F-19 SPIN-ROTATION CONSTANTS AND SHIELDING TENSOR OF SULFUR DIFLUORIDE FROM ITS MICROWAVE-SPECTRUM

The microwave spectrum of the unstable molecule sulphur difluoride, SF 2, prepared via a pulsed electric discharge in an SF6/OCS mixture, has been measured using a pulsed jet cavity Fourier transform spectromete r. F-19 hyperfine structure has been resolved, and unusually small, ne gative values have been determined for two of the spin-rotation coupli ng constants. These constants have been used to calculate the 19F prin cipal, inertial axis nuclear shielding components; the determined para magnetic shieldings are also unusual and produce a large positive aver age shielding, sigma(av). The shieldings have been compared with the c orresponding constants obtained for OF2, SiF2, and GeF2. The results f or SF2 are consistent with values determined from earlier NMR experime nts and ab initio calculations. The origin of the unusual shieldings h as been found, and is discussed, using a semi-quantitative model, in t erms of the geometry and the electronic energy level structure of the molecule. (C) 1997 American Institute of Physics.