羰基硫(COS)在麦田土壤中的地气交换

利用动态箱法研究了COS在麦田土壤中的地气交换,结果表明麦田土壤为COS的重要汇.COS在麦田土壤中的沉降速率随COS的浓度增加而增大,与温度密切相关,在15~20℃范围内和实际大气情况下,COS在麦田土壤中的沉降速率达到最大值约为0.6×10^-12mol·g^-1·h^-1.实验测量COS在麦田土壤表面的沉降速率与温度的关系与Guenther简化算式计算结果十分吻合.     

[Li…X]e-[1](X=FH,OH2, NH3)的光电性质从头算

在MP2理论水平上采用6-311G基组系列计算了一价阴离子van der Waals复合物[Li…X]e-[1](X=FH, OH2, NH3)的偶极矩(μ)、平均极化率(α)以及平均一阶超极化率(β), 讨论了基组效应和电子相关效应对计算结果的影响, 比较了价电子对复合物一阶超极化率的贡献. 在MP4(SDQ)/6-311++G(2df, 2pd)水平上计算得到[Li…FH]e-[1]的μ=2.5633 a.u., α=1.0476×10³ a.u., β=1.0948×10⁵ a.u.；[Li…OH2]e-[1] 的μ=2.3204 a.u., α=1.2201×10³ a.u., β=2.1410×10⁵ a.u.；[Li…NH3]e-[1]的μ=2.4687 a.u., α=1.4817×10³ a.u., β=3.4040×10⁵ a.u.. 计算结果表明, 三种一价阴离子复合物分子均具有非常大的一阶超极化率, 而一个价电子对复合物的一阶超极化率的贡献超过1.0×10⁵ a.u..     

冷却速率对液态金属Zn快速凝固过程中微观结构的影响

用分子动力学模拟方法研究了六种不同冷却速率对液态金属Zn凝固过程微观结构的影响. 采用双体分布函数g(r)曲线、平均原子总能量、Honeycutt-Andersen(HA)键型指数法和原子团类型指数法(CTIM-2)对凝固过程中微观结构的变化进行了分析. 结果表明, 冷却速率对微观结构的转变有决定性影响, 当冷却速率为1×10¹⁴、5×10¹³、2×10¹³、1×10¹³、5×10¹² K·s-1时, 系统形成以1551、1541、1431键型为主体的非晶态结构; 当冷却速率为1×10¹² K·s-1时, 系统形成以1421、1422键型为主或以密排六方(hcp)基本原子团(12 0 0 0 6 6)和面心立方(fcc)基本原子团(12 0 0 0 1 2 0)共存的部分晶态结构. 同时发现, 在形成非晶的五个系统中,玻璃化转变温度Tg随着冷速的降低而降低.     

DNA在Mg/聚2,6-吡啶二甲酸膜上的固定和杂交及其PAT基因片段的电化学阻抗谱测定

以静电吸附法使Mg²⁺修饰于玻碳电极(GCE)上电聚合的2,6-吡啶二甲酸膜(PDC)上, 制得的Mg/PDC/GCE电极, 成为DNA固定及杂交的良好平台. 应用微分脉冲伏安法和电化学阻抗谱对DNA的固定和杂交进行表征. 以电化学阻抗谱免标记法检测目标DNA比以亚甲基蓝为指示剂的微分脉冲伏安法有更高的灵敏度. 固定于电极表面的DNA探针与互补单链DNA杂交后使电负性的[Fe(CN)₆]^3-/4-的表面电子传递电阻值显著增大, 以此作为检测信号可以高灵敏度地测定目标DNA. 电化学阻抗谱检测转基因植物外源PAT基因片段, 线性范围为1.0×10^-9 ~ 1.0×10^-5 mol/L, 检测限为3.4×10^-10 mol/L.     

VP-16抗癌机理的脉冲辐解

运用脉冲辐解技术, 对用于肿瘤治疗的依托泊甙(VP-16)的抗癌机理进行探讨. 首次对过硫酸根氧化VP-16的过程进行了研究, 测得其绝对反应速率常数为4.04×10⁹ dm³·mol^-1·s^-1, 同时观测到VP-16与酪氨酸之间的电子转移, 两者绝对反应速率常数为1.1×10⁸ dm³·mol^-1·s^-1. 为医学工作者进一步了解、探讨VP-16的抗肿瘤机理提供科学的参考.     

