Masters Group Current Research
Projects are available in several different areas requiring different skill sets. Please contact Sarah Masters for further information.
Molecular Structures of Excited States
There is a lot of interest in the molecular structures of excited state systems, not least as they are, by nature, difficult to calculate and calibration for computational methods is required. This project would involve the implementation of methods to use laser excitation to study the molecular structure of systems in their 1st excited state. It would also involve data analysis and use of ab initio and DFT methods to calculate structure.
DYNAMITE/SEMTEX Software Development
The DYNAMic Interaction of Theory and Experiment (DYNAMITE) method has been recently developed to enable the complete molecular structure determination of large, asymmetric systems. In involves the synergistic interaction of computational methods (molecular mechanics) and structure refinement methods to extract all information from experimental data. The Structure Enhancement Method for Theory and Experiment (SEMTEX) method is an upgrade of this to incorporate information from ab initio methods. These methods now require implementation into the bespoke structure refinement software suite, and robust testing for systems including transition metals and cages. This project would involve some software development, data collection and analysis, in conjunction with use of ab initio and DFT methods to augment the structure investigation.
PP-MOCVD surface modelling
Metal-Organic Chemical Vapour Deposition is normally implemented to coat and protect flat surfaces. A new method (developed by Susan Krumdieck, Mechanical Engineering) led to an industrial process (pulsed-pressure MOCVD) which allows a ceramic coating on just about anything, particularly machine parts subject to extreme wear and tear. This project will involve modelling suitable precursors for the PP-MOCVD process to optimise the surface coating and minimise cost. Structural investigation will also take place for the precursors to gain understanding of their gas-phase behaviour.
Electron Density Data Extraction
Gas electron diffraction has been used for almost a century to obtain molecular structure and to provide insight into the bonding within molecules. However, we need to think carefully about what bonding between atoms actually means and what information about internuclear distances (from which we derive bond lengths) we are extracting from our experimental information. The beam of electrons is scattered by the electric field gradient between the nuclei and electrons in an atom. The scattering patterns obtained from a GED experiment mainly constitute scattering from pairs of atoms, however, we also obtain 3-atom, 4-atom up to N-atom scattering, where N is the number of atoms in the molecule. The assumption is generally made that internuclear distances obtained from GED are the same as bond lengths (for bonded atoms); however, due to the nature of the scattering this is not inherently true.
Firstly the assumption is made that scattering observed from individual atoms and atoms in molecules is the same [the Independent Atom Model (IAM)]. This is not the case as the electron density about an atom is not the same when the atom is bonded to another atom, as in molecules. Therefore there is a chemical binding effect that is present in the GED scattering data that we do not currently account for. Secondly, the total electron scattering is made up of elastic and inelastic contributions, which need to be accounted for in the GED data that we interpret. This project will involve data collection and analysis to understand chemical binding effects on the observed electron scattering data, understanding the elastic and inelastic contributions to the scattering data, rationalising the GED data to extract information about electron density around the nucleus and implementing derived methodology into the current Canterbury GED structure refinement software to improve the accuracy of gas phase structure determination.
Development of new CCD software
As part of the upgrade of the Canterbury GED apparatus, there is a desire to implement a CCD detection system to enable rapid, real-time data analysis. The incorporation of CCD technology will also minimise the potential of losing sample information due to fouling of the current photographic film or jamming of the films during the data collection process. This project will involve the implementation of CCD technology, writing the software to run the camera (in conjunction with researchers at the University of Edinburgh, UK), testing and calibrating the camera and exploring the capabilities with FVP-GED experiments at high temperatures.
FVP-GED generation of radicals from bulky precursors
Short-lived species are generated using flash vacuum pyrolysis (FVP) techniques. The methodologies of FVP and GED have been combined in a new inlet system, and the unstable species are passed into the diffraction chamber where structural data is collected. Very-high temperatures are required for this work, as the short-lived species are usually generated at temperatures between 500 and 900 K. A range of systems are available for study. This project involves data collection, analysis and use of ab initio and DFT methods to calculate structure and augment the experimental investigation.
Jack-in-the-box Systems
Generating stable radicals from sterically loaded systems is in the very early stages of experimental investigation. The systems Z2R4 / ZR2 [Z = P or As, R = CH(SiMe3)2] provided the first known examples of molecules with relatively normal strong Z-Z bonds, which required no additional energy to break. The driving force for dissociation is the conformational change, which allows relaxation of the steric strain upon dissociation. This led to the term 'jack-in-the-box' molecules being applied to these systems. Other systems have been predicted to behave in this manner, although no experimental work has been carried out on them. This project would involve synthesis of the proposed systems and the study of their chemistry by various structural methods (GED, X-ray crystallography) as well as full computational studies of the reaction pathways.
General Research Methods - Structures of small molecules
Methods that we use for determining structures, include for gases:
for solutions:
for solids:
and theoretical methods include:
Often one method will not provide sufficient information to give a complete and accurate structure for a molecule. To overcome this problem we have developed methods that allow us to analyse simultaneously data obtained by several techniques.
The STRADIVARIUS method uses data from different experimental methods, particularly electron diffraction, rotational spectroscopy and liquid crystal NMR spectroscopy.
The SARACEN method uses experimental data, and also uses flexible restraints derived from theoretical (ab initio or density functional theory) calculations. It thus gives structures which a based on the best available information, both experimental and theoretical.
The DYNAMITE method takes SARACEN one step further by dynamically linking experimental data and theoretical calculations within the least-squares refinement cycles. The core of the molecule is determined by GED and the periphery by theory, reducing the need to have lots of parameters to describe the ligands in C1 symmetry.
Comparison of structures in gaseous and crystalline phases is of particular interest, as we discover how interactions between neighbouring molecules can influence their structures.