First-principles theory for the H + H2O, D2O reactions

A full quantum dynamical study of the reactions of a hydrogen atom with water, on an accurate ab initio potential energy surface, is reported. The theoretical results are compared with available experimental data for the exchange and abstraction reactions in H + D2O and H + H2O. Clear agreement between theory and experiment is revealed for available thermal rate coefficients and the effects of vibrational excitation of the reactants. The excellent agreement between experiment and theory on integral cross sections for the exchange reaction is unprecedented beyond atom-diatom reactions. However, the experimental cross sections for abstraction are larger than the theoretical values by more than a factor of 10. Further experiments are required to resolve this.     

Experimental and theoretical differential cross sections for a four-atom reaction: HD + OH → H₂O + D

Quantum dynamical theories have progressed to the stage in which state-to-state differential cross sections can now be routinely computed with high accuracy for three-atom systems since the first such calculation was carried out more than 30 years ago for the H + H(2) system. For reactions beyond three atoms, however, highly accurate quantum dynamical calculations of differential cross sections have not been feasible. We have recently developed a quantum wave packet method to compute full-dimensional differential cross sections for four-atom reactions. Here, we report benchmark calculations carried out for the prototypical HD + OH → H(2)O + D reaction on an accurate potential energy surface that yield differential cross sections in excellent agreement with those from a high-resolution, crossed-molecular beam experiment.     

Theoretical and experimental rate constants for two isotopic modifications of the reaction h + h2

Theoretical rate constants for two isotopic modifications of the simplest possible chemical reaction, namely, H + D(2) --> HD + D and D + H(2) --> HD + H, are presented. Experimental results, which have previously been obtained in the higher temperature regime by a shock tube technique, are combined with lower temperature results to give an experimental determination of the rate behavior over the large temperature range approximately 200 to 2000 K. It is now possible to assess the accuracy of ab initio potential energy surface calculations and to judge theoretical chemical kinetic methods.     

The transition state of the f + h2 reaction

The transition state region of the F + H(2) reaction has been studied by photoelectron spectroscopy of FH(2)(-). New para and normal FH(2)(-)photoelectron spectra have been measured in refined experiments and are compared here with exact three-dimensional quantum reactive scattering simulations that use an accurate new ab initio potential energy surface for F + H(2). The detailed agreement that is obtained between this fully ab initio theory and experiment is unprecedented for the F + H(2) reaction and suggests that the transition state region of the F + H(2) potential energy surface has finally been understood quantitatively.     

Mode-specific energy disposal in the four-atom reaction OH + D2 --> HOD + D

Experiments, employing crossed molecular beams, with vibrational state resolution have been performed on the simplest four-atom reaction, OH + D2 --> HOD + D. In good agreement with the most recent quantum scattering predictions, mode-specific reaction dynamics is observed, with vibration in the newly formed oxygen-deuterium bond preferentially excited to v = 2. This demonstrates that quantum theoretical calculations, which in the past decade have achieved remarkable accuracy for three-atom reactions involving three dimensions, have progressed to the point where it is now possible to accurately predict energy disposal in four-atom reactions involving six dimensions.     

Theoretical study of geometric phase effects in the hydrogen-exchange reaction

The crossing of two electronic potential surfaces (a conical intersection) should result in geometric phase effects even for molecular processes confined to the lower surface. However, recent quantum simulations of the hydrogen exchange reaction (H + H2 --> H2 + H) have predicted a cancellation in such effects when product distributions are integrated over all scattering angles. We used a simple topological argument to extract reaction paths with different senses from a nuclear wave function that encircles a conical intersection. In the hydrogen-exchange reaction, these senses correspond to paths that cross one or two transition states. These two sets of paths scatter their products into different regions of space, which causes the cancellation in geometric phase effects. The analysis should generalize to other direct reactions.     

