No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

Gas-phase electron diffraction (GED) is the method of choice for determining the structures of molecules containing between two and 100 atoms, free from intermolecular interaction. However, for many molecules it becomes necessary to augment the experimental GED data with information from other sources. The SARACEN method, used routinely at Edinburgh when determining structures, allows computed parameters from ab initio and density functional theory (DFT) calculations to be used as extra data in the GED refinement process. This thesis describes the determinations of the gas-phase structures of molecules that contain heavy p-block elements, including examples from Groups 13, 14, 15 and 16. Each of the compounds studied was solid at room temperature, requiring heating to produce a suitable vapour pressure and vaporisation rate and testing the existing electron diffraction apparatus to its limits. Use was made of a new heated reservoir, recently developed in Edinburgh by a previous PhD student, which has allowed compounds to be studied that were previously inaccessible. The molecules that were studied during the course of this degree are: In(P3C2But2), In(P2C3But3), Sn(P2C2But2), Sb2(C6F6)3, Bi2(C6F6)3, Se(SCH3)2 and Te(SCH3)2. While determining the structures of these molecules, accurate theoretical geometries have been obtained using both ab initio and DFT methods. As a result a better understanding has been achieved of which methods are suitable for use in calculating the structures of molecules with heavy p-block elements. The use of pseudopotentials as opposed to all-electron basis sets proved necessary when performing calculations on such large molecules with heavy atoms. The extent to which these pseudopotentials, especially ones that consider very few electrons to be in the valence shell of an atom, can affect the calculated geometries has been shown to be considerable. In addition, methods being developed to compute vibrational corrections for gas-phase structure determination have been extended to the crystalline phase. Molecular dynamics simulations have been used to derive the effects of vibrations on average nuclear positions, relative to equilibrium positions. The differences, when applied to coordinates obtained experimentally by neutron diffraction yield experimental equilibrium structures.

Part I Electrochemical charge transfer at a metallic electrode: a simulation study The factors which affect the rate of heterogeneous electron transfer at a metallic electrode in the context of Marcus theory are investigated through molecular dynamics simulations. The system consists of the ionic melt K3Eu2+ 0:5Eu3+ 0:5Cl5:5 sandwiched between two parallel plate platinum electrodes held at a preset electrical potential. The charges on the electrode atoms are variationally obtained through the method of Siepmann and Sprik [J. Chem. Phys. 102, 511 (1995)] which models the polarization of the electrode by the melt and maintains the condition of constant potential. A two-dimensional Ewald summation is employed to ensure that the absolute value of the potential is known, and the expressions derived by Kawata and Mikami [Chem. Phys. Lett. 340, 157 (2001)] are extended to allow for induced dipoles on the melt ions by their mutual interaction and the interaction with the electrode surface. The Marcus free energy curves are calculated for electron transfer events between a europium ion and the metallic electrode, and their dependence on the position of the redox ion and the applied potential examined. The system is consistently found to be in accord with the linear response regime. A moderately-ranged oscillatory character in the mean electrical (Poisson) potential is observed extending into the fluid, which is in marked disagreement with the predictions of existing mean-field (Gouy-Chapman) predictions. These oscillations are found not to be reflected in the calculated Helmholtz reaction free energy, which indicates that the Poisson potential is not the appropriate potential for discussions of the kinetics of electrode processes. The strong dependence of the reorganization energy on the position of the redox ion is traced to the image charge effect, and appears insensitive to the polarizability of the anion. Following the evolution of the Eu{Cl radial distribution function throughout a redox process reveals that the bond length in the transition complex is exactly in between those of the ground state reactant and product complexes. The potentials of mean force for the approach of a Eu2+ and Eu3+ ion to the electrode calculated through umbrella sampling are found to be in quantitative agreement with those calculated through the position-dependence of the respective concentration profiles. A method to parameterize a model of the interactions between the melt ions and the electrode surface from ab initio density functional theory calculations is described. The method is used to obtain a suitable interaction model for a system consisting of a LiCl liquid electrolyte and a solid aluminium electrode. The electrolyte is found to exhibit a potential-driven phase transition which involves the commensurate ordering of the electrolyte ions with the electrode surface; this leads to a maximum in the differential capacitance as a function of applied potential. Away from the phase transition the capacitance was found to be independent of the applied potential. Part II Are dipolar liquids ferroelectric? The observation of a very sharp low frequency spike in the hyper-Rayleigh spectrum (HRS) of strongly dipolar fluids, such as acetonitrile and water, has been interpreted as reflecting a very slowly relaxing component in the transverse dipole density. This suggestion is at variance with the expectation of dielectric theory for an isotropic fluid and has led to the speculation that the slow relaxation is associated with the reorganization of ferroelectric domains. Very large-scale molecular dynamics simulation ( 28000 molecules) have been carried out using a 3-site potential model of acetonitrile. The simulated fluid shows no suggestion of strong dipole correlations and domain structure. The dipole density correlations behave as predicted by normal dielectric theory and their spectra do not show the low-frequency feature seen in the HRS. In order to examine the characteristics of the spectra which would be seen in a ferroelectric domain, the acetontrile model was transmuted to more closely resemble a Stockmayer-like fluid with the same dipole density and a ferroelectric phase was observed. In this phase the dielectric spectra show (i) a high-frequency spectral feature due to librational motion of the molecules within a domain, and (ii) slowly-relaxing longitudinal and transverse polar modes, again at variance from the experimental HRS characteristics.

