This work presents first-principles calculations of long-range intermolecular dispersion energies between two atoms or molecules and of electronic circular dichroism spectra of chiral molecules. The former is expressed in terms of the C6 dipole-dipole dispersion coefficients Δε, and the latter is given in terms of the extinction coefficient. In a series of publications, the complex linear polarization propagator method has been shown to be a powerful tool to provide accurate ab initio and first-principles density functional theory results. This was the case not only for the C6 dispersion coefficients but also for the electronic circular dichroism at an arbitrary wavelength ranging from the optical to the X-ray regions of the spectrum. The selected samples for the investigation of dispersion interactions in the electronic ground state are the noble gases, n-alkanes, polyacenes, azabenzenes, alkali-metal clusters, and C60. It is found that the values of C6 for the sodium-cluster-to-fullerene interactions are well within the error bars of the experiment. The proposed method can also be used to determine dispersion energies for species in their respective excited electronic states. The C6 dispersion coefficients for the first π → π* excited state of the azabenzene molecules have been obtained with the adopted method in the multiconfiguration self-consistent field approximation. The dispersion energy of the π → π* excited state is smaller than that of the ground state. It is found that the characteristic frequencies ω1 defined in the London approximation of n-alkanes vary in a narrow range which makes it possible to construct a simple structure-to-property relationship based on the number of π-bonds for the dispersion interaction in these saturated compounds. However, this simple approach is not applicable to the interactions of the π-conjugated systems since, depending on the systems, their characteristic frequencies ω1 can vary greatly. In addition, an accomplishment of calculations of the electronic circular dichroism spectra in the near-edge X-ray absorption has been demonstrated.

The studies of the dynamics, phase behavior, interparticle interactions, and hydrodynamics of stimuli-responsive pNIPAm-co-AAc microgels were described in this thesis. Due to their responsiveness to external stimuli, these colloidal particles serve as excellent model systems to probe the relationship between colloidal interactions and phase behavior. As a first step, we established our core experimental methodology, by demonstrating that particle tracking video microscopy is an effective technique to quantify various parameters in colloidal systems. Then we used the technique in combination with a microfluidic device that provides in situ control over sample pH to probe the phase behavior of pNIPAm-co-AAc microgel suspensions. In essence, the experimental set-up enables changes in effective particle volume fractions by changing pH, which can be used to construct the phase diagram. In order to explain the unique features of the microgel phase diagram, we measured the underlying pairwise interparticle potential of pNIPAm-co-AAc microgels directly in quasi-2D suspension and proved that the interactions are pH dependent and can range from weakly attractive to soft repulsive. Finally, the hindered Brownian diffusion due of colloidal particles confined by hard walls was investigated systematically and striking differences between hard sphere and soft sphere were found, with soft pNIPAm-co-AAc microgels showing surprising mobility even under strong confinement.

Thèse doctorat : Chimie Informatique et Théorique : Nancy 1 : 2005. / Titre provenant de l'écran-titre.

This thesis details research undertaken in the investigation of the open-shell open-shell Van der Waals complexes between nitric oxide (NO) and oxygen (O₂) and between nitrogen dioxide (NO₂ ) and oxygen (O</sub>2</sub> ). The Fourier transform microwave spectroscopy technique was used during the experimental work. Spectra were recorded for a mixture of NO and O₂ over the frequency range 7.0 to 18.2 GHz. Detailed ab initio calculations have been performed on the potential energy surface of both NO-O₂ and NO₂-O₂ using Multi-reference Rayleigh Schrodinger Second Order Perturbation Theory. These calculations were performed for the four distinct states of NO-O₂, ²A", ⁴A", ²A' and ⁴A', and for the two distinct states of NO,sub>2</sub>-O₂, ²A and ⁴A. Predicted rotational spectra have been generated by modelling the systems with a rigid-rotor Hamiltonians. The effective Hamiltonians included the rotational Hamiltonian, ℋ_rot, fine structure terms and hyperfine structure terms. It was not possible, for rationalizable reasons, to fit the predicted spectra for NO-O₂ to the experimental observations. Experimental details, calculation methods and rotational theory are discussed in the main body of the thesis. Predicted spectra, relevant calculations and other data is presented in the appendices.

The potential of filling carbon nanotubes in polymers has been widely acknowledged for composites with exceptional new properties owing to the high strength of carbon nanotubes. In the thesis, by employing Materials Studio 4.0 software, the buckling behaviour and vibration of polyethylene and carbon nanotube matrix composites are first discussed using molecular mechanics simulations. The research is aimed to acquire a high strength design of PE-CNT matrix with proper PE/CNT ratio as well as discovering the dynamic characteristics of the PE-CNT composites. Investigation results show that as the number of PE chains increases, the buckling strain and the resonance frequency will decrease. Van der Waals forces are collected to explain the relation of the PE chains to the buckling strain and the resonance frequency of the composites.

The potential of filling carbon nanotubes in polymers has been widely acknowledged for composites with exceptional new properties owing to the high strength of carbon nanotubes. In the thesis, by employing Materials Studio 4.0 software, the buckling behaviour and vibration of polyethylene and carbon nanotube matrix composites are first discussed using molecular mechanics simulations. The research is aimed to acquire a high strength design of PE-CNT matrix with proper PE/CNT ratio as well as discovering the dynamic characteristics of the PE-CNT composites. Investigation results show that as the number of PE chains increases, the buckling strain and the resonance frequency will decrease. Van der Waals forces are collected to explain the relation of the PE chains to the buckling strain and the resonance frequency of the composites.

