Using Density Functional Theory To Interpret the Infrared Spectra of Flexible Cyclic Phosphazenes

The cyclic phosphazene trimer P3N3(OCH2CF3)(6) and the related cyclic tetramer P4N4(OCH2CF3)(8) have been synthesized, isolated and their vapor-phase absorption spectra recorded at moderate resolution using an FTIR spectrometer. The interpretation of these spectra is achieved primarily by comparison with the results of high-precision density functional calculations, which enable the principal absorption features to be assigned and conclusions to be drawn regarding the geometries and conformations adopted by both molecules. These in turn allow interesting comparisons to be made with analogous cyclic halo-phosphazenes (such as P3N3Cl6) and other related ring compounds. The highly flexible nature of the two cyclic phosphazenes precludes a complete theoretical study of their potential energy hypersurfaces and a novel alternative approach involving the analysis of a carefully selected subset of the possible molecular conformations has been shown to produce satisfactory results. The two cyclic phosphazene oligomers have been proposed as the major low-to-medium temperature pyrolysis products of the parent polyphosphazene (PN(OCH2CF3)(2))(n), and the identification of vibrational absorption features characteristic of each molecule will enable future studies to test the validity of this proposition.