Development and Application of Dual-Level Dynamics Methods

• Main Authors: Yung-Lung Chen, 陳永隆
• Other Authors: Wei-Ping Hu
• Format: Others
• Language: zh-TW
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/25157284212356505013

博士 === 國立中正大學 === 化學研究所 === 92 === This thesis consists of five chapters and focuses on two subjects. The first is the theoretical study on the small clusters of metal hydride in chapter 1. The second is the development and application of dual-level dynamics methods in chapter 2 - 5. In chapter 1, high-level ab initio molecular orbital theory is used to calculate the geometries, vibrational frequencies, atomic charges, and binding energies of the small clusters (LiH)n, (NaH)n, (BeH2)n, and (MgH2)n (n = 1 - 4). For (LiH)n and (NaH)n, planar cyclic structures were found for n = 2 - 4. We have also found the cubic structure (Td symmetry) in addition to the planar cyclic D4h for n = 4. For (BeH2)n and (MgH2)n, there are three kinds of structures, chain C2v, planar cyclic D3h, and hat-like C2v, for n = 3. There are four kinds of structures, chain D2h, cubic Td, string-like C2, and cubic transformation C1, for n = 4. In chapter 2, we use the dual-level variational transition state theory with multidimensional tunneling to calculate the rate constants of E2 reactions for C2H5Cl + OF(H2O) and C2H5Br + CN. The calculated temperature range is from 200 to 800 K. It is the first time for using more reliable methods to calculate full reaction path. We also investigate various dynamics properties of chemical reactions, such as tunneling effects, variational effects, kinetic isotope effects, and solvent kinetic isotope effects. The calculated results were compared to the earlier work of our laboratory using semi-empirical methods to calculate the low-level PES. In chapter 3, a new dual-level correction scheme for variational transition state theory calculation has been developed to better estimate the tunneling effects on the reaction rate constants. The correction points were chosen in a self-consistent fashion based on the representative tunneling energy at a particular temperature. The new scheme was tested for two unimolecular and two bimolecular reactions against the schemes developed previously. The test results showed that the new scheme performed at least as good as the SIL-2 scheme for bimolecular reactions, and it provides significant improvement in accuracy and in computational cost for unimolecular reactions. In chapter 4, the rate constants for the gas-phase dissociation of HArF and HKrF through the bending coordinates have been calculated using the dual-level variational transition state theory with quantized reactant state tunneling(QRST) from 20 to 600 K. Tunneling was found to dominate the reaction below 250 K. The calculated KIEs showed dramatic increases below 250 K. Compared to the conventional tunneling method, the QRST predicted appreciably higher rate constants below 50 K and makes the transition from the temperature dependent domain to the temperature independent domain more sharply. In chapter 5, the rate constants for the gas-phase dissociation of HArF to constituent atoms have been calculated using the dual-level variational transition state theory with quantized reactant state tunneling from 20 to 600 K. We make detailed comparisons of the tunneling effects and kinetic isotope effects of reactions using QRST by using harmonic approximation and WKB approximation methods to obtain the quantized reactant energy levels. These results were compared to the calculations using conventional tunneling method. Tunneling was found to dominate the reaction below 300 K. The calculated KIEs showed dramatic increases below 400 K. Compared to the conventional tunneling method, the QRST by WKB approximation predicted appreciably higher rate constants below 70 K and higher KIEs below 40 K. The rate constants became higher than the “bending” pathway except at low temperature. Thus, except at low temperature in the gas phase, the reaction to constituent atoms may become the important dissociation pathway.