Theoretical study for degradation of Captan in gas phase Using DFT


Abstract Captan Photodegradation have been theoretically studied in gas phase through reacting with hydroxyl free radical simulating their degradation in aqueous solution using UV light and some oxidants. Quantum calculation likes Semi-empirical and ab initio and DFT have been carried out by using two reliable well-known programs Hyperchem7.5 and Gaussian03W. Chemical reactivity, activation energy and rate constant of cleavage reaction step are calculated for Captan . Total energy, heat of formation and vibration spectrum has been calculated for all reaction components that is probable to be formed during the complete degradation of Captan into essential minerals. The activation energy of peroxide dissociation into its two moieties of hydroxyl radical is equal to 224.304 kJ mol-1. The first degradation reaction step of Captan is endothermic reaction have been carried out through nitrogen-sulphur bond to give up three major component THID, SCCl2 and OCl*. The activation energy of this reaction step is 406.852 kj.mol-1. Rate constant of cleavage step reaction is 2.50 x 108 s-1. Enthalpy change value of overall reaction for Captan is -109089.432 kj.mol-1. Forty one moles of hydroxyl radical are needed to convert Captan completely into simple moieties produced from this reaction like H2CO3, H2O, H2SO4, HOCl, H2, NH3 and OCl*.