Application Of CIDNP To The Research On The Mechanism Of Free Radical Reactions

CIDNP technique has improved the sensitivity of NMR greatly. It can detect minim instantaneous free radicals in solutions. It has become a powerful means to study the mechanism of photo-induced electron transfer reactions and the structure of free radical intermediates.In this dissertation, three different kinds of reactions were studied by CIDNP technique and were given reasonable mechanistic explanation of corresponding reaction systems.Investigation on photo-induced reactions of triethylamine, tri-n-butylamine with 2-chloro-5-methoxy-p-benzoquinone was carried out in C6D6 and CD3CN. In CD3CN, triethylamine donated electron to 2-chloro-5-methoxy- p-benzoquinone to form charge transfer complex (CTC). Under UV irradiation, charge separation of CTC occurred, leading to ion pairs. The back electron transfer caused the methylene of triethylamine to give emission polarization. Because of the higher oxidation potential and steric hindrace of tri-n-butylamine, more difficult to form CTC, tri-n-butylamine and 2-chloro-5-methoxy- p-benzoquinone gave no polarization. In C6D6, although CTC of both reactions eixisted, neither triethylamine nor tri-n-butylamine gave polarization signals. It might be due to unpolar solvent C6D6 disfavoring the charge separation of CTC, leading to no ion radical and no polarization either.The photo reactions of β-diketones compounds with chloranil in C6D6 were through free radical intermediates. In the process, chloranil firstly was excited to singlet state and then a labile hydrogen abstraction interaction between singlet chloranil andβ-diketones was involved to form singlet stable neutral radical pairs (RP). As intermediate, the RP went outcage and conmbined to produce a series of CIDNP polarizations. The different substitutes had an effect on the main form of β-diketones, making the hydrogen donors to chloranil different correspondingly.The reactions of threeα,β-unsaturated aldehydes with different substitutes with 1-acetylindole-2, 3-didone and 2-chloro-5-methoxy-p-benquinone in different solvents under UV irradiation were investigated by CIDNP. The results showed they had different reaction mechanisms. The reactions with 1-acetylindole-2,3-didone was electron transfer reaction, while the reaction with 2-chloro-5-methoxy-p-benquinone was through 1,4-biradicals intermediate.Different substitutes of α,β-unsaturated aldehydes made electron density of double bond different and had an important effect on the raction mode of α,β-unsaturated aldehydes. The polarity of solvent disfavored the produce of triplet carbonyl compounds, restraining the CIDNP polarizations.