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81.
The strong drive to commercialize fuel cells for portable as well as transportation power sources has led to the tremendous
growth in fundamental research aimed at elucidating the catalytic paths and kinetics that govern the electrode performance
of proton exchange membrane (PEM) fuel cells. Advances in theory over the past decade coupled with the exponential increases
in computational speed and memory have enabled theory to become an invaluable partner in elucidating the surface chemistry
that controls different catalytic systems. Despite the significant advances in modeling vapor-phase catalytic systems, the
widespread use of first principle theoretical calculations in the analysis of electrocatalytic systems has been rather limited
due to the complex electrochemical environment. Herein, we describe the development and application of a first-principles-based
approach termed the double reference method that can be used to simulate chemistry at an electrified interface. The simulations mimic the half-cell analysis that is
currently used to evaluate electrochemical systems experimentally where the potential is set via an external potentiostat.
We use this approach to simulate the potential dependence of elementary reaction energies and activation barriers for different
electrocatalytic reactions important for the anode of the direct methanol fuel cell. More specifically we examine the potential-dependence
for the activation of water and the oxidation of methanol and CO over model Pt and Pt alloy surfaces. The insights from these
model systems are subsequently used to test alternative compositions for the development of improved catalytic materials for
the anode of the direct methanol fuel cell. 相似文献
82.
Pier Luigi Silvestrelli Mario Gleria Roberto Milani Angelo Boscolo Boscoletto 《Journal of Inorganic and Organometallic Polymers and Materials》2006,16(4):327-341
The interaction of hexachlorocyclophosphazene (HCCP) with the hydroxylated silicon-based surfaces is studied by a combination
of XPS experimental analysis and theoretical ab initio simulations, including the estimate of the energy barriers via CI-NEB method to determine the minimum energy reaction paths.
Several possible, stable chemisorbed configurations are found to exist, whose structural, energetic, and electronic properties have been analyzed in detail. The
theoretical results are in good qualitative agreement with available experimental data. In particular, we get indications
that water plays a crucial role in the surface functionalization with HCCP, although the presence of a suitable solvent (e.g.
tetrahydrofuran, THF) seems to be essential.
This paper is dedicated to Prof. Harry R. Allcock and the pioneering discoveries of his research group particularly in polyphospazenes
and generally in materials science. 相似文献
83.
S2分子是一种在能源和激光领域有应用前景的物质,几十年来,人们对S2分子的电子结构与光谱的研究不断深入。应用量子化学从头算方法(完全活性空间多组态自洽场方法和多参考二阶微扰方法)计算了S2分子的11个电子态,并与其他文献结果作了比较,得到了与其他文献符合较好的结果;讨论了不同方法对激发态计算的适用性的问题:完全活性空间多组态自洽场方法适用于分子平衡态几何构形的优化,考虑到电子的动力学相关,能量的计算多参考二阶微扰方法更好。计算了S2分子X3Σ-g(v)→B3Σ-u的垂直吸收谱,给出了速度规范下基态电子态各振动级X3Σ-g(v)垂直跃迁到B3Σ-u的振子强度,结果表明S2分子是一种泵浦范围较宽(350~600 nm)的良好的激光工作物质。 相似文献
84.
Takahiro Ohkubo Eiji Tsuchida Kenzo Deguchi Shinobu Ohki Tadashi Shimizu Toshiya Otomo Yasuhiko Iwadate 《Journal of the American Ceramic Society》2018,101(3):1122-1134
It remains a challenge to establish structural models of multicomponent oxide glass systems. In this study, we have investigated 68.3SiO2–16.1B2O3–4.2Al2O3–11.4Na2O glass and melt structures by ab initio molecular dynamics (AIMD) simulations. The atomic configurations obtained from AIMD simulations were validated against 17O solid‐state NMR spectrum under 24.0 T and neutron diffraction data, and excellent agreement was achieved. The bond lengths, angles, and coordination geometries were statistically analyzed for each atomic species. Here we particularly address the role of minor atomic species such as five‐coordinate Si (SiV) and Al (AlV). The SiV–O bond lengths and O–SiV–O angle distribution in the glass indicated 1.718 Å and three peaks at 90°, 120°, and 175°, which are assigned to a coordination geometry of the trigonal bipyramidal structure. Ring statistic analysis revealed that SiV and AlV were found to preferentially contribute to the formation of small ring sizes. 相似文献
85.
