(AB)8(AB=BN，AlP，GaAs，InSb)团簇环状结构与性质的密度泛函理论研究

用杂化密度泛函B3LYP方法研究了(AB)₈(AB=BN，AlP，GaAs，InSb)团簇环形结构的平衡几何构型、电子结构、振动特性以及极化率。计算结果表明，(AB)₈团簇的双层环状结构中，每个A(B)原子都与3个B(A)原子成键，且Ⅴ族元素的原子比Ⅲ族元素的原子更接近团簇中心，(BN)₈、(AlP)₈、(GaAs)₈、(InSb)₈的平均极化率依次增大，IR和Raman谱峰发生红移。另外，讨论了热力学稳定性和动力学稳定性的变化。     

Charge donation to and dearomatization of benzene attending complexation: DFT estimates of binding energies of TpMXO(L) with benzene, for Tp = hydridotris(pyrazolyl) borate, MXO = MoNO, ReCO, and WNO, and L = ammonia, N-methylimidazole, pyridine, phosphine, methyl isocyanide, and carbon monoxide

By density functional methods we characterize the bonding and charge distribution in complexes of benzene with dearomatizing agents tpReCO(L), tpMoNO(L), and tpWNO(L), where tp = hydrido Tris (pyrazolyl)borate), for a range of ligands L. Our LSDA and B3LYP density functional calculations use the Spartan LACVP+ basis and pseudopotential on Re, Mo, and W and 6-31G* on light atoms. The binding energy is strongly dependent on the nature of the ligand L, being greatest for L = ammonia and N-methylimidazole and weakest for CH3NC and CO. We find a correlation between strength of binding and electron transfer from the dearomatizing agents toward benzene. For the most strongly bound systems we find substantial (up to 500 millielectrons) charge transfer towards benzene, while for the most weakly bound systems charge is withdrawn from benzene. Structural details illustrate the ability of Re, Mo, and W species to dearomatize complexed benzene, which is extensive for all but the most weakly bound species with L = MeNC and CO. Re and W dearomatizing agents, which are computed and observed to form stable complexes with benzene, may be economic alternatives to osmium dearomatizing agents.     

The "invisible" 13C NMR chemical shift of the central carbon atom in [(Ph3PAu)6C]2+: a theoretical investigation

The experimental 13C NMR chemical shift of the central carbon atom in the octahedral [(Ph3PAu)6C]2+ cluster was investigated on the basis of relativistic density functional calculations. In order to arrive at independent model conclusions regarding the value of the chemical shift, a systematic study of the dependence of the cluster structure on the phosphine ligands, the chosen density functionals, and the basis set size was conducted. The best structures obtained were then used in the NMR calculations. Because of the cage-like cluster structure a pronounced deshielding of the central carbon nucleus could have been expected. However, upon comparison with the 13C NMR properties of the related complex [C{Au[P(C6H5)2(p-C6H4NMe2)]}6]2+, Schmidbaur et al. have assigned a signal at delta=135.2 ppm to the interstitial carbon atom. Our calculations confirm this value in the region of the aromatic carbon atoms of the triphenylphosphine ligands. The close-lying signals of the 108 phenyl carbon atoms can explain the difficulties of assigning them experimentally.     

Tetra- and hexaatomic cyclic clusters of main-group elements (XY)2 and (XY)3: an ab initio and density functional study

The electronic and molecular structure of planar (cyclic and linear) tetra- and hexaatomic clusters (XY) _n (XY = CC, BN, BeO, LiF; n = 2, 3) was studied using the ab initio CCD(full)/6-311+G** method and density functional approach (B3LYP/6-311+G**). The stability of cyclic clusters C₆, B₃N₃, and Be₃O₃ with D_3h symmetry is mainly determined by the aromaticity of their -electron systems.     

