链烃,环状烃和多面体烃的定域分子轨道研究

采用GAUSSAN 80程序对一系列直链烷烃、环状烃和多面体烷烃进行STO-3G基组下的曲initio计算,在所得非定域分子轨道自洽场结果的基础上,利用Boys方法进行定域化,进而研究了这些分子的定域分子轨道的键弯曲性质、轨道能量和集居数与几何结构之间的关系,用上述结果对一些典型分子的稳定性进行了讨论。     

定域分子轨道中的σ—π分离

对含重键的分子体系,分子轨道定域化会涉及到两种完全不同的重键描述,即等价的重键或“香蕉”键和不等价的σ和π键,文献曾利用杂化轨道法对此进行过讨论。对于定域分子轨道,具体得出哪种描述取决于所采用的定域准则及计算中采用的近似方法。对于从头算法的定域化研究,Boys定域准则强烈地趋向于等价重键描述;Ruedenberg定域准则只是对未共轭的重键体系有较强的等价重键描述倾向,对共轭的重键体系,这种倾向性明显减     

定域分子轨道及其通过空间和通过键相互作用

在分子轨道理论中,定域分子轨道模型很受化学家的欢迎,这是因为定域分子轨道对应于化学家长期使用的化学键的概念,比较直观,容易掌握。五十年代初,Hall和 Lennard-Jones提出了等价键轨道模型,这个模型简单、,形象,得到了进一步的发展,主要用于整理归纳分子的单电子性质,例如电离势等。六十年     

定域分子轨道重心和分子结构的关系

本文讨论了定域分子轨道（ＬＭＯ）的重心和分子中化学键之间的关系，建立了利用定域分子轨道重心判断分子中成键情况及其化学键类型的方法，进而在ａｂ ｉｎｉｔｉｏ所得正则分子轨道（ＣＭＯ）定域化的基础上，计算了一些常见分子的ＬＭＯ重心；由此讨论了这些分子的化学键性质及其成键规律，探讨了一些化合物的反应性能。     

含双键有机小分子的定域分子轨道的研究

选择22种具有双键的有机小分子(含C、H、O和N原子)作模型分子,应用ab initioSTO-3G和Foster-Boys定域化分子轨道程序,对它们的定域分子轨道进行了研究,得到了定域分子轨道能量,确定了相应分子轨道间的相互作用。对烯类分子的光电子能谱进行了分析,结果较为满意。     

含羟基镱单酞菁配合物的定域化分子轨道研究

用定域化INDO法讨论了含羟基镱单酞菁配合物(YbPcOH)分子的几何构型和电子结构。YbPcOH分子为金字塔形结构,Yb突出在酞菁平面外的中心轴上(h=1.8),Yb-O键长约为2.5,O-H也沿中心轴取向。正则波函数的前线轨道呈现π键性质。定域分子轨道分析表明,酞菁骨架中16轮烯型的共轭体系在配位后共轭程度降低,形成多个较小的共轭结构。金属和配体之间的化学键主要为共价性质,Yb的5d轨道对共价键有主要贡献,4f轨道则不参与成键。     

线形碳元素簇合物的成键性质

在ab initio 3-21G水平上, 用能量梯度法优化了线性碳元素簇合物C_n~e(n为成簇原子个数, e为电荷)的平衡几何结构. 所得的电离势随成簇原子个数的改变, 呈现出不同程度的奇偶交替变化趋势. 在ab initio计算基础上, 用Boys方法, 对其占据正则分子轨道进行定域化变换, 得到了它们的定域分子轨道. 对定域分子轨道性质的分析表明, 线性碳元素簇合物中, 主要键型有双中心σ和π健, 双中心弯键和三中心香蕉健, 以及多中心σ和π健. 这种键型的多样化可视为小元素簇的成健特征. 此外, 通过对其成键性质的分析, 讨论了线性碳元素簇的稳定性. 对于小碳元素簇, 化学键的共轭性对其稳定性具有十分显著的作用.     