Ion cyclotron resonance spectroscopy. Cyclotron double resonance provides a new technique for the study of ion-molecule reaction mechanisms

Ion cyclotron resonance spectroscopy yields information on many aspects of ion-molecule chemistry. The method is ideally suited for experiments involving ion energies below several electron volts, and hence provides a valuable complement to other techniques (27). eyclotron double resonance is uniquely suitable for establishing relationships between reactant ions and their product ions in complex ion-molecule reaction sequences. The double-resonance experiments with isotopic species yield information on reaction mechanisms and the nature of intermediate species. Ion-molecule reactions which occur at low energies are quite sensitive to the nature of functional groups and the details of molecular structure (28). Reactions of ions or neutral molecules with specific reagents in the cyclotron spectrometer can thus be used to characterize unknown species. Once the systematic ion-molecule chemistry of useful reagents has been worked out, it should be possible to proceed in a manner directly analogous to classical chemical methods. Suppose, for example, that reagents A(+), B(+), C(+), and D(+) each have characteristic reactions with different functional groups. Then these reagents can all be mixed with an unknown neutral species, X, and each of the reactions, X + A(+) --> ?, X + B(+) --> ?, . . . . can be examined. In contrast to solution chemistry, all the reagents can be added simultaneously to the unknown, since each of the specific reactions can be examined by cyclotron double resonance. The reactions which occur, the species synthesized , and the products of degradation then characterize X. The same methodology can be applied to characterize an unknown ionic epecies X(+), through use of neutral reagents A, B, C, and D. For example, proton transfer reactions to neuteal species have been applied in studying ions of mass 45 produced from various sources (29). The order of the proton affinities of the neutral reagent molecules are as follows: NH(3) isobutylene propene. Ions of mass 45 can be produced by the protonation of ethylene oxide (see structure III), the protonation of acetaldehyde (see structure IV), and the fragmentation of dimethyl ether (see structure V). Those ions might be expected to have, respectively, the three structures: Proton transfer from the mass-45 ions from sources III and IV to NH(3) and to isobutylene occurs readily, but not proton transfer to propene. For the ion from source V, proton transfer to NH3 occurs, but not proton transfer to isobutylene or propene. Thus the proton transfer reactions to various neutral reagents demonstrate that the mass-45 ions from the various sources are different. This example is only a rudimentary version of an approach to the characterization of unusual ionic species; niore sophisticated applications can follow when the systematic chemistry of more reagents is available. This approach should be ideal for comparing nonclassical carbonium ions produced by different routes. Some very interesting ionic species are produced by rearrangements in the fragmentation of molecules, following electron impact. Such molecular rearrangements frequently result in the fragmentation of an ion radical to another ion radical with the elimination of a small neutral species (30). It should be possible to run these reactions in reverse to check the postulated mechanisms. An interesting result of the systematic study of proton transfer to various functional groups is the finding that the proton affinity of various amines and pyridine is extremely high (31). Species such as VI and VII: might be expected to be very stable; they are in fact so stable that they are unreactive with respect to subsequent chemistry at the charge center. Thus, if there are other functional groups on the ion, the important reactions should occur at these functional groups. It should be possible to design species for which the presence of the charge has little influence on the reactivity of a neutral functional group. In this case the charge functions simply as an inert label which makes the study of neutral-neutral reactions accessible by cyclotron resonance: Various routes for development of the basic technique also appear to be very promising. Echo phenomena following sequences of pulsed excitation have been observed in electron cyclotron resonance (32). Analogous transient phenomena should also occur in ion cvclotron resonances (33). Pulsed-cyclotron-resonance techniques of course have intriguing analogies to nuclear-magnetic-resonance spin-echo experiments (34) and may be the technique of choice for making accurate measurements of ion-molecule-reaction cross sections as a function of energy for low ion energies. Finally, many ion-molecule reactions yield products in excited electronic states (35). For example, the reaction N(2)- + CO --> N(2) + CO- (46) has been studied by beam techniques (36). A straightforward procedure is to observe optical emission from the cyclotron spectrometer by placing a window at the end of the cyclotron cell (37). The emission can be analyzed with a crude set of optical filters, or with a high-speed spectrograph. Optical emission from the cyclotron cell can of course originate from many sources. The radiation from a specific excited product ion can be selected by a radio-frequency-optical double-resonance experiment. If, in the generai reaction A+ + B --> *C+ + D, (47) ion A+ is irradiated at its cyclotron resonance frequency, the number density of optical emitters *C+ is changed. If the irradiating frequency is modulated, then the number of optical emitters will be modulated, so that the intensity of emission from *C+ will also be modulated. When the optical emission from *C+ is analyzed in a spectrograph with a photoelectric cell, the output of the photoelectric cell can be detected with a phase sensitive detector referenced to the modulation frequency. This highly specific modulation-detection scheme should discriminate against other sources of light in the cyclotron cell.     

