Low-temperature structures and Raman spectra of crystalline modifications of selenium difluoride dioxide

Abstract

Low-temperature Raman spectra of solid SeO₂F₂ indicated the existence of two crystalline modifications between 174 and 100 K, the crystal and molecular structures of which have been determined by single-crystal X-ray diffraction techniques at 150 and 120 K. At about 130 K the higher-temperature tetragonal α form undergoes a first-order phase transformation to give the lower-temperature monoclinic β form. In both modifications the SeO₂F₂ molecules have similar distorted tetrahedral geometries: the weighted averages of the bond parameters are Se–O 1.57(1), Se–F 1.66(1)Å, O–Se–O 126.7(9), F–Se–F 94(2) and O–Se–F 107.9(2)°. The molecules are only very weakly associated through intermolecular Se–O ⋯ Se contacts: in the α form each selenium atom is co-ordinated by two oxygen atoms from two different neighbouring molecules, while the oxygen atoms have contacts to two other molecules forming a three-dimensional network. In the β form these Se–O ⋯ Se contacts give rise to a structure consisting of corrugated layers of SeO₂F₂ molecules. Raman spectra of both modifications are consistent with their crystal structures and a factor-group correlation analysis confirmed the observed splittings of the Se–F stretching modes.