Oscillatory kinetics and spatio-temporal self-organization in reactions at solid surfaces

Chemical reactions far from equilibrium on solid surfaces may exhibit typical phenomena of nonlinear dynamics, as exemplified by the catalytic oxidation of carbon monoxide on a platinum(110) single-crystal surface. Depending on the external parameters (temperature and partial pressures of the reactants), the temporal variation of the reaction rate may become oscillatory or even chaotic. In a parallel way, the concentration distributions of the adsorbed species on the surface form spatio-temporal patterns including propagating and standing waves, rotating spirals, as well as irregular and rapidly changing structures denoted "chemical turbulence."     

Thermophysiology of Tyrannosaurus rex: Evidence from Oxygen Isotopes

The oxygen isotopic composition of vertebrate bone phosphate (delta(p)) is related to ingested water and to the body temperature at which the bone forms. The delta(p) is in equilibrium with the individual's body water, which is at a physiological steady state throughout the body. Therefore, intrabone temperature variation and the mean interbone temperature differences of well-preserved fossil vertebrates can be determined from the deltap variation. Values of delta(p) from a well-preserved Tyrannosaurus rex suggest that this species maintained homeothermy with less than 4 degrees C of variability in body temperature. Maintenance of homeothermy implies a relatively high metabolic rate that is similar to that of endotherms.     

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.     

氨基酸叶面肥对砂梨叶片光合作用的促进效应

以8个砂梨品种叶片为材料,研究了含有0.05、0.5和5 mg.L-15-氨基乙酰丙酸(ALA)的"爱乐壮"氨基酸叶面肥处理对砂梨叶片叶绿素相对含量(SPAD)、净光合速率(Pn)和叶绿素快速诱导荧光参数的影响。结果表明:3个处理均能不同程度地提高砂梨叶片SPAD值,增加Pn,提高PSⅡ反应中心供体侧与受体侧光合能量转换活性,减少热能量耗散。相关性分析表明,砂梨叶片Pn与光合性能指数(PIABS)、放氧复合体活性(Wk)、PSⅡ反应中心最大关闭速率(Mo)、捕获激子将电子传递到Q-A下游其他电子受体的概率(Ψo)、PSⅡ反应中心用于电子传递的量子产额(φEo)以及能量热耗散比率(φDo)等荧光参数密切相关,说明"爱乐壮"氨基酸叶面肥可以通过提高砂梨叶片光化学能量转换效率促进光合作用。从农业生产成本上看,添加0.05～0.5 mg.L-1ALA性价比更高。     

Fractal reaction kinetics

Classical reaction kinetics has been found to be unsatisfactory when the reactants are spatially constrained on the microscopic level by either walls, phase boundaries, or force fields. Recently discovered theories of heterogeneous reaction kinetics have dramatic consequences, such as fractal orders for elementary reactions, self-ordering and self-unmixing of reactants, and rate coefficients with temporal "memories." The new theories were needed to explain the results of experiments and supercomputer simulations of reactions that were confined to low dimensions or fractal dimensions or both. Among the practical examples of "fractal-like kinetics" are chemical reactions in pores of membranes, excitation trapping in molecular aggregates, exciton fusion in composite materials, and charge recombination in colloids and clouds.     

Oscillatory mass transport in vapor-liquid-solid growth of sapphire nanowires

In vapor-liquid-solid (VLS) growth, the liquid phase plays a pivotal role in mediating mass transport from the vapor source to the growth front of a nanowire. Such transport often takes place through the liquid phase. However, we observed by in situ transmission electron microscopy a different behavior for self-catalytic VLS growth of sapphire nanowires. The growth occurs in a layer-by-layer fashion and is accomplished by interfacial diffusion of oxygen through the ordered liquid aluminum atoms. Oscillatory growth and dissolution reactions at the top rim of the nanowires occur and supply the oxygen required to grow a new (0006) sapphire layer. A periodic modulation of the VLS triple-junction configuration accompanies these oscillatory reactions.     