State-to-State Rates for the D + H2(v = 1, j = 1) rarr HD(v', j') + H Reaction: Predictions and Measurements

A fully quantal wavepacket approach to reactive scattering in which the best available H(3) potential energy surface was used enabled a comparison with experimentally determined rates for the D + H(2)(v = 1, j = 1) --> HD(v' = 0, 1, 2; j') + H reaction at significantly higher total energies (1.4 to 2.25 electron volts) than previously possible. The theoretical results are obtained over a sufficient range of conditions that a detailed simulation of the experiment was possible, thus making this a definitive comparison of experiment and theory. Good to excellent agreement is found for the vibrational branching ratios and for the rotational distributions within each product vibrational level. However, the calculated rotational distributions are slightly hotter than the experimentally measured ones. This small discrepancy is more marked for products for which a larger fraction of the total energy appears in translation. The most likely explanation for this behavior is that refinements are needed in the potential energy surface.     

Interference of quantized transition-state pathways in the H + D2 -> D + HD chemical reaction

The collision-energy dependence of the state-resolved differential cross section at a specific backward-scattering angle for the reaction H + D2 --> D + HD is measured with the D-atom Rydberg "tagging" time-of-flight technique. The reaction was modeled theoretically with converged quantum scattering calculations that provided physical interpretation of the observations. Oscillations in the differential cross sections in the backward-scattering direction are clearly observed and are attributed to the transition-state structures that originate from the interferences of different quantized transition-state pathways.     

Reaction product imaging: the h + d2 reaction

The differential cross section for the H + D(2) --> HD + D reaction has been measured using a technique called reaction product imaging. In this experiment, a photolytically produced beam of hydrogen (H) atoms crossed a beam of cold deuterium (D(2)) molecules. Product D atoms were ionized at the intersection of the two particle beams and accelerated toward a position-sensitive detector. The ion images appearing on the detector are two-dimensional projections of the three-dimensional velocity distribution of the D atom products. The reaction was studied at nominal center-of-mass collision energies of 0.54 and 1.29 electron volts. At the lower collision energy, the measured differential cross section for D atom production, summed over all final states of the HD(v,J) product, is in good agreement with recent quasi-classical trajectory calculations. At the higher collision energy, the agreement between the theoretical predictions and experimental results is less favorable.     

Carbon monoxide and nitric oxide consumption in polluted air: the carbon monoxide-hydroperoxyl reaction

A recent laboratory measurement of the ratio of the rate constant for the reaction CO + HO (2) --> C0(2) + OH relative to that for H + HO(2) --> 2OH indicates that the former reaction is probably faster than CO + OH --> CO(2) + H. On this basis a simple analysis is given showing that the calculated lifetime of nitric oxide in polluted atmospheres would be appreciably longer than that estimated on the assumption that the carbon monoxide-hydroperoxyl reaction may be neglected. A fast carbon monoxide-hydroperoxyl reaction implies that the cyclic consumption of carbon monoxide (an atmospheric sink) could occur even with no nitric oxide present.     

