Study On The Triplet Protonation And Hydrogen Abstraction Reaction Of Nitro Polycyclic Aromatic Hydrocarbon Derivatives

Nitro-polycyclic aromatic hydrocarbons?NPAHs?are one of harmful pollutants in the atmosphere,mutagenic and carcinogenic to organisms.The main degradation channel for NPAHs in the atmosphere is photo-degradation,which can generate their lowest excited triplets?T₁?with high yields for most of NPAHs.The microsecond time scale lifetime of these NPAHs triplets provides much opportunity for NPAHs to be protonated by acids,undergo intermolecular hydrogen transfer reactions with alcohols,phenols and other hydrogen donors in the atmosphere.At present,the hydrogen abstraction reaction of triplets based on studies on carbonyl compounds can be divided into two categories:one is the one-step hydrogen transfer reaction with alcohol,tetrahydrofuran and other hydrogen donors;the other is electron coupled proton transfer reactions with phenols.In order to investigate effects of heteroatoms,electron-withdrawing and electron-donating substituents such as nitro,amino groups etc.,hydrogen ions and solvent polarity and solvent abilities of hydrogen bonding on properties of T₁ state and hydrogen abstraction reactions in T₁of NPAHs,in this thesis,5-nitroisoquinoline?NO₂-IQ?,4-amino-4'-nitrobiphenyl?NO₂-BP-NH₂?,2-nitrofluorene?2NF?and 2,7-dinitrofluorene?DNF?were selected as model molecules.Nanosecond transient absorption spectroscopy was used to detect T₁ states and short-lived intermediates.Under the help of time-dependent density functional theory?TDDFT?calculation,structures of intermediates were identified and their electron configurations were revealed.As a result,the mechanisms of hydrogen transfer and protonation reactions at T₁ state were elucidated.The detailed results are as follows:?1?N atom in isoquinoline ring for both ground?S₀?and T₁ state?³NO₂-IQ?of NO₂-IQ can be protonated to produce their corresponding cations,NO₂-IQH⁺and ³NO₂-IQH⁺,with pK_a of3.90 and 2.96,respectively.The intermediate formed in the reaction of hydrogen abstraction by³NO₂-IQ from alcohols and phenols was characterized by transient absorption spectrum,and identified to have the radical structure.And the second-order reaction rate constants are 10⁴M^-1s^-1 and 10⁹ M^-1s^-1,respectively.Similarly,³NO₂-IQH⁺can also undergo hydrogen transfer reaction with alcohols and phenols,and second-order reaction rates are faster than those of³NO₂-IQ,but the mechanisms of acceleration are different.For one-step hydrogen transfer reaction,the acceleration is due to the partial ³?n,?^*?characteristic in T₁ state of ³NO₂-IQH⁺,which is supported by the TDDFT calculation results.For electron coupled proton transfer reaction,the acceleration is due to the lower charge density of nitro group after protonation,which is beneficial to electron transfer.?2?Both of amino and nitro groups of NO₂-BP-NH₂ can be protonated.In S₀ state,the amino group is preferentially protonated with pK_a of 4.37,while in T₁ state,the nitro group is preferentially protonated with pK_a^*of 3.09.Further reaction of T₁ cation in aqueous solution generates a radical anion.The hydrogen bond configurations of NO₂-BP-NH₂ in alcoholic solvents were analyzed with Kamlet-Taft model.The results suggest that in S₀ state,the hydrogen bond is mainly formed between O of alcohol hydroxyl group and H of amino group in NO₂-BP-NH₂,while in T₁ state,there are two hydrogen bond contributions:one is hydrogen bond between H of alcohol hydroxyl group and O of nitro group in ³NO₂-BP-NH₂,the other is hydrogen bond between O of alcohol hydroxyl group and H of amino group in ³NO₂-BP-NH₂.At the same time,the radicals generated by ³NO₂-BP-NH₂ hydrogen abstraction from alcohols,phenols are also captured by transient absorption spectroscopy.?3?Both ³2NF and ³DNF can abstract hydrogen atom from alcohols and phenols to form corresponding radicals,but ³2NF and ³DNF exhibit different properties in different solvents.In nonpolar solvents,the T₁ state of ³2NF is a mixture state of ³?n,?^*?and ³??,?^*?.In middle polar solvents,T₁ and T₂ states can coexist.While in strong polarity and strong hydrogen bonding supplying solvents,the T₁ state is ³??,?^*?,the inversion is regulated by solvent polarity and hydrogen bonding.The different is that the inversion does not occur in ³DNF because of its small dipole moment,weak charge separation and weak interaction with solvent polarity.