500,000-year stable carbon isotopic record from devils hole, nevada

The record of carbon-13 (delta(13)C) variations in DH-11 vein calcite core from Devils Hole, Nevada, shows four prominent minima near glacial terminations (glacial-interglacial transitions) V to II. The delta(13)C time series is inversely correlated with the DH-11 oxygen isotope ratio time series and leads it by as much as 7000 years. The delta(13)C variations likely record fluctuations in the delta(13)C of dissolved inorganic carbon of water recharging the aquifer. How such variations are transported 80 kilometers to Devils Hole without obliteration by water-rock reaction remains an enigma. The record may reflect (i) global variations in the delta(13)C of atmospheric CO(2) and, hence, the delta(13)C of continental biomass or (ii) variations in extent and density of vegetation in the southern Great Basin. In the latter case, delta(13)C minima at 414, 334, 246, and 133 thousand years ago mark times of maximum vegetation.     

Interception of excited vibrational quantum states by O2 in atmospheric association reactions

Bimolecular reactions in Earth's atmosphere are generally assumed to proceed between reactants whose internal quantum states are fully thermally relaxed. Here, we highlight a dramatic role for vibrationally excited bimolecular reactants in the oxidation of acetylene. The reaction proceeds by preliminary adduct formation between the alkyne and OH radical, with subsequent O(2) addition. Using a detailed theoretical model, we show that the product-branching ratio is determined by the excited vibrational quantum-state distribution of the adduct at the moment it reacts with O(2). Experimentally, we found that under the simulated atmospheric conditions O(2) intercepts ~25% of the excited adducts before their vibrational quantum states have fully relaxed. Analogous interception of excited-state radicals by O(2) is likely common to a range of atmospheric reactions that proceed through peroxy complexes.     

Mechanisms of nitrogen dioxide reactions: initiation of lipid peroxidation and the production of nitrous Acid

The reactions of nitrogen dioxide with cyclohexene have been studied as a model for the reactions that occur between nitrogen dioxide in smoggy air and unsaturated fatty acids in pulmonary lipids. As predicted from earlier studies at high nitrogen dioxide concentrations, this gas reacts with cyclohexene predominantly by addition to the double bond at nitrogen dioxide concentrations of 1 percent (10,000 parts per million) to 40 percent in nitrogen; in the presence of air or oxygen, this reaction initiates the autoxidation of the alkene. However, at concentrations below 100 parts per million in nitrogen, nitrogen dioxide reacts with cyclohexene almost exclusively by abstraction of allylic hydrogen; this unexpected reaction also initiates the autoxidation of the alkene in the presence of oxygen or air, but it leads to the production of nitrous acid rather than of a product containing a nitro group attached to a carbon atom. The nitrous acid can react with amines to produce nitrosamines. Moreover, the nitrite ion produced by the hydrogen abstraction mechanism would be expected to diffuse throughout the body, unlike nitrated lipids that would be confined to the pulmonary cavity. These findings have been confirmed with methyl oleate, linoleate, and linolenate; some of the kinetic features of the nitrogen dioxide-initiated autoxidation of these unsaturated fatty acids have been studied.     

Oxygen isotope zoning in garnet

Oxygen isotope zoning was examined within garnet with the use of the stable isotope laser probe. Four metamorphic garnets from the regional metamorphic terrane in Vermont and the skarn deposit at Carr Fork, Utah, were examined and were found to be concentrically zoned in delta(18)O values. The largest variations in delta(18)O values were observed in the regional metamorphic garnets, where delta(18)O values change by 3 per mil from core to rim. These oxygen isotope zoning profiles reflect the changes in the delta(18)O values of the rocks during garnet growth, which are caused by infiltration of fluids and by dehydration reactions during metamorphism.     

