This thesis presents the results of a numerical calculation of the light hadron spectrum in the lattice formulation of Quantum Chromodynamics. Results were obtained in both the quenched approximation, where the effects of quark loops in the QCD vacuum were neglected, and in "full" QCD, where two degenerate flavours of dynamical fermions were included in the simulation. All numerical simulations employed the standard Wilson gauge action with an <i>O</i>(a) improved Wilson fermion action. This study confirms that the quenched light hadron mass spectrum agrees with experiment at the 10% level. Finite size effects at one value of the coupling were investigated and an improved scaling behaviour arising from the implementation of the <i>O</i>(a) improvement programme was observed for the quenched simulations. With the aim of observing effects in the spectrum due to the inclusion of fermion loops in the QCD vacuum, simulations in "full" QCD forming a matched ensemble were compared with a quenched simulation at the same lattice spacing. Each simulation in the method ensemble was selected to have approximately the same lattice spacing as defined with respect to a physical observable in order to investigate chiral extrapolations independently from continuum extrapolations. A further simulation with a lighter sea quark mass at a smaller lattice spacing was included in the analysis for comparison. Evidence for small yet significant dynamical effects arising from the comparison with the quenched data were observed in the hyperfine splitting and partially quenched chiral extrapolations. Results obtained from the matched ensemble displayed a reduced residual dependence upon lattice artifacts.

The laser-desorption jet-cooling (LD-SJ) method has been applied successfully to the acquisition of LIF excitation spectra for a number of conformationally flexible molecules. The molecules 4-cyanobiphenyl, 4-hydroxybipenyl, tyramine and 4-t-butyl phenol were studied and well-resolved vibronic spectra obtained for each molecule. Modification to the experimental set-up have allowed the collection of dispersed fluorescence spectra for the first time. The dispersed emission spectra from both conformer bands of 4-t-butyl phenol, and several of the tyramine conformer bands were recorded. In the dispersed fluorescence spectrum of 4-hydoxylbiphenyl, a high enough resolution to observe individual torsional levels in the ground state could not be obtained. <i>Ab initio</i> methods have been used in tandem with the LD-SJ work to study the structural and electronic behaviour of each molecule studied experimentally. Several similarities were observed in the electronic properties of each molecule. Interactions in the HOMO were influential in determining the structure of each compound. In addition, substituent effects on the electron density in the HOMO were shown to influence the energetic stability of the various different molecular conformers. Calculations show that conjugative interactions in the HOMO also play an important role in the stability of the twisted molecular structure of biphenyl and derivatives. Substituent induced perturbations to the electron density in the lowest unoccupied molecular orbital (LUMO) can be directly linked to the curvature of the S₁ torsional potential. Substituents which are either strongly electron donating or withdrawing introduce a large asymmetry in the electron density across the inter-ring bond resulting in a reduced S₁ torsional frequency.

The gas-phase and solid-phase structures of <I>trans-</I>3,4-dibromotetrahydrothiophene-1,1-dioxide were determined and are compared with the <I>ab initio </I>study in chapter 3, as part of a study into the effect of remote SO<SUB>2</SUB> groups on the bromination of alkenes. The ring puckering parameters show that the in solid phase it adopts an envelope conformation, whilst in the gas phase it adopts a twist conformation. In the solid phase the bromine atoms occupy positions <I>equatorial </I>to the ring, whereas in the gas phase the optimum fit is for a mixture of <I>axial </I>and <I>equatorial </I>conformers. The tetranitriles are poly(ether imide) precursors, and poly(ether imides) are important as new gas separation membranes. Chapter 4 is an introduction to the work carried out on the tetranitriles with a review of the gas permeability data on poly(ether imides) in the literature. The structures of the tetranitriles were studied in an attempt to find a structure-property relationship with the gas-separation properties of the poly(ether imides). Chapter 5 describes the crystal structure determinations of the tetranitriles, whilst chapters 6 and 7 describe the molecular geometries and crystal packing of the tetranitriles. The analysis of the crystal structures of the tetranitriles reveals that the intermolecular interactions between aromatic rings is important in the packing schemes adopted by the tetranitriles. The results show how the packing is affected by the presence and position of various structural features. Chapter 8 is an analysis of the holes found in the crystal structures of the tetranitriles and whether these are related to pores in the poly(ether imides). The results give an insight into the causes of pores within the poly(ether imides). The crystal structures of a series of oxazolidin-2-one chiral auxiliaries based on derivatives of D-fructose are analyzed in chapter 9 to investigate if there is molecular self-recognition by the parent group. Chiral auxiliaries are used in asymmetric synthesis. They impact asymmetry to the diastereomers formed, to aid in their separation by crystallization. The results show that the parent molecule does not recognize itself during crystallization, thus they are likely to be useful chiral auxiliaries.

No description available.

The <I>gerade</I> Rydberg states of jet-cooled iodine have been thoroughly investigated in the region between 53000 and 70000 cm<SUP>-1</SUP> using mass resolved (2+1) resonantly enhanced multiphoton ionisation (REMPI) spectroscopy. All the <I>n</I>d Rydberg states were found to be heavily perturbed through vibronic coupling with ion-pair states in contrast to most of the <I>n</I>s Rydberg states which were found to be unperturbed. The characterisation of these Rydberg states provided the basis for a comprehensive study of the two spin-orbit components of the ground state of the cation using two colour (2+1') ZEKE-PFI photoelectron spectroscopy. The ZEKE-PFI spectrum recorded for the lower spin-orbit state exhibits non Franck-Condon behaviour in contrast to that recorded for the upper spin-orbit state. This was interpreted in terms of an autoionisation mechanism involving Rydberg states converging on the upper spin-orbit component. Several new Rydberg states of the iodine-argon van der Waals complex have been investigated using mass resolved (2+1) REMPI spectroscopy in the region between 53000 and 69000 cm<SUP>-1</SUP>. The complex spectra exhibit anharmonic vibrational progressions in the I<SUB>2</SUB> Ar van der Waals stretching mode in combination with the I<SUB>2</SUB> stretching progression. The I<SUB>2</SUB> Ar stretching frequencies and binding energies were found to increase with the principal quantum number of the Rydberg state which is consistent with a progressive strengthening of the van der Waals bond. This agrees with the expected reduction in effective shielding of the ion core by the Rydberg electron as it is excited into higher principal quantum number orbits. A (2+1') ZEKE-PFI investigation of the van der Waals complex cation revealed a very small binding energy increase in the ion compared to that determined for the <I>n</I>=8 Rydberg state. This small increase indicates that for Rydberg orbitals with n≥8 the effective shielding of the ionic core by the Rydberg electron is minimal.

No description available.

No description available.

No description available.

No description available.

No description available.