Hyperfine constants and nuclear shieldings from the microwave spectra of FBO, CIBO, and FBS

Rotational spectra of the unstable molecules FBO, CIBO, and FBS have been measured in the 8-26 GHz frequency range using a pulsed jet cavity Fourier transform microwave spectrometer. The samples were prepared by passing electric discharges through mixtures of O-2/BF3, O-2/BCl3, or OCS/BF3, respectively, entrained in Ne. Hyperfine structures in the observed transitions have been resolved and analyzed in terms of nuclear quadrupole, spin-rotation, and spin-spin interactions. The quadrupole and spin-rotation coupling constants have been used to show that the electronic environment surrounding the boron nucleus of all three species is very similar. The spin-rotation constants have been used to determine, for the first time, experimental average shieldings of the Cl, F, and B nuclei of these molecules. Ab initio Values for the nuclear shielding parameters have been calculated for comparison. (C) 1998 Academic Press.