Reactions of oriented molecules

Beams of oriented molecules have been used to directly study geometrical requirements in chemical reactions. These studies have shown that reactivity is much greater in some orientations than others and demonstrated the existence of steric effects. For some reactions portions of the orientation results are in good accord with traditional views of steric hindrance, but for others it is clear that our chemical intuition needs recalibrating. Indeed, the information gained from simultaneously orienting the reactants and observing the scattering angle of the products may lead to new insights about the detailed mechanism of certain reactions. Further work must be done to extend the scope and detail of the studies described here. More detailed information is needed on the CH(3)I reaction and the CF(3)I reaction. The effects of alkyl groups of various sizes and alkali metals of various sizes are of interest. In addition, reactions where a long-lived complex is formed should be studied to see if orientation is important. Finally, it would be of interest to apply the technique to the sort of reactions that led to our interest in the first place: the S(N)2 displacements in alkyl halides where the fascinating Walden inversion occurs.     

The role of pisigma* excited states in the photodissociation of heteroaromatic molecules

High-resolution measurements of the kinetic energies of hydrogen atom fragments formed during ultraviolet photolysis of imidazole, pyrrole, and phenol in the gas phase confirm that N(O)-H bond fission is an important nonradiative decay process from their respective 1pisigma* excited states. The measurements also reveal that the respective cofragments (imidazolyl, pyrrolyl, and phenoxyl) are formed in very limited subsets of their available vibrational states. Identification of these product states yields uniquely detailed insights into the vibronic couplings involved in the photoinduced evolution from parent molecule to ultimate fragments.     

Correlating structural dynamics and function in single ribozyme molecules

We have studied the correlation between structural dynamics and function of the hairpin ribozyme. The enzyme-substrate complex exists in either docked (active) or undocked (inactive) conformations. Using single-molecule fluorescence methods, we found complex structural dynamics with four docked states of distinct stabilities and a strong memory effect where each molecule rarely switches between different docked states. We also found substrate cleavage to be rate-limited by a combination of conformational transitions and reversible chemistry equilibrium. The complex structural dynamics quantitatively explain the heterogeneous cleavage kinetics common to many catalytic RNAs. The intimate coupling of structural dynamics and function is likely a general phenomenon for RNA.     

Anatomy of the photodissociation region in the orion bar

Much of the interstellar gas resides in photodissociation regions whose chemistry and energy balance is controlled by the flux of far-ultraviolet radiation upon them. These photons can ionize and dissociate molecules and heat the gas through the photoelectric effect working on dust grains. These regions have been extensively modeled theoretically, but detailed observational studies are few. Mapping of the prominent Orion Bar photodissociation region at wavelengths corresponding to the carbon-hydrogen stretching mode of polycyclic aromatic hydrocarbons, the 1-0 S(1) line of molecular hydrogen, and the J = 1-0 rotational line of carbon monoxide allows the penetration of the far-ultraviolet radiation into the cloud to be traced. The results strongly support the theoretical models and show conclusively that the incident far-ultraviolet radiation field, not shocks as has sometimes been proposed, is responsible for the emission in the Orion Bar.     

Probing proton dynamics in molecules on an attosecond time scale

We demonstrate a technique that uses high-order harmonic generation in molecules to probe nuclear dynamics and structural rearrangement on a subfemtosecond time scale. The chirped nature of the electron wavepacket produced by laser ionization in a strong field gives rise to a similar chirp in the photons emitted upon electron-ion recombination. Use of this chirp in the emitted light allows information about nuclear dynamics to be gained with 100-attosecond temporal resolution, from excitation by an 8-femtosecond pulse, in a single laser shot. Measurements on molecular hydrogen and deuterium agreed well with calculations of ultrafast nuclear dynamics in the H2+ molecule, confirming the validity of the method. We then measured harmonic spectra from CH4 and CD4 to demonstrate a few-femtosecond time scale for the onset of proton rearrangement in methane upon ionization.     

Electron transfer-induced dynamics of oxygen molecules on the TiO2(110) surface

Diffusion of oxygen molecules on transition metal oxide surfaces plays a vital role for the understanding of catalysis and photocatalysis on these materials. By means of time-resolved scanning tunneling microscopy, we provide evidence for a charge transfer-induced diffusion mechanism for O2 molecules adsorbed on a rutile TiO2(110) surface. The O2 hopping rate depended on the number of surface donors (oxygen vacancies), which determines the density of conduction band electrons. These results may have implications for the understanding of oxidation processes on metal oxides in general.     

Experimental test of self-shielding in vacuum ultraviolet photodissociation of CO

Self-shielding of carbon monoxide (CO) within the nebular disk has been proposed as the source of isotopically anomalous oxygen in the solar reservoir and the source of meteoritic oxygen isotopic compositions. A series of CO photodissociation experiments at the Advanced Light Source show that vacuum ultraviolet (VUV) photodissociation of CO produces large wavelength-dependent isotopic fractionation. An anomalously enriched atomic oxygen reservoir can thus be generated through CO photodissociation without self-shielding. In the presence of optical self-shielding of VUV light, the fractionation associated with CO dissociation dominates over self-shielding. These results indicate the potential role of photochemistry in early solar system formation and may help in the understanding of oxygen isotopic variations in Genesis solar-wind samples.     

