Theoretical Study on the Structure and Stability of Si4X（X=Li, Be, B, C, N, O, F） Clusters

1 INTRODUCTION Silicon is an important kind of semiconductormaterial having been used to produce many sorts ofapparatus, digital and linear integrated circuit andLarge Scale Integrated circuit (LSI), and its clustershave drawn many scientists’ atten…     

对取代苯Ph一X（X＝F，OH，NH_2）的价键理论研究

采用键表酉群方法对Ｃ６Ｈ５Ｆ、Ｃ６Ｈ５ＯＨ和Ｃ６Ｈ５ＮＨ２中的电子离域现象进行了计算和分析，讨论了取代苯的价键描述特性，并计算了取代基的π电子离域能．结果表明离子结构成分与离域能有直接关系，即离子成分越多，电子离域能越大．／６－３１Ｇ基组及“分子中的原子”方法将电荷密度分区积分得到各原子上的电荷集居数，并将此结果与取代苯的反应性能进行了比较。为在价键意义上分析和理解取代基对苯环电子结构及其反应性能的影响，本文对３个典型的取代苯Ｐｈ－Ｘ（Ｘ＝Ｆ，ＯＨ，ＮＨ２）进行了初步的价键计算和讨论．１计算方法及构型在键表酉群方法中［５］，体系的一个共振结构可用一个价键结构函数即键表ψ（ｋ）来描述，相应的体系总波函数Ψ可表示为Ｍ个正则键表的线性组合：式（１）便构成了键表相互作用（ＢＴＩ）计算方法［６］的基础．键表对体系的结构贡献定义为：原子轨道ｑ上的电荷集居数定义为：式中ｍｑ（ｋ）可取０、１或２，分别对应于键表ψ（ｋ）中原子轨道ｑ出现０、１或２次．为简比计算，我们将取代苯的σ骨架用ＨＦ分子轨道固定［７］，这样仅需考虑π电子及轨道．原子轨道积分及ＨＦ－ＳＣＦ计算采用Ｇａｕｓｓｉａｎ８０程序．联系人及第一作者：莫亦荣，男，２９     

HCCO自由基与LiX(X=F,Cl,Br)锂键复合物的从头算研究

在MP2/6-311++G**水平上优化乙烯酮自由基与LiX(X=F,Cl,Br)形成锂键复合物.当卤素的电负性很强(如F元素),使得Li原子处于缺电子状态,此时,电子给体会把电子偏移向锂,形成共价性较强的锂键.而当卤素的电负性减弱时,锂键中主要成分逐渐变为离子键,并且此时锂键性质还要受电子给体影响.另外,由于HCCO为缺电子结构,电负性较弱且体积较大的卤素中的孤对电子会与HCCO之间通过静电相互作用,使得HCCO…Li—X键夹角变小,接近120°.锂键性质对HCCO…LiX(X=F,Cl,Br)复合物中Li—X的伸缩振动频率有直接影响.当锂键表现为共价性时,该频率红移,而当锂键表现为离子性时,该频率蓝移.但是,由于Cl的电负性与O的接近,C的电负性与Br接近所以,在O…Li…Cl和C…Li…Br中容易形成共振结构,导致远大于在其他复合物中的红移.     

