Ab initio molecular modeling of ~(13)C NMR chemical shifts of polymers. 1. Ethylene-norbornene copolymers

摘要

Cycloolefin copolymers (COC) have recently raised much interest because of their excellent thermal and optical properties, largely determined by the chain composition and stereochemistry. Previous force-field computations allowed us to define the main conformational characteristics of ethylene-norbornene (E-N) copolymers and to contribute to the elucidation of their microstructure on the basis of empirical relationships between conformation and ~(13)C nuclear magnetic resonance (NMR) chemical shifts. A thorough test of ab initio ~(13)C chemical shifts computations [gauge-invariant atomic orbitals (GIAO)] on known cases shows that the agreement with experimental data is quite good, especially with the MPW1PW91 density functional theory (DFT), using the 6-311+G(2d,p) basis set on properly energy-minimized structures. We applied this method on proper model compounds to confirm the signal assignment of the spectra of E-N copolymers in the presence of norbornene microblocks, where strong effects arising from ring distoritions are expected to occur. The results nicely confirm the latest assignment of norbornene signals belonging to ENNE sequences. This shows that great potentialities of GIAO/DFT computations with regard to complex spectra interpretation and polymer microstructural investigations.