DNA纤维的X射线衍射分析与双螺旋结构的发现

概述了DNA化学结构式确立的过程,介绍了x射线衍射分析的概念、原理和方法,阐述了物理学家威尔金斯和物理化学家富兰克林在用实验事实证明DNA双螺旋结构方面所作出的巨大贡献。     

有机分子超薄膜的结构对摩擦的影响

采用分子动力学方法, 模拟了由脂肪酸C_nH_2n+1COOH}和C₁₇H₃₁COOH (n=12,13,14,15,16,17)组成的混合单层Langmuir-Blodgett(LB)膜间的摩擦特性, 探究了膜结构的变化对超薄膜的摩擦的影响. 结果显示. 在滑动过程中, 随着n的增加, 膜内分子的运动受到邻近分子的约束逐渐增加, 膜结构的稳定性也逐渐增加, 其剪切压逐渐减小, n=17时的剪切压最小. 在两单层膜之间无氢键形成; 而混合膜内的分子之间形成的氢键是单层膜结构稳定的主要因素, 其中n=16时形成的氢键最稳定, 但全部由相同C₁₇H₃₁COOH分子组成的单层膜的滑动效果最好. 分子的弯曲形变能对剪切压影响非常小.     

The basicity of sulfonamides and carboxamides. Theoretical and experimental analysis and effect of fluorinated substituent

The basicity of a series of sulfonamides and carboxamides with respect to protonation and hydrogen‐bonded complex formation with phenol was investigated by calculations using the Becke three‐parameter hybrid functional combined with Lee–Yang–Parr correlation functional with the 6‐311G** and 6‐311++G** basis sets and by infrared spectroscopy. The effect of fluorinated substituent was studied for the two series. The proton affinity of nitrogen in sulfonamides is higher than oxygen, in contrast to carboxamides, which are protonated at oxygen. The phenyl group in benzenesulfonamide increases the basicity of both heteroatoms, but more strongly of the nitrogen, whereas in benzamide the effect on the two heteroatoms is about the same. The CF₃ group equally decreases the basicity of nitrogen and oxygen atoms in sulfonamides and carboxamides. The second fluorinated substituent decreases the basicity of oxygen in (CF₃CO)₂NH more strongly than of nitrogen. For sulfonamides, the same effect results in the reverse of the center of basicity from nitrogen in (MeSO₂)₂NH to oxygen in (CF₃SO₂)₂NH. All studied carboxamides give H‐complexes via the carbonyl oxygen, whereas for sulfonamides two types of H‐complexes, with the OH···N and OH···O=S, were found theoretically, the latter being more stable. The exception is bisimide (CF₃SO₂)₂NH, for which only the OH···O=S complex is stable. Experimentally, only the oxygen‐bound complexes are observed. Analysis of the natural charges revealed an ‘abnormal’ increase of the electron density on the NH group by electron‐acceptor substituents in CF₃SO₂NHR, which was explained using the natural bond orbital analysis by loosening of the S–N bond because of orbital interactions with the σ*_S?N orbital. Copyright © 2012 John Wiley & Sons, Ltd.     

Nuclear Overhauser Effect Spectroscopy (NOESY) and 3JHH Coupling Measurements in the Determination of the Conformation of the Sesquiterpene Antimalarial Arteether in Solution

The twisting of the A-, B-, and D- rings of β-arteether were determined from dihedral angles derived from ³J_HH coupling constants and a Karplus-type equation that takes into account heteroatom substitution. The angles obtained by NMR were also compared to the angles obtained by molecular modeling and to those obtained from the methyoxyl analog (artemether) using x-ray crystallography. The NMR results showed that the A-ring was a relatively undistorted chair conformation with the 14-methyl group in an equatorial position. The NMR data indicated that the B-ring was very distorted and the D-ring was twisted outward because of the steric interactions with the ethoxy group. Two-dimensional NOESY data was used to measure selected proton-proton distances for arteether in solution and these distances were also compared to those obtained by molecular modeling and from the x-ray data for artemether. The NOESY data also indicated that the conformation of the A-ring was fairly normal, the B-ring was twisted, and the ethoxy group was in an extended conformation (C₁₁-C₁₂-O-C₁₆=+170°, and C₁₂-O-C₁₆-C₁₇=+169°)     

