近年来，随着国民经济的发展，能源、环境与健康逐渐成为人们关注的焦点。直接利用取之不尽的太阳能或将其转化为绿色环保的氢能，成为人们开发利用太阳能、解决能源环境问题的主要方式之一。光催化反应具有洁净、高效等优点，已经引起了化学和材料学家的广泛关注，其中，基于廉价易得的铜金属化合物的光催化反应更是备受关注。此外，近年来大气学家对自然界普遍存在的水气界面上发生的光/热化学反应越发重视，是因为该类反应的产物能够影响区域新颗粒的形成，且很可能是二次有机气溶胶形成的前驱体。在健康方面，紫外线和电离辐射等都会损伤生物体内的DNA，而DNA的损伤可导致细胞死亡、基因突变甚至癌症。但科学家对DNA损伤的化学机理尚不清楚，而DNA损伤机制的阐明是治疗癌症、修复DNA损伤的重要基础。然而，光催化、光生物或水气界面发生的反应过程往往较为复杂，仅依靠实验手段难以弄清其微观反应机理，需要借助于理论与计算化学的手段。 本论文围绕均相金属有机光催化、伽马射线辐射引发的DNA损伤和水气界面的大气化学反应等研究方向，采用电子结构计算及量子力学/分子力学（QM/MM）的方法，探究这些反应过程详细的微观反应机理，旨在为实验者提供有价值的理论指导和机理见解。主要研究内容概述如下： 1. 最近实验化学家研究了可见光诱导的苯酚与端炔的氧化偶联反应，该反应可以在温和的条件下以廉价的CuCl为光催化剂，合成出重要的有机中间体——羟基化的芳基酮。我们采用高精度的MS-CASPT2//CASSCF、DFT方法并结合能量分解、电子结构片段分解分析，详细地研究了整个可见光诱导的铜催化的苯酚-端炔偶联反应的微观反应过程。计算发现：（a）由于苯乙炔和氯化亚铜复合物PhCCH-CuCl的激发单态能通过超快的非辐射跃迁过程失活回到基态，因此可见光驱动苯乙炔转化为炔铜的反应本质上倾向于热反应过程；（b）由于苯炔铜PhCCCu(I)的系间窜跃过程效率较高，它的激发三态寿命较长，因此可以与氧气分子发生分子间的单电子转移（SET）而生成PhCCCu(II)；（c）在苯酚的氧化过程中，O2更容易进攻苯酚自由基中间体的对位，从而产生了对苯醌，解释了实验观察到的区域选择性的起源；（d）苯乙炔的C≡C三键的活化并不在T1态与O2之间发生SET过程，而是与苯醌的进行[2+2]环加成反应的过程；（e）底物苯酚在光催化反应的氢转移过程中，扮演着比传统水分子的“氢桥”更为有效的催化作用，很好地解释了实验上观测到的现象，即为什么加入额外的水并未提高光催化反应的产率。计算模拟结果支持了实验推测机理的基本框架，但解释了许多实验上并不清楚的机理细节，并提出了详细的机理见解，为实验上进一步优化反应条件、提高光催化性能提供了有价值的理论参考。 2. 实验上报道了一种新颖的可见光诱导[Cp*Ir(bpy)Cl)]+催化甲酸（HCO2H）产生氢气的反应，该反应为CO2/HCO2H光催化储氢提供了广阔的应用前景。然而，其反应机理并不清楚。该工作使用密度泛函理论（DFT）电子结构计算方法，详细探究了该反应的微观反应过程，并提出了一条可能的光催化反应机理。计算发现，在金属氢化物[Cp*Ir(bpy)(H)]+（5）形成的过程中，底物甲酸作为“氢桥”协助质子的迁移。一旦被光照激发，金属氢化物5很快从其最初布居的单重激发S1态经由两条非辐射的衰退途径布居到T1态。在T1态，水分子和甲酸分子促进了激发态H-/H+从铱金属中心迁移到配体Cp*和bpy上，可以产生多种能量较低的金属氢化物的异构体。然而，5*及其异构体在T1态上产生氢气的路径能量上均十分不利。计算表明，这些异构体应先经T1/S0交叉点通过T1 → S0系间窜跃过程回到S0基态，继而在基态与水反应产生氢气。在反应中，溶剂（水）作为助剂和催化剂可以降低反应活化能垒，从而加速产生氢气的过程，最终提高整个光催化反应的效率。该工作的计算结果为可见光诱导含金属铱配合物的光催化反应提供了有价值的理论指导。 3. 2017年，Greenberg等人发现经γ射线电离辐射产生的以氮为中心的嘌呤自由基（dA·），可以引发一连串的DNA损伤过程。近期，该课题组进一步对合成的寡核苷酸链中独立生成的嘌呤自由基dA·的反应活性进行探究，揭示了之前未被认识的dA·在DNA损伤中的潜在作用，并对其引发的一连串DNA损伤过程，提出可能的反应机理。该工作使用高精度的NEVPT2和DFT电子结构计算方法，结合QM/MM模型，探究了dA·引发一连串DNA损伤的微观机制。计算发现：（a）碱基A11上的HN·自由基很容易拔取邻位T12上甲基的氢原子（QM(NEVPT2)/MM水平计算的能垒为12.2 kcal/mol），从而形成T12·自由基，且该过程大量放热（相同计算水平下，大于-35 kcal/mol）；（b）随后，氧气分子的两个氧原子以分步的机理分别与碱基T12·和G13发生C-O键偶联，将这两个碱基通过“氧桥”连接，此时自由基的单电子重新排布并布居在鸟嘌呤G13碱基上；（c）中间体4与氧气分子的单电子转移（SET）过程热力学允许，随后的H迁移和O-O键断裂是个协同的过程，并且水分子的参与通过降低活化能垒加速了该过程的进行，使得阳离子型机理比自由基型机理更为有利；（d）在最后的去质子化过程中，水分子是质子的受体，形成的水合质子通过与磷酸根阴离子的静电相互作用而稳定存在。本工作在原子水平上对DNA的一连串的损伤过程进行了详细的理论计算模拟，使得实验上提出的机理合理化并提出许多新的机理见解。这些新的发现将对理解DNA损伤的新机制、指导修复DNA损伤有着重要意义。 4. Colussi等人发现在水/乙腈溶液表面，由α-草烯和臭氧产生的克里吉（Criegee）中间体不仅可以与水反应，还可以与脂肪酸（CnH2n+1COOH, n = 1-7）反应。我们采用QM/MM模型结合CASPT2和DFT静态电子结构计算方法，探究了空气-水/乙腈界面上α-草烯的臭氧化和后续与酸或水的克里吉反应机理。计算发现，α-草烯与臭氧的1,3-环加成反应以协同的机理发生，且在QM(CASPT2)/MM计算水平能垒小于2.5 kcal/mol。所产生的臭氧化产物的五元环断裂是决速步，能垒大于10.0 kcal/mol；这些开环过程也是协同发生的，并可以产生6种克里吉中间体。此外，这些克里吉中间体在空气-水/乙腈界面与水和酸发生加成反应中，与酸的加成过程能垒更低（在从乙酸的2.7 kcal/mol到辛酸的5.6 kcal/mol的能量范围内）。本工作还探究了不同个数的水分子与克里吉中间体的加成反应，发现随水分子个数的增加，加成的能垒逐渐降低。因此，除了水气界面较低的水浓度外，克里吉中间体与酸和水的反应活性的差异也是决定克里吉中间体命运的另一重要因素。总之，该工作的研究结果为空气-水/乙腈界面发生的的臭氧化和克里吉反应提供了新的机理见解，将为海洋边界层新颗粒以及二次有机气溶胶的形成机制提供了重要的理论参考。 5.实验者在Science杂志上报道了一个有趣的太阳光诱发的光解现象，发现气相光化学相对稳定的壬酸（C9脂肪酸）在水气界面竟然可以发生光解反应。本工作应用密度泛函方法，结合团簇模型，探究了壬酸在水气界面光解生成饱和及不饱和C9/C8醛的反应机理。计算发现，C9醛的生成并不是由两个壬酸分子间的Norrish II型反应引发，而是由壬酸分子在T1态分子内的C?O键断裂产生羟基和酰基引发；产生的酰基自由基被另一壬酸分子氢化而获得饱和的C9醛。随着两步连续的脱水过程，饱和的C9醛转化为不饱和的C9醛。另一方面，C8醛的产生由壬酸分子在T1态上分子内的C?C键断裂产生辛基和羰基引发，辛基与羟基结合得到辛醇。随着两步连续的脱水和一步壬酸分子协助的氢迁移过程，获得了饱和的C8醛。最后，饱和的C8醛经过两步脱水过程转化为不饱和的C8醛。在这些反应中，水分子和壬酸分子通过降低反应的活化能垒、促进反应的进行，而扮演着重要作用。本工作中，我们也探究了壬酸和氧气分子间的氧化反应以及自由基-自由基的二聚反应。

