POLYCARBONITRILE - A SEMIEMPIRICAL, AB-INITIO AND DENSITY-FUNCTIONAL STUDY OF MOLECULAR-STABILITY

The theoretical literature data with respect to the electronic propert ies of this compound is quite scarce and makes use of the planar all-t rans structure as the most stable for the calculations. In this work s emiempirical (AMI and PM3), ab initio (at the Hartree-Fock level) and density functional theory (using the correlation functional of Vosko, Wille and Nussair) were used to analyse the conformational stability o f the all-trans and all-cis dimers, trimers and tetramers of polycarbo nitrile. The semiempirical and ab initio calculations at the Hartree-F ock level showed in general that the all-trans structure with respect to other conformers is the most unstable structure. The inclusion of e lectronic correlation energy through the MP2 calculations or the VWN f unctional method suggest that the trans structure is the most stable. The relative energies calculated at the correlated level presented dif ferences around 2 kcal/mol among the different conformers. While the a ll-cis compounds presented a planar structure for any of the three met hods, the all-trans polymer showed a strong deviation of planarity wit h a set of local minima in its energy surface. These results suggest t hat further calculations on the electronic properties of this polymer can be significantly different of those actually available in the lite rature.