脉冲辐解研究类胡萝卜素与CCl3OO 的反应

通过脉冲辐解研究CCl₃OO×与胭脂树橙(BIX), β-胡萝卜素(b-C)及番茄红素(LYC)在水/醇均相混液中反应. 在与CCl₃OO×反应过程中, 类胡萝卜素本底吸收位置(BIX在500 nm左右, b-C在450 nm左右, LYC在500 nm左右)有明显的漂白, BIX与b-C在650 nm左右有反应产物生成过程的吸收峰出现. BIX和b-C与CCl₃OO×反应的二级反应速率常数分别为1.78 ´ 10⁸与7.8 ´ 10⁷ mol^-1·dm³·s^-1 , LYC与CCl₃OO ×反应只观察到漂白过程, 表明LYC与CCl₃OO×反应产物在检测范围内没有特征吸收, 根据漂白过程求得二级反应速率常数为4 ´ 10⁷ mol^-1·dm³·s^-1.     

含三苯胺聚希夫碱的三阶光学非线性极化率

设计合成并表征了新颖的具有三阶非线性光学性能的含有富电子结构单元的共轭聚希夫碱(PolySchiff base, PSB), 用Z-扫描技术研究了目标产物的三阶非线性特性, 测定了目标产物吡咯烷酮(NMP)溶液的三阶非线性折射率及非线性吸收率, 在波长为532 nm的Nd-YAG激光作用下, 得到非线性折射率n₂ = -1.23×10^-10 esu; Reχ⁽³⁾ = -3.06×10^-12 esu. 非线性吸收系数b = 3.63 10^-10 m/W, Imχ⁽³⁾ = 1.15 10^-11 esu, χ⁽³⁾ = 1.19 10^-11 esu, 在已知的材料中显示出较大的值. 由于n₂值为负数, PSB是自散焦材料. 表明PSB具有较强的三阶非线性光学性能, 有较大的应用前景. 用量子化学计算的方法对其电子结构, 前线分子轨道, χ⁽³⁾进行了理论计算, 并讨论了之间的关系.     

铁原子与氮分子间的相互作用——单端位构型

用杂化密度泛函B3LYP方法在6-311+G(d)基组水平上研究了Fe 原子与N2分子相互作用的单端位构型的直线形和弯曲形两种结构的平衡几何结构、电子结构、轨道布局及红外光谱等性质. 计算结果表明, 由于强的σ-σ电子对互斥作用, 基组态4s²3d⁶的Fe原子不能与N2分子发生化学作用; 当Fe 原子呈现可与N2之间发生σ-π授予反馈作用的激发组态时, Fe 与N2分子之间可形成稳定的结构; 在得到的多个电子态中, 能量最低的是直线形的1³∑^-, 比Fe(a⁵D)和N2(¹∑⁺g )能量高21.6 kJ·mol-1, 同时存在几个能量相近的电子态, 如1³∏、1³Φ; 弯曲形都是不稳定态, 可能是连接直线形和单侧双配位构型的过渡态; 单端位构型产物相对于基态的反应物均是热力学不稳定的; 单端位构型中Fe对N2的活化作用很小, N—N 键长增加不超过7 pm.     

Co2+OH2/Co3+OH2 电子转移反应性的理论研究

提出了研究Co²⁺OH₂/Co³⁺OH₂反应体系电子转移反应性的接触距离依赖关系的分析方案和ab initio计算的应用方法,并讨论验证了此方案及其相应模型的可行性,分析了有关动力学量的接触距离依赖关系.详细的结果表明,用精确PES法得出的活化能与用非谐振子势得出的活化能吻合较好,它们明显优于谐势模型.对分布函数随接触距离从1.20~0.35nm改变而从10^-2变到10^-5.偶合矩阵元随接触距离的增加呈指数性降低.有效电子偶合要求接触距离＜0.75nm.在0.50~0.75 nm范围内,相应的电子发射系数值在1.0~10^-6之间.电子因子使得定域ET速率也指数性的随接触距离的增加而降低,而对分布函数对总电子转移速率的贡献与电子因子的贡献则相反.球平均ET速率随接触距离的变化呈抛物线变化,并在接触距离为0.5 nm时有最大值.此最大值与总观测ET速率非常接近.对于此偶合体系,气态时ET速率是10⁶L·mol^-1·s^-1.进一步来说,实验上难于确定此类水合体系尤其是未饱和中间组分的电子结构和PES,ab initio算法在讨论其ET反应性方面能起到一个有效的辅助作用.     

