Neutron scattering from molecular metallic compounds

Thesis



The alkali metal graphite intercalates and the conducting polymers polyacetylene and polypyrrole are reviewed and compared.

Polyacetylene was synthesized by Luttinger's method and doped by exposure to caesium vapour. The doped and undoped polyacetylene were characterized by neutron powder diffraction. Hydrogen absorption and subsequent pyrolysis of caesium doped polyacetylene were studied. Mass spectrum analysis of the gases evolved at 300°C showed methane and other alkanes, indicating H₂ addition to the polymer chain followed by methane elimination. The graphite intercalates C₈Cs and C₂₄Cs reacted irreversibly with acetylene, neutron powder diffraction indicating graphite and two new substances. Mass spectrum analysis of gases evolved on heating showed ethylene, butene and butadiene, suggesting a polymerization.

Polypyrrole was synthesized electrochemically and characterized by cyclic voltammetry. Gram quantities of polypyrrole doped with BF^-₄ and C10^-₄ were prepared electrochemically under high vacuum conditions to achieve high purity and avoid oxygen doping. Perchlorate doped polypyrrole prepared in an oxygen-free environment proved dangerously unstable, especially when dry.

The neutron scattering law for a harmonic oscillator and the energy level splitting pattern of a 1-D rotor in a cos3θ hindering potential are reviewed and applied to solid acetonitrile. Intense lines in the neutron spectrum at 140 and 190 cm^-1 are assigned to methyl group torsions with barriers to rotation of 400 and 734 cm^-1.

Hydrogen is reversibly physisorbed by intercalates C₂₄K, C₂₄Rb and C₂₄Cs at 100K to form compounds C₂₄M(H₂)₂. Neutron spectra of H₂, HD and D₂ physisorbed by C₂₄Rb and C₂₄Cs at energy transfers between 0.3-3 meV and 10-200 meV (1 meV = 8.065 cm^-1) were measured and assigned to rotational tunnelling and librations of hydrogen molecules in two sites of different dimensions. The cos2θ barriers to rotation were 57 and 78 meV respectively in C₂₄Rb(H₂)_x and 51 and 78 meV in C₂₄Cs(H₂)_x. A structural model is proposed for the metal layers with the hydrogen molecules in the layers and perpendicularly oriented.

Authors: Stead, W; Stead, William John

• Publication date: 1986
• DOI: 10.5287/ora-zr9mroyxw
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Neutrons; Molecular dynamics; Polyacetylenes; Metallic compounds; Scattering