Hydrogen Transfer between Sulfuric Acid and Hydroxyl Radical in the Gas Phase:Competition among Hydrogen Atom Transfer,Proton-Coupled Electron-Transfer,and Double Proton Transfer

摘要

In an attempt to assess the potential role of the hydroxyl radical in the atmospheric degradation of sulfuric acid,the hydrogen transfer between H_2SO_4 and HO~(centre dot)in the gas phase has been investigated by means of DFT and quantum-mechanical electronic-structure calculations,as well as classical transition state theory computations.The first step of the H_2SO_4 + HO~(centre dot)reaction is the barrierless formation of a prereactive hydrogen-bonded complex(Cr1)lying 8.1 kcal mol~(-1)below the sum of the(298 K)enthalpies of the reactants.After forming Cr1,a single hydrogen transfer from H_2SO_4 to HO~(centre dot)and a degenerate double hydrogen-exchange between H_2SO_4 and HO~(centre dot)may occur.The single hydrogen transfer,yielding HSO_4~(centre dot)and H_2O,can take place through three different transition structures,the two lowest energy ones(TS1 and TS2)corresponding to a proton-coupled electron-transfer mechanism,whereas the higher energy one(TS3)is associated with a hydrogen atom transfer mechanism.The double hydrogen-exchange,affording products identical to reactants,takes place through a transition structure(TS4)involving a double proton-transfer mechanism and is predicted to be the dominant pathway.A rate constant of 1.50 X 10~(-14)cm~3 molecule~(-1)s~(-1)at 298 K is obtained for the overall reaction H_2SO_4 + HO~(centre dot).The single hydrogen transfer through TS1,TS2,and TS3 contributes to the overall rate constant at 298 K with a 43.4%.It is concluded that the single hydrogen transfer from H_2SO_4 to HO~(centre dot)yielding HSO_4~(centre dot)and H_2O might well be a significant sink for gaseous sulfuric acid in the atmosphere.