Theoretical Studies On The Degradation Mechanisms For Several Fuoirnated Organic Compounds In The Atmosphere

In1985, the discovery of the annual depletion of ozone above the Antarctic was first announced by British scientists. In the past three decades, The atmospheric pollution problems such as acid rain, photochemical smog and gobal warming have become increasingly prominent, and the environment issue has been one of the important issues of common concern. The chlorofluorocarbons (CFCs) has been phased out because of the adverse environmental effects in stratospheric ozone depletion and global warming. The fluorochemicals such as Hydrofluorocarbonns (HFCs) and Hydrofluoroalchohols (HFAs) containing neither chlorine nor bromine do not cause ozone depletion, and thus have been proposed as a new generation alternatives for CFCs and are used in many industrial applications (e.g. cleaning of electronic components, paints, and polymers). Nevertheless, due to the strong infrared radiation (IR) activity of the C-F bonds, they may still contribute to the global warming potentials (GWPs). Short-lived species with low GWPs are regarded as good candidates for CFC replacements. Thus, to evaluate their potential as candidates, the knowledge about their atmospheric lifetimes is very essential. In atmosphere, their main degradation processes are the reactions with OH radical and Cl atom. Thus, to better assess the environmental impact of such species, a theoretical study for their degradation reactions in the atmosphere is very desirable. In this thesis, we have conducted theoretical calculations on the kinetics of the reactions of OH and Cl atom with fluorochemicals. Ab initio and density functional theory combined with the direct dynamics methods have been used to study the following reactions:CF3CH2CH2OH+OHâ†'productsCF3CH2CF2CH3+Clâ†'productsCH3OCF2CF2OCHO+Clâ†'productsThe main results are summarized as follows:(1) The hydrogen abstraction reaction of CF3CH2CH2OH+OH has been studied theoretically by dual-level direct dynamics method. The required potential energy surface information for the kinetic calculation was obtained at the MCG3-MPWB//M06-2X/aug-cc-pVDZ level. Five stable conformers of CF3CH2CH2OH have been located. For each conformer, there are three potential H-abstraction sites (Ca, Cβ and-OH), and some of the H atoms can be abstracted by more than one abstraction channel due to the different attack orientations of the incoming OH radical. As a result, thirty-one distinct H-abstraction channels have been identified for the reaction. The individual rate constants for each H-abstraction channel were calculated by the improved canonical transition-state theory with small-curvature tunneling correction (ICVT/SCT), and the overall rate constant was evaluated by the Boltzmann distribution function. It is shown that the calculated rate constant is in good agreement with the available experimental data at298K, and exhibits negative temperature dependence with200-350K. H-abstraction from the α site dominates the reaction at low temperatures, while the contributions from the β and OH abstractions should be taken into account as temperature increases. The fitted four-parameter expressions within200-1000K for the overall rate constants as well as the rate constants from the α, β and OH abstractions were given to provide good estimation for future laboratory investigations. In addition, because of the lack of available experimental data for the product radicals involved in the reactions, their enthalpies of the formation (â–³Hf,298°) were predicted via isodesmic reaction at the MCG3-MPWB//M06-2X/aug-cc-pVDZ level.(2)The mechanism of the CF3CH2CF2CH3+Cl reaction was investigated by the M06-2X method combining with the6-31+G(d,p) basis set (M06-2X/6-31+G(d,p)). There are two distinguishable stable conformers (RC1and RC2) for the reactant CF3CH2CF2CH3, and eight H-abstraction channels as well as two substitution channels were located associated with them. The rate constants for each of the H-abstraction channels were evaluated by the improved canonical variational transition theory (ICVT) with the small-curvature tunneling (SCT) approximation at the M06-2X/6-31+G(d,p) level. The overall rate constant (KT) was obtained by considering the weight factor of each conformer from the Boltzmann distribution function, and the calculated values agree well with the available experimental values. Moreover, the contribution of the two conformers to the whole reaction as well as the site selectivity for each of the conformers were discussed. A three-parameter rate constant-temperature expression for the total reaction within200-1000K was fitted to:KT=1.88×10-22T376exp(-1780.69/7). In addition, because of the lack of available experimental data for the reactant as well as the corresponding product radicals involved in the reactions, their enthalpies of the formation (AHf,298°) were predicted via isodesmic reaction at the M06-2X/6-31+G(d,p) level. The reaction of CH3OCF2CF2OCHO with Cl atom has been investigated theoretically by direct dynamics method. The BB1K hybrid functional in conjunction with the6-31+G(d,p) basis set has been used to optimize the geometries for the stationary points and explore the potential energy surface of the reaction. Four rotation conformers (RC1-4) of CH3OCF2CF2OCHO are identified, and they are all considered in the kinetic calculation. For each conformer, there are two kinds of H-abstraction channels and one displacement channel, and the latter one should be negligible due to involving much higher energy barrier than the former two. The individual rate constants for each H-abstraction channel are evaluated by the improved canonical variational transition-state theory with a small-curvature tunneling correction. The overall rate constant is evaluated by the Boltzmann distribution function, and a fitted four-parameter rate constant expression is obtained over a wide temperature range of200-2,000K. The agreement between the calculated and available experimental value at296K is good. The contribution of each conformer to the title reaction is discussed with respect to the temperature. In addition, because of the lack of available experimental data for the species involved in the reactions, the enthalpies of the formation (â–³Hf,298°) for he reactant and its product radicals are predicted via isodesmic reaction at the BB1K/6-31+G(d,p) level.