双环戊二烯-氯醌体系的CIDNP研究

利用Nd/YAG激光器的三倍频激光脉冲(355nm),对双环戊二烯的光敏化反应进行了CIDNP方面的研究,结合CIDNP理论模拟谱的计算,证实了Roth等人所推测的此光敏化反应自由基中间体结构为双重的烯丙基物种,通过计算,确定了自由基各质子的超精细偶合常数hfc的相对大小和绝对符号以及各碳原子上的未偶电子自旋密度,从而对这个特殊的自由基中间体结构有了更清晰的认识.     

CIDNP及其在光化学中的应用

化学诱导动态核极化(Chemically Induced Dynamic Nuclear Polarization,简称CIDNP)是一种在化学反应体系中观察到的NMR谱线强度反常的现象(常称之为极化谱,包括增强吸收和发射等)。1967年由Bargon和Fischer^[1] (西德)以及Ward和Lawler^[2] (美国)首先发现的。由于他们所研究的反应体系中涉及自由基,因此最初认为这种谱线强度反常现象是由于自由基中间体中的电子-核交叉弛豫引起的。这种机理通常称为:Overhauser型的动态核极化(Dynamic Nnclear Polarization简称DNP)。但很快就发现它不足以解释实验得到的结果,而CIDNP的命名却是这样因袭下来了。     

双自由基CH燃烧反应机理的研究

应用量子理论从头算和密度泛函理论(DFT)对双自由基CH(X2Π)与O2(X3∑g-)的反应机理进行了研究.在B3PW91/6-311G**水平上优化了反应通道上各驻点(反应物、中间体、过渡态和产物)的几何构型,并计算了零点能和过渡态的虚频率.并由B3PW91/6-311G**给出了各物种的总能量.计算表明,反应物中自由基CH与O2反应主要在二重态势能面上进行,CH中的C原子可以插在O2分子中两个氧原子中间形成中间体1(2HCO2),中间体1(2HCO2)可以经过不同的反应通道形成不同的产物P1(1CO2 2H)和P2(1CO 2OH),各反应通道的反应热的计算与实验值吻合较好.     

丙烯腈单体自由基反应及光谱分析的研究

采用密度泛函理论对以偶氮二异丁腈为引发剂形成的丙烯腈单体自由基(CH₃)₂(CN)C—CH₂—(CN)CH的终止方式反应机理进行研究。先用B3LYP/6-31G(d)基组对反应物、偶合中间体、过渡态、歧化产物的几何构型进行优化,并计算了各物种经过零点能校正后的总能量,红外频率和电子结构,同时对过渡态结构进行了验证。研究结果表明,反应物先通过自由基的偶合终止方式形成偶合中间体a,再由此经过氢迁移、裂解等机理得到歧化终止产物P[p₁(CH₃)₂(CN)CCHCHCN+p₂(CH₃)₂(CN)C—CH₂—CH₂CN]。双自由基发生偶合和歧化终止反应分别形成偶合中间体a和歧化终止产物P均为放热反应,且偶合产物有着更低的能量。反应过程的速控步骤a→TS→P在常温下的速率常数表示为k(298.15 K)=2.71×10-59。歧化终止反应随着反应温度的升高易于进行,歧化产物所占的比例也增加。通过对反应过程各物种红外谱图的分析,说明自由基在整个终止反应的化学变化,同时分析了HOMO-LUMO电子密度分布的信息,验证丙烯腈双自由基偶合终止方式和各物种结构的准确性。     

FC(O)O2的结构及其自由基与NO反应的微观机理研究

用密度泛函理论(DFT)和哈特里-福克(HF)从头计算方法和半经验势方法等研究了FC(O)O₂ 的结构和振动性质.在DFT中采用B3LYP方法,在6-311G(d)基组上对FC(O)O₂自由基与NO反应的微观过程进行了分析.首先给出了各反应物、中间体、过渡态和生成物的几何构型,然后计算了它们的能量和频率,通过频谱分析得到反应的中间体和过渡态信息,即FC(O)O₂与NO反应为多反应通道,势垒高度和反应速度给出主要通道是FC(O)O₂     

C2（a 3Πu）+NO 的反应机理研究

在G2 (CC ,MP2 )理论水平上研究了C2 (a3 Πu)自由基与NO分子的反应 .计算了反应体系最低二重态势能面上各驻点的构型参数、振动频率和能量 ,揭示了此反应存在两种反应机理 :由NO中的O原子进攻C2 自由基形成CCON中间体的CCON机理和NO中的N进攻C2 自由基形成CCNO中间体的CCNO机理 ,分析了对应与这两种反应机理的五个可能的反应通道 ,得出了由NO中的N原子进攻3 C2 自由基 ,生成中间体CCNO自由基 ,最终得到产物CN +CO的通道是最有利的通道 .     

