On the structure of hexahydroxocyclotriphosphazene Marián Gall, Martin Breza * Department of Physical Chemistry, Slovak Technical University, Radlinskeho 9, SK-812 37 Bratislava, Slovakia article info Article history: Received 10 February 2008 Received in revised form 3 April 2008 Accepted 4 April 2008 Available online 11 April 2008 Keywords: Geometry optimization Conformational analysis Non-planar cyclotriphosphazene ring Electron structure Lone pair conjugation p Bonding abstract Hexahydroxocyclotriphosphazene geometry optimization at DFT level of theory produces 15 stable con- formational isomers whereas at MP2 level only 14. Unlike other cyclotriphosphazenes, only one of them has planar cyclotriphosphazene ring. The preferred anti-planar conformation of O–H and P–N bonds con- nected with shorter and stronger P–N bonds and increased electron density transfer from oxygen and nitrogen to phosphorus atoms suggests that this feature originates in the interaction between oxygen lone pairs and out-of-plane p PN bonds. This feature cannot be explained by negative hyperconjugation which is connected with P–N bonds weakening. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction A phosphazene backbone consists of alternating phosphorus and nitrogen atoms with formally alternating single and double P–N bonds. The electronic structure of the phosphonitrile skeleton has been an object of many controversies. The results of X-ray crys- tal structure analysis of cyclic phosphazenes imply that all P–N distances in the cyclophosphazene ring are equal and the perhalo- genated cyclotriphosphazenes are planar [1]. The generally accepted ‘‘island model” [2,3] describes the bond- ing between the phosphorus and nitrogen atoms in terms of r bonding and additional (out-of-plane and in-plane) p bonding aris- ing from the overlap of 3d orbitals of the phosphorus with nitrogen p orbitals being combined into sets of three-center p molecular orbitals (‘islands’ of electron density over P–N–P units with nodes at the phosphorus center). Each nitrogen is attached by equally strong p bonds to both neighboring phosphorus atoms. The break in conjugation at each phosphorus atom implies that adjacent three-center p bonds need not be coplanar. However, the group- theoretical analysis of molecular orbitals in planar cyclophospha- zenes of D nh symmetry with odd n [4] excludes any d P –p N bonding and it makes this bonding scheme doubtful in other phosphazenes as well. More recent bonding models [5] are based on prevailingly ionic P–N bonding (involving substantial charge transfer from phospho- rus to nitrogen) with negative hyperconjugation contributions resulting from the donor–acceptor interactions of (out-of-plane and in-plane) nitrogen lone pairs into strongly polarized antibond- ing r * PN and r * PX orbitals. The extent of negative hyperconjugation depends significantly on the nature of the X substituents on phosphorus atoms. Unequal negative hyperconjugation contribu- tions provide a previously lacking and compelling explanation for the P–N bond-length alternation observed in some linear polyphosphazenes. Halocyclophosphazenes are the most known phosphazene derivatives. They hydrolyze slowly under aqueous solutions. The rate of this reaction depends on the halogens attached to the phosphorus centers, on the ring size and on the pH [6]. The hydrolysis of the chlorophosphazene cyclic trimer has long been of interest because of its possible relevance to polymeriza- tion catalysis, as well as the undesirable role of water in the cross-linking of poly(dichlorophosphazene). The dihydroxotetrachlo- rocyclotriphosphazene cyclo-N 3 P 3 Cl 4 (OH) 2 and the hexahydroxocy- clotriphosphazene (trimetaphosphimic acid) cyclo-[NP(OH) 2 ] 3 in tautomeric equilibrium with cyclo-[HNP(O)(OH)] 3 has been reported [7]. At low concentrations, water either acts as a poly- merization catalyst or reacts with the trimer to generate the active catalytic species. At water contents greater than 1 mol%, an insoluble cross-linked polymer is formed, which cannot be derivatized to form useful materials. Allcock and co-workers [8] have proposed that water catalyzes the polymerization indirectly by the formation of hydrolyzed trimer species, some of them have been identified by 31 P NMR spectroscopy [9]. The lack of interest of quantum chemists in hydroxophosphazenes is thus very surprising. We have found only one HF/6-31G ** study of cyclotriphosphazenes [10] where a single stable confor- mational isomer of hexahydroxo derivative is mentioned as well. 0166-1280/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2008.04.004 * Corresponding author. Tel.: +421 2 59325 482; fax: +421 2 52493198. E-mail address: martin.breza@stuba.sk (M. Breza). Journal of Molecular Structure: THEOCHEM 861 (2008) 33–38 Contents lists available at ScienceDirect Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem