活性中间体是理解化学反应机制的中枢物质和实现有机化学合成的结构基础， 亦是现代合成化学和生物化学分子不可或缺的重要组成部分。在有机化学中常见的活性中间体有碳正离子、碳负离子、自由基、氮烯、苯炔以及卡宾等，通过对这些中间体的组成、结构、性质以及寿命等的剖析，能够帮助人们更好地理解化学反应进行的过程、原理和机制等问题。在加热或者光照条件下，反应物的弱化学键发生断裂，能够产生高能量、高活性、短寿命的中间体。这些生成的活性中间体极其不稳定，在反应过程中会快速地转变为更稳定的分子。因此，在实验上捕获或检测这些活性中间体非常困难。而高精度的理论计算是得到其电子结构信息的重要手段，也是研究其产生和淬灭机制的有效途径。 目前，相对于研究较多的热反应产生途径来说，对活性中间体的光产生机制研究相对滞后。这是由于活性中间体的光产生机制涉及复杂的激发电子态、多种光淬灭通道以及副产物。在本论文中，我们应用高精度的电子结构计算方法对以下重要的活性中间体（氧正离子、苯氧基自由基、鸟嘌呤自由基阳离子）的光致生成机理、电子转移反应以及淬灭机制进行了研究。 （1）氧正离子的光致生成机制。氧正离子是具有7电子壳层结构的开壳层活性中间体，因其缺电子特性，作为强亲电试剂被广泛地应用于重要的工业有机反应中。虽然经历了40多年的研究，相对于常见的碳正离子和氮正离子来说，人们对氧正离子的产生机制、电子结构和相关反应特性知之甚少。为此，我们采用CASPT2//IRC/CASSCF/6-31G^**理论计算方法，对一系列的苯基羟胺及其衍生物光解生成单-三态氧正离子和苯氧基自由基的反应机理进行研究。计算结果表明：（i）在紫外光照射下，苯基羟胺及其衍生物被激发布居在由苯环上的π电子跃迁到N-O键之间的σ^*轨道的长程电荷迁移S_CT(¹πσ^*)明态，触发并控制了后续的N-O键断裂和氧正离子的生成。（ii）随着N-O键断裂，S_CT(¹πσ^*)态能量急剧下降，很快弛豫到S_CT(¹πσ^*)与基态的交叉区域，生成基态双自由基中间体。富电子的苯基羟基自由基很容易将电子转移到缺电子中心氨基自由基阳离子（·NH3⁺）基团上，从而生成带正电荷的闭壳层单态氧正离子和中性的氨气分子。（iii）取代基效应计算显示，在苯基羟胺间位引入二甲基氨基供电子基团，能够显著降低¹πσ*态的能量，进而使得¹πσ*/³nπ*/³ππ*的三态交叉变得能量可及。同时，间位取代显著提升了自旋-轨道相互作用。因此，光解间位取代苯基羟胺主要生成开壳层三态氧正离子，闭壳层单态氧正离子变为次要产物。（iv）借助能量可及的¹nσ*和³σσ*中间态及两次单占电子的自旋翻转，苯基羟胺和对位苯基取代的苯基羟胺前驱体能够生成少量的自由基产物。 （2）鸟嘌呤自由基阳离子的光致生成和淬灭机制的理论研究。理解光照和氧化环境中DNA碱基的去质子化反应，是探索DNA双螺旋结构稳定性的重要途径，对研究遗传信息的正确转录具有重要的理论意义。我们通过应用CASPT2//CASSCF/AMBER QM/MM方法对G-四联体G₂T₂G₂TGTG₂T₂G₂ (TBA)序列的单电子氧化反应进行研究，并对鸟嘌呤自由基阳离子（G^+·）的5条去质子化路径进行了测试，以此来探究不同氢键和碱基环境下鸟嘌呤自由基阳离子的光致生成和淬灭机理。研究发现：（i）在紫外光照射下，光敏化剂Na₂S₂O₈被激发布居在由O原子上的一个电子从n轨道跃迁到O-O键之间的σ*轨道的长程电荷迁移S_NΣ(¹nσ*)态，触发并控制了后续的O-O键断裂和强氧化剂SO₄^-·的生成。（ii）强氧化剂SO₄^-·一经产生，就会与TBA中的G发生单电子氧化反应，G碱基因失去电子形成G^+·（空穴）。同时，与氢键相连的SO₄^-·因得到电子而形成SO₄^2-- SO₄^-·离子对，完成还原反应。（iii）碱基及氢键环境能够显著影响G的还原能力，进而控制脱质子反应的活化位点。在负电荷中心SO₄^2-的强吸引作用下，G-四联体TBA中Loop的自由亚氨基质子（N₁-H₁）最易发生去质子反应，生成HSO₄^-并淬灭G^+·。同时，发现G-quartet的自由氨基质子(N₂-H_2b)表现一定的脱质子活性，协助实现鸟嘌呤自由基阳离子的淬灭。

Reactive intermediates are the central substance for understanding the mechanism of chemical reactions and the structural basis for the organic synthesis. They are also an indispensable part of modern synthetic chemistry and biochemical molecules. There are some common active intermediates in organic chemistry, such as carbocations, carbanions, free radicals, nitrene, phenylyne and cabin and so on. By analyzing the compositions, structure and properties as well as the lifetime of these reactive intermediates, the usufull informations about highly reactive intermediates can help people better understand the process, principles, and mechanisms of chemical reactions. Under heat or light conditions, intermediates are produced during the processes of a chemical reactions and are molecules with a high energy, high reactivity, and short lifetime due to the cleavage of a weak chemical bond in the reactants. Owing to its short lifetime, it will quickly change to a more stable molecule in the reaction processes. Therefore, it is very difficult to experimentally capture or detect these reactive intermediates. The high-precision theoretical calculation is an important means to obtain information of its electronic structure, and it is also an effective way to study its photogeneration and quenching mechanism. At present, the research on the photogenic mechanism of reactive intermediates is fall behind compared to the thermolytic generation path. This is mainly attributable to the fact that the photogenerated mechanism of reactive intermediates involves complex excited electron states, multiple photo-quenchable channels, and by-products. In this work, I focused on the use of high-precision electronic structure calculation methods for photogeneration and quenching of the following important highly active intermediates containing oxenium ion, phenoxy radical, guanine radical cation. The following is a theoretical study of the photolysis for some precursors leading to the reactive intermediates and the mechanistic insights into the photogeneration and quenching for reactive intermediates. (1)The photogeneration mechanism of the oxenium ions. Oxenium ions are key intermediates in synthetic chemistry and biological processes in which a positively charged oxygen atom has an incomplete (heptad) electron shell with a formally monovalent oxygen