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

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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.

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