In recent years, with the development of national economy, energy, environment and health have aroused people's wide concern. Directly using inexhaustible solar energy or converting it into green hydrogen energy has become one of the main ways for people to develop and utilize solar energy and to solve energy and environmental issues. Visible light induced reactions are relatively clean and efficient, hence extensive research interests have been attracted in fields of chemistry and materials, especially for the photocatalytic reactions based on cheap and readily available metallic compounds. In addition, in recent years, atmospheric scientists have paid more and more attentions into the photo-/thermal-chemical reactions occurred at the water-air interface. Because the products of such chemical reactions can affect the formation of new particles in local region and may be precursors for the formation of secondary organic aerosols. In terms of health, ultraviolet radiation, ionizing radiation and so on could damage the DNA in the organism. While DNA damage is harmful to cells, which is likely to cause cell death, gene mutation even cancer. The elucidation of DNA damage mechanism provides the important basis for treating cancer and repairing DNA damage, however, scientists have not yet understood its chemical mechanism. However, the reactions in photocatalysis, photobiology or at the water-air interface are relatively complex. It is difficult to understand the detailed mechanism merely by means of experimental methods, therefore theoretical and computational chemistry are required. Focusing on the typical chemical reactions on homogeneous metal organic photocatalysis, DNA tandem damage in organisms and those occured at water-gas interface, we adopt high-level electronic structure meathods and QM/MM theory to explore the detailed mechanism, in order to provide valuable theoretical guidance and mechanistic insights for experiments. The mainly researched systems are as follows: (1) A recent experimental study reported a visible-light-mediated aerobic oxidative coupling reaction of phenol with alkynes that produces hydroxyl-functionalized aryl ketones using inexpensive CuCl as catalyst under mild conditions. Here we apply the complete active space self-consistent field (CASSCF) method and multistate second-order perturbation (MS-CASPT2) theory in combination with density functional theory (DFT) to explore the entire photocatalytic reaction between phenol and phenylacetylene in acetonitrile solution in the presence of molecular oxygen and CuCl. Our main findings are as follows: (a) The visible-light-driven conversion of phenyl-acetylene to PhCCCu(I) occurs thermally because of efficient excited-state deactivation to the S0 state. (b) The single electron transfer from PhCCCu(I) to molecular oxygen that leads to the PhCCCu(II) cation takes place in the T1 state after an efficient S1 → T1 intersystem crossing. (c) During the initial oxidation of phenol, molecular oxygen prefers to attack the para position of the phenol radical intermediate to produce 1,4-benzoquinone, which further reacts with PhCCCu(II) to generate para-hydroxyl-substituted aryl ketones; this is the origin of the experimentally observed regioselectivity. (d) The C-C bond of the phenylacetylene moiety is not activated by the triplet-state single electron transfer from PhCCCu(I) to molecular oxygen but is cleaved at a later stage, in the [2+2] cycloaddition between PhCCCu(II) and 1,4-benzoquinone. (e) The substrate phenol plays an active role in several hydrogen transfer and decarboxylation reactions; the barriers of these phenol-assisted reactions are lower than those of the corresponding direct or water-assisted reactions, which explains the experimental finding that adding water does not enhance the photocatalytic reaction yield. In summary, while supporting