Static and dynamic effects of sterically demanding ligands

• Main Author: Hinchley, Sarah L.
• Published: University of Edinburgh 2000
• Subjects: 547.1223
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.652455

This thesis is concerned with the determination of the gas-phase structures of large, asymmetric, sterically crowded molecules, with bulky alkyl ligands. Gas-phase electron diffraction is the best fluid phase technique available for the determination of structure. However, the many assumptions needed to refine these sterically encumbered molecules made complete structural determination impossible. In these cases, other fluid phase experimental data are called upon to fill in the structural detail, the main ones being liquid crystal nuclear magnetic resonance (LCNMR) and microwave spectroscopy. However, for the kind of systems under study in this thesis, it is not possible to collect data from these types of experiment. The combination of gas-phase electron diffraction data and <i>ab initio</i> calculations, called the SARACEN method, has overcome these problems. It has opened up the possibility of studying compounds previously beyond our capabilities. Advances in computational power mean that we can now calculate the structures of larger molecules with ~ 70 atoms to reasonable accuracy, and that our computers can now refine experimental structures up to 100 atoms in size. Previously, the SARACEN method has been applied to smaller systems of ~20 atoms. In this thesis, the method has been applied to much larger systems and many interesting features of the ligands, the effects they have on each other and the overall structures of molecules have been revealed. A series of disilanes with increasing steric bulk have been studied: 1,2-di-<i>tert</i>-butyltetrachlorodisilane, 1,1,2-tri-<i>tert</i>-butyldisilane, and 1,1,2,2-tetra-<i>tert-</i>butyldisilane. The structural results are surprising. The three-coordinate systems of tri(<i>tert</i>-butyl)sulfurtriimide and bis(trichlorosilyl)<i>tert-</i>butylphosphine have also been studied. Finally, the very unusual bis[bis(trimethylsily)methyl]phosphine and arsine radicals and the related dimers in the crystalline phase have been studied. Detailed analysis of the steric crowding of the solid structures has revealed the interesting new concept of ligands as energy reservoirs, which facilitate bond dissociation in the dimer.