Diamonds, Eclogites, and the Oxidation State of the Earth's Mantle

The reaction dolomite + 2 coesite --><-- diopside + 2 diamond + 2O(2) defines the coexistence of diamond and carbonate in mantle eclogites. The oxygen fugacity of this reaction is approximately 1 log unit higher at a given temperature and pressure than the oxygen fugacities of the analogous reactions that govern the stability of diamond in peridotite. This difference allows diamond-bearing eclogite to coexist with peridotite containing carbonate or carbonate + diamond. This potential coexistence of diamond-bearing eclogite and carbonate-bearing peridotite can explain the presence of carbon-free peridotite interlayered with garnet pyroxenites that contain graphitized diamond in the Moroccan Beni Bousera massif at the Earth's surface and the preferential preservation of diamond-bearing eclogitic relative to peridotitic xenoliths in the Roberts Victor kimberlite.     

Pore Fluid Constraints on the Temperature and Oxygen Isotopic Composition of the Glacial Ocean

Pore fluids from the upper 60 meters of sediment 3000 meters below the surface of the tropical Atlantic indicate that the oxygen isotopic composition (delta18O) of seawater at this site during the last glacial maximum was 0.8 ± 0.1 per mil higher than it is today. Combined with the delta18O change in benthic foraminifera from this region, the elevated ratio indicates that the temperature of deep water in the tropical Atlantic Ocean was 4°C colder during the last glacial maximum. Extrapolation from this site to a global average suggests that the ice volume contribution to the change in delta18O of foraminifera is 1.0 per mil, which partially reconciles the foraminiferal oxygen isotope record of tropical sea surface temperatures with estimates from Barbados corals and terrestrial climate proxies.     

Effects of progressive drought on photosynthesis and partitioning of absorbed light in apple trees

To understand how drought stress affects CO2 assimilation and energy partitioning in apple(Malus domestica Borkh.), we investigated photosynthesis and photo-protective mechanisms when irrigation was withheld from potted Fuji trees. As the drought progressing, soil relative water content(SRWC) decreased from 87 to 24% in 15 d; this combined the decreasing in leaf relative water content(LRWC), net photosynthesis rate(P n) and stomatal conductance(G s). However, the concentrations of chlorophylls(Chl) remained unchanged while P n values were declining. Photochemistry reactions were slightly down-regulated only under severe drought. Rubisco activity was significantly decreased as drought conditions became more severe. The actual efficiency of photosystem II(ΦPSII) was diminished as drought became more intense. Consequently, xanthophyll-regulated dissipation of thermal energy was greatly enhanced. Simultaneously, the ratio of ΦPSII to the quantum yield of carbon metabolism, which is measured under non-photorespiratory conditions, increased in parallel with drought severity. Our results indicate that, under progressive drought stress, the reduction in photosynthesis in apple leaves can be attributed primarily to stomatal limitations and the inhibited capacity for CO2 fixation. Xanthophyll cycle-dependent thermal dissipation and the Mehler reaction are the most important pathways for dispersing excess energy from apple leaves during periods of drought stress.     

From force fields to dynamics: classical and quantal paths

Reaction path methods provide a powerful tool for bridging the gap between electronic structure and chemical dynamics. Classical mechanical reaction paths may usually be understood in terms of the force field in the vicinity of a minimum energy path (MEP). When there is a significant component of hydrogenic motion along the MEP and a barrier much higher than the average energy of reactants, quantal tunneling paths must be considered, and these tend to be located on the corner-cutting side of the MEP. As the curvature of the MEP in mass-scaled coordinates is increased, the quantal reaction paths may deviate considerably from the classical ones, and the force field must be mapped out over a wider region, called the reaction swath. The required force fields may be represented by global or semiglobal analytic functions, or the dynamics may be computed "directly" from the electronic structure results without the intermediacy of potential energy functions. Applications to atom and diatom reactions in the gas phase and at gas-solid interfaces and to reactions of polyatomic molecules in the gas phase, in clusters, and in aqueous solution are discussed as examples.     

