Quantum theory of chemical reaction dynamics

It is now possible to use rigorous quantum scattering theory to perform accurate calculations on the detailed state-to-state dynamics of chemical reactions in the gas phase. Calculations on simple reactions, such as H + D2 --> HD + D and F + H2 --> HF + H, compete with experiment in their accuracy. Recent advances in theory promise to extend such accurate predictions to more complicated reactions, such as OH + H2 --> H2O + H, and even to reactions of molecules on solid surfaces. New experimental techniques for probing reaction transition states, such as negative-ion photodetachment spectroscopy and pump-probe femtosecond spectroscopy, are stimulating the development of new theories.     

Laser probing of chemical reaction dynamics

Lasers are used in increasingly sophisticated ways to carry out reactions between molecules in selected vibrational, rotational, and electronic states and to probe the product states of chemical reactions. Such investigations are providing unprecedented insights into chemical reaction dynamics, the study of the detailed motions that molecules undergo in simple chemical reactions. In many cases it is possible to describe the influence that specific types of molecular excitation have on reactive events. Experiments are also being carried out to leam about chemical reactivity as a function of the alignment of reagents. There is increasing excitement concerning the potential of laser methods to interrogate the transition states of molecular reactions.     

Regulation of microtubule dynamics by reaction cascades around chromosomes

During spindle assembly, chromosomes generate gradients of microtubule stabilization through a reaction-diffusion process, but how this is achieved is not well understood. We measured the spatial distribution of microtubule aster asymmetry around chromosomes by incubating centrosomes and micropatterned chromatin patches in frog egg extracts. We then screened for microtubule stabilization gradient shapes that would generate such spatial distributions with the use of computer simulations. Only a long-range, sharply decaying microtubule stabilization gradient could generate aster asymmetries fitting the experimental data. We propose a reaction-diffusion model that combines the chromosome generated Ran-guanosine triphosphate-Importin reaction network to a secondary phosphorylation network as a potential mechanism for the generation of such gradients.     

Conformationally controlled chemistry: excited-state dynamics dictate ground-state reaction

Ion imaging reveals distinct photodissociation dynamics for propanal cations initially prepared in either the cis or gauche conformation, even though these isomers differ only slightly in energy and face a small interconversion barrier. The product kinetic energy distributions for the hydrogen atom elimination channels are bimodal, and the two peaks are readily assigned to propanoyl cation or hydroxyallyl cation coproducts. Ab initio multiple spawning dynamical calculations suggest that distinct ultrafast dynamics in the excited state deposit each conformer in isolated regions of the ground-state potential energy surface, and, from these distinct regions, conformer interconversion does not effectively compete with dissociation.     

Deciphering the reaction dynamics underlying optimal control laser fields

Femtosecond high-resolution pump-probe experiments have been used together with theoretical ab initio quantum calculations and wave packet dynamics simulations to decode an optimal femtosecond pulse that is generated from adaptive learning algorithms. This pulse is designed to maximize the yield of the organometallic ion CpMn(CO)3 while hindering the competing fragmentation. The sequential excitation and ionization of the target ion are accomplished by an optimized field consisting of two dominant subpulses with optimal frequencies and time delays.     

The hydroperoxyl radical in atmospheric chemical dynamics: reaction with carbon monoxide

From measurements of the photochemical rate of production of CO(2)(16,18) and CO(2)(16,16), produced from the low intensity photolysis of mixtures of CO, H(2)O, Ar, and O(2)(18,18), the rate constant for the reaction HO(2) + CO --> CO(2) + OH has been determined at 300 degrees K to be less than or equal to 10(-20) cubic centimeter per molecule per second. These measurements indicate that the reaction of thermalized HO(2) is of negligible importance as a sink mechanism for converting CO to CO(2) in either the troposphere or the stratosphere.     

Vibrationally quantum-state-specific reaction dynamics of H atom abstraction by CN radical in solution

Solvent collisions can often mask initial disposition of energy to the products of solution-phase chemical reactions. Here, we show with transient infrared absorption spectra obtained with picosecond time resolution that the nascent HCN products of reaction of CN radicals with cyclohexane in chlorinated organic solvents exhibit preferential excitation of one quantum of the C-H stretching mode and up to two quanta of the bending mode. On time scales of approximately 100 to 300 picoseconds, the HCN products undergo relaxation to the vibrational ground state by coupling to the solvent bath. Comparison with reactions of CN radicals with alkanes in the gas phase, known to produce HCN with greater C-H stretch and bending mode excitation (up to two and approximately six quanta, respectively), indicates partial damping of the nascent product vibrational motion by the solvent. The transient infrared spectra therefore probe solvent-induced modifications to the reaction free energy surface and chemical dynamics.     

我国主要松树诱导抗虫性的一些规律比较

该文以马尾松、油松、华北落叶松3种我国主要针叶树为对象,分析比较了3种松树在受松毛虫危害后,所产生的迅速诱导抗性和滞后诱导抗性;研究了取食相应的受害针叶后,其生长发育情况的变化;探讨了松树诱导抗性对松毛虫种群动态的作用过程,总结了我国主要松树诱导抗性的一般规律,为分析松毛虫发生机理及松毛虫综合管理提供理论依据.     

The chemistry of water on alumina surfaces: reaction dynamics from first principles

Aluminas and their surface chemistry play a vital role in many areas of modern technology. The behavior of adsorbed water is particularly important and poorly understood. Simulations of hydrated alpha-alumina (0001) surfaces with ab initio molecular dynamics elucidate many aspects of this problem, especially the complex dynamics of water dissociation and related surface reactions. At low water coverage, free energy profiles established that molecularly adsorbed water is metastable and dissociates readily, even in the absence of defects, by a kinetically preferred pathway. Observations at higher water coverage revealed rapid dissociation and unanticipated collective effects, including water-catalyzed dissociation and proton transfer reactions between adsorbed water and hydroxide. The results provide a consistent interpretation of the measured coverage dependence of water heats of adsorption, hydroxyl vibrational spectra, and other experiments.     

Data transfer: Basic concepts

Experimental methods often utilize computers to acquire, then transfer data to more convenient facilities for storage, processing, and analysis. Knowledge of the basic concepts of digital data transmission is often essential in the proper set up and operation of microprocessor-based instrumentation. This paper discusses methods of electronically transferring data from laboratory or field facilities to a central computer.     

几个遗传学概念的辨析--兼与《面向21世纪课程教材·遗传学》作者商榷

就农业院校普遍使用的<面向21世纪课程教材*遗传学>(朱军主编,第3版)的几个概念论述上存在的问题进行了探讨,并提出了相应的建议.这些概念包括交换值、重组率和遗传图距,同(共)线基因与连锁基因,伴性遗传、限性遗传与从性遗传,细胞质遗传与母性遗传,显性-隐性与野生型-突变体等.