The Quantum Chemical Study On Excited State Electronically Of Small Molecule

The calculation of the electronically excited state is still a challenge in theoretical chemistry. In recent years, the high accuracy in calculation of quantum chemistry has been able to come true with the development of the technology about computer and the improvement on the computational strategy. The quantum chemical ab initio has been used to investigate systematically a series of meaningful small molecules.This work is incorporated into five chapters: Chapter One, we intend to introduce briefly the situations of the study upon small molecular nowadays and the prominent aspects of our concern and interest. Chapter Two, we are going to introduce the theoretical background and the computational methods of excited state.Chapter Three, the C3Cl and its ions that exist probably in interstellar space will be investigated. The vertical excitation energy, the geometry of excited state, the effect of spin-orbital coupling (SOC) electronically and the Cl-loss dissociate reactions will be taken into account.Chapter Four, the HOOOH which is very important in circumstance chemistry and biochemistry will be the research target. This chapter is devoted to the calculations of cis- and trans- forms and electronic structure, equilibrium geometry, harmonic vibrational frequency, isomerization and vertical excited state. The results suggest that the trans- form is more stable than the cis- isomer, that there are two pathways for the isomerization reaction, and that for the lowest vertical excitation energy of single and triplet state, the trans- isomer is lower than that in cis isomer.Chapter Five, we are going to concentrate on the metal-thiols MSH (M=metal) which play a very important role in the industrial catalytic hydrodesulfurization process and in the reaction mechanism on metalloenzyme of life science. The alkali and alkaline earth metal MSH were studied extensively, including equilibrium geometry, vertical excitation energy and SOC effect. The results show that, the variation of bond length of M?S in MSH follows as the metal radii; the vertical excitation energy decreases with weight increasing.