啤酒花茎多酚的提取优化及其抗氧化活性

为啤酒花加工废弃物的综合开发利用提供参考,以啤酒花茎为试验材料,采用超声辅助丙酮法提取多酚,通过单因素试验考察料液比、提取时间、丙酮体积分数和提取次数4个因素对啤酒花茎多酚含量的影响,并利用响应面法优化其提取工艺。结果表明:通对响应面法建立的啤酒花茎多酚提取回归方程为Y=106.5+1.64A+2.13B+2.63C+11.12D+0.51AB-2.53AC+0.19AD+4.37BC-0.87BD-0.24CD-3.85A~2-2.41B~2+1.04C~2-3.65D~2,模型决定系数R2=0.933 8,该回归模型的拟合程度良好;最佳提取工艺为提取时间20min、料液比1∶12、丙酮体积分数60%、提取4次,此条件提取啤酒花茎多酚的含量为115.89mg/g,与理论预测值116.56mg/g接近。啤酒花茎多酚对DPPH自由基的清除率为90.70%,半清除浓度(SC_(50))为22.81g/mL。采用响应面法优化的啤酒花茎多酚超声辅助提取工艺准确可靠,可用于啤酒花茎多酚的实际提取;啤酒花茎多酚具有较强的抗氧化活性,可作为天然抗氧化资源进行开发利用。     

A Quantum State-Resolved Insertion Reaction: O((1)D) + H(2)(J = 0) --> OH((2) product operator product operator product operator, v, N) + H((2)S)

The O((1)D) + H(2) --> OH + H reaction, which proceeds mainly as an insertion reaction at a collisional energy of 1.3 kilocalories per mole, has been investigated with the high-resolution H atom Rydberg "tagging" time-of-flight technique and the quasiclassical trajectory (QCT) method. Quantum state-resolved differential cross sections were measured for this prototype reaction. Different rotationally-vibrationally excited OH products have markedly different angular distributions, whereas the total reaction products are roughly forward and backward symmetric. Theoretical results obtained from QCT calculations indicate that this reaction is dominated by the insertion mechanism, with a small contribution from the collinear abstraction mechanism through quantum tunneling.     

苦竹各器官生物量模型

调查了杭州市余杭区中泰乡苦竹Pleioblastus amarus 林生物量, 并采用回归分析的方法探讨了苦竹各变量的相关性, 建立并选择出苦竹各器官生物量与胸径、秆高或枝下高等因子的最佳相关数学模型:m秆=13.439 5 D 2.0048 H0.442 5 ;m枝=2 956.359 8 D 1.992 9 H-0.641 0 ;m叶=43.746 7 -30.541 2 D +53.759 7 D2 ;m篼=270.956 0D 2.357 9 H -0.399 5 ;m鞭=512.436 1 -175.936 0D +2.907 8H0 ;m地上=432.446 8 -479.307 5D +422.828 5D2 ;m地下=396.622 3 -53.286 9 D +2.877 5H0 ;m总=191.038 0D 1.198 6 H0 0.296 2 。应用上述模型估算出苦竹单株各器官生物量和苦竹林分产量。表4 参12     

Growth of SiO2 at room temperature with the use of catalyzed sequential half-reactions

Films of silicon dioxide (SiO2) were deposited at room temperature by means of catalyzed binary reaction sequence chemistry. The binary reaction SiCl4 + 2H2O --> SiO2 + 4HCl was separated into SiCl4 and H2O half-reactions, and the half-reactions were then performed in an ABAB ellipsis sequence and catalyzed with pyridine. The pyridine catalyst lowered the deposition temperature from >600 to 300 kelvin and reduced the reactant flux required for complete reactions from approximately 10(9) to approximately 10(4) Langmuirs. Growth rates of approximately 2.1 angstroms per AB reaction cycle were obtained at room temperature for reactant pressures of 15 millitorr and 60-second exposure times with 200 millitorr of pyridine. This catalytic technique may be general and should facilitate the chemical vapor deposition of other oxide and nitride materials.     