自然定域轨道方法研究

本文报导我们按自然定域轨道原理编制的NLMO程序以及对自然定域轨道计算方法的一些改进。用NLMO程序计算出七个具有不同成键特性分子(包括是否含有孤对或π键,是否含有多中心键等不同情况)的自然定域轨道,结果与用其它定域化方法得出的基本一致。由于自然定域轨道方法基本上具有内禀定域性质,而计算量明显小于其它定域化方法,可以认为它是有效和方便的。     

分子轨道对称守恒原理与等电子分子的反应

1965年 Woodward-Hoffman 提出了分子轨道对称守恒原理,经 Pearson、唐敖庆等人工作,又有了进一步发展。Pearson 在微扰论基础上,得出反应对称允许的条件是:诸断裂键的对称性必须等于诸生成键的对称性。这与能级相关图实质上是一致的,但简单多了。可是无机物的最高填充轨道与最低空轨道的能级很靠近,它们的能级顺序不像有机共轭分子那样容易确定,应用时受到了很大的限制。     

单占据片断轨道的组合式(Kost+Perkin)定域化

为了使开壳层片断轨道具有正确的集居数,正则轨道必须定域化.但是,在确保目标轨道Φ_sdan单占据性的同时,Kost定域化也破坏了Φ_s特定的对称性.实际计算表明,Perkin定域化可以弥补Kost程序的缺陷,将单占据的Φ_s转化成高度定域的、对称的片断轨道.在片断分子中,C-H_R键长r的选择和Kost定域化的方式对Perkin定域化的成败具有重大的影响.当Gaussian基组为STO-3G3-21G和4-31G时,r应为0.1nm;但在6-31G水平下,必须r=0.09nm.     

Symmetric halogen bonding is preferred in solution

Halogen bonding is a recently rediscovered secondary interaction that shows potential to become a complementary molecular tool to hydrogen bonding in rational drug design and in material sciences. Whereas hydrogen bond symmetry has been the subject of systematic studies for decades, the understanding of the analogous three-center halogen bonds is yet in its infancy. The isotopic perturbation of equilibrium (IPE) technique with (13)C NMR detection was applied to regioselectively deuterated pyridine complexes to investigate the symmetry of [N-I-N](+) and [N-Br-N](+) halogen bonding in solution. Preference for a symmetric arrangement was observed for both a freely adjustable and for a conformationally restricted [N-X-N](+) model system, as also confirmed by computation on the DFT level. A closely attached counterion is shown to be compatible with the preferred symmetric arrangement. The experimental observations and computational predictions reveal a high energetic gain upon formation of symmetric, three-center four-electron halogen bonding. Whereas hydrogen bonds are generally asymmetric in solution and symmetric in the crystalline state, the analogous bromine and iodine centered halogen bonds prefer symmetric arrangement in solution.     

Fuzzy symmetries of two classes of linear polyacene molecules

The fuzzy symmetries of two kinds of linear polyacene molecules are probed into in the paper. In these molecules, any one of the benzene rings abreast connects at most other two rings in two ways: either its two opposite C–C bonds combine with two other rings, respectively, or its two meta-position C–C bonds connect two rings in cis- and trans-form, respectively. The former is called p-polyacenes (or straight polyacenes), and the latter is m-polyacenes (or kinked ones). It can be thought as the planar molecule with approximate one-dimensional space periodic transformation (parallel translation) symmetry, namely, group G₁²{{\rm G}_{1}^{2}} symmetry, when the number of its benzene ring is very large; on the other hand, it can be considered as the fuzzy group G₁²{{\rm G}_{1}^{2}} symmetry, if the benzene ring number is not large enough. The p-polyacene and m-polyacene with 20 benzene rings are analyzed as typical examples, and the energies of the π-molecular orbital (MO) and the fuzzy symmetry characters related to the space symmetry transformations are carefully examined. Moreover, the π-MOs of the p-polyacenes and m-polyacenes with different numbers of benzene ring are investigated to obtain the related rules.     