A Valence Bond Study of the Low‐Lying States of the NF Molecule

The electronic structures of the three lowest‐lying states of NF are investigated by means of modern valence bond (VB) methods such as the VB self‐consistent field (VBSCF), breathing orbital VB (BOVB), and VB configuration interaction (VBCI) methods. The wave functions for the three states are expressed in terms of 9–12 VB structures, which can be further condensed into three or four classical Lewis structures, whose weights are quantitatively estimated. Despite the compactness of the wave functions, the BOVB and VBCI methods reproduce the spectroscopic properties and dipole moments of the three states well, in good agreement with previous computational studies and experimental values. By analogy to the isoelectronic O₂ molecule, the ground state ³Σ^? possesses both a σ bond and 3‐electron π bonds. However, here the polar σ bond contributes the most to the overall bonding. It is augmented by a fractional (19 %) contribution of three‐electron π bonding that arises from π charge transfer from fluorine to nitrogen. In the singlet ¹Δ and ¹Σ⁺ excited states the π‐bonding component is classically covalent, and it contributes 28 % and 37 % to the overall bonding picture for the two states, respectively. The resonance energies are calculated and reveal that π bonding contributes at least 24, 35 and 42 kcal mol^?1 to the total bonding energies of the ³Σ^?, ¹Δ and ¹Σ⁺ states, respectively. Some unusual properties of the NF molecule, like the equilibrium distance shortening and bonding energy increasing upon excitation, the counterintuitive values of the dipole moments and the reversal of the dipole moments as the bond is stretched, are interpreted in the light of the simple valence bond picture. The overall polarity of the molecule is very small in the ground state, and is opposite to the relative electronegativity of N vs F in the singlet excited states. The values of the dipole moments in the three states are quantitatively accounted for by the calculated weights of the VB structures.     

Photochemical Generation of Chiral N,B,X‐Heterocycles by Heteroaromatic C−X Bond Scission (X=S,O) and Boron Insertion

Chiral organoboron compounds with a chelate backbone and mesityl/heterocycle substituents (thienyl, furyl, and derivatives thereof) undergo a quantitative phototransformation that yields rare, chiral N,B,X‐containing heterocycles, such as base‐stabilized 1,2‐thiaborinines and 1,2‐oxaborinines. Boriranes were observed as intermediates in some of these transformations. The oxaborinines display further reactivity, generating 4a,12b‐dihydrobenzo[h][1,2]oxaborinino[4,3‐f]quinolines through a sequential conrotatory electrocyclization and a [1,5]‐H shift. The N,B,X‐containing heterocycles display strong blue‐green to orange‐red emission in the solid state. Combined DFT//CASP2T calculations suggest that a common biradical intermediate is responsible for the formation of these compounds as well as their interconversion.     

Theoretical Study on the Structure and Stability of Si5X （X = Li, Be, B, C, N, O, F, Na, Mg, Al, Si, P, S, Cl） Clusters

1 INTRODUCTION In the later 60s of last century, silicon substituted for germanium to present as mainstream in semicon- ductor. The semi-conductive devices made by silicon have many advantages, for example, refractory pro- perty, high radioresistance, simple and stable process- ing technic, high machinability and low cost. So it was widely used to manufacture large power appara- tuses, for instance, digit and linear integrated circuit, large scale integrated circuit (LSI), etc. Thus, th…     

The Influence of Bond Valence on Bond Covalency in RMn_2O_5 (R=La, Pr, Nd, Sin, Eu)

Itisnowwellestablishedthattheconceptofbondcovalencyisveryimportantfor explainingandclassifyingmanybasicpropertiesindiverseareasincludingchemisedand condensedmatterphysics.Reviewsconcerningbondcovalencyanditsapplicationhave beenmadebyPhillips"',Van...     

C59XH（X=N,B）与1,3-丁二烯Diels-Alder环加成反应的区域选择性

应用半经验的AMI和密度泛函B3LYP/6-31G*方法对1,3-丁二烯与C59XH(X=N,B)Diels-Alder环加成反应的区域选择性进行理论研究,选择一些有代表性的C59XH(X=N,B)的6-16键探讨环加成反应的机理.1,3-丁二烯与C59NH进行的Diels-Alder反应,随着加成位置远离C59NH的N原子,活化能越来越低,但都比1,3-丁二烯与C60相应反应的活化能高.与此相反,对于1,3-丁二烯与C59BH进行的环加成反应.加成位置最靠近B原子的2,12/r-和2,12/f-过渡态的势垒最低,并且比1,3-丁二烯与C60进行环加成反应的活化能约低18 kJ·mol-1,其产物也是热力学最稳定的.与C60相应的反应相比,C59NH和C59BH中N和B原子不同的电子性质对其邻位双键进行Diels-Alder环加成反应的活性产生了不同影响,前者使反应活性降低,后者使反应活性增强.     

