Ab initio molecular modeling of C-13 NMR chemical shifts of polymers. 2. Propene-norbornene copolymers

A methodology based on ab initio DFT computations as well as on molecularmechanics has been devised for helping the elucidation of the microstructure of polymers through the interpretation of their C-13 NMR spectra and has been applied to the case of propene-norbornene (P-N) copolymers. In a first step, a rotational-isomeric-state model of the chain of these copolymers has been achieved. The results of molecular mechanics calculations on various model compounds were checked and corrected by means of ab initio DFT computations at high level of theory. In general, good agreement was found between the results obtained with Allinger's MM2 force field and with the quantum-mechanical method B3LYP/ 6-31G**. Then, theoretical C-13 chemical shifts for two basic compounds were obtained according to the GIAO (gauge including atomic orbitals) method, using a large basis set and Adamo and Barone's functional. The theoretical chemical shifts averaged over the RIS populations provided rather unambiguous indications for interpreting the C-13 NMR signals observed in spectra of isotactic (P-N) copolymers with mid-low norbornene content. Thus, only with the help of these calculations was it possible to achieve a first unambiguous assignment of each of the seven major N signals in the complex spectra of P-N copolymers. This methodology may turn out to be of general utility, in particular for those macromolecular systems where empirical relationships between conformation and chemical shifts are not available.