烷氧自由基RO·是大气中挥发性有机物（Volatile organic compounds, VOCs）氧化降解过程的重要中间体，它具有一个单电子，化学性质较为活泼。烷氧自由基的大气反应路径主要有单分子分解反应、氢迁移引起的异构化反应以及与O₂发生的双分子反应等，反应路径的不同决定了产物的多样性，因此研究烷氧自由基的性质对于建立健全的大气监测机制具有重要意义。早期关于烷氧自由基的研究主要集中在小分子烷氧自由基，随着研究的不断深入，人们逐渐认识到取代基的引入可以改变烷氧自由基的分子结构和性质，由此含各类取代基的烷氧自由基走进了人们的研究视野。近年来，人们陆续报道了羟基（-OH）、卤素（-X）、甲基（-CH₃）及甲氧基（-OCH₃）等基团取代烷氧自由基的性质。其中，在甲氧基取代链状烷氧自由基的能级结构中预测到了新出现的电荷转移激发态，体现出甲氧基给电子基团的性质，但是由于链状烷氧自由基碳链可发生自由旋转的特点，其分子的基态和电荷转移激发态都存在分子内氢键，难以直接对电荷转移的发生机制进行解释。环己烷独特的椅式结构使得不同位点的甲氧基取代均难以在基态结构中形成甲氧基与烷氧自由基之间的分子内氢键，可以帮助我们确定电荷转移的途径；同时环状结构提供的不同取代位点有助于我们研究甲氧基取代基效应和结构效应对烷氧自由基光谱和反应动力学性质的影响。本论文以2位、3位和4位-甲氧基取代的环己烷氧自由基为研究对象，采用激光诱导荧光光谱技术（Laser-induced fluorescence, LIF）和理论计算进行相关研究，具体内容如下：

(1)合成了前体物质2位、3位和4位-甲氧基取代的亚硝酸环己酯，通过结构表征和自然键轨道NBO电荷分布分析确定了不同位置甲氧基取代对亚硝酸酯结构及光谱性质的影响。我们利用不同位置甲氧基取代的环己醇、亚硝酸钠和浓硫酸，在冰浴的条件下获得了2位、3位和4位-甲氧基取代的亚硝酸环己酯。通过紫外-可见吸收光谱、红外吸收光谱和核磁共振氢谱对亚硝酸酯进行了表征，紫外-可见吸收光谱在250 nm左右及300-400 nm之间出现了亚硝酸酯N=O π-π*跃迁及n-π*跃迁造成的强吸收峰；红外吸收光谱在1640 cm^-1附近出现了N=O伸缩振动吸收峰，同时也观察到N-O对称伸缩振动和O-N=O弯曲振动在800 cm^-1和600 cm^-1附近的吸收峰；在甲氧基取代环己醇的核磁共振氢谱中，与羟基（-OH）相连碳原子αH的化学位移约在3.5 ppm，在合成产物酯的核磁共振氢谱中没有杂质峰出现且与酯基（-ONO）相连碳原子αH的化学位移约在5.3 ppm，与醇相比αH的化学位移显著增大。以上光谱现象均说明我们合成出了高纯的亚硝酸酯，能够满足后续光谱实验的需要。基于红外光谱N=O伸缩振动吸收峰、紫外光谱N=O n-π*跃迁吸收峰随着取代基靠近氧自由基逐渐蓝移的现象，结合NBO电荷分析确定了取代基位置的不同对分子自身性质（键长、跃迁能、电荷分布、键的极性等）的影响。我们还通过对比αH的NBO电荷指认了甲氧基取代亚硝酸环己酯核磁共振氢谱中的顺反异构体，为后续LIF光谱的标识奠定了基础。

(2)研究了甲氧基取代环己烷氧自由基的能级结构及电荷转移的发生机制。理论计算结果表明，甲氧基（-OCH₃）取代会引入一个电荷转移激发态，自然跃迁轨道NTO分析结果表明电荷是从甲氧基氧原子O的p轨道转移到氧自由基O∙的p轨道。在含时密度泛函理论（Time dependent density theory，TDDFT）计算中我们发现，2位、3位、4位-甲氧基取代环己烷氧自由基最低能量构象电荷转移激发态的振子强度分别为0.0386、0.0067和0.0004，随着取代基甲氧基与自由基C-O∙基团的远离逐渐减小，说明取代基位点的改变对电荷转移的发生产生了影响。我们尝试对不同构象的电荷转移激发态进行结构优化，通过TD-CAM-B3LYP/6-311++G(d,p)的方法获得了3-甲氧基环己烷氧自由基cis (a,a)构象的电荷转移激发态结构，结合分子结构变化及自然键轨道NBO电荷分布分析总结了电荷转移的发生机制为通过s键的电荷转移，而氢键可以起到辅助作用。