Kinetic isotope effects for the reactions of muonic helium and muonium with H2

The neutral muonic helium atom may be regarded as the heaviest isotope of the hydrogen atom, with a mass of ~4.1 atomic mass units ((4.1)H), because the negative muon almost perfectly screens one proton charge. We report the reaction rate of (4.1)H with (1)H(2) to produce (4.1)H(1)H + (1)H at 295 to 500 kelvin. The experimental rate constants are compared with the predictions of accurate quantum-mechanical dynamics calculations carried out on an accurate Born-Huang potential energy surface and with previously measured rate constants of (0.11)H (where (0.11)H is shorthand for muonium). Kinetic isotope effects can be compared for the unprecedentedly large mass ratio of 36. The agreement with accurate quantum dynamics is quantitative at 500 kelvin, and variational transition-state theory is used to interpret the extremely low (large inverse) kinetic isotope effects in the 10(-4) to 10(-2) range.     

Biogenic methane, hydrogen escape, and the irreversible oxidation of early Earth

The low O2 content of the Archean atmosphere implies that methane should have been present at levels approximately 10(2) to 10(3) parts per million volume (ppmv) (compared with 1.7 ppmv today) given a plausible biogenic source. CH4 is favored as the greenhouse gas that countered the lower luminosity of the early Sun. But abundant CH4 implies that hydrogen escapes to space (upward arrow space) orders of magnitude faster than today. Such reductant loss oxidizes the Earth. Photosynthesis splits water into O2 and H, and methanogenesis transfers the H into CH4. Hydrogen escape after CH4 photolysis, therefore, causes a net gain of oxygen [CO2 + 2H2O --> CH4 + 2O2 --> CO2 + O2 + 4H(upward arrow space)]. Expected irreversible oxidation (approximately 10(12) to 10(13) moles oxygen per year) may help explain how Earth's surface environment became irreversibly oxidized.     

杉木种源地理位置的数学模型

<正>杉木种源研究始于一九五七年。20多年来,不少专家致力于杉木种源的研究工作。地理种源试验的重要目的之一:通过种源试验来合理区划种源区域和种子调拨界限,但是由于种源试验规模和数量总是有限的。为了达到上述地理种源试验的目的,就必须研究种源与地理位置的关系。在研究杉木种源与地理位置之间的关系过程中,早期的研究多以定性的方式描述其间关系,而近几年来,多用各种数学模型定量地描述其间关     

Control of electron localization in molecular dissociation

We demonstrated how the subcycle evolution of the electric field of light can be used to control the motion of bound electrons. Results are presented for the dissociative ionization of deuterium molecules (D2 --> D+ + D), where asymmetric ejection of the ionic fragment reveals that light-driven intramolecular electronic motion before dissociation localizes the electron on one of the two D+ ions in a controlled way. The results extend subfemtosecond electron control to molecules and provide evidence of its usefulness in controlling reaction dynamics.     

Time-Resolved X-ray Diffraction Study of Solid Combustion Reactions

Real-time synchrotron diffraction has been used to monitor the phase transformations of highly exothermic, fast self-propagating solid combustion reactions on a subsecond time scale down to 100 milliseconds and in some instances to 10 milliseconds. Three systems were investigated: Ti + C --> TiC; Ti + C + xNi --> TiC + Ni-Ti alloy; and Al + Ni --> AlNi. In all three reactions, the first step was the melting of the metal reactants. Formation of TiC in the first two reactions was completed within 400 milliseconds of the melting of the Ti metal, indicating that the formation of TiC took place during the passage of the combustion wave front. In the Al + Ni reaction, however, passage of the wave front was followed by the appearance and disappearance of at least one intermediate in the afterburn region. The final AlNi was formed some 5 seconds later and exhibited a delayed appearance of the (210) reflection, which tends to support a phase transformation from a disordered AlNi phase at high temperature to an ordered CsCl structure some 20 seconds later. This new experimental approach can be used to study the chemical dynamics of high-temperature solid-state phenomena and to provide the needed database to test various models for solid combustion.