Theoretical Studies On Infrared Spectra Of Molecular Crystals And Van Der Waals Clusters And Analysis On Collective Vibrational Modes

With the rapid development of supercomputer hardware and the continuous improvement of numerical methods, the first-principle quantum chemistry simulation, especially the density functional theory (DFT), has been widely used in the research fields of materials science and nanotechnology. Theoretical simulations provide a great deal of complements, analysis and mechanism explanations for experimental research. However, the theoretical simulation of weak intermolecular interactions is still a challenging task. In recent years, a series of new methods for accurately simulating intermolecular interactions have been developed by combining theoretical deductions and experimental data to make the theoretical studies on the relevant systems feasible, Based on these new developments of DFT, we carry out the simulations on the structure and infrared spectra of molecular crystals and oligomers, and surface adsorption. Effects of intermolecular interactions on the material properties are also analyzed. The structure of the thesis is as follows.In Chapter 1, we briefly introduces DFT and intermolecular interactions. The introduction of DFT covers the origin of theory, the development of exchange-correlation functionals simulating real systems, the theoretical methods for structural optimization and the density functional perturbation theory for analyzing vibrational properties. The intermolecular forces part covers several types of intermolecular interactions with examples, and their effects in molecular crystal. At the end of this chapter, we introduce several computational softwares used in this thesis.In Chapter 2, we investigate the collective vibrational modes of aniline crystal and analyze the infrared spectra to study influences of the intermolecular interactions on the molecular crystals. We find that the intermolecular van der Waals interactions not only lead to large frequency shifts in some infrared vibrational modes and change the intensities of their absorption peaks, but also induce some long-range delocalized collective vibrational modes in the aniline crystal.In Chapter 3, we further simulate the aniline oligomers, acetanilide crystals and the isotopic substitution of aniline crystals to make comparison to the results of aniline crystals in Chapter 2. By analyzing and comparing their vibrational patterns and infrared spectra, we find that the large frequency shifts and intensity changes of absorption peaks are similar to those of aniline crystal, but there is no collective vibrational mode induced by the intermolecular forces. It is therefore revealed that in the aniline crystals, the special hydrogen atoms chain structure under the influence of intermolecular interactions is the key to the long-range delocalized vibrational modes.In Chapter 4, we study the adsorption properties of Pb ions on different facets of the Cu2O crystal. Based on the theoretical simulation of the geometrical structure and adsorption energy of several facets and the comparison with the experimental results, we find the optical material with the best adsorption.In Chapter 5, we summarize the thesis and make prospect on future research.