Neuronal coherence as a mechanism of effective corticospinal interaction

Neuronal groups can interact with each other even if they are widely separated. One group might modulate its firing rate or its internal oscillatory synchronization to influence another group. We propose that coherence between two neuronal groups is a mechanism of efficient interaction, because it renders mutual input optimally timed and thereby maximally effective. Modulations of subjects' readiness to respond in a simple reaction-time task were closely correlated with the strength of gamma-band (40 to 70 hertz) coherence between motor cortex and spinal cord neurons. This coherence may contribute to an effective corticospinal interaction and shortened reaction times.     

Observation of individual chemical reactions in solution

Discrete chemical reaction events occurring in solution have been observed by single photon detection of a bimolecular, chemiluminescent reaction. The reactants were generated from 9,10-diphenylanthracene in acetonitrile with potential pulses applied to an ultramicroelectrode. Electrogenerated radical ions of opposite sign react to yield the excited singlet state of the parent compound. The chemical reactions were restricted to a 20-femtoliter volume adjacent to the electrode by the use of rapid potential pulses. Individual chemical reaction events were stochastic and followed the Poisson distribution, and the interarrival time between successive reaction events was exponentially distributed.     

A methanotrophic marine molluscan (bivalvia, mytilidae) symbiosis: mussels fueled by gas

An undescribed mussel (family Mytilidae), which lives in the vicinity of hydrocarbon seeps in the Gulf of Mexico, consumes methane (the principal component of natural gas) at a high rate. The methane consumption is limited to the gills of these animals and is apparently due to the abundant intracellular bacteria found there. This demonstrates a methane-based symbiosis between an animal and intracellular bacteria. Methane consumption is dependent on the availability of oxygen and is inhibited by acetylene. The consumption of methane by these mussels is associated with a dramatic increase in oxygen consumption and carbon dioxide production. As the methane consumption of the bivalve can exceed its carbon dioxide production, the symbiosis may be able to entirely satisfy its carbon needs from methane uptake. The very light (delta(13)C = -51 to -57 per mil) stable carbon isotope ratios found in this animal support methane (delta(13)C = -45 per mil at this site) as the primary carbon source for both the mussels and their symbionts.     

The 100,000-year ice-Age cycle identified and found to lag temperature, carbon dioxide, and orbital eccentricity

The deep-sea sediment oxygen isotopic composition (delta(18)O) record is dominated by a 100,000-year cyclicity that is universally interpreted as the main ice-age rhythm. Here, the ice volume component of this delta(18)O signal was extracted by using the record of delta(18)O in atmospheric oxygen trapped in Antarctic ice at Vostok, precisely orbitally tuned. The benthic marine delta(18)O record is heavily contaminated by the effect of deep-water temperature variability, but by using the Vostok record, the delta(18)O signals of ice volume, deep-water temperature, and additional processes affecting air delta(18)O (that is, a varying Dole effect) were separated. At the 100,000-year period, atmospheric carbon dioxide, Vostok air temperature, and deep-water temperature are in phase with orbital eccentricity, whereas ice volume lags these three variables. Hence, the 100,000-year cycle does not arise from ice sheet dynamics; instead, it is probably the response of the global carbon cycle that generates the eccentricity signal by causing changes in atmospheric carbon dioxide concentration.     

Atmospheric influence of Earth's earliest sulfur cycle

Mass-independent isotopic signatures for delta(33)S, delta(34)S, and delta(36)S from sulfide and sulfate in Precambrian rocks indicate that a change occurred in the sulfur cycle between 2090 and 2450 million years ago (Ma). Before 2450 Ma, the cycle was influenced by gas-phase atmospheric reactions. These atmospheric reactions also played a role in determining the oxidation state of sulfur, implying that atmospheric oxygen partial pressures were low and that the roles of oxidative weathering and of microbial oxidation and reduction of sulfur were minimal. Atmospheric fractionation processes should be considered in the use of sulfur isotopes to study the onset and consequences of microbial fractionation processes in Earth's early history.