Signature of the hydrogen-bonded environment of liquid water in X-ray emission spectra from first-principles calculations

Based on ab initio molecular dynamics simulations and density functional theory, we performed a systematic theoretical study to elucidate the correlation between the H-bonded environment and Xray emission spectra of liquid water. The spectra generated from excited water molecules embedded in an intact H-bonded environment yield broader spectral peaks and a larger spectral range than the spectra generated from water molecules in a broken H-bonded environment. Such differences are caused by the local electronic structures on the excited water molecules within the core-hole lifetime that evolve differently through the rearrangement of neighboring water molecules in different H-bonded environments.     

The x-ray LIII absorption discontinuity of ytterbium in the metal and in some of its compounds

The shape and fine structure of the x-ray L_III absorption discontinuity of ytterbium (Z=70) has been studied in the pure metal and in several of its compounds. The shape and the near edge structure of the discontinuity in the metal reflects the band structure. A molecular orbital interpretation for the fine structure near the absorption edge has been proposed for the spectrum of the sesqui-oxide. The average bond lengths for the systems studied have been determined from the measurements on the fine structure employing Lytle’s and Levy’s methods and are compared with the available crystallographic data in the case of the metal, oxide and fluoride.     

The evaluation of some integrals that arise in the perturbation theory of liquids

ABSTRACT

The venerable, but still very useful, perturbation theory of liquids of Barker and Henderson involves some integrals that must be obtained numerically. There is no avoiding this for potentials such as the Lennard–Jones potential. However, to obtain the pressure, Barker and Henderson also obtained the necessary density derivatives numerically. This is cumbersome and care must be used to avoid excessive error. In this paper, it is noted that the earlier and less successful perturbation theory of Frisch et al. employed an alternative method, all but forgotten, for the evaluation of similar integrals. Their method still requires numerical integration. However, it allows an investigator to obtain the integrals for the pressure by analytical differentiation. The resultant integrals must still be evaluated numerically, but the extra step and annoyance of numerical differentiation is avoided. It is noted here that this method of using density derivatives can be incorporated into the Barker–Henderson theory.     

Expansions for the eigenvalues of three-term recurrence relations. Two applications in molecular physics

Approximate expressions for the eigenvalue of a three-term recurrence relation with a general form describing various physical problems are proposed. Their range of availability is examined by comparison with exact values for two different problems: the bound and continuum states of monoelectronic diatomic ions and the Schr?dinger equation describing molecular alignment in intense laser fields. For each case, very good predictions have been obtained, which may be useful as initial values in iterative procedures for deriving exact solutions. Received: 30 January 1998 / Received in final form: 10 April 1998 / Accepted: 25 May 1998     

Determination of bond lengths in some compounds of rhenium from the x-ray LIII absorption discontinuity

The shapes and the fine structure of the x-ray L_III absorption discontinuity of rhenium have been studied in seven octahedral (K₂ReCl₆, K₂ReBr₆, (pyH)₂ReCl₆, (dipyH)₂ReCl₆, Re(dipy)Cl₄, K₃ReO₂(CN)₄ and ReO₃) and three tetrahedral (KReO₄, NH₄ReO₄ and NaReO₄) compounds. The bond lengths in these compounds have been determined by applying Levy’s theory to the data on the fine structure associated with the edge beyond the molecular orbital region.     

物质结构在夸克-轻子层次上的动力学规律和研究发展趋势

文章综述了粒子物理中标准模型理论的历史发展、面临挑战以及未来的发展趋势.目前阶段物质结构最小组成单元是夸克和轻子,量子色动力学是描述夸克-胶子之间强相互作用的基本理论,它具有渐近自由和夸克禁闭的特点.量子色动力学和电弱统一理论一起构成粒子物理中标准模型理论.标准模型理论成功同时也面临两大挑战：对称性破缺的本质和夸克禁闭难题,这意味着标准模型理论需要发展和突破.人们期望粒子物理学、天文学和宇宙学交叉发展联手解决物质结构和早期宇宙研究中面临的难题,最终揭示超出标准模型的新物理规律.