植物挥发物在昆虫定向行为中的作用

综述了植物挥发物在昆虫定向行为中的作用,包括它对植食性昆虫、捕食性昆虫和寄生性昆虫定向行为的影响。     

Directional specificity in the regeneration of lamprey spinal axons

After spinal transection in ammocoetes (lamprey larvae) 4 to 5 years old, functional recovery is accompanied by a limited regeneration in which axons grow as far as 5 millimeters beyond the scar. In axotomized giant interneurons labeled intracellularly with horseradish peroxidase 16 to 120 days after transection, 74 percent of regenerating neurites grew in their normal projection pattern, rostal and contralateral to the cell body. One third of the neurites originated anomalously from posterior dendrites. Despite their initial abnormal orientation, 80 percent of these neurites looped contralaterally and rostrally to assume the normal projection path. The directional specificity persisted when giant interneurons were located in islands formed by double simultaneous cord transection. This limited regeneration seems to be characterized by directional selectivity that cannot be attributed to nonspecific influences, such as a tendency of neurites to grow in an already established direction or a trophic effect of the zone of injury.     

Directional selection and the evolution of breeding date in birds

In many bird species, those pairs that breed earlier in the season have higher reproductive success than those that breed later. Since breeding date is known to be heritable, it is unclear why it does not evolve to an earlier time. Under assumptions outlined by Fisher, a model is developed that shows how breeding date may have considerable additive genetic variance, appear to be under directional selection, and yet not evolve. These results provide a general explanation for a persistent correlation of fitness with a variety of traits in natural populations.     

基于点的方向图算法

提出了一种基于曲率的点方向初始值算法．该算法根据像素点在不同方向上的曲率变化确定该点的初始方向值，并明确提出了点方向的概念．提出了一种点方向图的新算法并研究了相应的快速算法．基于块划分的传统块方向图有时是不连续的．与块方向图相比，点方向图保留了块方向图的统计特性，又克服了块方向图的缺陷，不需要先进行背景分离，点方向计算与背景分离同步进行．     

Signatures of H2CO photodissociation from two electronic states

Even in small molecules, the influence of electronic state on rotational and vibrational product energies is not well understood. Here, we use experiments and theory to address this issue in photodissociation of formaldehyde, H2CO, to the radical products H + HCO. These products result from dissociation from the singlet ground electronic state or the first excited triplet state (T1) of H2CO. Fluorescence spectra reveal a sudden decrease in the HCO rotational energy with increasing photolysis energy accompanied by substantial HCO vibrational excitation. Calculations of the rotational distribution using an ab initio potential energy surface for the T1 state are in very good agreement with experiment and strongly support dominance of the T1 state in the dynamics at the higher photolysis energies.     

制动时汽车方向稳定性运动分析

简单回顾了汽车制动方向稳定性的一般原理和规律，着重分析了在跑偏前提下，汽车前轴两轮先抱死及后轴两轮先抱死两种情况下的汽车运动状态     

红外定向强辐射器特性分析

首先概述了红外辐射器的演进过程，进而对近年来出现的第3代红外加热器产品，红外定向强辐射器的性能，特点进行了分析研究，以探究其实现高效、节能之机理和途径。     

Directional switching of the kinesin Cin8 through motor coupling

Kinesin motor proteins are thought to move exclusively in either one or the other direction along microtubules. Proteins of the kinesin-5 family are tetrameric microtubule cross-linking motors important for cell division and differentiation in various organisms. Kinesin-5 motors are considered to be plus-end-directed. However, here we found that purified kinesin-5 Cin8 from budding yeast could behave as a bidirectional kinesin. On individual microtubules, single Cin8 motors were minus-end-directed motors, whereas they switched to plus-end-directed motility when working in a team of motors sliding antiparallel microtubules apart. This kinesin can thus change directionality of movement depending on whether it acts alone or in an ensemble.     

基于教学ERP的自学习系统分析与设计

随着信息时代的来临和ERP在中国的复苏,教学ERP作为培养ERP专业人才的教学ERP软件的产生已经到了刻不容缓的地步。本文从教学ERP背景入手,结合Agent的概念和思想,着重讲述了教学ERP成功构建和实施的一个关键——自学习子系统的分析与设计及其主要的功能和工作原理。     

Directional hearing: effect of unilateral change of the sound duration

Monaural change of the duration of a binaural acoustic stimulus within the range of 0.5 to 1.5 milliseconds caused a considerable shift of the sound image. This could be counterbalanced by a unilateral change of the signal intensity. Further unilateral lengthening of the stimulus did not affect the sound localization, but it revealed a masking aftereffect (4 to 10 milliseconds) of the binaural signal. The data define the temporal characteristics of binaural effect influenced the binaural interaction.