The microwave and infra-red spectra of some unstable gaseous molecules

The microwave spectra of three unstable molecules, bromine isocyanate (BrNCO), iodine isocyanate (INCO), and bromine thiocyanate (BrSCN), have been observed and analyzed in the frequency range 16-54 GHz. The infra-red spectrum of aminodifluoroborane (BF₂NH₂) has been investigated in the region 3700-400 cm⁻¹, and one band, the 2¹₀ vibrational band has been recorded at a resolution of 0.004 cm⁻¹, and the rotational structure analyzed. BrNCO: BrNCO was generated from the flow reaction of Br₂ with silver cyanate. The spectra of two isotopic species were observed: ⁷⁹BrNCO and ⁸¹BrNC0. They contain strong a-type transitions as well as some weaker b-type transitions which could not initially be assigned. In addition, the transitions show both Br and N quadrupole hyperfine structure. A novel method has been developed which uses perturbations in the Br structure to evaluate all the rotational constants, as well as the Br quadrupole tensor, entirely from a-type R branch transitions using a global least-squares fitting programme. This has allowed some b-type transitions to be assigned. The molecule has been shown to be planar and a partial r₀ structure was determined. The principal values of the Br quadrupole tensor have been evaluated and have provided some information about the type of bonding involved in the Br-N bond. INCO: INCO was observed in the flow reaction of I₂ with silver cyanate. Many strong a-type transitions were observed as well as some very weak b-type transitions. These transitions showed I and N quadrupole hyperfine structure; many perturbations in the I structure were observed. The method developed in the analysis of the spectrum of BrNCO using such perturbations to obtain otherwise unobtainable rotational constants was extended to include centrifugal distortion constants. The planarity of INCO has been confirmed and a partial r₀ structure was determined. The principal values of the I quadrupole tensor have been evaluated and have provided some information about the type of bonding involved in the I-N bond. BrSCN: BrSCN was generated in the flow reaction of Br₂ with silver thiocyanate. The spectra contained both a- and b-type transitions, all of which showed Br quadrupole hyperfine structure. The rotational constants and all five quartic centrifugal distortion constants were obtained for two isotopic species: ⁷⁹BrSCN and ⁸¹BrSCN. The rotational constants confirm the thiocyanate configuration, and a partial r₀ structure has been obtained. Also, the principal values of the Br quadrupole tensor were evaluated which show that the Br-S bond is essentially covalent, with small amounts of π and ionic character. BF₂NH₂ : BF₂NH₂ was generated by heating solid BF₃ NH₃. The wavenumbers of 7 of the vibrational fundamentals have been obtained, and from the analysis of the 2¹₀ band, an accurate set of rotational and quartic centrifugal distortion constants of the ground vibrational state have been obtained and the rotational and quartic centrifugal distortion constants of the 2¹ level have been evaluated. Also a Coriolis type perturbation was observed, probably due to the combination level v₇ + v₁₁ , which has provided an estimate of the wavenumber of the v₁₁ fundamental which has not yet been observed.Science, Faculty ofChemistry, Department ofGraduat