Theoretical Study On Decomposition Mechanism And Kinetics Properties Of Trifluoroacetic Acid

Perfluorinated carboxylic acids （PFCAs） such as perfluorooctanoic acid,PFOA, have recently received much attention as ubiquitous environmentalcontaminants. Trifluoroacetic acid （TFA, the lowest PFCAs homolog） is alsoa class of persistent environmental pollutants and its potential accumulation insome aquatic ecosystems is a matter of environmental concern. TFA is anatmospheric degradation product of Freon substitutes and it does not degradeunder normal environmentally conditions. Hence, in this paper, theapplication of quantum methods calculate the decomposition mechanism ofTFA in the gas phase. The main contents of the paper are as follow.（1）Theoretical investigations are performed on mechanism forthermolysis of TFA in the gas phase. The electronic structure information onthe potential energy surface is obtained at the density functional theory （DFT）and the energetics along the reaction path is refined by coupled cluster[CCSD（T）] method. The rate constants of primary decomposition pathway iscalculated using transition state theory in the temperature range of300-2000K.The results show that TFA has five pathways and the lowest energy reactionpathway to dissociation of TFA is the molecular elimination of HF to formCF₂COO diradical. The high temperature is greatly accelerated the reactionrates of TFA decomposition.（2）The reaction mechanism of the singlet CF₂reacting with CF₂O hasbeen studied by using the density functional theory （DFT）, coupled clustersingle and double excitation[CCSD（T）], natural bond orbital （NBO） andatoms in molecules （AIM） methods. The calculated results indicated that theaddition reaction pathway of the singlet CF₂reacting with CF₂O involve twointermediates and two transition states. Under thermochemical condition, thedecomposing of P1（CF3COF） to CO and CF4is difficult to occur for itslargest activation energy.（3）Astudy was carried out for the gas-phase reaction of TFAwith OH radical at CCSD（T）/6-31++G（d,p）//B3LYP/6-31G（d,p） level theory. The rateconstants of TFA with OH radical reaction is calculated using transition statetheory in the temperature range of300-1200K. In addition, the importantgeometries in the pathway has been studied by AIM and NBO analysis. It isshows that the hydrogen bond is an important role to form complex.（4）The time-dependent density functional theory （TD-DFT） has beenused to investigate the electronic spectra of TFA and its complexes ligandedwith Fe atom. The results show that adulteration with the Fe atom can makethe energy gaps of complexes smaller and the largest absorption wavelengthred-shift. In addition, the frontier molecular orbital energy gap anaysisindicates that the smaller energy gaps due to HOMO energy increasing.