(3)获得了不同位置甲氧基取代环己烷氧自由基的LIF光谱并分析了甲氧基（-OCH₃）的取代基效应。我们在超声射流的条件下首次获得了2位、3位和4位-甲氧基取代环己烷氧自由基的激光诱导荧光光谱。2-甲氧基环己烷氧自由基没有观察到光谱信号，3位和4位甲氧基取代环己烷氧自由基在26140-29200 cm^-1范围内光谱信号丰富且存在着明显的C-O伸缩振动序列，经激发态能级跃迁和势能面交叉分析，确定观察到的光谱来源于烷氧自由基基态至C-O激发态的跃迁，即C-O∙基团s键上的一个电子被激发到自由基O∙的p轨道上。通过对比顺反异构体的光谱，我们将3-甲氧基环己烷氧自由基的光谱载体归属于cis (e,e)构象，4-甲氧基环己烷氧自由基的光谱载体指认为能量最低的trans (e,e)构象，同时结合CASSCF（9,7）计算方法的频率分析结果对LIF光谱进行了标识。通过对光谱的特征进行分析并与甲基取代环己烷氧自由基的光谱进行比较，总结了甲氧基的取代基效应：自由基C-O∙基团相隔两个C-C键以内的甲氧基取代将使光谱发生显著红移，LIF光谱的红移程度与取代基的给电子能力密切相关。

综上所述，本论文工作获得了甲氧基取代环己烷氧自由基的激光诱导荧光光谱，研究了甲氧基取代对环己烷氧自由基的LIF光谱、电子能态以及激发态分子结构的影响，提出了甲氧基给电子基团向烷氧自由基分子内电荷转移的发生机制并总结了甲氧基的取代基效应。研究结果为大气化学中甲氧基取代烷氧自由基的检测和动力学行为研究提供了理论和实验依据，也为探究非共轭结构的分子内电荷转移提供了基础。

Alkoxy radicals (RO∙) are important intermediates in the process of oxidation degradation of volatile organic compounds (VOCs) in the atmosphere. It has a single electron and its chemical properties are relatively active. Atmospheric reaction paths of alkoxy radicals mainly inculde unimolecular dissociation, isomerization via hydrogen shift and bimolecular reaction with O₂. Different reaction paths of alkoxy radicals determine the diversity of products, so understanding the properties of alkoxy radicals is of great significance for establishing a sound atmospheric monitoring mechanism. Early research on alkoxy radicals mainly focused on small molecule alkoxy radicals. With the deepening of research, people gradually realized that introduction of substituents can change the molecular structure and properties of alkoxy radicals, so alkoxy radicals containing various substituents came into people's research field. In recent years, people have reported the properties of alkoxy radicals substituted by hydroxyl (-OH), halogen (-X), methyl (-CH₃) and methoxy (-OCH₃). Among them, a new charge-transfer excited state was predicted in the energy level structure of methoxy-substituted chain alkoxy radicals, which reflected the electron-donating property of methoxy. However, due to the free rotation of carbon chain of chain alkoxy radicals, there are intramolecular hydrogen bonds in both the ground state and the charge transfer excited state of this molecule, so it is difficult to directly explain the mechanism of charge transfer. Due to the unique chair structure of cyclohexane, it is difficult for methoxy substitution at different sites to form intramolecular hydrogen bonds between methoxy and alkoxy radicals in the ground state structure, which can help us determine the way of charge transfer. At the same time, different substitution sites provided by the cyclic structure are helpful for us to study the influence of methoxy substituent effect and structural effect on the spectra and reaction kinetics of alkoxy radicals. Therefore, in this paper, the 2-, 3- and 4-methoxy-substituted cyclohexoxy radicals are taken as the research objects, the related research is carried out by using laser-induced fluorescence (LIF) spectroscopy and theoretical calculation. The specific contents are as follows:

