共查询到20条相似文献，搜索用时 31 毫秒
1.
A full-dimensional quantum dynamics simulation of a hydrogen atom reacting with methane on an accurate ab initio potential energy surface is reported. Based on first-principles theory, thermal rate constants are predicted with an accuracy comparable to (or even exceeding) experimental precision. The theoretical prediction is within the range of the significantly varied experimental rate constants reported by different groups. This level of accuracy has previously been achieved only for smaller, three-or four-atom reactive systems. Comparison with classical transition state theory confirms the importance of quantum mechanical tunneling for the rate constant below 400 kelvin. 相似文献
2.
Zuev PS Sheridan RS Albu TV Truhlar DG Hrovat DA Borden WT 《Science (New York, N.Y.)》2003,299(5608):867-870
We observed ring expansion of 1-methylcyclobutylfluorocarbene at 8 kelvin, a reaction that involves carbon tunneling. The measured rate constants were 4.0 x 10(-6) per second in nitrogen and 4 x 10(-5) per second in argon. Calculations indicated that at this temperature the reaction proceeds from a single quantum state of the reactant so that the computed rate constant has achieved a temperature-independent limit. According to calculations, the tunneling contribution to the rate is 152 orders of magnitude greater than the contribution from passage over the barrier. We discuss environmental effects of the solid-state inert-gas matrix on the reaction rate. 相似文献
3.
Nuñez-Regueiro M Tholence JL Antipov EV Capponi JJ Marezio M 《Science (New York, N.Y.)》1993,262(5130):97-99
The recently discovered homologous series HgBa(2)Can-1 Cun O2n+2+delta possesses remarkable properties. A superconducting transition temperature, T(c), as high as 133 kelvin has been measured in a multiphase Hg-Ba-Ca-Cu-O sample and found to be attributable to the Hg-1223 compound. Temperature-dependent electrical resistivity measurements under pressure on a (> 95%) pure Hg-1223 phase are reported. These data show that T(c) increases steadily with pressure at a rate of about 1 kelvin per gigapascal up to 15 gigapascals, then more slowly and reaches a T(c) = 150 kelvin, with the onset of the transition at 157 kelvin, for 23.5 gigapascals. This large pressure variation (as compared to the small effects observed in similar compounds with the optimal T(c)) strongly suggests that higher critical temperatures could be obtained at atmospheric pressure. 相似文献
4.
The elusive protonated ozone ion (O(3)H(+)) has been long postulated as a reactive intermediate but never experimentally observed. This ion has been detected here in mass spectrometric experiments with the use of Fourier transform ion cyclotron resonance. In these experiments, ozone (O(3)) was protonated by strong acids-for example, H(3)(+), KrH(+), XeH(+), and CH(5)(+). The hitherto experimentally unknown proton affinity of O(3) was evaluated by a "bracketing" technique and determined to be 148 -/+ 3 kilocalories mole(-1) at 298 kelvin, in excellent agreement with a value determined in a recent theoretical study of the O(3)/O(3)H(+) system, which was 148 kilocalories mole(-1) at zero temperature ( approximately 149.5 kilocalories mole(-1) at 298 kelvin). 相似文献
5.
Sundström G Mowat JR Danared H Datz S Broström L Filevich A Källberg A Mannervik S Rensfelt KG Sigray P Ugglas MA Larsson M 《Science (New York, N.Y.)》1994,263(5148):785-787
Knowledge of the abundance of H(3)(+) is needed in interstellar and planetary atmospheric chemistry. An important destruction mechanism of H(3)(+) is low-energy electron impact followed by dissociation, but estimates of the reaction rate span several orders of magnitude. As an attempt to resolve this uncertainty, the cross section for dissociative recombination of vibrationally cold H(3)(+) has been measured with an ion storage ring down to collision energies below 1 millielectron volt. A rate coefficient of 1.15 x 10(-7) cubic centimeters per second at 300 kelvin was deduced. The cross section scaled with collision energy according to E(-1.15), giving thee rate a temperature dependence of T(-0.65). 相似文献
6.
The Density of Hydrous Magmatic Liquids 总被引：2，自引：0，他引：2
Density measurements on several hydrous (相似文献
7.
Wang ZR Wang XL Fernandez-Baca JA Johnston DC Vaknin D 《Science (New York, N.Y.)》1994,264(5157):402-404
Magnetization and neutron diffraction measurements on polycrystalline BaCuO2+x revealed a combination of magnetic behaviors. The Cu(6) ring clusters and Cu(18) sphere clusters in this compound had ferromagnetic ground states with large spins 3 and 9, respectively. The Cu(6) rings ordered antiferromagnetically below the Néel temperature T(N) = 15 +/- 0.5 kelvin, whereas the Cu(18) spheres remained paramagnetic down to 2 kelvin. The ordered moment below T(N) was 0.89(5) Bohr magnetons per Cu in the Cu(6) rings, demonstrating that quantum fluctuation effects are small in these atomic clusters. The Cu(18) clusters are predicted to exhibit ferromagnetic intercluster order below about 1 kelvin. 相似文献
8.