Electrostatic calculation of the substituent effect: an efficient test on isolated molecules

The energy of a disubstituted molecule has often been approximated by simple electrostatic formulas that represent the substituents as poles or dipoles. Herein, we test this approach on a new model system that is more direct and more efficient than testing on acid-base properties. The energies of 27 1,4-derivatives of bicyclo[2.2.2]octane were calculated within the framework of the density functional theory at the B3LYP/6-311+G(d,p) level; interaction of the two substituents was evaluated in terms of isodesmic homodesmotic reactions. This interaction energy, checked previously on some experimental gas-phase acidities, was considered to be accurate and served as reference to test the electrostatic approximation. This approximation works well in the qualitative sense as far as the sign and the order of magnitude are concerned: beginning with the strongest interaction between two poles, a weaker interaction between pole and dipole, and the weakest between two dipoles. However, all the electrostatic calculations yield energies that are too small, particularly for weak interaction, and this fundamental defect is not remedied by some possible improvements. In particular, variation of the effective permittivity would require a physically impossible value less than unity. The explanation must lie in a more complex distribution of electron density than anticipated in the electrostatic model. It also follows that possible conclusions about the transmission of substituent effects "through space" have little validity.     

Oxygenolysis of flavonoid compounds: DFT description of the mechanism for quercetin.

Flavonoids are naturally occurring phenol derivatives present in substantial amounts in a large variety of plants, fruits and vegetables daily eaten by humans. Most of these compounds exhibit several interesting biological activities, such as antiradical and antioxidant actions. Indeed, by complexation with specific enzymes, flavonoids are notably liable to metabolize molecular dioxygen. On the basis of experimental results describing oxygenolysis of the flavonoid quercetin, activated by the enzyme quercetin 2,3-dioxygenase (2,3-QD),ur attention has focused on the role of metal center in the activation of the substrate quercetin. Thus, in the present study, by means of DFT calculations at the B3LYP/ 6-31(+)G* level on model molecular systems, we describe different mechanisms for dioxygen metabolization by quercetin. Stationary points are described, and energetic and structural analyses along the reaction paths are reported. Our calculations show that the copper cation must act as an oxidant towards the substrate and that the reaction proceeds through a 1,3-cycloaddition.     

3-(3′-吡啶基)-6-芳基-1，2，4-三唑并[3，4-b]-1，3，4-噻二唑衍生物基态和激发态性质

用密度泛函B3LYP方法对3-(3'-吡啶基)-6-芳基-1,2,4-三唑并[3,4-b]-1,3,4-噻二唑衍生物(芳基为苯基、3-吡啶基和苯乙烯基)进行基态几何构型全优化,计算分子的电离势I_p和电子亲和势E_A等相关能量,并用Zerner间略微分重叠(ZINDO)和含时密度泛函(TDDFT)方法计算吸收光谱,用单组态相互作用方法(CIS)优化三种化合物分子的S_1激发态结构,分析其能量与发射光谱的关系,计算溶剂中分子的吸收和发射光谱,并与实验结果对照.计算结果表明,从3-(3'-吡啶基)-6-苯基-1,2,4-三唑并[3,4-b]-1,3,4-噻二唑分子(化合物A)到3-(3'-吡啶基)-6-(3'-吡啶基)-1,2,4-三唑并[3,4-b]-1,3,4-噻二唑分子(化合物B)以及3-(3'-吡啶基)-6-对乙烯苯基-1,2,4-三唑并[3,4-b]- 1,3,4-噻二唑分子(化合物C)的电子亲和势依次增大,愈来愈容易接受电子,吸收光谱和发射光谱红移.     

Transition state geometry in radical abstraction reactions: comparison of interatomic distances in the intersecting parabolas and Morse curves models with quantum-chemical calculations

Interatomic distances in the transition state were estimated for the reactions of radical abstraction: H^· + H₂, H^· + HCl, H^· + CH₄, N^·H₂ + NH₃, HO^· + H₂O, HO₂ ^· + HOOH, and C^·H₃ + SiH₄. The calculation was performed by the quantum-chemical density functional method or coupled clusters method (QCH), as well as by the methods of intersecting parabolas (IPM) and Morse curves (IMM), using experimental data (activation energies and reaction enthalpies). The results of the latter two methods are close to the quantum-chemical calculation and differ only by the increment a: r(IPM or IMM) = a + r(QCH), where a = –4.5·10^–12 m for IPM and a = +1.9·10^–12 m for IMM.