安息香及其衍生物光反应的CIDNP研究

本文用光照的¹H化学诱导动态核极化技术,研究了安息香及其衍生物的光化反应。并通过不同浓度的自由基捕获剂(PBN)的加入,对所研究体系的CIDNP效应的影响,分析推断了安息香等的光解及其光解自由基的复合的反应机理。并讨论了双取代安息香醚的光反应机理随取代基的变化而有所不同。     

C2（a^3Пu）＋NO的反应机理研究

在G2(CC，MP2)理论水平上研究了C2(a^3Пu)自由基与NO分子的反应．计算了反应体系最低二重态势能面上各驻点的构型参数、振动频率和能量，揭示了此反应存在两种反应机理：由NO中的O原子进攻C2自由基形成CCON中间体的CCON机理和NO中的N进攻C2自由基形成CCNO中间体的CCNO机理，分析了对应与这两种反应机理的五个可能的反应通道，得出了由NO中的N原子进攻^3C2自由基，生成中间体CCNO自由基，最终得到产物CN+CO的通道是最有利的通道．     

激光引发自由基反应磁效应的光谱学研究

“动态自旋化学”(dynamic spin chemistry)作为一门新兴的交叉研究领域,其重要性已得到广泛的共识。涉及的研究内容包括： 化学反应的磁效应(MFE)、同位素效应(MIE)、化学诱导动态核极化(CIDNP)和化学诱导动态电子极化(CIDEP)。文章简要介绍了激光引发自由基反应的磁效应发展历史及其光谱学研究方法。分析并总结了自由基反应磁效应产生的原因、单-三转换理论及磁效应机理。同时,也为国内同行介绍了自由基反应磁效应研究新的发展动态。     

NH自由基与臭氧反应机理的理论研究

用量子化学从头计算方法,在HF／6－31＋＋G**水平研究了臭氧与NH三线态活性自由基反应的微观机理,优化得到反应物、过渡态、中间体和产物的几何构型。用MP2／6－31＋＋G**／／HF6－31＋＋G**方法计算能量,同时进行零点能校正。研究结果表明： NH三线态活性自由基与O3反应首先成稳定中间体NHO3,然后中裂解生成HNO和O2。     

The CIDNP effect in plastic crystals

The CIDNP effects in the photolysis reactions of some aldehydes and ketones (including linear, aromatic and cyclic) in plastic crystals of cyclohexane have been discovered and studied. In going from liquid to solid solutions, the change of polarization sign is observed for some substances investigated. The CIDNP effect for a simple model for a relative motion of atoms of radicals possessing spin density (translational diffusion in a restricted volume) has been calculated. The results observed are interpreted in the framework of the radical pair mechanism of CIDNP effect formation.     

1-乙酰基-2,3-吲哚二酮的光诱导氢转移反应的CIDNP研究

采用CIDNP方法对UV光照条件下的1-乙酰基-2, 3-吲哚二酮与几类氢给体的光诱导氢转移反应进行了研究.     

Coherent Polarization Transfer Effects Are Crucial for Interpreting Low-Field CIDNP Data

In this work we demonstrate that low-field chemically induced dynamic nuclear polarization (CIDNP) is strongly affected by re-distribution of polarization, which is formed in the course of spin evolution in transient radical pairs, in diamagnetic reaction products. This phenomenon is of importance when the spins of the reaction product are coupled strongly meaning that spin–spin interactions between them are comparable to the differences in their Zeeman interactions with the external magnetic field. In this case, polarization transfer relies on a coherent mechanism; as a consequence, spins can acquire significant polarization even when they have no hyperfine coupling to the electron spins in the radical pairs, i.e., cannot be polarized directly by CIDNP. This is demonstrated by taking CIDNP of n-butylamine as an example: in this case only the α-CH₂ protons are polarized directly, which is confirmed by high-field CIDNP, whereas the β-CH₂, γ-CH₂ and δ-CH₃ protons get polarized only indirectly due to the transfer of polarization from the α-CH₂ protons. These results show that low-field CIDNP data should be interpreted with care to discriminate between the effects of spin evolution in transient radical pairs and in diamagnetic reaction products.     

