Estudo computacional da fotodissociação do 1-cloro-1-flúor-metano (HCFC-31) utilizando dinâmica não-adiabática

Abstract

Many compounds directly interfere with the O2 ⇌ O3 chemical balance present in the Stratosphere. One of the classes responsible for this is known as hydro-chloro-fluorocarbons (HCFCs), formed by hydrogen, chlorine, fluorine and carbon. An example is 1-chloro-1-fluoromethane (CH2FCl or HCFC-31) which acts in the stratosphere through a mechanism similar to chlorofluorocarbons (CFCs) and reduces the concentration of atmospheric ozone through the release of the Cl radical. • after absorption of radiation. The study of the photodissociation mechanism of CFCs and HCFCs is essential to understand how these molecules are activated in the Stratosphere. To obtain a complete theoretical understanding of photochemical mechanisms, it is necessary to determine all important non-adiabatic and adiabatic pathways available after photoexcitation. For this, excited state dynamics calculations are necessary. The objective of this research is to investigate the photodissociation mechanisms of CH2FCl, using single and multireference electronic structure methods (TDDFT, MCSCF and MR-CISD(+Q)) and non-adiabatic dynamics through the Surface Hopping (SH) method. . The ion pairs [FHC+F]Cl- e FHC+H•••Cl-, resulting from the decay to the minimum of the 31A' state in the potential energy curves along the C–Cl bond, were completely characterized at the level MR-CISD/aug-cc-pVTZ, in which the second structure has μ = 9.57 D, configuration 0.65 ionic + 0.20 biradical, is bonded by 4.72 eV and has strong to moderate H-bonding characteristics. The benchmarking carried out showed that the TD-M06-2X functional had the best performance when compared with the MR-CISD+Q results. The theoretical TD-M06-2X and MR-CISD spectra presented a correct energetic order (nσ*, n3s and n3p states) in the assignments of the experimental transitions, unlike the CASSCF spectrum. However, shifts were found in the energies of the spectral bands, although the MR-CISD spectrum showed good agreement with the experimental one. In all excitation windows studied, the dynamics simulations via TDDFT and CASSCF had chlorine as the main photoproduct. For the first, the yield of S3, the surface responsible for the formation of the H2C+F (1A')/Cl– (1S) ion pair, was 18%. While for the second method this value varied according to each window and number of states used, with the highest yield of S3 being equal to 55% for the spectral range 7,69 ± 0,10 eV. The energetic separation of this state is also necessary for the ionic configuration to be observed on the third excited surface, regardless of the depopulation of S3.