the general features of the experimentally proposed mechanism, our computational study provides detailed mechanistic insights that should be useful for understanding and further improving visible-light-induced copper-catalyzed coupling reactions. (2) A novel light-triggered hydrogen evolution reaction from formic acid mediated by an Ir(III) photocatalyst has been experimentally reported recently. However, its reaction mechanism remains elusive. Herein, we have employed the density functional theory (DFT) method to explore this photocatalytic reaction in detail. On the basis of the results, we have proposed a possible photocatalytic reaction mechanism. In the formation of the metal hydride [Cp*Ir(bpy)(H)]+(5), formic acid acts as a bridge assisting proton shuttling. Upon irradiation, two nonadiabatic excited-state decay pathways quickly populate the lowest triplet T1 state of the metal hydride from its initially populated excited singlet S1 state. In the T1 state, water and formic acid facilitate excited-state hydride/proton transfers from the Ir center to Cp* and bpy ligands producing several energetically lower triplet-state isomers demonstrating that the triplet-state metal hydride 5* could not be the only precursor for the photocatalysis. Adiabatic H2 evolution in the T1 state is energetically unfavorable. These T1 isomers hop, through radiationless T1 → S0 intersystem crossings via T1/S0 crossing points, to the S0 state in which H2 evolution takes place. In these reactions, solvents acting as assistants and catalysts reduce reaction barriers, thereby accelerating H2 release. Our current work provides significant mechanistic insights into light-induced hydrogen-evolution reactions of iridium-containing photocatalysts. (3) In 2017, Greenberg group reported a tandem lesion formation from a neutral purine radical, 2'-deoxyadenosin-N6-yl radical (dA·), which is an important type of DNA damage. Recently, they reported the characterization and mechanism of formation of this tandem lesion again, delivering a possible general role of dA· radical in DNA damage that was never recognized previously. But, the detailed mechanism at the atomic level remains unclear. Herein, we employed combined high-level NEVPT2 and DFT electronic structure methods using the QM/MM model to explore the mechanism of this tandem lesion formation from a neutral purine radical dA·. Based on the calculated results, we found that: (1) the initial hydrogen abstraction of CH3 in T12 by the -NH· group of 2?-deoxyadenosin-N6-yl radical (dA·) is feasible. Because the activation energy is only 12.2 kcal/mol at the QM(NEVPT2)/MM level and a large energy is released during the direct product formation (> -35 kcal/mol at the same calculation level). (2) with the add of oxygen molecule, the bases T12 and G13 are linked by “oxygen bridge” through two stepwise C-O bond couplings; herein the single electron is repopulated and delocalized upon G13 base. (3) SET process of generated intermediate 4 with oxygen molecule is allowed to occur thermodynamically. The presence of water molecule accelerates the H transfer coupled O-O bond cleavage process by lowering the corresponding activation energies, dramatically making cation mechanism more favorable than the radical mechanism. Lastly, (4) water molecule in the system serves as the proton acceptor for the last deprotonation, forming the hydrated proton that is stabilized through electrostatic interaction with sideward phosphate group. Our work for the first time presents a detailed theoretical calculation to rationalize the experimentally proposed