bearing two nonbonding electron pairs. As analogs of nitrenes, the electron-deficient nature of these intermediates makes them powerful electrophiles. Although the original investigation of aryloxenium ions first emerged more than 40 years ago, the studies of oxenium ions concerning their generation mechanism, the electronic structure and related properties, and reactivities have seriously lagged behind other activated intermediate congeners such as carbenium and nitrenium. Therefore, we employed the CASPT2//IRC/CASSCF/6-31G** level of theory along the unbiased reaction coordinates to understand how the products of oxenium ions and radical were generated from the reaction precursor states. The photolysis of photoprecursor to produce oxenium ion has been subject of extensive experimental studies from the femtosecond to the microsecond time scale, however mechanistic insights to the generation of activated intermediate species remain largely elusive. We present a theoretical investigation to elucidate comprehensively the possible reaction channels for the formation of oxenium ion and radical intermediates at the multi-configuration perturbation level of theory. Computational results reveal that the photo-initiated electron promotion from the phenyl moiety to the repulsive N-O σ* orbital leads to the formation of diradical intermediate in ground state, and further induces the intramolecular electron transfer from the phenyl part to ammonia radical cation (·N〖H_3〗^+), thereby producing the major product of closed-shell singlet oxenium ion and the neutral species of :NH_3. Whereas, the generation of open-shell triplet outcomes were demonstrated to rely on the energetically accessible single-triplet crossings and spin-orbital interaction among the involved electronic states. Taken together, these data can be used to uncover the electronic structures and related properties, as well as reactivities for the generated oxenium ions and radicals through the photolysis of phenylhydroxylamine and its substitutes. (2) Mechanistic insights into the photogeneration and quenching of guanine radical cation. Generally, the DNA double helix exhibits a high degree of intrinsic stability through the base pairing between complementary strands and stacking between adjacent bases with the aid of inter- or intra- nucleotide hydrogen bonding, thereby ensuring the transcription of genetic information. However, in some cases, the DNA damage caused by oxidative stress and UV irradiation occurs to generate the highly reactive species, leading to a number of deleterious effect including the induction of mutations. The mechanistic aspects of the photogeneration and quenching of guanine radical cation through the one-electron oxidization of the G-quadruplex of G2T2G2TGTG2T2G2 (TBA) sequence were investigated by a combined quantum mechanical/molecular mechanical (QM/MM) approach, at the CASPT2//CASSCF/AMBER level of theory. Several deprotonation paths for the generated G hole were examined to disclose the mechanistic alterations with the varied hydrogen bonding and base surroundings. One electron promotion of oxygen lone pair of the photo-excited photosensitizer peroxydisulfate to its O-O σ* orbital was first demonstrated to become tunable through the varied reduction ability of G base in presence or absence of interbase hydrogen bonding, thereby dynamically controlling the deprotonation site in G-quadruplex TBA. The quenching of G radical cation mediated by the formation of SO42? via the photoinduced electron transfer can be triggered effectively by the deprotonation reaction of free proton rather than the hydrogen-bonded proton in G-G (G-quartet) and G-T (loop) aqueous surrounding. By calculating the deprotonation paths for G radical cation, the deprotonation reactions in G-quadruplex TBA are verified to proceed predominantly along the site of imino proton (N1?H) in the loop moiety showing the coexistent occurrence of amino (N2?H) deprotonation in G-quartet part. The mechanistic features discussed in this work represent considerable advances in understanding DNA radical chemistry.