift as a source of structural information are addressed. Methodical aspects, including preconditions and limitations, are covered in a separate chapter. To check the range of applicability, the method is applied to various polymer systems of different structural classes (poly(ethylene), poly(propylene), poly(isobutylene), poly(butadiene), poly(isoprene), poly(vinyl chloride), poly(methyl methacrylate)).