非经典碳正离子简介

非经典碳正离子作为一种特殊的反应中间体,具有重要的理论和实际研究的意义。本文以碳正离子理论的发展为主线,以历史上非经典碳正离子(特别是降冰片基正离子)之争为中心,简要综述了Winstein、Brown和Olah三人的观点,并结合最新进展进行评述。此外,还对除降冰片基正离子之外的非经典碳正离子进行了适当的讨论。     

环庚烷正离子重排反应的从头算研究

先采用HF方法,基组采用STO-3G,对环庚烷正离子的重排机理进行了初步粗略的从头算研究,较快地找到了反应过程中的部分过渡态。然后再采用MP2/3-21G方法精确计算了整个重排过程中的各个过渡态的几何构型、零点能,同时对反应路径也进行了计算,以作进一步的过渡态验证。得出的结论是:环庚烷正离子的重排是环的缩小过程,在生成甲基环己烷叔正碳离子的过程中,经历了2个过渡态;首先是C(1)C(7)的键长变长、C(1)C6的键长变短,β位H(20)逐渐远离与之相连的C(1),与C(7)形成化学键;然后是与C+相连的H(16)逐渐远离C+,与β位的C(1)形成化学键,产生稳定的甲基环己烷叔正碳离子椅式结构,甲基环己烷叔正碳离子还有可能进一步重排为一个含伯正碳离子的甲基环己烷结构,计算了每一步重排反应所需的活化能。     

溴与不同取代C=C双键反应的机理和立体选择性

溴与C=C双键的反应是有机化学中一类常见的基元反应。溴与烯烃的C=C双键发生亲电加成反应,脂肪烯烃经过三元环正离子中间体再亲核开环机理,形成立体专一的反式加成产物;而芳基烯烃形成的三元环中间体,由于芳基对碳正离子的稳定作用,其苯甲位C―Br键容易断裂,会得到顺式和反式加成产物的混合物。溴与酮和酰氯形成的烯醇或烯醇负离子发生亲电取代,反应中溴不会与其C=C双键形成三元环正离子中间体。类似地,溴与烯醇醚和烯胺的反应也不经过三元环正离子中间体,分别生成非立体专一的加成和取代产物。本文通过参与该步基元反应的分子轨道合理地解释了溴与这两类底物反应的机理和立体选择性的区别,并总结了溴与不同C=C双键反应机理的判断方法,便于教师讲授和学生理解。     

第30届中国化学奥林匹克(初赛)试题解析(二)

正第4题固体电解质以其在电池、传感器等装置中的广泛应用而备受关注。现有一种由正离子An+、Bm+和负离子X-组成的无机固体电解质,该物质50.7°C以上形成无序结构(高温相),50.7°C以下变为有序结构(低温相),二者结构示意见下图。图中,浅色球为负离子;高温相中的深色球为正离子或空位;低温相中的大深色球为An+离子,小深色球为Bm+离子。     

十氢萘在酸性催化剂上的反应路径及生焦研究

采用小型固定流化床装置（ACE Model C），研究了在反应温度460-540℃下，模型化合物十氢萘在Y分子筛催化剂上的裂化反应路径及生焦机理。结果表明，十氢萘裂化的初始阶段，H+进攻十氢萘上与叔碳原子相连的C-H键和C-C键形成非经典五配位叔正碳离子是其最主要的引发反应；十氢萘裂化产物主要是丙烯、丙烷、异丁烷、异戊烷、甲基环戊烷、甲苯、二甲基苯等；产物的收率在催化剂上由大到小为，非芳烃、单环芳烃、双环芳烃；十氢萘催化生焦的机理是碳正离子机理，随反应温度和分子筛酸量的升高，双分子氢转移以及脱氢缩合能力增强，焦炭产率和转化率也随之升高。     

生物抗氧化剂的研究——Ⅴ.维生素E、维生素C及其脂溶性衍生物在胶束中对亚油酸自动氧化的抗氧化作用

本文研究了维生素E、维生素C及其脂溶性衍生物L-抗坏血酸-6-辛酸酯、6-月桂酸酯和6-棕榈酸酯在负离子胶束SDS,非离子胶束Triton X-100和正离子胶束CTAB中对亚油酸自动氧化的抗氧化作用,讨论了抗氧化剂的亲脂性及胶束对抗氧化活性的影响。     

CuCl对Et3NHCl/AlCl3离子液体催化性能的影响

 用CuCl对Et3NHCl/AlCl3离子液体进行改性,并考察了其对异丁烷-丁烯烷基化反应的催化性能. 结果表明,烷基化油的收率达到178%,C8组分的含量达到85%,辛烷值(RON)达到94.8. 通过结构组成及电荷分布分析可知,在CuCl改性的Et3NHCl/AlCl3离子液体中形成了新的配位结构AlCl4CuCl-,它作为更好的碳正离子受体,在反应过程中可降低碳正离子的浓度,进而降低烯烃的聚合程度,抑制较长碳链烷烃的生成,改善烷基化油的组成.     

