| In this thesis, quantum chemical theoretical investigations on the reaction mechanisms between oxygenous radicals and unsaturated hydrocarbons have been carried out. Important information of potential energy surfaces such as structures and energies of intermediate isomers and transition states, possible reaction channels, reaction mechanisms and major products are obtained. The results obtained in the present thesis may be helpful for further theoretical and experimental studies of these kinds of reactions. The main results are summarized as follows:1. The UB3LYP/6-311++G(d,p)//UB3LYP/6-31G(d) study on the complex reaction potential energy surfaces of H2NO·radical with cis-2-butene revealed three types, five distinct elementary channels: L4-I and L4-II (abstraction-addition), L4-III and L4-IV (addition- addition-elimination), L4-V (addition-addition-elimination-catalyzed conversion). The kinetic analysis demonstrates that the title reaction undergoes the addition-addition-elimination mechanism (L4-IV). The quantum chemical investigation on the complex potential energy surface of this reaction is expected to provide useful information for understanding the selective oxidation of unsaturated hydrocarbons by such type of oxygen-mediated radicals. Our study would also be helpful for future mechanistic study and selective synthesis on the kinetically stabilized nitroxyl radicals like R2NO·and R2C=NO·towards unsaturated alkenes. 2. The reaction mechanism between (Me)3CO·radical and trans-3-hexene in benzene was studied for the first time at the B3LYP/6-311++G(d,p)//B3LYP/6-31G(d)+ZPVE level. Two distinct elementary channels were identified as: (1) abstraction-addition; (2) addition-addition-elimination. Analysis of the potential energy surface demonstrates that for the title reaction, the channels (1) and (2) have the major and minor contribution, respectively. Our calculated results can well explain the recently observed product distribution by Coseri et al. However, we found that the addition-abstraction channel proposed by Coseri et al. is kinetically infeasible.3. At the B3LYP/6-311++G(d,p) level, we performed the first computational study on the interconversion mechanism of a series of double-substituted ammonium oxide (R2HNO) and double-substituted hydroxylamine (R2NOH) isomers with (R=CH3, NH2, OH, F, CH2CH3, CH(CH3)2, C(CH3)3, N(CH3)2, N(CH2CH3)2, N(C(CH3)3)2, OCH3, OCH2CH3, OC(CH3)3). Comparisons were made with the mechanism of H3NO and H2NOH. This indicates that all of them might be characterized in gas-phase.4. We performed the first ab initio study on the potential energy surfaces of the R2NO·-radical reactions towards cyclopropene, cyclobutene, cyclopentene, cyclohexene, cycloheptene and cyclooctene. The solvent effect of benzene on several important reaction channels is calculated. It was shown that there are three important channels and two distinct mechanisms for these reactions. The mechanism of the reactions of nitroxyl radical with cyclopropene and cyclobutene is addition-addition-elimination-catalyzed conversion, but the mechanism of the reactions of nitroxyl radical with cyclopentene, cyclohexene, cycloheptene and cyclooctene is abstraction-addition. Our research of these reactions is expected to be helpful for resolving the recent mechanistic controversy on the reactions of nitroxyl radicals with cycloalkenes. Moreover, our newly proposed mechanism"addition-addition-elimination-catalyzed conversion mechanism"should broaden our understanding of these types of radical-alkene reactions.
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