Theoretical Studies On The Photodissociation Mechanism Of Cyanogen Azide And Several Carbonyl Compounds

Photodissociation and the reaction with OH radicals are the two major degradation processes of the most atmospheric species. Thus, the studies of these two kinds of reactions are of great significance for evaluating the atmospheric lifetimes and the environmental impact of these species. In this thesis, the photolysis mechanisms and kinetics of some species such as, N3CN, HOCH2CHO, CH3COCHO, and CF3CH2CHO have been studied theoretically in detail by using the multi-reference state methods (CASSCF, CASPT2, CIPT2, and MRCI), density dunctional theory (DFT) and variational transition stste theory (VTST). The main contents are summarized as follows:(1). Cyanogen azide (N3CN) as the cyano-group substituted compounds of azide has proven to be a useful highly active reagent in synthetic chemical. Cyanogen azide undergoes photodissociation to form radicals such as NCN, CN, and N3. In recent years, these products have been acctracted many experimental and theoretical attentions due to the important nature of these radicals. Therefore, in view of the importance of photolysis products, we investigated the photodissociation mechanism of N3CN using the multi-reference state method. The minima, transition states were obtained at the CAS(10,9)/6-311+G(2df) level, and the singlet/singlet conical intersection and singlet/triplet crossing points of the ground and low-lying excited states were located by the state-average CAS(10,9)/6-311+G(2df) method. The single point energies have been calculated at the MRCI+Q level, and the potential energy surfaces of N3CN have been constructed. It is shown that N-N bond fission to form N2+NCN is the predominant dissociation pathway on the So, S1, S2 and T1 surfaces whereas the C-N bond fission channel is the minor pathway. The 220 nm absorption peak observed experimentally corresponds to an excitation from the So to the S1 state leading to the major photodissociation product NCN [a1Δg]. The 275 nm absorption peak corresponds to the So-T1 transition leading to the formed ground-state product NCN [X3Σg-] via the barrierlessly direct dissociation in the T1 state.(2). Glycolaldehyde (HOCH2CHO) is the simplest a-hydroxy carbonyl compound. Since HOCH2CHO represents a common structure feature of sugar, it plays an important role in the biochemistry process and the formation of natural products such as O3 and HOx. Photolysis reaction has been identified as one of the major gas-phase loss processes for HOCH2CHO. In present study, the electronic structures of the stationary points and dissociation potential energy surfaces (PES) of glycolaldehyde (HOCH2CHO) in the lowest three electronic states (So, S1 and T1) have been calculated at the CASPT2//CASSCF/6-311+G(2df,2p) level of theory by using the multi-reference state method. Combining with the surface intersection points, the wavelength-dependent photolysis mechanisms leading to probable photolysis products have been elucidated. It is shown that in the experimental photolysis wavelength range of 240-400 nm, the HOCH2CHO molecule mainly occurs the dissociation reactions on the S1 surface or decays to the ground state via the So and S1 vibronic interaction, followed by So dissociation reactions. The C-C bond fission to yield ground-state products HOCH2 (2A')+HCO (2A') is the dominant pathway, while in the certain wavelength range, the So concerted channel to produce CH3OH (A')+CO (A1), H-elimination of the aldehyde group and OH-elimination channels are also energetically accessible. Our calculations are in good agreement with experimental results.(3). Methylglyoxal (CH3COCHO) is one of the important decarbonyl compounds. In troposphere, one of the most efficient degradation pathways for CH3COCHO is photodissociation reaction. The photodissociation products of CH3COCHO have been determinated experimentally by several groups, and the photodissociation mechanisms of CH3COCHO in different wavelength range have also been proposed. However, the detailed photodissociation mechanism is still unclear. In present study, by exploring the detailed potential energy surfaces and surface intersections for several low-lying excited states (S0, S1 and T1), we have been able to provide a description on the photochemical and photophysical behavior of CH3COCHO. To obtain more accurate energetic information, the single point energies have been calculated at the CIPT2+Q//CAS(14,12)/6-311G(d, p) level. The calculated vertical excited energy was in good agreement with the available experimental data. The calculated results show that the energies of the intersysterm crossing point, S1/T1 and the internal conversion point, S1/So are higher than all energy barrier heights in S1 states, and as a results, CH3COCHO mainly occurs via the dissociation reactions on the S1 surface. In S1 state, the fission of CH3C(O)-CHO bond to form ground state products CH3CO (2A')+HCO (2A') is the primary pathway, while the adiabatic cleavage of CH3-COCHO bond to yield CH3 (2A')+ COCHO (2A") is a competitive pathway. The dissociate products CH3CO(2A'), HCO(2A'), and COCHO(2A') produced from above two pathways can undergo further secondary dissociation reactions leading to final products H(2S), CH3(2A'), CO(2A'), and HCO(2A'). In addition, because of the higher barrier height, the adiabatic fission of CH3COC(O)-H bond to form CH3COCO(22A')+H(2S) is a minor pathway. Also, the CH3COCHO can dissociate to CO+CH3CHO by intersystem crossing process, however, due to the complexity of this reaction channel, it is a minor channel too. The present calculations are in good agreement with the available experimental results.(4). Partially fluroalchohols (FAs) (with the structure of CF3(CH2)xCH2OH) have been proposed as a new generation of CFCs used in many industrial applications. CF3CH2CH2OH is the one among the family of CF3(CH2)xCH2OH, and its major atmospheric photooxidation product is CF3CH2CHO. The major degradation process of CF3CH2CHO in atmosphere is initiated by the OH radicals. Thus, in order to ascertain the environmental impact of these fluorinated alcohols released into the troposphere, a mechanistic and kinetic study of the resulting secondary oxidation product CF3CH2CHO with OH is very desirable. Trans-CF3CH2CHO (denoted as t-CF3CH2CHO) and cis-CF3CH2CHO (denoted as c-CF3CH2CHO) were the two stable conformers of CF3CH2CHO obtained at the M06-2X/aug-cc-pVDZ level, with the O-C-C-C dihedral angles of 149.9°and 0°, respectively. There were four distinct H-abstraction channels located for t-CF3CH2CHO+OH, inwhich two distinct H-abstraction transition states from -CHO and the other two from -CH2- are located, and two for c-CF3CH2CHO+OH, inwhich only one distinct H-abstraction channel is feasible from the two groups, respectively. In order to acquired the accurate energy, the single-point energies of the stationary and nonstationary points were calculated at the MCG3-MPWB level based on the M06-2X geometries. The rate constants were calculated using improved canonical transition-state theory with small-curvature tunneling correction (ICVT/SCT). It is shown that the reaction proceeds predominantly via the H-abstraction from the -CHO group over the temperature range 200-2000 K. The calculated rate constants were in good agreement with the experimental data between 263 and 358 K, within a factor of 0.3. In addition, the four-parameter rate-temperature expression for the ICVT/SCT rate constants (kT) within 200-2000 K is given as...