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 considered as a source of structural information. To this end, C-13 chemical shifts are calculated on an ab initio level for the central carbons of a tetramer model molecule, employing the IGLO method. Remarkable agreement between experimental and simulated spectra is obtained for polyisobutylene using the conformational statistics as obtained by Vacatello and Yoon. The observed experimental spread of almost-equal-to 20 ppm for the CH2 resonance is quantitatively reproduced in the calculations as is the gamma-gauche effect. Correlations of the chemical shift with specific geometrical aspects as C-C bond lengths are established.