Hydrogen (H2) was produced by aqueous-phase reforming of biomass-derived oxygenated hydrocarbons at temperatures near 500 kelvin over a tin-promoted Raney-nickel catalyst. The performance of this non-precious metal catalyst compares favorably with that of platinum-based catalysts for production of hydrogen from ethylene glycol, glycerol, and sorbitol. The addition of tin to nickel decreases the rate of methane formation from C-O bond cleavage while maintaining the high rates of C-C bond cleavage required for hydrogen formation. 相似文献
9.
Hydrogen tunneling in enzyme reactions 总被引：2，自引：0，他引：2
Primary and secondary protium-to-tritium (H/T) and deuterium-to-tritium (D/T) kinetic isotope effects for the catalytic oxidation of benzyl alcohol to benzaldehyde by yeast alcohol dehydrogenase (YADH) at 25 degrees Celsius have been determined. Previous studies showed that this reaction is nearly or fully rate limited by the hydrogen-transfer step. Semiclassical mass considerations that do not include tunneling effects would predict that kH/kT = (kD/kT)3.26, where kH, kD, and kT are the rate constants for the reaction of protium, deuterium, and tritium derivatives, respectively. Significant deviations from this relation have now been observed for both primary and especially secondary effects, such that experimental H/T ratios are much greater than those calculated from the above expression. These deviations also hold in the temperature range from 0 to 40 degrees Celsius. Such deviations were previously predicted to result from a reaction coordinate containing a significant contribution from hydrogen tunneling. 相似文献
10.
Johnson RD Yannoni CS Dorn HC Salem JR Bethune DS 《Science (New York, N.Y.)》1992,255(5049):1235-1238
The rotational dynamics of C(60) in the solid state have been investigated with carbon-13 nuclear magnetic resonance ((13)C NMR). The relaxation rate due to chemical shift anisotropy (1/9T1(CSA)(1)) was precisely measured from the magnetic field dependence of T(1), allowing the molecular reorientational correlation time, tau, to be determined. At 283 kelvin, tau = 9.1 picoseconds; with the assumption of diffusional reorientation this implies a rotational diffusion constant D = 1.8 x 10(10) per second. This reorientation time is only three times as long as the calculated tau for free rotation and is shorter than the value measured for C(60) in solution (15.5 picoseconds). Below 260 kelvin a second phase with a much longer reorientation time was observed, consistent with recent reports of an orientational phase transition in solid C(60). In both phases tau showed Arrhenius behavior, with apparent activation energies of 1.4 and 4.2 kilocalories per mole for the high-temperature (rotator) and low-temperature (ratchet) phases, respectively. The results parallel those found for adamantane. 相似文献
11.
The cross pressure (P) and temperature (T) dependence of the elastic moduli (Cij) of single-crystal samples of periclase (MgO) from acoustic wave travel times was measured with ultrasonic interferometry: partial differential2C11/ partial differentialP partial differentialT = (-1.3 +/- 0.4) x 10(-3) per kelvin; partial differential2C110/ partial differentialP partial differentialT = (1. 7 +/- 0.7) x 10(-3) per kelvin; and partial differential2C44/ partial differentialP partial differentialT = (-0.2 +/- 0.3) x 10(-3) per kelvin. The elastic anisotropy of MgO decreases with increasing pressure at ambient temperature, but then increases as temperature is increased at high pressure. An assumption of zero cross pressure and temperature derivatives for the elastic moduli underestimates the elastic anisotropy and overestimates the acoustic velocities of MgO at the extrapolated high-pressure and high-temperature conditions of Earth's mantle. 相似文献
12.
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. 相似文献
13.
Ultrasonic shear wave velocities of MgSiO3 perovskite at 8 GPa and 800 K and lower mantle composition 总被引：1，自引：0，他引：1
YD Sinelnikov G Chen DR Neuville MT Vaughan RC Liebermann 《Science (New York, N.Y.)》1998,281(5377):677-679
Ultrasonic interferometric measurements of the shear elastic properties of MgSiO3 perovskite were conducted on three polycrystalline specimens at conditions up to pressures of 8 gigapascals and temperatures of 800 kelvin. The acoustic measurements produced the pressure (P) and temperature (T) derivatives of the shear modulus (G), namely ( partial differentialG/ partial differentialP)T = 1.8 +/- 0.4 and ( partial differentialG/ partial differentialT)P = -2.9 +/- 0.3 x 10(-2) gigapascals per kelvin. Combining these derivatives with the derivatives that were measured for the bulk modulus and thermal expansion of MgSiO3 perovskite provided data that suggest lower mantle compositions between pyrolite and C1 carbonaceous chondrite and a lower mantle potential temperature of 1500 +/- 200 kelvin. 相似文献
14.
