Matrix Isolation Infrared Spectroscopic And Theoretical Studies On The Reactions Of Simple Hydrocarbon Free Radicals

Simple hydrocarbon free radicals play important pole in Organic Chemistry. The investigation on their spectra, structures and reactivities have gained intensive attentions. Due to their high reactivities, most of free radicals are short-lived species. Their reactions proceed very fast. Conventional gas phase end product analysis aids in the prediction of suitable mechanisms, but generally does not yield a complete description of the reaction. Monitoring the important reaction intermediates is necessary in understanding the radical reaction mechanisms.Matrix isolation has been proved to be a powerful method in trapping reactive intermediates for spectroscopic studies. In this thesis, the reactions of free radicals are investigated by matrix isolation infrared spectroscopy and quantum chemical calculations. The reaction intermediates and products are identified by isotopic substitution and quantum chemical frequency calculations. The reaction mechanisms are discussed.The reactions of simple hydrocarbon free radicals (including formyl radical and vinyl radical) with dioxygen were investigated, and the major results are as follows:1,The reaction of formyl radical with dioxygen in solid argon was studied. The formyl radical was produced through high frequency discharged methanol. The formyl radical reacted with dioxygen spontaneously on annealing to form the formylperoxy radical with the O-O bond in a cis- position relative to the C-H bond, which was characterized by C=O and C-O stretching vibrations at 1821.5 and 957.3 cm^-1, respectively. The formylperoxy radical dissociated to give CO and CO₂ underUV-visible light irradiation. The reactions are summarized as follows:2,The reaction of C₂H₃ radical with O₂ has also been studied. The vinyl radical was produced through high frequency discharge of ethylene. The vinyl radical reacted with oxygen spontaneously on annealing to form the vinylperoxy radical C₂H₃OO with the O-O bond in a trans- position relative to the C-C bond, which is characterized by O-O stretching and out-of-plane CH₂ bending vibrations at 1140.7 and 875.5cm^-1.The vinylperoxy radical underwent visible light-induced dissociation to the CH₂OH(CO) complex, which was characterized by O-H, C≡O and C-O stretching, HCOH deformation and torsion vibrations at 3604.5, 2155.8, 1188.2, 1063.9 and 494.4cm^-1.The overall reaction can be expressed as below:The reaction of vinyl radical with oxygen has been studied in the gas phase. The major end products were determined to be HCO and H₂CO. While the reaction channel for the formation of CH₂OH + CO has never been considered in the previous studies. This channel was predicted to be thermodynamically more favorable than the previously reported major HCO + H₂CO channel. The CH₂OH(CO) product is most likely produced by hydrogen atom transfer from the firstly-formed H₂CO-HCO pair in solid argon.Our results indicate that the formation of CH₂OH + CO could be a major channel for the reaction of vinyl radical with oxygen at low temperatures, suggesting it could also be a major channel in the gas phase.In summary, simple hydrocarbon free radicals (formyl and vinyl radical) are produced via high frequency discharge and are trapped in solid argon matrix. Their reactions with dioxygen were investigated The reaction mechanisms can be described as:R·+O = O→R-O-O…(?)…-ProductsThe initial reaction step is the formation of the R-O-O·radical intermediate, in which the unpaired electron is mainly located on the terminal oxygen atom. The ROO intermediate undergoes photon-induced dissociation reaction to form the final products. If the R fragment involves C=C double bond, the photon-induced dissociation reaction proceeds with the initial weakening of the C-C bond following the intramolecular hydrogen atom transfer to the terminal oxygen atom to give the final dissociation products.Another topic of the present thesis was focused on the photoisomerization process of nitrobenzene. The products from UV photon induced dissociation or isomerization of nitrobenzene in solid argon were studied using infrared absorption spectroscopy as well as density functional calculations. The weakly bounded phenoxyl radical-nitric oxide complex (C₆H₅O-NO) was produced upon UV irradiation of nitrobenzene. The complex rearranged to the more stable phenyl nitrite molecule upon sample annealing. The C₆H₅O-NO complex was predicted to have a triplet ground state with a planar C_S symmetry, while the phenyl nitrite molecule was predicted to have a non-planar structure with the C-O bond in a trans- position relative to the N-O bond. The C₆H₅O-NO complex and the C₆H₅ONO molecule can be interconverted in solid argon matrix via UV irradiation and sample annealing.