羟基-氧钼(Ⅴ)[5,10,15,20-四(4-磺酸基苯基)]卟啉配合物在溶液中的二聚平衡

在25±0.02℃、0.025mol·dm^-3KCl介质及磷酸盐缓冲条件下,用光度滴定法研究了羟基-氧钼(V)[5,10,15,20-四(4-磺酸基苯基)]卟啉配合物在水溶液中的二聚平衡,用线性最小平方法确定了平衡模型,求得二聚反应平衡常数pK_el=4.34±0.28,单体的摩尔吸收ε_m(447um)=1.27×10⁴ε_m(475um)=2.01×10⁴;二聚体的摩尔吸收ε_d(447nm)=3.17×10⁴,ε_d(475nm)=1.40×10⁴。     

10-30T恒定磁场下碱金属原子的边界轮廓

孤立原子是具有球对称性的.如果将孤立原子置于磁场中,原子的边界半径必然会发生变化.根据原子边界轮廓理论模型,研究和计算了在10~30 T磁场强度下碱金属原子的边界轮廓,可供有关参考.     

A fast algorithm to compute atomic charges based on the topology of the electron density

This work proposes a novel algorithm to compute atomic charges as defined by the theory of “atoms in molecules” (AIM). Using the divergence theorem it is possible to express the 3D volume integral over an atomic basin purely in terms of 2D surface integrals. Hence, it can be proven that an atomic charge is equal to the flux of the electric field of the whole molecule through the atom's complete boundary. This boundary consists of the interatomic surfaces and the so-called outeratomic surface, which is the open side of the atom. When fine-tuned the algorithm can generate atomic charges in the order of minutes without introducing any approximations. Moreover, the problem of the geometrical cusp occurring in atomic basins and that of multiple intersections is also eliminated. The computational overhead of computing the electric field (which is analytical) is compensated by the gain in computing time by eliminating one dimension of quadrature. The proposed algorithm opens an avenue to invalidate the oft-quoted drawback that AIM charges are computationally expensive. We explain the details of the implementation in MORPHY01 and illustrate the novel algorithm with a few examples. Received: 1 June 2000 / Accepted: 4 October 2000 / Published online: 23 January 2001     

Atomic absorption with ultracold atoms

Recent advances in laser-atom cooling techniques and diode-laser technology now allow one to conduct an idealised atomic absorption experiment comprising a sample of ultracold, quasi-stationary absorbing atoms and a source of near-monochromatic resonant light. Under such conditions, the atomic absorption coefficient at line centre is independent of the oscillator strength of the atomic resonance line. This offers the prospect of ‘oscillator-strength-free’ atomic absorption spectroscopy in which the absorption signal is equally large for both strong and weak (closed) transitions of the same wavelength and in which absolute atomic absorption could be performed without knowledge of the oscillator strength. Moreover, the resolution and sensitivity for a given atom density are greatly enhanced, typically by approximately three orders of magnitude (and even more for weak transitions), compared with conventional flame or graphite-furnace atomic absorption. We describe an atomic absorption experiment based on samples of ultracold, laser-cooled caesium atoms and a narrow-bandwidth diode laser source that approximates the idealised conditions for oscillator-strength-free atomic absorption. The absorption measurements are used to determine the number density and temperature (approx. 6 μK) of the sample of ultracold atoms. Some of the technical obstacles that would have to be overcome before samples of ultracold atoms and diode laser sources could be used in analytical atomic absorption spectroscopy are discussed.     

