CONFORMATIONAL EFFECTS ON C-13-NMR CHEMICAL-SHIFTS OF AN AMORPHOUS POLYMER - AN AB-INITIO STUDY BY THE IGLO METHOD

The spread in C-13-NMR chemical shifts m solid amorphous polymers is c onsidered as a source of structural information. To this end, C-13 che mical shifts are calculated on an ab initio level for the central carb ons of a tetramer model molecule, employing the IGLO method. Remarkabl e agreement between experimental and simulated spectra is obtained for polyisobutylene using the conformational statistics as obtained by Va catello and Yoon. The observed experimental spread of almost-equal-to 20 ppm for the CH2 resonance is quantitatively reproduced in the calcu lations as is the gamma-gauche effect. Correlations of the chemical sh ift with specific geometrical aspects as C-C bond lengths are establis hed.