Experimental And Theoretical Study On Physical And Chemical Properties Of Atmospheric Oxygen-containing Hydrogen Bonded Complexes

Aerosol particles in the atmosphere contribute to air pollution and affect human health.Atmospheric aerosols may interact with solar radiation directly and indirectly,and are the factors with the largest uncertainty in current global warming and climate change predictions.The driving force and mechanism for the formation of atmospheric aerosols are not well understood.The preliminary steps of atmospheric aerosol formation are forming of molecular complexes through molecular interactions,such as hydrogen bonding,proton transfer,etc.The study of these steps is the key to understand atmospheric new particle formation.However,currently there is no effective experimental measurement method for potential molecular complexes in the process of new particle formation.The uncertainty of the basic physical and chemical parameters of atmospheric molecular complexes will lead to the uncertainty in global and regional climate model simulations.The stability of the hydrogen bond complex depends on the hydrogen bond donor or acceptor.Thus,the physicochemical properties of the hydrogen bond complex can be explored by changing the donor or acceptor.After the hydrogen bond complex formation,it will collide with the third substance,and triggers a chemical reaction.For radical reactions,the first step is to form a complex through the interaction between the reactant molecules,and then usually undergo a higher energy transition state,eventually producing a product,so the formation of a hydrogen bond complex plays an important role in the free radical reaction.Here,the role of hydrogen bonding on atmospheric molecular complexes and radical reaction was determined by gas phase and matrix isolation FTIR spectroscopy combined with high level quantum chemical calculations.Important physical and chemical parameters such as Gibbs free energy,equilibrium constant,and rate constant can be used in aerosol growth models to help improve the accuracy of model predictions.The main conclusions of this study are as follows:(1)In the polluted atmosphere,alcohols are one of important oxygen-containing volatile organic compounds.Three alcohols,methanol(MeOH),ethanol(EtOH)and 2,2,2-trifluoroethanol(TFE)were used as hydrogen bond donor,which bonded with several oxygenated volatile organic compounds to form hydrogen bond complexes.The equilibrium constant for complex formation was determined from the experimental integrated absorbance and the calculated oscillator strength of the OH-stretching fundamental transition.The TFE complexes are more stable and form stronger hydrogen bonds compared to complexes with MeOH and EtOH,which are comparable,and only for the stronger hydrogen bond donor(TFE)are the small differences in acceptor molecules highlighted.The hydrogen bonds involving sulfur are generally regarded as weak hydrogen bonds in comparison with the conventional ones.The O-H…O and O-H…S hydrogen bonds in the alcohol-ethylene oxide(EO)and alcohol-ethylene sulfide(ES)complexes in the gas phase have been investigated by FTIR spectroscopy,and comparable OH-stretching red shifts were observed for the complexes with EO and ES as hydrogen bond acceptors.Therefore,the O-H…O and O-H…S hydrogen bonds were found to be of similar strength.Combined with the TFE-EO,trimethylene oxide(TMO),tetrahydrofuran(THF)and tetrahydropyran(THP)as hydrogen bond acceptors were interacted with TFE,we found that the difference of red shifts is so small(<7 cm-1)for TFE-TMO/THF/THP complexes.Their stabilities and the strength of the hydrogen bonds are nearly similar and do not show any marked dependence with the ring size of the hydrogen bond acceptor.In addition,propylene oxide(PO)and isobutylene oxide(IBO)were chosen as acceptors to explore the effect of methylation on the stability and spectral shift of hydrogen bond complexes.The comparable OH-stretching red shifts were observed upon complexation,and an enhancement of the OH-stretching band is shown with the partial pressure of monomers increasing.The OH-stretching frequency of TFE is red shifted by 180 and 201 cm-1 with PO and IBO,respectively.Compared with the TFE-EO complex,the strength of the hydrogen bond in complex increases with the addition of methyl group,which likely results from the increase in basicity of the hydrogen bond acceptor.(2)Nitrite(HONO)is widely present in the atmosphere and is an important intermediate product in atmospheric photochemical reactions.The photolysis of HONO under sunlight is an important source of atmospheric OH radicals.Due to the two HONO configurations(cis-and trans-)existing in the atmosphere,the water-free HONO+OH reaction has two major elementary channels,both based on the HONO hydrogen abstraction by the hydroxyl radical.In the presence of water,the reaction becomes more complex,each starting with the formation of a binary complex between water and one reactant followed by its interaction with the third species.The products are similar to those of the water-free reaction.At 298 K,the rate constants of water-free cis-HONG+OH and trans-HONO+OH reactions are 1.34×10-12 and 1.00×10-15 cm3 molecule-1 s-1,respectively.The calculated rate constants for H2O-complexed HONO or OH increase by one to two orders of magnitude,but weighted for their relative abundances,the H2O-complexed fractions of the reactants in the atmosphere are so small that the effect of H2O on the overall reaction rate is minor.(3)ClO radicals play an important role in the oxidation of inorganic and organic pollutants.Two reaction pathways,cis-HONO+CIO and trans-HONO+ClO,were also identified for the HONO+ClO reaction.When considering the concentration of water,it shows that the effective rate constants of water-assisted cis-HONO+ClO pathway are much smaller than that of naked reaction,whereas the presence of water accelerates the trans-HONO+CIO at room temperature.This study demonstrates that water has a positive role in the pathway of trans-HONG+ClO by modifying the stabilities of reactant complexes and transition states through the hydrogen bonds formation,which contributes to the sink of atmospheric HONO.Moreover,the CH3OH+ClO reaction proceeds through two channels:abstraction of the hydroxyl H-atom and methyl H-atom of CH3OH by ClO,leading to the formation of CH3O+HOCl(+H2O)and CH2OH+HOCl(+H2O),respectively.Results indicate that the formation of CH2OH+HOCl(+H2O)is predominant over the formation of CH3O+HOCl(+H2O).Over the temperature range 216.7?298.2 K,the presence of water is seen to effectively lower the rate constants for the most favorable pathways by 4-6 orders of magnitude in both cases.It is therefore concluded that water plays an inhibitive role on the CH3OH+ClO reaction under tropospheric conditions.(4)The ozonolysis of oxygen-containing volatile organic compounds in the atmosphere also plays an important role in atmospheric chemical reactions.The matrix isolation technique combined with infrared spectroscopy has been used to study the ozonolysis mechanism of 2,5-dihydrofuran(2,5-DHF).A new reaction pathway that is different from the widely accepted Criegee mechanism has been found.Experimental and theoretical results show the evidence of the formation of a furan-H2O3 complex through a dehydrogenation process.The complex is trapped in the argon matrix and stabilized through hydrogen bonding interaction.Meanwhile,the conventional ozonolysis intermediates were also observed,including the primary ozonide,the Criegee intermediate and the secondary ozonide.The present study highlights the cases in which the Criegee mechanism is not the dominant pathway for the reactions of cyclic alkenes with ozone.The cyclic alkenes that can form an aromatic conjugated system by the dehydrogenation process may follow the new mechanism when react with ozone in the atmosphere.Hydrogen bond plays an important role in the new mechanism.