SF4 and SOF2: Infrared Evidence for Dimer Formation at Low Temperatures

Abstract

The infrared spectrum of SF₄ trapped in an argon matrix at ∼4°K shows characteristic changes when the sample is warmed to allow diffusion and recooled. These changes are discussed in terms of association of the SF₄ molecules, and their simplicity and stability suggest a simple model for the dimer and more highly associated species. Weak bonding appears to exist between the pair of equatorial F atoms on one SF₄ molecule and the sulfur lone pair of electrons on an adjacent SF₄ molecule. The spectrum of SOF₂, present as an impurity, shows changes suggestive of the formation of an SF₄–SOF₂ complex. The observations, together with the NMR data of Muetterties and Phillips indicate that the mechanism for the rapid exchange of axial and equatorial fluorines in liquid SF₄ at room temperature is not intermolecular. A low barrier for the inversion of isolated SF₄ is also ruled out by the apparent absence of inversion effects in the microwave spectrum of the vapor, studied by Tolles and Gwinn. The conclusion is drawn that there are two possible paths for the dimerization or association reactions, and that one of these paths involves inversion of the SF₄ molecule. The activation energy of the latter reaction corresponds closely to the inversion energy of SF₄, which must be markedly smaller in this dimeric transition state than in an isolated SF₄ molecule.