The Quantum Chemical Studies On The Structures And Spectroscopic Properties Of Carbon Chain Clusters

Carbon clusters have been the subject of extensive experimental and theoretical investigations due to their importance in astrophysics, cosmic chemistry, combustion, molecular electronics and material science. So studies on them have attracted much attention. Carbonaceous molecules are widely present in interstellar and circumstellar mediums. They play an important role in the formation of stars, protoplanetary disks, planetesimals, and in the study of the origin of life. Most of these molecules are carbon clusters, including pure carbon clusters (C₄, C₅, etc.) and substituted carbon clusters (C₈H, C₂S, HC₁₁N, etc.). Carbon clusters are also found to be present in hydrocarbon flames and other soot-forming systems. They are thought to be intermediates for many chemical reactions. Some kinds are reported to have extraordinary electronic properties.Carbon chain clusters can be divided into two types, cumulene and polyyne. However, carbon clusters have high reactivity, which are difficult to produce in laboratory. Computational chemistry doesn't need to synthesize or extract matter in laboratory, but can predict various properties of molecules. And the results usually show reliable agreement with the experiment. Quantum chemistry, as a part of computational chemistry, can do high accuracy calculations on small systems. A thorough understanding of the excited-state properties is important for material science, luminescence, and chemistry of the interstellar medium. As a result, the aim of this dissertation is to study the structures and electronic spectra of carbon chain clusters by quantum chemical calculations, and supply theoretical basis to experiments and astronomical observations. The computational methods are as follows:The geometries of the ground states are optimized with density functional theory. The vibrational frequencies and IR intensities are also calculated after the geometry optimization at the same level of theory. The geometries of the selected excited states are optimized with Complete Active Space Self Consistent Field (CASSCF) method. After the geometries are located, the vertical excitation energies are computed with Complete Active Space Second order Perturbation Theory (CASPT2), which is known as one of the most accurate methods to calculate excited states. The corresponding transition dipole moments are calculated to obtain the oscillator strengths. It reveals that theoretically predicted values show good agreement with the available observed values. The calculations not only give the assignment of the absorption peaks in previous experiments, but also predict other new peaks and the electronic spectra of the systems that haven't been observed in lab. See below in detail:In the first chapter, we introduced the research background and significance of the carbon chain clusters, and summarized the computational methods of many excited states.In the second chapter, we performed the extensive ab initio studies on a set of H₂C_2nH⁺ (n=2-6) molecules, including the density functional theory (DFT). The equilibrium geometries, vibrational frequencies, and electronic spectra of these species were illuminated. Based upon the present calculations the explicit expressions between the vertical transition energies and n were discussed.In the third chapter, we investigated the excited-state properties of the carbon chain clusters HC_2nH⁺ (n=2-7) and estimated their vertical excitation energies and corresponding oscillator strengths. The studies indicate that the energy levels of the excited states for each species have a similar pattern and the distribution of the spectral bands in the experiments is reliable. What is more, the CASPT2 method is well suited to model the system.In the fourth chapter, we optimized the geometries of the pure carbon clusters C₅ in their ground states using the DFT method with the cc-pVTZ basis set and obtained five stable isomers: l-C₅, r-C₅, t1-C₅, t2-C₅, and b-C₅. Here, t2-C₅ and b-C₅ are two new structures. In addition, also we determined their electronic spectra at the CASPT2/cc-pVTZ level and evaluated the vertical excitation energies of a series of excited states.In the fifth chapter, we explored the vertical excition energies and corresponding oscillator strengths of the NC₁₆N system at the CASPT2 and EOM-CCSD levels of theory. On the basis of the present calculations and available experimental results, we found that the CASPT2 approach is very suitable to study the largeÏ€-conjugated system like NC₁₆N, while the EOM-CCSD method leads to the larger deviation.