Electron Diffraction by Gases

This thesis contains an account of six gas phase electron diffraction molecular structure determinations carried out at the University of Glasgow for various oxygen-containing compounds of chlorine and sulphur. The apparatus used to record diffraction patterns was a modern, commercial, high accuracy, electron diffraction machine, and scattered electron intensities were measured using an automatic microdensitometer. Normal coordinate calculations, based on published infrared spectroscopic data, are also reported for most of the molecules studied. Of the first five chapters of the work, Chapter One is devoted to a review of the electron diffraction technique, Chapter Two to a description of the theory on which this technique is based, Chapter Three to a discussion of the experimental methods followed, Chapter Four to an outline of the data processing procedures adopted, and Chapter Five to a summary of the theory of molecular vibrations. Chapters Six and Seven describe normal coordinate, and root mean square amplitude of vibration, calculations for perchloryl fluoride (FCIO3) and perchloric acid (HCIO4), whilst Chapter Eight, in addition to providing an introduction to the diffraction work discussed in subsequent chapters, includes a table of vibrational amplitudes calculated for dichlorine monoxide (Cl2O), chlorine dioxide (ClO2), and sulphur dioxide (SO2), from published force constant data. In Chapters Nine to Fourteen structural results, obtained by gas phase electron diffraction experiments, are presented for the molecules of dichlorine monoxide, perchloric acid, perchloryl fluoride, chlorine dioxide, sulphur dioxide and sulphur trioxide (SO3), values being obtained, not only for the intranuclear distances present in these systems, but also for the corresponding root mean square amplitudes of vibration. In the cases of dichlorine monoxide, chlorine dioxide, sulphur dioxide and sulphur trioxide, force constant and thermal data are used to correct rg(1) bond lengths obtained by least squares refinement, to the more fundamentally significant re values. For all six molecules studied, qualitative remarks are made concerning the chemical bonding present, the dn-pn bonding theory, described by Cruickshank for tetrahedrally coordinated second row elements, being invoked to help rationalise the structural parameters obtained for perchloric acid and perchloryl fluoride. In Chapter Fifteen, sources of systematic error liable to have affected the accuracy of the electron diffraction results of Chapters Nine to Fourteen, are considered, and future improvements to the experimental technique, intended to reduce such errors, are suggested. It is concluded, however, that these uncertainties must be fairly small, since in most cases where checks are possible, the best determined molecular dimensions and root mean square amplitudes of vibration obtained, agree to within a few thousandths of an Angstrom unit with corresponding results determined by other independent physical methods. In Chapter Sixteen, certain force constant, bond length, and force constant, bond order, relationships, originally published for the Cl-O bond by Robinson, are redetermined, this revision being worthwhile, not only in the light of the present work, but also in view of recently published results for other Cl-O containing compounds. The revised relationships obtained should be of considerable value in predicting Cl-O bond lengths for molecules whose infrared and Raman spectra have been thoroughly investigated, and for which there is therefore a possibility of calculating force constants. Finally, five appendices are included which describe computer programmes written to carry out the vibrational calculations of Chapters Six, Seven and Eight