The pure rotational spectrum of sulfuryl chloride fluoride, SO,CIF, has been investigated in the frequency range 5.6-24.0 GHz using a pulsed molecular beam microwave Fourier-transform spectrometer. Between 48 and 170 lines of 8 to 26 rotational transitions have been observed for the six isotopomers SO,CIF, SO, 37CIF, 34S0,CIF, 34S0, 37CIF, SO'80CIF and SO " 0 37CIF (unlabelled atoms indicate l60, "F, 32S and 35CI) in natural isotopic abundance. The rotational and quartic centrifugal distortion constants have been determined. ro I T A P and r,-type structural parameters have been evaluated. A harmonic force field has been calculated to derive ground-state average and estimated equilibrium geometries. The description of the normal modes in terms of internal coordinates is discussed. Chlorine and fluorine hyperfine structures have been resolved, allowing quadrupole coupling (including zbc for the isotopomers containing " 0 ) and spin-rotation constants to be determined. Variations in the chlorine quadrupole coupling constants with different isotopomers have allowed the CI quadrupole tensor to be diagonalized and indicate that its z-axis coincides with the SCI bond.Although sulfuryl chloride fluoride, SO,ClF, has been known for more than 50 years,' there is only one study, by Holt and Gerry, reporting its structure in the gas phase., Using a conventional Stark modulated microwave spectrometer, they determined rotational constants, chlorine quadrupole coupling constants and an effective distortion constant for each of the two most abundant isotopomers, S0,ClF and SO, 37ClF (unlabelled atoms indicate l60, 19F, 32S and 35Cl). By assuming values for r ( S 0 ) and L(C1SF) from a comparison with the related molecules sulfuryl fluoride, SO,F,, and sul-fury1 chloride, S02C12, they were able to derive the remaining geometrical parameters.Recently Gombler investigated 32/34S isotopic shifts of the "F NMR frequency of sulfur-and fluorine-containing comp o u n d ~. ~ From these results he suggested that the SF bond in S 0 2 F 2 should be longer than that in SO,ClF, in contrast to the conclusions from the microwave studies. More recently a single-crystal X-ray diffraction study of S0,XY (X, Y = C1, F) at low temperatures has yielded the precise geometrical parameters of these compounds in the solid state.4Raman and IR spectra have also been re~orded.'.~ In addition, Pfeiffer obtained a force field of S0,ClF by transferring force constants and structural parameters from S02C1, and S02F2.7 These force constants reproduced most of the observed vibrational wavenumbers moderately well. Better agreement was obtained when the assignments of two pairs of fundamentals in ref. 5 were exchanged. In addition, the earlier assignment of the v6 band' was rejected and it was assumed that this fundamental is overlapped by v9. The assignments of Pfeiffer have been confirmed in a recent reinvestigation of the Raman and IR spectra.'At room temperature the microwave spectrum of S0,ClF is very dense, with many transitions between rotational levels at ...