电场对(4, 0)Zigzag模型单壁碳纳米管的影响

The structural and electronic properties of a (4, 0) zigzag single-walled carbon nanotube (SWCNT) under parallel and transverse electric fields with strengths of 0-1.4×10~(-2) a.u. Were studied using the density functional theory (DFT) B3LYP/6-31G~* method. Results show that the properties of the SWCNT are dependent on the external electric field. The applied external electric field strongly affects the molecular dipole moments. The induced dipole moments increase linearly with increase in the electrical field intensities. This study shows that the application of parallel and transverse electric fields results in changes in the occupied and virtual molecular orbitals (Mos) but the energy gap between the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO) of this SWCNT is less sensitive to the electric field strength. The electronic spatial extent (ESE) and length of the SWCNT show small changes over the entire range of the applied electric field strengths. The natural bond orbital (NBO) electric charges on the atoms of the SWCNT show that increase in the external electric field strength increases the separation of the center of the positive and negative electric charges of the carbon nanotube.     

Electric field effects on the performance of a candidate multipole molecular switch: a quantum computational study

Structural and electronic responses of the organic molecule di(4-nitro-2-methylenamine phenyl) diazene a candidate molecular switch, as an active device in a nanoelectronic circuit, to the external electric fields with strengths 5 x 10(-4) - 1.8 x 10(-2) a.u. included explicitly in the Hamiltonian are studied using B3LYP/6-31G* method. This study shows that thermodynamic formation functions are not affected significantly by the applied field. Electronic spatial extent show a negligibly small change (<2%) over the studied range of the electric field strength. Calculated electric dipole moments show significant sensitivity to the external electric field, which result consequently in much stronger interactions with the electrodes (poles) of the mother nanoelectronic circuit at higher electric field strengths. Natural bond orbital atomic charges analysis shows different field effects on different atoms depending on their positions with respect to the direction of the field. The applied field increases HOMO, LUMO, and the Fermi level energies; however, decreases the HOMO-LUMO gap (HLG) values. Results of this study show that it is possible to control field-induced charge redistribution over the molecule by using push-pull effects of different substitution via their connection points to the extended pi-system.     

Spatial relaxation of electrons in nonisothermal plasmas

The spatial relaxation of electrons to homogeneous states under the action of space-independent electric fields is investigated in helium, krypton, and N₂ plasmas for various electric field strengths. These investigations are based on a new method recently developed for solving the one-dimensional inhomogeneous electron Boltzmann equation in weakly ionized, collision-dominated plasmas. Elastic as well as conservative inelastic collisions of electrons with gas atoms have been included in the kinetic treatment. The spatial relaxation is caused by an imposed direct disturbance in the velocity distribution of the electrons on a spatial boundary. A pronounced dependence of the relaxation structure and the resultant relaxation length on the atomic data of the electron collision processes in different gases has been found. Furthermore the relaxation process sensitively depends on the electric field strength in the region of medium field values.     

Molecular dynamics simulations of ice nucleation by electric fields

Molecular dynamics simulations are used to investigate heterogeneous ice nucleation in model systems where an electric field acts on water molecules within 10-20 ? of a surface. Two different water models (the six-site and TIP4P/Ice models) are considered, and in both cases, it is shown that a surface field can serve as a very effective ice nucleation catalyst in supercooled water. Ice with a ferroelectric cubic structure nucleates near the surface, and dipole disordered cubic ice grows outward from the surface layer. We examine the influences of temperature and two important field parameters, the field strength and distance from the surface over which it acts, on the ice nucleation process. For the six-site model, the highest temperature where we observe field-induced ice nucleation is 280 K, and for TIP4P/Ice 270 K (note that the estimated normal freezing points of the six-site and TIP4P/Ice models are ～289 and ～270 K, respectively). The minimum electric field strength required to nucleate ice depends a little on how far the field extends from the surface. If it extends 20 ?, then a field strength of 1.5 × 10(9) V/m is effective for both models. If the field extent is 10 ?, then stronger fields are required (2.5 × 10(9) V/m for TIP4P/Ice and 3.5 × 10(9) V/m for the six-site model). Our results demonstrate that fields of realistic strength, that act only over a narrow surface region, can effectively nucleate ice at temperatures not far below the freezing point. This further supports the possibility that local electric fields can be a significant factor influencing heterogeneous ice nucleation in physical situations. We would expect this to be especially relevant for ice nuclei with very rough surfaces where one would expect local fields of varying strength and direction.     