Electron transport at low temperatures in two-dimensional electron systems is governed by two quantum corrections to the conductivity: weak localisation and electron-electron interaction in the presence of disorder. We present the first experimental observation of these quantum corrections in graphene, a single layer of carbon atoms, over a temperature range 0.02 - 200 K. Due to the peculiar properties of graphene, weak localisation is sensitive not only to inelastic, phase-breaking scattering events, but also to elastic scattering mechanisms. The latter includes scattering within and between the two valleys (intra- and inter-valley scattering, respectively). These specifics make it possible, for example, to observe a transition from weak localisation to antilocalisation. Our work reveals a number of surprising features. First of all the transition occurs not only as the carrier density is varied, but also as the temperature is tuned. The latter has never been observed in any other system studied before. Second, due to weak electron-phonon interaction in graphene, quantum interference of electrons survives at very high temperatures, up to 200 K. For comparison, in other two-dimensional (2D) systems the weak localisation effect is only seen below 50 K. The electron-electron interaction correction is also affected by elastic scattering. In a two-valley system, there are two temperature regimes of the interaction correction that depend on the strength of inter-valley scattering. In both regimes the correction has its own expression. We show that because of the intra-valley scattering, a third regime is possible in graphene, where the expression for the correction takes a new form. The study of weak localisation demonstrates that the third regime is realised in our experiments. We use the new expression to determine the Fermiliquid parameter, which turns out to be smaller than in other 2D systems due to the chirality of charge carriers. At very low temperatures (below 100 mK) we observe a saturation of the electron dephasing length. We study different mechanisms that could be responsible for the saturation and discuss in detail one of them – spin-orbit interaction. We determine the spin coherence length from studies of weak localisation and the temperature dependence of the conductivity and found good agreement between the two types of experiments. We also show the way to tune the spin coherence length by an order of magnitude by controlling the level of disorder. However, experiment shows contradictions with theory both in values of the spin coherence length and the type of spin relaxation. We speculate about another spin-related mechanism, spin flip by vacancies, which to some extent could also explain our observations. We also present electron transport in graphene irradiated by gallium ions. Depending on the dosage of irradiation the behavior of electrons changes. Namely, electron localisation can be tuned from weak to strong. At low dosages we observe the weak localisation regime, where the mentioned quantum corrections to the conductivity dominate at low temperatures. We found the electron scattering between the valleys to be enhanced, attributing it to atomically sharp defects (kicked out carbon atoms) produced by ion irradiation. We also speculate that gallium ions can be embedded in the substrate or trapped between silica and graphene. We draw this conclusion after investigation of the spin-orbit interaction in irradiated samples. At high dosages electrons become strongly localised and their transport occurs via variable-range hopping.