mechanism of DNA tandem lesion at the atomic level and provides some new mechanistic insights. These findings will be helpful for understanding the new DNA damage pathway and for guiding the development of photosensitized DNA-directed therapies. (4) We employed combined CASPT2 and B3LYP electronic structure methods in the framework of the quantum mechanics/molecular mechanics (QM/MM) approach to explore the ozonolysis of α-humulene and subsequent Criegee reactions with acids and water at the air-water/acetonitrile interface as a surrogate for atmospheric aqueous organic media. First, we found that the 1,3-cycloaddition reactions of ozone on α-humulene proceed concertedly and have small barriers (less than 2.5 kcal/mol) at the QM(CASPT2)/MM level. Second, the five-membered ring cleavage reactions of the generated ozonides are rate-limiting steps and have considerable barriers (more than 10.0 kcal/mol). Third, although these Criegee intermediates can react with water and acids near the air-water/acetonitrile boundary, the addition reactions with acids have smaller barriers, which range from 2.7 kcal/mol of R1-COOH to 5.6 kcal/mol of R7-COOH. In contrast, in water addition reactions, several different water-mediated reaction pathways have been disclosed. Their reaction barriers are found to decrease remarkably with an increase in the number of water molecules involved in the reactions. Finally, we found that in addition to low water concentration near the air-water/acetonitrile boundary, distinct reactivities of Criegee intermediates with acids and water play very important roles in the determination of the fates of the Criegee intermediates. Our present QM/MM study provides new mechanistic insights into ozonolysis and Criegee reactions at air-water/acetonitrile interfaces and gives important implications for new particle formation and secondary organic aerosol formation near the marine boundary. (5) Photoinduced chemical reactions of organic compounds at the marine boundary layer have recently attracted signi?cant experimental attention because this kind of photoreactions has been proposed to have substantial impact on local new particle formation and their photoproducts could be a source of secondary organic aerosols. We have employed ?rst-principles density functional theory method combined with cluster models to explore photochemical reaction pathways of nonanoic acids (NAs) to form volatile saturated and unsaturated C9 and C8 aldehydes at air?water interfaces. Based on the results, we have found that the formation of C9 aldehydes is not initiated by intermolecular Norrish type II reaction between two NAs but by intramolecular T1 C-O bond ?ssion of NA generating acyl and hydroxyl radicals. Subsequently, saturated C9 aldehydes are formed through hydrogenation reaction of acyl radical by another intact NA. Following two dehydrogenation reactions, unsaturated C9 aldehydes are generated. In parallel, the pathway to C8 aldehydes is initiated by T1 C-C bond ?ssion of NA, which generates octyl and carboxyl radicals; then, an octanol is formed through recombination reaction of octyl with hydroxyl radical. In the following, two dehydrogenation reactions result into an enol intermediate from which saturated C8 aldehydes are produced via NA-assisted intermolecular hydrogen transfer. Finally, two dehydrogenation reactions generate unsaturated C8 aldehydes. In these reactions, water and NA molecules are found to play important roles in reducing barriers. Our work has also explored oxygenation reactions of NA with oxygen and radical-radical dimerization reactions.