Bonding and aromaticity of cyclic phosphazenes viewed as interaction of Dnh fragments.pdfVIP

Journal of Molecular Structure: THEOCHEM 820 (2007) 148–158 /locate/theochem Bonding and aromaticity of cyclic phosphazenes viewed as interaction of Dnh fragments Libor Kapicˇka a, Pavel Kuba′cˇek a,*, Petr Holub b a Department of Theoretical and Physical Chemistry, Faculty of Science, Masaryk University, Kotla′ rˇska′ 2, 61137 Brno, Czech Republic b Supercomputing Center Brno, Institute of Computer Science, Masaryk University, Botanicka′ 68a, 60200 Brno, Czech Republic Received 20 March 2007; accepted 19 June 2007 Available online 29 June 2007 Abstract The qualitative molecular orbital approach based on orbital interactions was used to explore the nature of bonding in cyclic ?uorophosphazenes (F2PN)n, where n is 2–6. Besides the classical skeleton of r-bonds, only two, one radial and one axial, 2n-center two-electron p-bonds signi?cantly participate in the extra stabilization of the (PN)n ring. The p-radial interaction is more e?ective and comparable by size with the r ones. Additional slight stabilization of a (PN)n ring is achieved by nonbonding p-radial and p-axial molecular orbitals (MOs) which are mainly localized on nitrogen atoms. The orbital interactions have a hyperconjugation character. The bonding energy decomposition analysis showed that the cyclic interactions are about half covalent and half electrostatic. The covalent bonding is dominated by radial interactions. The aromaticity concept is not appropriate for description of bonding in cyclophosphazenes. The contribution of phosphorus d atomic orbitals to the concept of chemical bonding in phosphazenes is negligible, but the inclusion of d phosphorus functions in a basis set is appropriate for a correct quantitative description of electronic and geometric structure. Extended Hu¨ ckel (EHT), ab initio and density functional (DFT) calculations provide the same qualitative picture of the bonding. The very ?at B3LYP/6-311+G(3df)//B3LYP/6-31G(3df) potential energy surface (PES) with the low energy barrier (