The Effect Of （H₂O）_n（n=1-3） Clusters On The Reaction Of HONO With HX（X=F,Cl,Br）:Mechanistic And Kinetic Study

Water and water clusters exist widely in the atmosphere with high concentration,while nitrous acid（HONO）has multiple hydrogen bonding sites and can form hydrogen bonding complexes with water,which has an important impact on atmospheric chemical reactions.The reaction of HONO with HX（X=F,Cl,Br）can produce nitrite halide（XNO）.This product is easy to photolysis to produce halogen radicals,which affects the generation and destruction of ozone and accelerates the oxidation of volatile organic compounds in the atmosphere.However,the current experiments and theories are all focused on anhydrous reactions,and there is a big gap between the reaction rate constants obtained from theory and experiment in the reaction of HONO with HCl.Therefore,it is necessary to study the influence of hydrate on the reaction mechanism and kinetics of HONO and HX and find out the reasons for the large difference in reaction rate constants.In order to investigated the effects of single water molecule as well as water clusters on the gas-phase reactions of HONO with HF,HCl,and HBr,this paper used quantum chemical calculations with Gaussian 09calculation software.The reaction of HONO with HX（X=F,Cl,Br）in the presence of a single water molecule,water dimer and water trimer were studied in detail from the perspective of reaction mechanism and kinetics at the theoretical level of CCSD（T）/aug-cc-p VTZ//M06-2X/6-311+G（2df,2p）.Meanwhile,the effects of HCl excess and HONO excess on the reaction of HONO with HCl were studied.The possible reaction channels were proposed and the detailed potential energy surface information was given.The thermodynamic and kinetic properties were analyzed and compared to solve the inconsistency between theoretical and experimental rate constants.Meanwhile,the influence of water clusters on HONO and HX reactions was systematically evaluated to provide theoretical guidance for exploring and solving reaction-related problems.In the absence of water,the reaction between HONO and HX（X=F,Cl,Br）proceeded mainly through the direct extraction of H atoms from HX by HONO,resulting in the formation of the products XNO and H₂O.The reaction of trans-HONO+HX was more favorable than cis-HONO+HX in terms of potential energy surface.When a single water molecule was participated in the reaction,HONO and HX can proceed through direct hydrogen extraction and double hydrogen transfer mechanisms,where the reaction path of HONO first forming a hydrogen bonding complex HONO…H₂O with water followed by direct extraction of H atoms in HX.This reaction path had a low activation energy and was the main reaction path.The presence of a single water molecule reduced the reaction energy barrier and increased the reaction rate constant by about 1-4 orders of magnitude.Although the rate constant of HONO+HCl reaction increased,there was still a gap between the theoretical and experimental result.Under atmospheric conditions,considering the concentration of water molecules,a single water molecule only had positive catalytic effect on HONO+HBr reaction.When water dimer participated in HONO and HX reaction,the optimum reaction channels were cis-HONO…（H₂O）₂-a+HX and trans-HONO…（H₂O）₂-b+HX,the reaction mechanism was direct hydrogen extraction.According to energy analysis,the reaction barrier in the presence of water dimer was significantly lower than that in the absence of water,in the presence of single water molecule and in the presence of water trimer.According to kinetic analysis,the presence of water dimer increased the reaction rate constant of HONO and HX by 4-8 orders of magnitude compared with that without water,and the reaction rate was positively correlated with temperature.The rate constant of HONO and HCl reaction with water dimer was close to the experimental results,which was the reason for the great difference between the experimental and theoretical rate constants.The water trimer had a negative catalytic effect on the trans reactions of HONO with HF and HONO with HCl and HBr,while the reaction barrier decreases and the reaction rate constant increases in the cis reaction,but the catalytic effect was not obvious compared with the water dimer.In actual atmospheric environment,water dimer and water trimer have negative effect on HONO+HX reaction.When HONO reacted with HCl in a ratio of 1:2,the reactions can be processed by hydrogen transfer between HCl molecules,with HONO extracting H atom from one of the HCl molecules,where the reaction of trans-HONO+HCl…HCl had the lowest reaction potential barrier.When HONO reacted with HCl in a ratio of 2:1,Cl NO can be generated by direct hydrogen extraction mechanism,where the O atom of trans-HONO extracted the H atom in HCl as the main reaction path.According to the energy analysis,both the HCl excess and the HONO excess can lead to the reduction of the reaction potential.According to the kinetic analysis,although the HCl excess and the HONO excess increase the reaction rate constant,they are still far from the experimental data and are not the main influencing factors.