四氢叶酸辅酶模型1, 2, 3-三取代咪唑啉盐的质谱裂解行为

讨论了１,２,３－不对称取代咪唑啉盐各种离子的生成途径以及取代基对裂解方式的影响并分析了该类化合物的主要裂解方式。在所有中的均ｍ／ｚ＝１２７（１＾＋）的离子峰和有机正离子Ａ峰存在,表明化合物１和２均有机正离子和碘离子形成的盐。化合物１和２的质谱有相似的裂解方式;有机正离子发生失掉氮原子上的一个取代基而保留咪唑啉环产生Ｄ,Ｅ,Ｆ离子的裂解方式。更重要的是咪唑啉环有相同的裂解方式：Ｃ＾２－Ｎ＾３键和Ｃ     

自由基正离子盐促进的化学转化研究进展

自由基正离子盐早在一百多年前就被制备出来,近年来,随着自由基化学的复兴,自由基正离子盐促进的有机化学转化受到了广泛地关注.在早期的研究中,自由基正离子盐主要被用于单电子氧化反应,可以高效地实现环加成、碎裂、重排等反应.与此同时,近期的研究表明,自由基正离子盐还可以促进C—H键的需氧氧化,实现一系列C—H键的直接官能团化反应.本综述按照自由基正离子盐促进的反应类型,总结了这一领域的最新进展.     

新型苯二氮卓类策划药氯地西泮异构体质谱识别

采用液相色谱高分辨质谱（LC-Q-Orbitrap/MS）和气相色谱质谱（GC-MS）技术对新型苯二氮卓类策划药2’-氯地西泮和4’-氯地西泮的碎裂途径进行研究。在LC-Q-Orbitrap/MS中,由于七元杂环的结构相较于苯环稳定性更差,2’-氯地西泮和4’-氯地西泮主要是通过七元环裂解生成特征碎片离子m/z 154。在GC-MS中,由于2’-氯地西泮和4’-氯地西泮的苯环上电子云密度显著高于七元杂环,经电子轰击后,苯环更容易发生电离。因此,2’-氯地西泮和4’-氯地西泮经电子轰击失去一分子氯自由基,形成具有苯基正离子的特征碎片离子m/z 283。由于2’-氯地西泮失去一分子氯自由基形成2’-苯基正离子,能与N4位上孤对电子相互作用形成场效应,使得正离子不易进一步发生离域。因此,2’-氯地西泮生成的碎片离子m/z 283丰度高,而生成的碎片离子m/z 282丰度相对较低。4’-氯地西泮失去一分子氯自由基形成4’-苯基正离子,由于无法与N4位上孤对电子形成场效应,使得正离子能在苯环上离域,生成的碎片离子m/z283和m/z 282的丰度基本一致,并且相对2’-氯地西泮,生成碎片离子m/...     

Improved virtual orbital multireference Møller-Plesset study of the ground and excited electronic states of protonated acetylene, C2H3+

The ground state geometries and associated normal mode frequencies of the classical and nonclassical protonated acetylene ion, i.e., the vinyl cation C(2)H(3) (+), are computed using the complete active space self-consistent field and improved virtual orbital (IVO) complete active space configuration interaction methods. In addition, the minimum-energy reaction path for the classical to nonclassical interconversion is determined (as are excitation energies) using the IVO modification of multireference Moller-Plesset (MRMP) perturbation theory. The IVO-MRMP treatment predicts the nonclassical structure to be 4.8 kcalmol more stable than the classical one, which is consistent with other high level theoretical estimates. The proton affinity of acetylene from the IVO-MRMP treatment (154.8 kcalmol) also agrees well with experiment (153.3 kcalmol) and with earlier CASPT2 calculations (154.8 kcalmol). We further report geometries and vibrational frequencies of low lying excited states of C(2)H(3) (+), which have not been observed and/or studied before. Comparisons with previous highly correlated calculations further demonstrate the computational efficiency of the IVO-MRPT methods.     

Homogeneous nucleation: classical formulas as asymptotic limits of the Cahn-Hilliard approach

Classical expressions for the critical cluster work of formation approximate the nonclassical expressions based in the density functional theory of capillarity for the limit of low supersaturation degrees. However, the ratio between classical and nonclassical expressions for nucleation rates grows as the supersaturation degree decreases. Here, with the aim to obtain simple and more accurate expressions that approximate the modern nucleation rate formulas, an asymptotic expansion of the Cahn-Hilliard expression of the critical work of formation is developed within the limit of low supersaturation. In such asymptotic expansion, terms up to third order are retained. The ratios between the corrected classical expressions and the nonclassical ones are now decreasing for supersaturation degrees tending to zero. However, the corrected approximate formulas are as difficult to handle as the exact Cahn-Hilliard expressions. When only the two first low-order terms of the asymptotic expansion are retained, a simpler corrected classical expression is obtained but it can only approximate nonclassical expressions up to order unity. Finally, using a Becker-Doring model of nucleation, the kinetic prefactor of the critical nuclei rate of formation is modeled consistently with the Cahn-Hilliard approach to the critical work of formation.     