(1) The 2-, 3- and 4-methoxy-substituted cyclohexyl nitrites were synthesized and the effects of methoxy substitution at different sites on the structural and spectral properties of nitrites were determined by structural characterization and natural bond orbitals (NBO) charge distribution analysis. We have obtained 2-, 3- and 4-methoxy-substituted cyclohexyl nitrites with 2-, 3- and 4-methoxycyclohexanols, sodium nitrite and concentrated sulfuric acid under the condition of ice bath. Nitrites were characterized by UV-Vis absorption spectroscopy, infrared absorption spectroscopy and ¹H NMR. The UV-Vis absorption spectra showed strong absorption peaks caused by nitrite N=O π-π* transition and n-π* transition at about 250 nm and wavelength range from 300 to 400 nm. In the infrared absorption spectra, the absorption peaks of N=O stretching vibration appeared at around 1640 cm^-1, the absorption peaks of N-O symmetric stretching vibration and O-N=O bending vibration at around 800 cm^-1 and 600 cm^-1 were also observed. In the ¹H NMR spectra of methoxy-substituted cyclohexanols, the chemical shift of αH of carbon atom connected to hydroxyl (-OH) is about 3.5 ppm, there is no impurity peak in the ¹H NMR spectra of nitrites, and the chemical shift of αH of carbon atom connected to ONO group is about 5.3 ppm, which is significantly higher than that of methoxy-substituted cyclohexanols. All the above spectral phenomena indicates that we have synthesized high-purity nitrites, which can meet the needs of subsequent LIF spectroscopy experiments. Based on the phenomenon that the absorption peak of N=O stretching vibration in infrared spectra and the absorption peak of N=O n-π* transition in ultraviolet spectra gradually shift blue with the substituents close to oxygen radicals, combined with NBO charge analysis, the influence of different substituent sites on the molecular properties (bond length, transition energy, charge distribution, bond polarity, etc.) is determined. According to the NBO charge of αH, we also identified the cis and trans isomers in the ¹H NMR spectra of methoxy-substituted cyclohexyl nitrites, which laid the foundation for the subsequent identification of LIF spectra.

(2) The energy level structure of methoxy-substituted cyclohexoxy radicals and the mechanism of charge transfer were studied. The theoretical calculation results show that methoxy substitution will introduce a charge transfer excited state, and the analysis of natural transition orbitals (NTO) shows that the charge is transferred from the Op orbital of methoxy to the radical Op orbital. In time dependent density theory (TDDFT) calculation, we found that the oscillator strengths of the lowest energy conformations of the 2-, 3- and 4-methoxy-substituted cyclohexoxy radicals were 0.0386, 0.0067 and 0.0004 respectively, which increased significantly with the proximity of the substituent methoxy group and the radical C-O∙ group, this can fully indicated that the change of the substituent sites had an impact on the occurrence of charge transfer. By TD-CAM-B3LYP/6-311++G(d,p) method, the charge transfer excited state of cis (a,a) conformation of 3-methoxycyclohexoxy radical was obtained, combined with the analysis of molecular structure and natural bond orbitals (NBO) charge distribution, it is concluded that the occurrence of charge transfer is through σ bonds, and hydrogen bonds can play an auxiliary role.

(3) The LIF spectra of 2-, 3- and 4-methoxy-substituted cyclohexoxy radicals were obtained and the substituent effect of methoxy (-OCH₃) was analyzed. We obtained the laser-induced fluorescence spectra of 2-, 3- and 4-methoxy-substituted cyclohexoxy radicals in supersonic jet-cooled conditions for the first time. We didn’t observe spectral signals in the LIF spectrum of 2-methoxycyclohexoxy radical, but the 3- and 4- methoxycyclohexoxy radicals were rich in spectral signals and had obvious C-O stretching vibration sequences in the range of 26140-29200 cm^-1. The excited state energy level transition and the potential energy surface cross analysis confirm that the observed spectra come from the transition from the ground state of alkoxy radical to the excited state of C-O, that is, an electron on the σ bond of C-O∙ group is excited to the P orbital of radical O∙. By comparing the spectra of cis and trans isomers, we determined that the spectral carrier of 3-methoxycyclohexoxy radical was cis (e,e) conformer, and the spectral carrier of 4-methoxycyclohexoxy radical was determined to be trans (e,e) conformer with the lowest energy. At the same time, the frequency analysis results obtained by CASSCF (9,7) calculation method were used to identify the LIF spectra. By analyzing the characteristics of the spectra and comparing them with the spectra of methyl-substituted cyclohexoxy radicals, we summarized the substitution effect of methoxy: Methoxy substitution within two C-C bonds from the C-O∙ group will cause a significant red shift in the spectra, and the degree of red shift in LIF spectra is closely related to the electron donating ability of substituents.

To sum up, the laser-induced fluorescence spectra of methoxy-substituted cyclohexoxy radicals were obtained in this paper, and the effects of methoxy substitution on LIF spectra, electronic energy states and excited molecular structure of cyclohexoxy radicals were studied. The mechanism of intramolecular charge transfer from methoxy electron-donating groups to alkoxy radicals was proposed and the substituent effect of methoxy group was summarized. The results provide theoretical and experimental basis for detection and kinetic behavior of methoxy-substituted alkoxy radicals in atmospheric chemistry, this can also provide a basis for exploring intramolecular charge transfer of non-conjugated structures.