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In this work three different approaches to the study of intermolecular interactions are shown. The aim is to further the understanding of specific intermolecular interactions with a view to eventually allowing the prediction and design of crystal structures from the initial molecular building blocks: crystal engineering. All three approaches make use of crystal structural information derived from X-ray and/or neutron diffraction studies. The three approaches are: • Data base approach. Specifically, the study of occurrence of hi- and tri-furcated hydrogen bonds in the Crystallographic Structural Database, and the analysis of the frequency with which they occur and the geometric restrictions of such interactions.• Analysis of a series of compounds, where there are small changes in the molecular structure as the series progresses. The influence of these changes in the molecular structure on the crystal structure is considered. The series studied was the 4-amino- 4'-hydroxydiphenylalkanes as well as some of the corresponding 4-amino-4'- hydroxydiphenylsulphides and -alkylsulphides.• Detailed analysis of individual structures to identify the intermolecular interactions that are influencing the structure. The compounds analysed in this part were 2,4,6- m.s-(4-chlorophenoxy)-l,3,5-triazene co-crystallised with tribromobenzene, triphenylisocyanurate co-crystallised with trinitrobenzene, 4,4'-dinitrotetrapheny 1 methane, 2,3-dichloro-l,4-diethynyl-l,4-dihydroxy-napthalene and, 4,4-diphenyl- 2,5-cyclohexadienone.

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The effect of anchimeric assistance by neighbouring groups on the rates and products for the solvolysis of organic halides and sulphonates has been widely studied,(^1) but the activation parameters of such internal nucleophilic substitutions (S(_N)i) have received relatively little attention. This thesis describes experiments aimed at obtaining this information for the effect of neighbouring hydroxyl, methoxyl and phenyl groups on reactions with aqueous acetone. Compounds containing these substituents but reacting by the normal bimolecular mechanism (s(_N)2) were also examined, and data(^2) for the unimolecular reactions (S(_N)l) of substrates with structurally similar reaction centres allowed a comparison to be made of the entropies and heat capacities of activation associated with these three mechanisms. The chlorohydrins, HO(CH(_2)(_n)CH(_2)Cl, (I; n = 1-5) showed the enhanced rates and the abnormal reaction products expected for the S(_N)i mechanism when n = 3,4, as previously reported for reaction in water(^3). Entropies and heat capacities of activation for these compounds were intermediate between the values of the parameters for the solvolysis of structurally similar substrates by mechanisms S(_N)l and S(_N). The corresponding methoxy-bromides, MeO(CH(_2))(_n)CH(_2)Br, (II; n = 1-5) showed very similar kinetic features when initial rates were measured, but subsequent reactions of the intermediate produced in the first step of the S(_N)i process (n = 3,4) made it difficult to obtain rate coefficients with sufficient accuracy for the determination of accurate activation parameters. Similar results had already been reported for the reactions of MeO(CH(_2))(_2)CH(_2)0Bs, (III; n = 1-5) but approximate rate coefficients could be assigned to the individual reaction steps in the present work. Anchimeric assistance by a phenyl group in the reactions of C(_6)H(_5))(_3)CCH(_2)Cl and (C(_6)H(_5))(_3)CCH(_2)OTs was accompanied by more positive activation parameters than in similar S(_N)l and 5(_N)2 reactions. The results allowed some estimate to be made of the effect of individual leaving groups on the values of the entropies and heat capacities of activation for reaction by mechanism S(_N)i.

This thesis presents work on the determination of both linear and non-linear optical properties of organic molecular crystals from high-resolution X-ray diffraction data. The eventual goal of this work is to obtain accurate and reliable estimates of the non-linear optical properties for these materials of proven technological importance and to further our understanding of the factors affecting the relationship between molecular structure and macroscopic properties in order to aide our quest in designing new and better non-linear optical materials. The basic theory of crystallography is discussed in Chapter 1, with a particular emphasis on obtaining accurate charge densities from high-resolution X-ray diffraction data. Chapter 2 summarises the theory behind modern quantum chemistry calculations for single molecules and periodic materials. Also introduced is the relatively new method for obtaining ‘experimental' wave functions by constraining the wavefunction with structure factors obtained from X-ray diffraction experiments. This technique, devised by D. Jayatilaka, is the basis for much of the work carried out in this thesis. In Chapter 3, the definitions of the dipole (hyper) polarisabilities and related bulk susceptibilities are given along with a scheme for the calculation of approximate dipole polarisabilities attributed to Sylvain and Csizmadia. Chapter 4 discusses the equations required for the calculation of dipole рolararisabilities and hyperpolarisabilities derived from coupled-perturbed Hatree-Fock theory (СРHF). In addition, a scheme was presented for the calculation of refractive indices proposed by Rohleder and Munn. Routines for the calculation of these quantitles were implemented in the Tonto quantum chemistry package. This has allowed us for the first time to determine CPHF polarisabilities and hyperpolarisabilities from constrained wavefunction calculations. Constrained wavefunction calculations were performed on three compounds, benzene, urea and 2-methyl-4-nitroaniline. CPHF polarisabilities and related refractive indices were calculated and compared with the Sylvain-Csizmadia values from the previous study by A.E. Whitten at the University of New England. The CPHF polarisabilities and refractive indices were comparable to experimental values and those obtained from the Sylvain-Csizmadia approach, but unfortunately no significant improvement was observed using the more rigorous CPHF approach. Similar constrained wavefunction calculations were performed on three well known organic non-linear optical materials, 4-(N, N -dimethylamino)-3-acetamidomtrobenzene (DAN), 2-(N-L-prolinol)-5-nitropyridine (PNP) and (S)-2-(α-methylbenzylamino)- 5-nitropyridine (MBANP), selected from the literature due to their importance in the field of non-linear optics. Enhancements in the calculated dipole polarisability, hyperpolarisability and refractive indices were observed after wavefunction fitting, which is attributed to the effects of the crystal field and intermolecular interactions included by way of the X-ray diffraction data. A comparison between the CPHF(Electric Field Induced Second Harmonic generation) experiments, showed that the former were underestimated by on average 16.7 X 10(^-51)Cm(^3)V(^-2). A comprehensive comparison of various properties determined from wavefunction fitting and multipole refinements of the same X-ray diffraction data, was reported in Chapter 6, in order to further our understanding of the effect of wavefunction fitting and the nature of the 'experimental' wavefunctions. Notable differences were observed between properties obtained from the multipole model and experimental wavefunction, with large differences observed for the electron densities of the atomic core regions. Finally, Chapter 7 presents a charge-density study on the non-linear optical prototype material N,N-dimethyl-4-nitroaniline (NNDPNA). The multipole model obtained suggests a dipole moment enhancement of some 24 Debyes over that of the isolated molecule. Unreasonable estimates for the electrostatic properties such as this, are thought to be the result of the limitations of using X-ray diffraction data alone to obtain accurate charge densities.