Time resolved CIDNP study of electron transfer reactions in proteins and model compounds

Intramolecular electron transfer (IET) from tyrosine to tryptophan cation radicals is investigated using time resolved chemically induced dynamic nuclear polarization (CIDNP) spectroscopy in combination with laser flash photolysis. In both the tryptophan-tyrosine dipeptide and the denatured state of hen lysozyme in aqueous solution, the transformation TrpH⁺ → TyrO by IET leads to an increase in the tyrosine radical concentration, growth in the tyrosine CIDNP signal, fast decay of the tryptophan CIDNP, and inversion of the phase of the CIDNP of the photosensitizing dye, 2,2′-dipyridyl. IET effects are not observed for mixtures of the amino acid or for the native state of lysozyme. The steady state CIDNP effects seen for denatured lysozyme thus depend not only on the accessibility of the amino acid residues on the surface of the protein but also on the reactivity of the radical intermediates.     

Chemically amplified 19F-1H nuclear Overhauser effects

Chemically induced dynamic nuclear polarisation (CIDNP) is explored as a source of nuclear hyperpolarisation in heteronuclear Overhauser effect experiments. A photochemical reaction proceeding through a radical pair intermediate is used to enhance (19)F nuclear magnetisation in 3-fluorotyrosine by more than an order of magnitude with a corresponding increase in the amplitudes of (19)F-(1)H cross-relaxation and cross-correlation effects. The reactions employed are cyclic and leave the sample chemically unchanged. The potential for enhancing the sensitivity of heteronuclear NOEs in (19)F-labelled proteins is discussed.     

Photo-CIDNP experiments with an optimized presaturation pulse train, gated continuous illumination, and a background-nulling pulse grid

Methods to record chemically induced dynamic nuclear polarization (CIDNP) spectra that are virtually free from background magnetization and avoid the sensitivity loss and subtraction artifacts of difference spectroscopy have been developed. Presaturation by a string of composite pi/2 pulses, each followed by a defocussing field gradient, is analyzed, and guidelines for the optimization of pulse phases and gradient strengths are derived. Subsequent gated illumination during a grid of pi pulses with a prescribed timing causes the background magnetization to vanish at those moments of a pulse sequence when CIDNP magnetization is to be sampled or transferred. By shifting the illumination intervals within such a grid, the sign of the polarizations can be inverted without influencing the development of the background magnetization, allowing a further strong suppression of residual background by a phase cycle. Experimental examples for the application of these methods to more complex CIDNP experiments (1D-CIDNP-COSY, 1D-CIDNP-TOCSY, CIDNP-induced heteronuclear Overhauser effects, water suppression in protein CIDNP) are given.     

The CIDNP kinetics in recombination of successive radical pairs

A theory of chemically induced dynamic nuclear polarization (CIDNP) formed in recombination of successive radical pairs (PRs) is developed. The theory is based on that of RP recombination with the spin Hamiltonian instantaneously changing in time. With kinematics approximation it is shown that general relations for CIDNP are fully expressed via the quadratures of Green functions, which characterize the molecular motion of reagents. Analytical formulae for the time dependence of CIDNP both of primary and secondary RPs have been derived in the strong magnetic field approximation (S-T₀ approximation); field dependences of stationary CIDNP effect for some model cases have been analyzed. For long-lived systems the sensitivity of secondary RP CIDNP to the singlet-triplet evolution of primary RP has been demonstrated. It is shown that sometimes the correct analysis of the effect calls for taking into account the reactivity anisotropy.     

安息香激光光解的CIDNP研究

采用Nd/YAG激光器的三倍频激光脉冲(355nm),研究了安息香的激光光解过程及其CIDNP效应,获得了不同动力学参数d₉下的CIDNP谱,由此对预测的反应机理作出了证实.通过自编程序,根据对两种光反应产物不同形成途径的CIDNP模拟谱的计算,对安息香光解过程CIDNP效应芳环上质子的极化情况作出了合理归属.