The dynamics of dangling bond (DB) diffusion was studied after deuterium desorption from the silicon (100)-2x1 surface. At elevated temperatures, paired DB sites produced after desorption unpaired as deuterium atoms hopped from adjacent dimers. Below 620 kelvin, the unpaired configuration most commonly observed corresponded to two DBs on adjacent silicon dimers. At higher temperatures, unpaired DBs executed one-dimensional walks along the dimer rows, and recombination was observed with the same partner after walks lasting many minutes. The frequency and extent of these excursions increased with temperature. Above 660 kelvin, complete dissociation was observed and was sometimes followed by recombination by means of partner exchange. The implications for low-temperature materials growth are discussed. 相似文献
15.
Accurate terrestrial glacial chronologies are needed for comparison with the marine record to establish the dynamics of global climate change during transitions from glacial to interglacial regimes. Cosmogenic beryllium-10 measurements in the Wind River Range indicate that the last glacial maximum (marine oxygen isotope stage 2) was achieved there by 21,700 +/- 700 beryllium-10 years and lasted 5900 years. Ages of a sequence of recessional moraines and striated bedrock surfaces show that the initial deglaciation was rapid and that the entire glacial system retreated 33 kilometers to the cirque basin by 12,100 +/- 500 beryllium-10 years. 相似文献
16.
Berndt T Böge O Stratmann F Heintzenberg J Kulmala M 《Science (New York, N.Y.)》2005,307(5710):698-700
We investigated the formation of new particles in a laboratory study, starting from H2SO4 produced in situ through the reaction of OH radicals with SO2. Newly formed particles were observed for H2SO4 concentrations above 7 x 10(6) per cubic centimeter. At 293 kelvin, a rough estimate yielded a nucleation rate of 0.3 to 0.4 particles per cubic centimeter per second for approximately 10(7) particles per cubic centimeter of H2SO4 (particle size >/= 3 nanometers). These findings are in agreement with observations from the atmosphere. The results demonstrate that under laboratory conditions similar to the atmosphere, particle formation occurs at atmospheric H2SO4 concentration levels. 相似文献
17.
The existence of a relatively long-lived doubly charged negative atomic ion H(2-) (and D(2-)), isoelectronic with the lithium atom, has been demonstrated by mass spectrometry through a combined analysis of ion energy, velocity, and momentum. This species, formed in a hydrogen plasma, has a half-life of 2.3 x 10(-8) seconds before it spontaneously dissociates to produce H(-) ions. 相似文献
18.
The molecular volume of crystals depends on their isotopic masses. This influence originates from the zero-point motion and the resulting small differences in lattice constants. This effect was measured with high precision by using an x-ray standing wave. The standing wave is generated during Bragg reflection and thus is in phase with the planes of the substrate crystal, which is covered with a homoepitaxial film that has a different isotopic composition than the substrate. The positions of the surface planes of the film with respect to the substrate planes are revealed by the photoelectrons excited by the maxima of the standing wave. For germanium-76 on natural germanium(111), a difference in lattice constant of -1.1 x 10(-5) and -2.5 x 10(-5) at 300 and 54 kelvin, respectively, was found. The results are in good agreement with theoretical predictions. 相似文献
19.
The puzzling mass-independent isotopic enrichment in ozone formation contrasts markedly with the more recently observed large unconventional mass-dependent ratios of the individual ozone formation rate constants in certain systems. An RRKM (Rice, Ramsperger, Kassel, Marcus)-based theory is used to treat both effects. Restrictions of symmetry on how energy is shared among the rotational/vibrational states of the ozone isotopomer, together with an analysis of the competition between the transition states of its two exit channels, permit the calculation of isotope effects consistent with a wide array of experimental results. 相似文献
20.
The responses of Earth's and Mars' thermospheres to the quasi-periodic (27-day) variation of solar flux due to solar rotation were measured contemporaneously, revealing that this response is twice as large for Earth as for Mars. Per typical 20-unit change in 10.7-centimeter radio flux (used as a proxy for extreme ultraviolet flux) reaching each planet, we found temperature changes of 42.0 +/- 8.0 kelvin and 19.2 +/- 3.6 kelvin for Earth and Mars, respectively. Existing data for Venus indicate values of 3.6 +/- 0.6 kelvin. Our observational result constrains comparative planetary thermosphere simulations and may help resolve existing uncertainties in thermal balance processes, particularly CO2 cooling. 相似文献