The effect of large size ratios of solute to solvent on the critical properties and phase behaviour of binary mixtures of spherical particles is investigated using an "augmented van der Waals" equation of state. The equation used is essentially a van der Waals equation with an improved hard sphere repulsive term. Molecular dynamics and constant-pressure Monte Carlo simulations of binary mixtures of hard spheres with different diameter ratios and mole fractions are undertaken to check the adequacy of the hard sphere equation. Good agreement is found, even for systems with large differences in size. Furthermore, many of the hard sphere mixtures exhibited a transition from a fluid to a solid phase at high densities. Phase boundaries are calculated for model mixtures comprising spheres of different sizes between which there are long-ranged attractive forces. Particular attention is paid to the case in which the ratio of sizes is infinite. The systems show a wide variety of behaviour that includes liquid-liquid and gas-gas immiscibility, and the formation of negative azeotropes. Calculations investigating the effect of different attractive interactions between the small and large spheres show that as the magnitude of this interaction is increased, liquid-liquid immiscibility becomes the dominant feature of the phase diagram at moderate temperatures. The extent of liquid-liquid coexistence is greatest at large size differences. These model systems are shown to reproduce some of the behaviour of aqueous solutions of surfactants if it is assumed that the large spheres are models of the micelles and the small spheres models of the solvent molecules. The properties of binary lattice mixtures of bifunctional molecules whose ends are chosen to mimic surfactant and solvent molecules are also briefly investigated, to determine the effect of the asymmetric surfactant molecule on the phase separation. Closed-loops emerge in the phase diagrams as the surfactant character of one of the species is increased.

This thesis describes the commissioning and development of a pulsed supersonic nozzle, Fourier-transform microwave spectrometer and its application to the study of several weakly bound van der Waals complexes. A pulsed supersonic expansion, Fourier-transform microwave spectrometer based on the Flygare design with a number of modifications has been constructed with an operating range of 6-18 GHz. A homodyne detection circuit mixing signals to modulus values between dc and 1 MHz is used, requiring two measurements to determine absolute transition frequencies. Transition frequencies are measured from the power spectrum by determining the first derivative zero crossing point in a least squares fitting procedure. Semiautomation of many of the spectrometer operations has been achieved allowing unattended data collection over scans of up to 300 MHz. The microwave spectrum of Ar₂-OCS and Ar₂-OC³⁴S has been observed and analysed using conventional Watson S reduction hamiltonian parameters. Effective structural parameters are derived and used in a harmonic force field analysis, based on the centrifugal distortion constants, to compare the trimer interations with a model based on the sum of dimer interactions. A series of complexes containing the nitrogen molecule undergoing tunnelling motions have been studied. Hyperfine matrix elements for the first order nuclear quadrupole interaction are derived for the coupled identical nuclei case appropriate to the rapid tunnelling motions observed. The microwave spectrum of N₂-OCS is described. Tunnelling and nuclear spin statistical effects for two symmetry states are observed arising from the interchange of nitrogen nuclei. Rotational and quadrupole constants are derived; an accidental near degeneracy of two rotational levels allows the off-diagonal quadrupole coupling constant to be determined from second order effects. A tunnelling hamiltonian fitting the quadrupole coupling constants to an angular potential has been used to calculate the tunnelling frequency and barrier to N₂ rotation. The microwave spectrum of N₂-O₃ and a preliminary spectrum of N₂-SO₃ have been observed. Rotation-inversion motions of the O₃ and SO₂ moieties must be considered in addition to the N₂ tunnelling to fit the spectrum. Tunnelling frequencies for the O₃/SO₂ and geared motions with the N₂ are derived as well as structural parameters. Modifications for production of refractory molecules and complexes by laser ablation have been made. A modified nozzle employing rods of material is used with the ablation process taking place in the nozzle throat. Modifications to obtain an expansion along the axis of the microwave cavity employ a hemispherical Fabry-Perot cavity configuration. The system has been tested on a number of diatomic molecules including PbS and CuCl.

The P22 c2 repressor protein (P22R) binds to DNA sequence-specifically and helps direct the temperate lambdoid bacteriophage P22 to the lysogenic developmental pathway. To gain insight into its DNA binding mechanism, we solved the 1.6 Å x-ray structure of the N-terminal domain (NTD) of P22R in a complex with a DNA fragment containing the synthetic operator sequence [d(ATTTAAGATATCTTAAAT)]2 This operator has an A-T at position 9L and T-A at position 9R and is termed DNA9T. Van der Waals interactions between protein and DNA appear to confer sequence-specificity. The structure of the P22R NTD – NA9T complex suggests that sequence-specificity arises substantially from interaction of a valine with a complementary binding cleft on the major groove surface of DNA9T. The cleft is formed by four methyl groups on sequential base pairs of 5' TTAA 3'. The valine cleft is intrinsic to the DNA sequence and does not arise from protein-induced DNA conformational change. Protein-DNA hydrogen bonding plays a secondary role in specificity.

Dissertation (Ph.D.)--Auburn University, / Abstract. Vita. Includes bibliographic references.