Investigation Into Hydrogen Abstraction Reaction Mechanism Of Triplet State Of Selected Nitronaphthalene Derivatives With Transient Spetroscopies

Nitro-polycyclic aromatic hydrocarbons?NPAHs?are organic pollutants in atmosphere and photo-degradation is their major degradation channel.Excitation of NPAHs populates the lowest singlet state?S₁?,which bifurcates into two decay channels.The first decay channel is generation of nitrogen oxide and aryloxy radicals;the second decay channel is the intersystem crossing?ISC?to the lowest excited triplet state?T₁?.The higher triplet state?T_n?with n?^*configuration as the receiver of ISC process,has a small energy gap below S₁ and results in the ultrafast ISC dynamics.The lifetime of T₁ is on the microsecond time scale,long enough to undergo intermolecular reactions.Especially in the polluted atmosphere aerosol,it is very easy for T₁ to react with phenols compounds which is present in diesel exhaust.In order to elucidate the mechanism of hydrogen abstraction reaction of T₁ of NPAHs from different hydrogen donors,as well as the influence of substitutents on the mechanism,in this thesis,1-chloro-4-nitronaphthalene?1Cl4NN??1-bromo-5-nitronaphthalene?1Br5NN?and4-nitro-1-naphthol?4NO₂1NpOH?were selected as model molecules.Nanosecond transient absorption was used to acquire the characteristic absorption of intermediates and the kinetic information in solutions containing alcohols and phenols.Helped with transient resonance Raman spectroscopic measurements and the density functional theory?DFT?calculations,the intermediate species were identified and the mechanisms of hydrogen abstraction were deduced.The results are as following:?1?Transient absorption spectra have captured and identified the radicals formed through H-atm abstraction by nitro oxygens of T₁ of 1Cl4NN and 1Br5NN from alcohols.The second order reaction rate constant was on the order of 10⁴ M^-1?s^-1.Density functional theory?DFT?calculation predicted that both oxygens of the nitro group are active sites for hydrogen abstraction,and the difference of activation barriers for formation of two radical isomers is only1.0kcal/mol.The T₁ of 1Cl4NN and 1Br5NN can undergo one-step H-atom abstraction from alkyl hydrogen in alcoholic solvents by comparing the results obtained in trifluoroethanol solution and tetrahydrofuran solutions.?2?The radicals formed through H-atm abstraction by nitro oxygens of 1Cl4NN and1Br5NN T₁ states from phenols was also captured and identified.The experimental results suggested that the hydrogen abstraction of T₁ form 1-naphthol takes palce through electron transfer followed by proton transfer through hydrogen bonding interaction,with rate constants of?10⁹ M^-1?s^-1.In this process,hydrogen bonding can reduce the oxidation-reduction potential of electron acceptor and donor,which is the key to the reaction.?3?The results for 1Br5NN were consistent with those of 1Cl4NN in the characteristic absorption and lifetime of transient species,and the reaction kinetics with phenols and alcohols.These results indicate that the different substitution of halogen atoms with similar electron withdrawing ability on aromatic rings has little influence on the properties and reactivities of T₁.?4?The photochemical reaction of 4NO₂1NpOH,which bears hydrogen donor and acceptor in the same molecule,in alcoholic solution was studied for the first time.By controlling the methanol concentration in acetonitrile solution,the transient species can be observed with current nansosecond time resolution spectroscopies.The experiment and calculation results show that T₁ of 4NO₂1NpOH undergoes deprotonation from hydroxyl group,resulting in anion triplet state.Than the oxygen on nitro group abstracts hydrogen atom from alcohols,to produce anion radical.Finally,the anion radical undergoes protonation to generate hydrogen abstraction radical.This result indicates that the intramolecular proton transfer reation through the solvent bridge can't take place in 4NO₂1NpOH.Abstracting hydrogen atom was the major reaction channel for the oxygen on nitro group,although the anion triplet state structure it has bigger electron density.