Fourier transform microwave spectroscopy of halogenated triatomic molecules

• Main Author: Gatehouse, Bethany
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/6786

Pure rotational microwave spectra of the following molecules have been obtained using a pulsed jet cavity Fourier Transform microwave (FTMW) spectrometer. Nitrosyl Chloride: C1NO Rotational transitions of five isotopic species of nitrosyl chloride were measured in the 4-26 GHz frequency range. From these transitions, rotational and quartic centrifugal distor-tion constants were obtained. The rotational constants were used in various structure determinations, and the centrifugal distortion constants were used to refine an already known harmonic general valence force field. Quadrupole and spin-rotation hyperfine structure in the measured transitions was analysed to yield diagonal and off-diagonal quadrupole coupling constants and diagonal spin-rotation coupling constants of both the chlorine and nitrogen nuclei. The principal quadrupole coupling constants were evaluated and used to calculate the approximate ionic character of the N-Cl bond. The spin-rotation coupling constants were used to calculate the nuclear shielding of the nitrogen and chlorine nuclei. Sulphur Difluoride: SF₂ The microwave spectrum of the unstable molecule sulphur difluoride, prepared via a pulsed electric discharge in an SF₆/OCS mixture, has been measured. ¹⁹F hyperfine structure has been resolved in some of the measured transitions, and unusually small, negative values have been determined for two of the three spin-rotation coupling constants. These constants have been used to calculate the ¹⁹F principal inertial axis nuclear shielding components; the determined paramagnetic shieldings are also un-usual and produce a large positive average shielding. The shieldings have been compared with the corresponding constants obtained for OF₂, SiF₂, and GeF₂. The results for SF₂ are consistent with values determined from earlier NMR experiments and ab initio calculations. The origin of the unusual shieldings is discussed, using a semi-quantitative model, in terms of the geometry and the electronic energy level structure of the molecule. Halogenated Oxo- and Sulphido-boranes: FBO, C1BO, and FBS The unsta-ble XBE species FBO, C1B0, and FBS have been prepared though the use of an electric discharge and their microwave spectra have been measured using FTMW spectroscopy. Hyperfine structure in the observed transitions has been resolved and analysed in terms of quadrupole, spin-rotation, and spin-spin interactions. The determined quadrupole coupling constants have been used to show that the electronic environment surrounding the boron nucleus of all three species is very similar, and the spin-rotation constants have been used to calculate the average shieldings of the CI, F, and B nuclei of these molecules; this is the first experimental determination of these parameters. Phosphenous fluoride: OPF Free gas phase OPF has been prepared here for the first time by passing an electric discharge through a mixture of PF₃ and O₂, and the microwave spectra of two isotopomers have been measured in the 4-26 GHz frequency range. These transitions were analysed in terms of rotational, centrifugal distortion, and spin-rotation coupling constants. The determined rotational constants have been used to calculate ro, r2, and approximate re molecular geometries, and the spin-rotation coupling, constants have been used to calculate the nuclear shielding parameters of the fluorine and phosphorus nuclei. Because both F and P are spin ½ nuclei, and because both sets of determined spin-rotation coupling constants had very similar values, assignment of these constants to a particular nucleus was not possible; the nuclear shielding parameters of this species have been calculated using both possible assignments.