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Work presented in this thesis details the development of new applications for molecular crystalline systems using first-principles simulation. In particular work has focused on the most important type of intermediate interactions-the hydrogen bond. A new computational procedure to more accurately mimic the crystalline environment has been developed and applied to two systems: the test system ammonia and the more unusual dihydrogen bonded system BH₃NH₃. Both generated surprising results, which challenged the conventional view of bonding in the solid state. Work has also focused on the dynamics of the hydrogen bond, resulting in the implementation of a constraint molecular dynamics (MD) algorithm for the popular simulation package, CASTEP. This code development allows molecular systems to be treated as rigid or semi-rigid bodies, thus allowing appreciable increase in the first-principles MD time step. It also allows interesting chemistry to be explored at the <i>ab-initio</i> level, which would be inaccessible by any other route. The method has been applied to the phase I structure of ammonia and a full vibrational analysis is reported.

This thesis is concerned with the application of the SARACEN method to a range of molecules with interesting structural properties and applications in industry. The gas-phase structures of a series of compounds have been obtained, combining gas-phase electron diffraction and <i>ab initio </i>molecular orbital theory. For each compound a series of the potential energy surface was carried out to investigate the number of conformational minima and to locate the global energy minimum. The gas-phase structures of <i>trans-</i>1,2-dichloro-1,2-disilylethene (1), 1,1-bromosilylethene (2) and 1,1,1,4,4,4-hexachloro-1,4-disilabutane (3) were studied and <i>ab initio </i>calculations on analogous compounds were carried out. This study revealed extreme asymmetry in coordination at carbon, and trends in bond lengths and angles were interpreted in terms of cumulative electronic and steric contributions. The structure of bistrichlorosilyldimethylgermane [(Me₂Ge(SiCl₃)₂] (4) was determined <i>ab initio </i>and by gas-phase electron diffraction and compared to that of a previously determined trimethyl analogue. An important difference between experimental and theoretical results was revealed. Three silylhydrazines, F₃SiN(Me)NMe₂ (5) and F₃SiN(SiMe₃)NMe₂ (6) ClH₂SiN(Me)NMe₂ (7), have been studied <i>ab initio </i>and by gas-phase electron diffraction. Previous solid- and gas-phase investigations of other substituted silylhydrazines revealed unusually narrow Si-N-N bond angles. Structural investigations were initiated for the halogen and bulky alkyl-substituted compounds to gauge the effects of these different substitutions on the Si-N-N bonds angles. Further calculations were carried out on similar compounds upon removing the “donor” and “acceptor” atoms, to verify the presence of suspected Si β-N interactions.

The room-temperature Raman spectra of bulk As-S and As-Se glasses have been recorded. The compositior studied range throughout the glass-forming regions but are mainly close to the stoichiometric composition As40X60 (X = S or S a): eleven compositions in the range As35X65 - As45S55 have been studied in each system. Polarisation measurements have been made on all the glasses and from them have been derived the depolarisation spectra. The frequencies and polarisation states of the bands in the a-As 40S60 and a-As 40Sefi0 spectra can be accounted for using the Lucovsky-Martin molecular model. It is found that the spectra of these two glasses scale by the same two factors that relate the spectra of the corresponding crystals. The applicability of the interlayer scale factor to the glass spectra supports the presence of layer regions in the glasses. The spectra on the chalcogen-rich side of the stoichiometric composition As40X60 have also been analysed in terms of the Lucovsky-Martin molecular model and indicate the replacement of the ASX-2.s links between the AsX3 pyramid units by AsX X-As links as the chalcogen content is increased. The presence of S8 rings is increasingly apparent in the sulphide compositions more S-rich than As 37S63. In the case of the As-rich sulphides sharp spectral features characteristic of the crystal p-As4S4 appear near stoichiometry and increase smoothly as the As content is increased through the limit of the glees-fo=ing region, -As 43S57, and on into the phase-separated region. The results also indicate that As4S3 or As4S5 molecules are present and that As-As bonds are formed in the glass network. In the case of the As-rich selenides, for the compositions between As40Se60 and As45Se55 the additional As atoms go mainly into the formation of As-As bonds in the network. For compositions containing more than 50 % As the production of As4Se4 molecules predominates over the formation of As-As bonds in the network, although these are still present.

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