Polycarbonitrile: A semiempirical, ab initio and density functional study of molecular stability

The theoretical literature data with respect to the electronic properties of this compound is quite scarce and makes use of the planar all-trans structure as the most stable for the calculations. In this work semiempirical (AM1 and PM3), ab initio (at the Hartree-Fock level) and density functional theory (using the correlation functional of Vosko, Wilk and Nussair) were used to analyse the conformational stability of the all-trans and all-cis dimers, trimers and tetramers of polycarbonitrile. The semiempirical and ab initio calculations at the Hartree-Fock level showed in general that the all-trans structure with respect to other conformers is the most unstable structure. The inclusion of electronic correlation energy through the MP2 calculations or the VWN functional method suggest that the trans structure is the most stable. The relative energies calculated at the correlated level presented differences around 2 kcal/mol among the different conformers. While the all-cis compounds presented a planar structure for any of the three methods, the all-trans polymer showed a strong deviation of planarity with a set of local minima in its energy surface. These results suggest that further calculations on the electronic properties of this polymer can be significantly different of those actually available in the literature.