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Title: Some spectroscopic studies of derivatives of the Group III elements
Author: Johnson, Eric Martin
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 1971
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In this thesis a systematic study of the structures adopted by the trimethyl compounds of the Group III elements under a variety of conditions is described. Oligomerization is a feature of common and apparently irrational occurrence amongst these compounds and the investigation was undertaken with the aim not only of determining the structures themselves but also of elucidating the factors influencing structure within the series. The investigation is presented in four main sections: Introduction (Chapters 1 and 2), Experimental (Chapters 3, 4 and 5), Results (Chapters 6, 7 and 8) and Discussion (Chapter 9). The background to the problem is discussed in Chapter 1. Previous investigations of the Group III trimethyl compounds have been haphazard and have concentrated, in general, on the elucidation of the structure of one compound in one phase. The fluid phases have received most attention and whilst trimethylboron, trimethylgallium and trimethylindium are all monomeric, the aluminium compound adopts a symmetrical dimeric structure with bridging methyl groups. In the former three compounds the MC3 skeleton is planar and, since the methyl groups are able to rotate freely, the overall point group of the molecules is D3h; the trimethylaluminium dimer belongs to the D2h point group. The structure of trimethylthallium in the fluid phases has not been reported. In the solid state trimethylaluminium has been shown by X-ray crystallography to retain the bridged, dimeric structure, whereas trimethylindium adopts an unusual structure with a polymeric network of tetramer units in which each indium atom is penta-coordinated by methyl groups. The tetrameric unit is held together by unsymmetrical methyl bridges and the tetramers also interact, but wore weakly. Within each tetramer nearly trigonal Me3In 'monomer' units can still be distinguished and the binding within these units is much stronger than the binding between them. Thus the strength of the methyl group to indium bonds in the trimethylindium crystal falls in the order: Intra-'monomer', intra-tetramer > inter-tetramer. Recently trimethylthallium has been reported to adopt a similar solid state structure, but with more nearly equivalent inter and intra tetrameric bonds. For both trimethylindium and trimethylthallium the point group of the tetrameric unit is S4 and the overall symmetry of the crystal is C44h. The structures of trimethylboron and trimethylgallium in the solid state have not been investigated. The concept of electron deficiency in the context of the present investigation is briefly discussed and the factors conceivably influencing the equilibrium: n (monomer) ? oligomer are considered in some detail. For the specific case of dimer formation five factors arc isolated and, as far as possible, their relative importance assessed for each of the Group III elements:

  1. the M-M distance in the dimer;
  2. the M-C bond enthalpy;
  3. the s→p promotion energy;
  4. the van der Waals repulsion between non-bonded atoms in the dimer;
  5. the stability of the monomer.

In the light of these factors it is concluded that the relative tendency amongst the trimethyl compounds of the Group III elements towards dimer formation in particular, and oligomer formation in general, is given: Al > In > Ga ? Tl ? U The requirements of the technique for structural elucidation are considered:

  1. applicability to all three phases;
  2. applicability over a very wide temperature range;
  3. feasibility of examining highly reactive compounds;
  4. adaptability to special circumstances (e.g. matrix isolation);

and it is concluded that only vibrational spectroscopy is able to meet all these stringent requirements. In consequence the techniques of Raman and infrared spectroscopy were used almost exclusively in the structural investigations described in the thesis. The interpretation of the spectroscopic results is briefly discussed and the value of vibrational spectroscopy as a structural tool critically reviewed in Chapter 2. The solid state is given particular attention and it is concluded that the level of sophistication appropriate to the interpretation of the spectra of solid compounds is dependent upon both the nature and degree of intermolecular interaction involved. The structure of solid trimethylindium is used to exemplify three distinct approaches to the interpretation of solid-phase spectra:

  1. the site symmetry approach;
  2. the aggregate unit approach;
  3. the factor group approach.

By way of introduction to the experimental section of the thesis, the general problems involved in the examination of compounds which react avidly with oxygen and moisture are discussed. The salient features of the instruments used in the investigation are then described. A Cary 81 spectrophotometer equipped with either a helium-neon or an argon-ion laser was employed to obtain the bulk of the Raman data. A Perkin-Elmer 225 spectrophotometer and a Beckman I.R.11 spectrophotometer were used to obtain the infrared spectra in the 4000-200 cm-1 and 600-50 cm-1 regions, respectively. The chapter is concluded by a description of the experimental assemblies developed during the course of the investigation for the examination of the Raman spectra of liquid samples over the temperature range -150 to +150°C and for the examination of the infrared and Raman spectra of solid samples at -196°C. The preparation and manipulation of the five compounds are described in Chapter 4, together with details of the simpler sampling devices developed during the course of the investigation. In Chapter 5 the nature, scope and validity of the matrix-isolation method in the context of the present investigation are discussed in SORO detail. It is concluded not only that the method provides a powerful and valid tool for the investigation of the structures of both stable and unstable species but also that, by allowing the matrix to become mobile, it is possible to use the method to follow oligomerization processes in a novel and potentially highly-informative way. The experimental assembly and technique for the examination of the infrared spectra of species isolated in noble gas or nitrogen matrices at 20°K are described. The assembly comprises in essence a Perkin-Elmer 225 spectrometer coupled to a miniature Joule-Thomson liquid hydrogen cryostat bearing a caesium iodide window and encased within a suitable vacuum system. The potential advantages of obtaining the Raman spectra of matrix-isolated species are discussed and the development of an assembly for use in conjunction with the Cary 81 instrument is described. The results of the fluid-phase investigations are presented in Chapter 6. The infrared and Raman spectra of pure trimethylboron wore fully examined at room temperature and, in the case of the Raman spectrum, at temperatures down to -150°C, no change in the spectrum being detected. Trimethylaluminium was examined both pure and in solution at room temperature and, in the case of the Raman investigation, at elevated temperatures both in solution and in the vapour phase. In particular, it proved possible, by examining the vapour at 170°C, to obtain the major features of the Raman spectrum of monomeric trimethylaluminium for the first time.

Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available