Search

Since the discovery of C60 fullerene, considerable efforts have been devoted to find other elements with similar hollow cage structures. However, search for hollow metallic cages with a diameter similar to that of C60 fullerene has been elusive. We describe a procedure for the rational design of metallic cages by suitably choosing their size, composition, and charge state. A 22 karat nanogolden cage with a diameter of about 8.5 Å and consisting of 12 Al and 20 Au atoms is found to be metastable, which can be stabilized by embedding a Mn4 cluster. In contrast to bulk Mn, which is antiferromagnetic, and isolated Mn4 cluster, which is ferromagnetic with a giant magnetic moment of 20μB, the Mn4@Al12Au20 endohedral complex exhibits magnetic bistability with 0μB and 14μB configurations being energetically nearly degenerate. These results, based on density functional theory, open the door to design a novel class of endohedral complexes with possible applications. [ABSTRACT FROM AUTHOR]

A first-principles calculation based on gradient corrected density functional theory reveals unusual properties of a Cu atom interacting with F. Up to six F atoms are bound to a single Cu atom with electron affinities steadily rising as successive F atoms are attached, reaching a peak value of 7.2 eV in CuF5. The large energy gaps between the highest occupied and lowest unoccupied molecular orbitals, both in neutral and anionic form, provide further evidence of their stability. These unusual properties brought about by involvement of inner shell 3d-electrons not only allow CuFn to belong to the class of superhalogens but also show that its valence can exceed the nominal value of 1 and 2. [ABSTRACT FROM AUTHOR]

Contrary to the conventional understanding that atomic clusters usually differ in properties and structure from the bulk constituents of which they are comprised, we show that even a dimer of tungsten oxide (WO3)2 possesses bulklike features and the geometry of a small cluster containing only 4 tungsten and 12 oxygen atoms bears the hallmarks of crystalline tungsten oxide, WO3. This observation, based on a synergistic approach involving mass distributions under quasisteady state conditions, photoelectron spectroscopy, and first principles molecular orbital theory, not only illustrates the existence of a class of strongly covalent or ionic materials whose embryonic forms are tiny clusters but also lends the possibility that a fundamental understanding of complex processes such as catalytic reactions on surfaces may be achieved on an atomic scale with clusters as model systems. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using density functional theory with generalized gradient approximation for exchange and correlation, we show that otherwise antiferromagnetically coupled chromium atoms in very small chromium clusters couple ferromagnetically when doped with a nitrogen atom, thus leading to giant magnetic moments. For example, the magnetic moment of Cr[sub 2]N is found to be 9μ[sub B] while that of Cr[sub 2] is 0μ[sub B]. Strong bonding between Cr and N atoms brings about this magnetic transition. The Cr atoms nearest neighbor to N couple ferromagnetically with each other and antiferromagnetically with nitrogen. The significance of these results in understanding the ferromagnetic order in Cr-doped GaN is discussed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The vibrational and electronic structures of p-phenylenediamine radical cation have been studied using time-resolved resonance Raman spectroscopy and molecular orbital calculations. The Raman spectra in aqueous solution show striking variations in intensity profile when excitation is varied from 340 to 480 nm. Excitation in resonance with the 460–480 nm absorption shows enhancement of only totally symmetric vibrations. Most prominent are the δNH2 scissor mode at 1658 cm-1 and the Wilson modes ν8a (CC stretch) at 1644 cm-1, ν7a (CN stretch) at 1423 cm-1, and ν9a (CH bend) at 1184 cm-1. Also observed are ν1 (ring breathe) at 840 cm-1 and ν6a (CCC bend) at 467 cm-1. With excitation in the 340–410 nm region the ν1 band becomes relatively stronger and an additional band at 1524 cm-1 appears. This latter band, which dominates the spectrum at 360 nm, is assigned to the nontotally symmetric vibration ν8b (CC stretch) having b3g symmetry that gains intensity through vibronic coupling. Raman spectra of ring- and amine-deuterated radicals allow distinction of overtone and combination bands enhanced by Fermi resonance from fundamentals due to δNH2 and ν8a (CC stretch) modes in the 1600–1700 cm-1 region. The theoretical calculations show reasonable agreement with the observed vibrational frequencies and lead to detailed information on the bond structure and normal modes of the radical. The calculated CN bond lengths of 1.33 Å are intermediate between lengths typical of single and double bonds in aromatic amines. The calculations also allow interpretation of the experimental absorption spectrum. The broad medium intensity absorption peaked at 460 nm with a shoulder at 480 nm is assigned to a 2B3u ←2B2g transition, the very weak 360 nm absorption to 2Au ← 2B2g and the very strong ∼325 nm absorption to a higher 2B3u ← 2B2g.Calculations with the simple relation Ik∝... [ABSTRACT FROM AUTHOR]

New experimental rate coefficients are reported for the H2(D2)+CN→H(D)CN+H(D) reactions over the temperature range 209 to 740 K for H2 and 250 to 740 K for D2. Previous reduced dimensionality reaction probabilities for the reaction with H2, and new ones for the reaction with D2 are used to obtain analogous rate coefficients. In addition, reaction probabilities and rate coefficients for vibrationally excited reactants H2(v=1), D2(v=1), or CN(v=1) are presented. Comparisons of the calculated rate coefficients are made with the new and previous experiments, especially those of Sims and Smith [Chem. Phys. Lett. 149, 565 (1988)]. [ABSTRACT FROM AUTHOR]

A hybrid stochastic simulation method is developed to study H2–O2 auto-ignition at the microscale. Simulation results show that the discrete and stochastic characteristics of reaction collisions have notable impacts on the ignition process, particularly in the early stages when only a few radicals exist. The statistical properties of ignition delay time, which reflect the accumulated stochasticity during ignition, are obtained and analyzed for different initial temperatures and total molecular numbers. It is found that the average and standard deviation of ignition delay time increase as the total molecular number decreases, with this phenomenon being particularly pronounced near the crossover temperature. When the total molecular number is sufficiently small, the chain initiation reaction becomes crucial to the stochastic properties, as its average firing time exhibits an inverse proportionality to the total molecular number. As the total molecular number increases, the influence of other chain reactions intensifies, causing the power law relation between standard deviation and total molecular number to shift from −1 power to −0.5 power. Owing to different chain reaction paths for high- and low-temperature auto-ignition, the strongest relative fluctuation occurs near the crossover temperature. A theoretical equation for the standard deviation of ignition delay time is obtained based on dimensional analysis, giving excellent agreement with the simulation results in both high- and low-temperature modes. [ABSTRACT FROM AUTHOR]

Presents a correction to the article "#x201C;Appearance of bulk properties in small tungsten oxide clusters”," previously published in the periodical.

First-principles calculations are used to systematically investigate the geometric and electronic structures of both pure TMn (n=2–4) and Ag-modulated AgTMn-1 (n=2–4; 3d-transition metal (TM): from Sc to Cu; 4d-TM: from Y to Ag elements) clusters. Some new ground state structures are found for the pure TMn clusters, such as a low symmetry configuration for Cr3, which is found to be about 0.20 eV more stable than the previously reported C2v symmetry. In the most cases, Ag-doping can significantly elongate the bond lengths of the clusters and induce geometric distortions of the small clusters from the high dimensional to the low dimensional configurations. Importantly, introduction of Ag significantly changes the electronic structures of the small clusters and modulates the density of states in the proximity of the Fermi levels, which also varies with the size and the type of the cluster. The results contribute to future design of effective bimetallic alloy Ag/TM catalysts. [ABSTRACT FROM AUTHOR]