A Valence Bond Model for Electron‐Rich Hypervalent Species: Application to SFn (n=1, 2, 4), PF5, and ClF3

Some typical hypervalent molecules, SF₄, PF₅, and ClF₃, as well as precursors SF (⁴Σ^? state) and SF₂ (³B₁ state), are studied by means of the breathing‐orbital valence bond (BOVB) method, chosen for its capability of combining compactness with accuracy of energetics. A unique feature of this study is that for the first time, the method used to gain insight into the bonding modes is the same as that used to calculate the bonding energies, so as to guarantee that the qualitative picture obtained captures the essential physics of the bonding system. The ⁴Σ^? state of SF is shown to be bonded by a three‐electron σ bond assisted by strong π back‐donation of dynamic nature. The linear ³B₁ state of SF₂, as well as the ground states of SF₄, PF₅ and ClF₃, are described in terms of four VB structures that all have significant weights in the range 0.17–0.31, with exceptionally large resonance energies arising from their mixing. It is concluded that the bonding mode of these hypervalent species and isoelectronic ones complies with Coulson’s version of the Rundle–Pimentel model, but assisted by charge‐shift bonding. The conditions for hypervalence to occur are stated.     

C59XH(X=N,B)自由基加成区域选择性的理论研究

用半经验的AMl方法,对C59XHCl2n(X=N,B;n=1～2)和C60H2Cl2n(n=1～2)的异构体进行几何构型全优化和振动频率计算,结合密度泛函B3LYP／6—31G^*单点能计算确定各异构体的相对稳定性．对比C59XH(X=N,B)和C60H2的H2加成方式,计算结果表明H2或Cl2加在碳笼官能化部分的邻近位置在能量上都是有利的：C59NH和C59BH自由基多加成物区域选择性的差别可归因于N原子和B原子电子性质的不同;立体效应是导致H2和Cl2加成方式不同的主要原因．     

Ab initio investigation of water clusters (H2O)n (n = 2–34)

Various properties of typical structures of water clusters in the n = 2–34 size regime with the change of cluster size have been systematically explored. Full optimizations are carried out for the structures presented in this article at the Hartree–Fock (HF) level using the 6‐31G(d) basis set by taking into account the positions of all atoms within the cluster. The influence of the HF level on the results has been reflected by the comparison between the binding energies of (H₂O)_n (n = 2–6, 8, 11, 13, 20) calculated at the HF level and those obtained from high‐level ab initio calculations at the second‐order Møller–Plesset (MP2) perturbation theory and the coupled cluster method including singles and doubles with perturbative triples (CCSD(T)) levels. HF is inaccurate when compared with MP2 and CCSD(T), but it is more practical and allows us to study larger systems. The computed properties characterizing water clusters (H₂O)_n (n = 2–34) include optimal structures, structural parameters, binding energies, hydrogen bonds, charge distributions, dipole moments, and so on. When the cluster size increases, trends of the above various properties have been presented to provide important reference for understanding and describing the nature of the hydrogen bond. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Identity SN2 reactions X- + CH3X --> XCH3 + X- (X=F, Cl, Br, and I) in vacuum and in aqueous solution: a valence bond study

The recently developed (L. Song, W. Wu, Q. Zhang, S. Shaik, J. Phys. Chem. A 2004, 108, 6017) valence bond method coupled with a polarized continuum model (VBPCM) has been applied to the identity SN2 reaction of halides in the gas phase and in aqueous solution. The barriers computed at the level of the breathing orbital VB method (P. C. Hiberty, J. P. Flament, E. Noizet, Chem. Phys. Lett. 1992, 189, 259), BOVB and VBPCM//BOVB, are comparable to CCSD(T) and CCSD(T)//PCM results and to experimentally derived barriers in solution (W. J. Albery, M. M. Kreevoy, Adv. Phys. Org. Chem. 1978, 16, 85). The reactivity parameters needed to apply the valence bond state correlation diagram (VBSCD) method (S. Shaik, J. Am. Chem. Soc. 1984, 106, 1227), were also determined by VB calculations. It has been shown that the reactivity parameters along with their semiempirical derivations provide a satisfactory qualitative and quantitative account of the barriers.     

