Kinetics And Mechanism On Intermolecular Proton Transfer And Hydrogen Transfer Of Nitro-Naphthol And Its Derivatives

Nitro-polycyclic aromatic hydrocarbons?NPAHs?are highly carcinogenic and mutagenic organic pollutants.They are derived from incomplete combustions of fossil fuels and photochemical reactions of polycyclic aromatic hydrocarbons in the atmosphere.Irradiation of NPAHs populates to their excited singlet states,which usually undergo two deactivation channels.One is the generation of nitrogen oxide radical and aryloxyl radical;the other is the ultrafast intersystem crossing?ISC?to the lowest excited triplet state?T₁?.Due to the existence of a higher triplet state?T_n?with the energy under and close to the lowest excited singlet state?S₁?,as well as T_n and S₁ have strong spin-orbital coupling effect,so that T₁ has a relatively high yield.Because of the long lifetime of T₁,reactions of intermolecular hydrogen transfer and proton transfer are easy to occur.In order to study the reaction mechanism of hydrogen transfer and proton transfer of NPAHs in T₁,especially the presence of both hydrogen donor group and hydrogen acceptor group in the molecule,4-nitro-1-naphthol?NO₂-NN-OH?and 8-hydroxy-5-nitroquinoline?HO-QN-NO₂?were selected as model molecules in this thesis.Ultraviolet and visible absorption spectroscopy,nanosecond transient absorption spectroscopy and transient resonance Raman spectroscopy with the help of density functional theory?DFT?calculation method were used to investigate the properties and reactivities of the ground state?S₀?,T₁ and short-lived transient species,and explore the detailed reaction mechanism of hydrogen transfer and proton transfer reaction in T₁.The study results are summarized as following.?1?As nitro group substituted at the para position of hydroxyl,the acidity of NO₂-NN-OH is stronger than that of 1-naphthol.In T₁,NO₂-NN-OH has three step-wise reactions in alcohol solution:NO₂-NN-OH loses the hydroxyl proton in T₁ to form anion triplet³NO₂-NN-O^-,and its life in methanol and acetonitrile mixed solvent is in microsecond timescale;The oxygen atom on the nitro group of³NO₂-NN-O^-abstracts the?-H of the alcohol molecule to form an anion radical HNO₂-NN-O^·-.Due to the reaction equilibrium,HNO₂-NN-O^·-takes the hydroxyl proton of the alcohol molecule again to form a neutral radical HNO₂-NN-OH^·.By plotting the reaction rate constant with a power function of the alcohol concentration,it can be obtained that the reaction order for NO₂-NN-OH in T₁ is 2-3,2-3,1-2,respectively,indicating that reactions of deprotonation,hydrogen abstraction and proton abstraction for NO₂-NN-OH need solvent molecular cluster to participate in the transfer.?2?The three step-wise reactions of protonation,hydrogen abstraction and proton abstraction of NO₂-NN-OH were carried out with the participation of the solvent molecular cluster,and the methanol solvent cluster was the most stable when the number of methanol molecules associated with the solvent was 3.Based on the linear fitting of the reciprocal of UV absorbance for NO₂-NN-OH in aceonitrile-methanol mixed solution and the reciprocal of the molar concentration of methanol to the n power,the number of solvent molecules in cluster in S₀is about 2-3.The optimized configuration of S₀ and T₁ can be obtained by comparing the binding energy for solvent cluster models with the number?n?of molecules n=1-3.According to the maximum binding energy of NO₂-NN-OH in S₀ and T₁,the number of cluster associating solvent can be known as 3,which is consistent with S₀ and T₁ reaction series fitted by the experiment data.By comparison of resonance Raman spectra with DFT calculations on vibration frequencies,the model of solvent molecular cluster in S₀ was determined.?3?HO-QN-NO₂ can undergo proton transfer in both S₀ and T₁,in which the proton transfer in T₁ is achieved by dimer.In S₀ of HO-QN-NO₂,there are four kinds of neutrals,cation and anion that are converted to each other with the changing of solvent pH.In pure acetonitrile,it is mostly the form of neutral HO-QN-NO₂ existent in S₀,and the transient absorption spectrum indicates that the T₁ in pure acetonitrile has undergone proton transfer.According to the comparison of single pulse resonance Raman spectra in acetonitrile and the normal Raman spectra predicted by DFT calculation,the vibrational frequency of T₁ dimer³NH-dimer at1632cm^-1was identified consistent with that in the experiment,indicating that the T₁ of HO-QN-NO₂ carried out intramolecular proton transfer in the form of dimer.The specific process is as follows:after HO-QN-NO₂ populates to S₁,ISC to forms T₁,and then generates the excimer³OH-dimer;through hydrogen bonds within the dimer,proton transfer occurs to form T₁state of isomeric dimer³NH-dimer.³NH-dimer is then decomposed to form ketone monomer,which can further undergo hydrogen transfer reaction.At the same time,the free radical species was also captured by the transient spectrum.