|Title:||A theoretical investigation of the effects of different levels of ab initio, density functional theory (DFT) and transition state theory (TST) calculations on computed reaction rate coefficients|
|Subject:||Chemical reactions -- Mathematical models.|
Hong Kong Polytechnic University -- Dissertations
|Pages:||xxxvi, 246 pages : color illustrations|
|Abstract:||Kinetic modeling of gas-phase reaction systems has improved enormously due to rapid advances in computational technology in the last decade. It becomes a powerful tool to understand complex chemical systems in a broad range of areas, including combustion, fire suppression, chemical vapor deposition (CVD) and atmospheric chemistry. Reliable thermochemical and kinetic data is required to incorporate accurate description of chemical reactions into kinetic models. However, this data may not be experimentally available due to some experimental limitations. In this connection, calculating these thermochemical constants and reaction rate coefficients in an ab initio manner has the advantage of not being restricted by any experimental conditions. However, the relationship between various levels of approximation in ab initio, density functional theory (DFT) and transition state theory (TST) is complicated and it is difficult to make the appropriate choices of levels of approximation in each of these areas in calculations. The aim of this work is to investigate the effects of different levels of ab initio, DFT and TST calculations on computed reaction rate coefficients, in order to ultimately establish a consistent, reliable and practical methodology for calculating reaction rate coefficients in an ab initio manner for the development of a reliably computed thermochemical and kinetic database, which complements available experimental databases, for kinetic modeling.|
State-of-the-art quantum chemical calculations were performed at different levels of theory to obtain reaction enthalpies, barrier heights, rate coefficients and branching ratios (if applicable) for the gas-phase reactions of importance in fire suppression, thin film preparation in the electronics industry and atmospheric chemistry. The three gas-phase reactions being presented in this work are (i) the reaction between hydrogen atom and a Halon-alternative fire suppression agent, heptafluoropropane (FM 200), (ii) the decomposition reactions of a feed gas, hexafluoropropylene oxide (HFPO), utilized in CVD for thin film preparation in the electronics industry and (iii) the Cl-initiated oxidation of formic acid (HCOOH). In particular, the effects of basis set size, the lower-level geometry and the complete basis set (CBS) extrapolation scheme on computed relative energies, the variational, tunneling and classical adiabatic ground-state (CAG) effects on computed rate coefficients, and the correlation of computed rate coefficients obtained at different TST levels with some computed critical quantities of the minimum energy paths (MEPs) were examined. Although no single level considered in this work gives good agreement with the theoretical benchmarks for all relevant stationary points of all the gas-phase reactions being studied, the BH&HLYP, MPW1PW91 and BMK functionals are recommended for reliable rate coefficients because of their good performance in reaction rate coefficient calculations. It was found that variational effect depends on both the barrier height of the classical potential energy (VMEP) curve, and the shape and position of the dip in the zero-point energy difference (ΔZPE) curve. The results also showed that multidimensional tunneling effects depend on both the barrier height and the imaginary vibrational frequency of the transition state, and CAG corrections at the TST level are similar to the variational effects at the canonical variational transition state theory (CVT) level. This work not only improves the understanding of the reaction mechanisms and the kinetics of the gas-phase reactions being studied, but also provides valuable data for building up a high-level ab initio thermodynamic and kinetic database on these reactions, and makes progress in calculating reliable reaction rate coefficients in an ab initio manner.
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