Ab initio band structure calculations on polymers (C5H3X)n (X=CH,N,P,As)

The ab initio crystal orbital calculations on conjugated aromatic six-membered rings polymers,namely,poly(p-phenylene) (PPP),poly(2,5-pyridinediyl) (PPD),poly(2,5-phosphabenzene) (PPB) and ploy(2,5-arsabenzene) (PAB) are reported.The comparison of the important electronic properties of these polymers,such as band gap,bandwidth,ionization potential and electron affinity,indicates that PPP is the best intrinsic semiconductor,and PPD has the best prospects for forming n-doped conducting materials.     

X-H (X = C, N, O, Si, P, S) 键离解能计算的密度泛函方法研究

使用了不同密度泛函方法计算X-H (X = C, N, O, Si, P, S) 键离解能，并分析不同密度泛函方法的计算精度。研究发现大多数密度泛函方法包括B3LYP, B3P86, B3PW91, G96LYP, PBE1PBE，和BH&HLYP都明显低估键离解能13-25 kJ/mol。该现象与是否使用无限基组无关，因为即使使用无限基组键离解能仍然被低估。因此密度泛函方法不适合用于键离解能的估算。其中B3P86方法的偏差最小。进一步分析表明，使用限制性开壳层计算并无任何优势，在大多数情况下非限制性开壳层计算实际上比限制性开壳层计算要好。最后，我们发现了密度泛函方法对键离解能的低估是系统的，因此建议利用校准后的UDFT/6-311++G(d, p)方法计算化学键离解能。     

ChemInform Abstract: Ab initio Study of Structure and Stability of LAB3 Hydride and Halide Complex Molecules (L: Li,Na; A: Be,Mg, B+, Al+; and B: F,Cl, H).

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

Photochemical Generation of Chiral N,B,X‐Heterocycles by Heteroaromatic C−X Bond Scission (X=S,O) and Boron Insertion

Chiral organoboron compounds with a chelate backbone and mesityl/heterocycle substituents (thienyl, furyl, and derivatives thereof) undergo a quantitative phototransformation that yields rare, chiral N,B,X‐containing heterocycles, such as base‐stabilized 1,2‐thiaborinines and 1,2‐oxaborinines. Boriranes were observed as intermediates in some of these transformations. The oxaborinines display further reactivity, generating 4a,12b‐dihydrobenzo[h][1,2]oxaborinino[4,3‐f]quinolines through a sequential conrotatory electrocyclization and a [1,5]‐H shift. The N,B,X‐containing heterocycles display strong blue‐green to orange‐red emission in the solid state. Combined DFT//CASP2T calculations suggest that a common biradical intermediate is responsible for the formation of these compounds as well as their interconversion.     

Ab initio quantum-mechanical calculations of magnetic shielding constants of 13C, 15N and 19F in CF 3X(X = H,F,CN,CI,NO)

The magnetic shielding constants for ¹³C, ¹⁵N and ¹⁹F nuclei are calculated in the title compounds with several GIAO basis sets. Better overall agreement with experimental data is obstained with the 6-311G basis set.     

X…H2O（X=Li，Na，K）非线性光学性质的从头算理论

在MP2水平上采用6-311G基组计算了van der Waals复合物X…H2O(X=Li, Na, K)的非线性光学性质(μ, α, β), 讨论了基组效应和电子相关效应对计算结果的贡献. 在MP2/6-311++G(2df, 2pd)水平上计算得到的三个复合物分子X(X=Li, Na, K)•••H2O的非线性光学性质. 结果表明, 三种复合物分子均具有巨大的一阶超极化率, 其中最外层电子的弥散特性对一阶超极化率有很大的影响.