Denatured state effects and the origin of nonclassical phi values in protein folding

Analysis of the phi value is one of the most powerful tools to understand the transition state for protein folding. In principle, phi values are expected to fall in the range of 0 to 1. However, a noticeable number of phi values have been observed which are either less than 0 or greater than 1. The origin of such phi values, sometimes referred to as noncanonical or nonclassical phi values, has been controversial. Here we show that mutational effects upon denatured state energetics can lead to nonclassical phi values.     

Semiempirical molecular-orbital studies on the problem of nonclassical resonance in the homoallylic cation

The quantum-mechanical methods (EHT and CNDO /2) have been applied to the homoallylic cation. Both methods successfully predict that the ion need not be represented as a nonclassical structure.     

Nonclassical Aryl Radicals: Intermediates or Transition States for the Hydrogen Shift Reactions?

Electronic properties of aryl radicals obtained by removing single hydrogen atoms from the sterically congested regions of benzo[c]phenanthrene, biphenyl, triphenylene, phenanthrene, and perylene are studied at the UBLYP/6-311G level of theory. Two structures are considered by each radical, the classical one involving a C-H.C arrangement of atoms and the nonclassical one possessing a three-center C-H-C linkage. The five nonclassical radicals under study are found to be transition states for degenerate 1,4- and 1,5-hydrogen shift reactions that interconvert the classical species. However, the results of the present calculations indicate that the nonclassical structures with the C-H distances in the C-H-C linkages shorter than 1.34 ? should be energy minima representing potentially observable chemical systems. The predicted energy barrier to the 1,5-hydrogen shift in the 1-benzo[c]phenanthrenyl radical is only 9.3 kcal/mol (6.1 kcal/mol with the zero-point energies included), making the hydrogen migration in this system facile at relatively low temperatures. Rigorous analysis of the computed electronic wave functions provides a clear-cut picture of bonding in both the classical and nonclassical aryl radicals.     

Mesocrystals: inorganic superstructures made by highly parallel crystallization and controlled alignment

Controlled self-organization of nanoparticles can lead to new materials. The colloidal crystallization of non-spherical nanocrystals is a reaction channel in many crystallization reactions. With additives, self-organization can be stopped at an intermediary step-a mesocrystal-in which the primary units can still be identified. Mesocrystals were observed for various systems as kinetically metastable species or as intermediates in a crystallization reaction leading to single crystals with typical defects and inclusions. The control forces and mechanism of mesocrystal formation are largely unknown, but several mesocrystal properties are known. Mesocrystals are exiting examples of nonclassical crystallization, which does not proceed through ion-by-ion attachment, but by a modular nanobuilding-block route. This path makes crystallization more independent of ion products or molecular solubility, it occurs without pH or osmotic pressure changes, and opens new strategies for crystal morphogenesis.     

IR spectra of C2H5(+)-N2 isomers: evidence for dative chemical bonding in the isolated ethanediazonium ion

The potential energy surface (PES) of C(2)H(5)(+)-N(2) is characterized in detail by infrared photodissociation (IRPD) spectroscopy of mass-selected ions in a quadrupole tandem mass spectrometer and ab initio calculations at the MP2/6-311G(2df,2pd) level. The PES features three nonequivalent minima. Two local minima, 1-N(2)(H) and 1-N(2)(C), are adduct complexes with binding energies of D(0) = 18 and 12 kJ/mol, in which the N(2) ligand is weakly bonded by electrostatic forces to either the acidic proton or the electrophilic carbon atom of the nonclassical C(2)H(5)(+) ion (1), respectively. The global minimum 3 is the ethanediazonium ion, featuring a weak dative bond of D(0) = 38 kJ/mol. This interaction strength is sufficient to switch the C(2)H(5)(+) structure from nonclassical to classical. The 1-N(2)(C) isomer corresponds to the entrance channel complex for addition of N(2) to 1 yielding the product 3. This reaction involves a small barrier of 7 kJ/mol as a result of the rearrangement of the C(2)H(5)(+) ion. The partly rotationally resolved IRPD spectrum of C(2)H(5)(+)-N(2) recorded in the C-H stretch range is dominated by four bands assigned to 3 and one weak transition attributed to 1-N(2)(H). The abundance ratio of 1-N(2)(H) and 3 estimated from the IRPD spectrum as ～1% is consistent with the calculated free energy difference of 12 kJ/mol. As the ethanediazonium ion escaped previous mass spectrometric detection, the currently accepted value for the ethyl cation affinity of N(2) is revised from -ΔH(0) = 15.5 ± 1.5 to ～42 kJ/mol. The first experimental identification and characterization of 3 provides a sensitive probe of the electrophilic character and fluxionality of the ethyl cation. Comparison of 3 with related alkanediazonium ions reveals the drastic effect of the size of the alkyl chain on their chemical reactivity, which is relevant in the context of hydrocarbon plasma chemistry of planetary atmospheres and the interstellar medium, as well as alkylation reactions of (bio)organic molecules (e.g., carcinogenesis and mutagenesis of DNA material).     

