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Fourier transform microwave spectroscopy of scandium monohalides Lin, Wei

Abstract

The microwave spectra of three scandium monohalides, ScF, ScCl and ScBr, have been measured and analysed for the first time, using Fourier Transform Microwave (FTMW) spectroscopy. The samples were studied while entrained in pulsed jets of argon. They were prepared by the reaction of a precursor gas (SF₆, Cl₂ , Br₂ ) in the argon with scandium plasma produced by laser ablation from a scandium rod. Scandium Monofluoride, ScF The J= 1-0 transition of ScF in the ground state was measured. The nuclear quadrupole coupling constant of Sc, the spin-rotation coupling constants of both Sc and F, and the nuclear spin-nuclear spin constant were determined for the first time. The hyperfine parameters have been interpreted to explain the bonding of ScF. Scandium Monochloride, ScCl The J= 1-0 and J= 2-1 transitions of two isotopomers of ScCl have been measured in the ground vibrational state, along with the same transitions for Sc³⁵Cl in the first excited vibrational state. Rotational constants, centrifugal distortion constants, nuclear quadrupole coupling constants and nuclear spin-rotation coupling constants for both nuclei, and nuclear spin-spin coupling constants have been determined. A new equilibrium bond length r[sub e] has been evaluated, along with the infrared vibration frequency. The dissociation energy, D[sub e], has also been estimated. Scandium Monobromide, ScBr The J= 1-0, 2-1 and 3-2 transitions of ⁴⁵Sc⁷⁹Br and ⁴⁵Sc⁸¹Br have been measured in the ground vibrational states, along with the latter two transitions in the first excited vibrational states. This is the first high resolution spectroscopy of any kind carried out for this molecule. Precise rotational constants and centrifugal distortion constants have been measured along with Sc and Br nuclear quadrupole coupling constants, spin-rotation constants and a Sc-Br spin-spin constant. An equilibrium bond length r[sub e] has been evaluated, along with the infrared vibration frequency. The dissociation energy, D[sub e], has also been estimated. A plausible rationale for the nuclear quadrupole coupling constants has been produced using the Townes-Dailey theory. The molecules are highly ionic, with ionic characters greater than 94% . Reasonable absolute ⁴⁵Sc quadrupole coupling constants and variations between molecules have been calculated using estimated constant values for eQq[sub n10] along with orbital populations based on ab initio values for ScO. These results have been found to be inconsistent with those based on populations from direct Hartree-Fock calculations for ScO, ScF and ScCl. However, since the calculation itself produced reasonable coupling constants, use of the Townes-Dailey theory for the Sc halides is questioned. Magnetic shieldings for all nuclei have been calculated from the spin-rotation constants. The nuclear spin-nuclear spin coupling constants are consistent with a direct magnetic dipole-dipole interaction between nuclei.

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