Effects of external electric fields on double proton transfer kinetics in the formic acid dimer

Molecules can be exposed to strong local electric fields of the order of 10(8)-10(10) V m(-1) in the biological milieu. The effects of such fields on the rate constant (k) of a model reaction, the double-proton transfer reaction in the formic acid dimer (FAD), are investigated. The barrier heights and shapes are calculated in the absence and presence of several static homogenous external fields ranging from 5.14 × 10(8) to 5.14 × 10(9) V m(-1) using density functional theory (DFT/B3LYP) and second order M?ller-Plesset perturbation theory (MP2) in conjunction with the 6-311++G(d,p) Pople basis set. Conventional transition state theory (CTST) followed by Wigner tunneling correction is then applied to estimate the rate constants at 25 °C. It is found that electric fields parallel to the long axis of the dimer (the line joining the two carbon atoms) lower the uncorrected barrier height, and hence increase the raw k. These fields also flatten the potential energy surface near the transition state region and, hence, decrease the multiplicative tunneling correction factor. The net result of these two opposing effects is that fields increase k(corrected) by a factor of ca. 3-4 (DFT-MP2, respectively) compared to the field-free k. Field strengths of ～3 × 10(9) V m(-1) are found to be sufficient to double the tunneling-corrected double proton transfer rate constant at 25 °C. Field strengths of similar orders of magnitudes are encountered in the scanning tunneling microscope (STM), in the microenvironment of a DNA base-pair, in an enzyme active site, and in intense laser radiation fields. It is shown that the net (tunneling corrected) effect of the field on k can be closely fitted to an exponential relationship of the form k = aexp(bE), where a and b are constants and E the electric field strength.     

Valence photoelectron spectroscopy of N2 and CO: recoil-induced rotational excitation, relative intensities, and atomic orbital composition of molecular orbitals

Recoil-induced rotational excitation accompanying photoionization has been measured for the X, A, and B states of N(2)(+) and CO(+) over a range of photon energies from 60 to 900 eV. The mean recoil excitation increases linearly with the kinetic energy of the photoelectron, with slopes ranging from 0.73×10(-5) to 1.40×10(-5). These slopes are generally (but not completely) in accord with a simple model that treats the electrons as if they were emitted from isolated atoms. This treatment takes into account the atom from which the electron is emitted, the molecular-frame angular distribution of the electron, and the dependence of the photoelectron cross section on photon energy, on atomic identity, and on the type of atomic orbital from which the electron is ejected. These measurements thus provide a tool for investigating the atomic orbital composition of the molecular orbitals. Additional insight into this composition is obtained from the relative intensities of the various photolines in the spectrum and their variation with photon energy. Although there are some discrepancies between the predictions of the model and the observations, many of these can be understood qualitatively from a comparison of atomic and molecular wavefunctions. A quantum-mechanical treatment of recoil-induced excitation predicts an oscillatory variation with photon energy of the excitation. However, the predicted oscillations are small compared with the uncertainties in the data, and, as a result, the currently available results cannot provide confirmation of the quantum-mechanical theory.     

Density matrix averaged atomic natural orbital (ANO) basis sets for correlated molecular wave functions

Summary Generally contracted basis sets for first row atoms have been constructed using the Atomic Natural Orbital (ANO) approach, with modifications for allowing symmetry breaking and state averaging. The ANOs are constructed by averaging over several atomic states, positive and negative ions, and atoms in an external electric field. The contracted basis sets give virtually identical results as the corresponding uncontracted sets for the atomic properties, which they have been designed to reproduce. The design objective has been to describe the ionization potential, the electron affinity, and the polarizability as accurately as possible. The result is a set of well-balanced basis sets for molecular calculations. The starting primitive sets are 8s4p3d for hydrogen, 9s4p3d for helium, and 14s9p4d3f for the heavier first row atoms.