Nonclassical fullerenes with a heptagon violating the pentagon adjacency penalty rule

Nonclassical fullerenes with heptagon(s) and their derivatives have attracted increasing attention, and the studies on them are performing to enrich the chemistry of carbon. Density functional theory calculations are performed on nonclassical fullerenes C_n (n = 46, 48, 50, and 52) to give insight into their structures and stability. The calculated results demonstrate that the classical isomers generally satisfy the pentagon adjacency penalty rule. However, the nonclassical isomers with a heptagon are more energetically favorable than the classical ones with the same number of pentagon–pentagon bonds (B₅₅ bonds), and many of them are even more stable than some classical isomers with fewer B₅₅ bonds. The nonclassical isomers with the lowest energy are higher in energy than the classical ones with the lowest energy, because they have more B₅₅ bonds. Generally, the HOMO–LUMO gaps of the former are larger than those of the latter. The sphericity and asphericity are unable to rationalize the unique stability of the nonclassical fullerenes with a heptagon. The pyramidization angles of the vertices shared by two pentagons and one heptagon are smaller than those of the vertices shared by two pentagons and one hexagon. It is concluded that the strain in the fused pentagons can be released by the adjacent heptagons partly, and consequently, it is a common phenomenon for nonclassical fullerenes to violate the pentagon adjacent penalty rule. These findings are heuristic and conducive to search energetically favorable isomers of C_n, especially as n is 62, 64, 66, and 68, respectively. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Ab initio/GIAO-CCSD(T) study of structures, energies, and 13C NMR chemical shifts of C4H7(+) and C5H9(+) ions: relative stability and dynamic aspects of the cyclopropylcarbinyl vs bicyclobutonium ions

The structures and energies of the carbocations C 4H 7 (+) and C 5H 9 (+) were calculated using the ab initio method. The (13)C NMR chemical shifts of the carbocations were calculated using the GIAO-CCSD(T) method. The pisigma-delocalized bisected cyclopropylcarbinyl cation, 1 and nonclassical bicyclobutonium ion, 2 were found to be the minima for C 4H 7 (+) at the MP2/cc-pVTZ level. At the MP4(SDTQ)/cc-pVTZ//MP2/cc-pVTZ + ZPE level the structure 2 is 0.4 kcal/mol more stable than the structure 1. The (13)C NMR chemical shifts of 1 and 2 were calculated by the GIAO-CCSD(T) method. Based on relative energies and (13)C NMR chemical shift calculations, an equilibrium involving the 1 and 2 in superacid solutions is most likely responsible for the experimentally observed (13)C NMR chemical shifts, with the latter as the predominant equilibrating species. The alpha-methylcyclopropylcarbinyl cation, 4, and nonclassical bicyclobutonium ion, 5, were found to be the minima for C 5H 9 (+) at the MP2/cc-pVTZ level. At the MP4(SDTQ)/cc-pVTZ//MP2/cc-pVTZ + ZPE level ion 5 is 5.9 kcal/mol more stable than the structure 4. The calculated (13)C NMR chemical shifts of 5 agree rather well with the experimental values of C 5H 9 (+).     

Luminescence of the nonclassical homoleptic metal carbonyl complex [Rh(CO4]+

The nonclassical [Rh(CO)(4)](+) cation is luminescent at low temperature with a peak maximum at 19640 cm(-1) and a full-width at half-maximum of 2900 cm(-1). Both the solution absorption and low-temperature solid-state luminescence spectra of the [Rh(CO)(4)](1-Et-CB(11)F(11)) salt are reported. The cesium salt of the [1-Et-CB(11)F(11)](-) counterion is also luminescent (with a peak maximum at 22650 cm(-1)), but the emission maximum of [Rh(CO)(4)](+) is lower in energy. The Rh-C bond length changes of approximately 0.07 A in the excited state are calculated. The calculated bond length changes in the nonclassical [Rh(CO)(4)](+) ion are not very different from those in classical complexes.