F 19 spin-rotation constants and shielding tensor of sulphur difluoride from its microwave spectrum

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Abstract
The microwave spectrum of the unstable molecule sulphur difluoride, SF 2 , prepared via a pulsed electric discharge in an SF 6 /OCS mixture, has been measured using a pulsed jet cavity Fourier transformspectrometer. 19 F hyperfine structure has been resolved, and unusually small, negative values have been determined for two of the spin-rotation coupling constants. These constants have been used to calculate the 19 F principal inertial axis nuclear shielding components; the determined paramagnetic shieldings are also unusual and produce a large positive average shielding, σ av . The shieldings have been compared with the corresponding constants obtained for OF 2 , SiF 2 , and GeF 2 . The results for SF 2 are consistent with values determined from earlier NMR experiments and ab initio calculations. The origin of the unusual shieldings has been found, and is discussed, using a semi-quantitative model, in terms of the geometry and the electronic energy level structure of the molecule.