Macrocyclic scaffold for the collagen triple helix

[structure: see text] Three strands of natural collagen are linked by covalent bonds prior to their folding into a triple helix. We report on a synthetic collagen in which the strands are pendent on a rigid macrocyclic scaffold of C(3) symmetry. The scaffold confers substantial conformational stability upon the collagen triple helix and makes its folding independent of concentration, both desirable attributes for exploring and exploiting synthetic collagens.     

Bond length alternation in infinitely long conjugated polyacenes

This paper concludes that whereas linear polyacenes are stable to distortions that preserve mirror symmetry with respect to the plane which is the perpendicular bisector of their interior bonds, they are unlikely to be stable with respect to distortions that do not preserve this symmetry.     

The Eu luminescence as a measure for chemical bond differences in solids

The well-known use of the emission of the Eu³⁺ ion as a probe for its site symmetry can be extended in suitable cases to give information on differences between chemical bonds in the neighbourhood of the Eu³⁺ ion.     

A new hydrogen-bonding motif for chiral recognition in the diastereomeric salts of racemic 1-phenylethylamine derivatives with enantiopure O-ethyl phenylphosphonothioic acid

An enantiopure phosphonothioic acid showed a unique and superior chiral recognition ability, arising from its P-stereogenicity, for racemic 1-phenylethylamine derivatives through diastereomeric crystallization. Spherical molecular clusters, associated by hydrogen bonds and CH/pi interactions, aggregated with high symmetry in the less-soluble diastereomeric salts.     

Author index to volume 60

Tantalum pentabromide has a trigonal bipyramidal structure (D_3h symmetry) with thermal average axial bonds longer than the equatorial ones by 0.061(10) A. The equatorial bonds are 2.412(4) A. The barrier to pseudorotation was estimated to be 1.3(0.6) kcal/mol from the electron-diffraction data.     

Theoretical Study of Structural Symmetry in Ternary Clusters

The geometrical symmetry presents an intriguing theoretical problem in many kinds of clusters. The diversity of geometrical structures is associated with cluster sizes, different model functions and potential parameters, and ternary clusters are investigated to study the relationship between geometrical symmetry and homotopic symmetry. Ternary Lennard-Jones model potential is studied with different parameters, and the putative global minimum structures of A₁₃B₃₀C₁₂ clusters are optimized using an adaptive immune optimization algorithm. The results show that there mainly exist five geometrical symmetry structures, i.e., Mackay icosahedral, fivefold partial Mackay icosahedral, sixfold pancake, partial double Mackay icosahedral, and amorphous structures. Furthermore, the number of bonds is used to distinguish the geometrical symmetry. The importance of geometrical symmetry and homotopic symmetry determined by potential parameters is discussed. It was found that in the optimization it is more important to generate geometrical symmetry than to optimize homotopic symmetry.     

Local symmetry and chirality of molecular faces

The concept of local symmetry has been applied to faces of planar sites such as carbon–carbon double bonds and aromatic rings with the principal results being as follows. The two faces of a planar site must have the same local symmetry group. This local symmetry group is limited to the polar point groups. For cyclic compounds, directed cycles must have chirotopic faces although the reverse is not necessarily true: chirotopic faces are possible for both directed and undirected cycles. A number of examples are provided to illustrate these results. Received: 30 June 1999 / Accepted: 4 October 1999 / Published online: 19 April 2000     

Stretching vibrations of CH bonds in spectra of dicarbaclosododecaboranes (p-, m-, and o-carboranes)

Parameters of bands corresponding to stretching vibrations of C-H bonds in spectra of o-, m-, and p-isomers of the icosahedral carborane B₁₀H₁₀C₂H₂ have been measured with high accuracy. The stretching vibrations of C-H bonds in spectra of o- and m-carborane (12), molecules which have C_2v point group symmetry, are demonstrated to appear the same as in the spectrum of the p-isomer with D_5d symmetry. The frequencies of Raman and IR lines practically coincide. This indicates the absence of interaction of the stretching vibrations of the two C-H bonds even in the o-carborane molecule.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2263–2267, October, 1989.