Ab initio molecular dynamics studies have been carried out on the room temperature ionic liquid, 1,n-butyl,3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and supercritical carbon dioxide mixture at room temperature and experimental density. Partial radial distribution functions
(RDF) for different sites have been computed to see the organization of CO2 molecules around the ionic liquid. Several partial RDFs around the carbon atom of CO2 molecule are compared to find out that the CO2 has specific interaction with a carbon atom present in the imidazolium ring. The CO2 is also found to be very well organized around the terminal carbon atom of the butyl chain. The partial RDFs for the oxygen
atoms around oxygen and carbon atoms of the CO2 suggests that there is very good organization of CO2 molecules around themselves even in the [bmim][PF6]-CO2 mixture. The instantaneous quadrupole moment tensor has been calculated for the anion and the cation. The ensemble average
of diagonal components of quadrupole moment tensor of the cation have finite values, whereas the off-diagonal components of
the cation and both the diagonal and off-diagonal components of the anion have the value of zero with a large standard deviation.
The CPMD studies performed on CO2 clusters reveals the greater tendency of the clusters with more CO2 units, to deviate from the linear geometry. 相似文献
86.
Friction between ice-Ih and hexagonal boron nitride (h-BN) has been determined by periodic ab initio calculations. Surfaces
of ice-Ih and h-BN were brought into sliding contact, and the interaction energies were calculated as a function of interplanar
distance and lateral displacement of the surfaces. The friction between the surfaces was calculated from the interaction energies,
producing a friction coefficient of 0.140. Friction is further influenced at high loads by a tribochemical reaction between
ice-Ih and h-BN. 相似文献
87.
A comparative study of the reported performance of ab initio protein structure prediction algorithms
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Glennie Helles 《Journal of the Royal Society Interface》2008,5(21):387-396
Protein structure prediction is one of the major challenges in bioinformatics today. Throughout the past five decades, many different algorithmic approaches have been attempted, and although progress has been made the problem remains unsolvable even for many small proteins. While the general objective is to predict the three-dimensional structure from primary sequence, our current knowledge and computational power are simply insufficient to solve a problem of such high complexity.Some prediction algorithms do, however, appear to perform better than others, although it is not always obvious which ones they are and it is perhaps even less obvious why that is. In this review, the reported performance results from 18 different recently published prediction algorithms are compared. Furthermore, the general algorithmic settings most likely responsible for the difference in the reported performance are identified, and the specific settings of each of the 18 prediction algorithms are also compared.The average normalized r.m.s.d. scores reported range from 11.17 to 3.48. With a performance measure including both r.m.s.d. scores and CPU time, the currently best-performing prediction algorithm is identified to be the I-TASSER algorithm. Two of the algorithmic settings--protein representation and fragment assembly--were found to have definite positive influence on the running time and the predicted structures, respectively. There thus appears to be a clear benefit from incorporating this knowledge in the design of new prediction algorithms. 相似文献
88.
89.
Mizuka Tagashira Satoru Yamazaki Shozo Yamanaka 《Computational Materials Science》1999,14(1-4):248-253
The 13C and 1H NMR shielding constants for -methylmannose and -methylmannobiose have been calculated using the ab initio gauge-including atomic orbital (GIAO) method to study the conformational dependencies of the NMR chemical shifts of the sugars. The molecular structures were fully optimized using B3LYP/6-31G* and the NMR shielding constants were calculated at both Hartree–Fock (HF) and density functional (DF) levels of theory with various kinds of basis sets. The values determined using the B3LYP hybrid functional were a little closer to those obtained experimentally than those determined at the HF level. Both, HF and B3LYP with the 6-311+G(2d,p) basis were found to give a very good correlation between the experimental and calculated shielding constants, especially for 13C. 相似文献
90.
The bonding of molecularly‐thin (10 Å) Zdol 4000 films to amorphous, hydrogenated carbon (CHx) was investigated as a function of the Zdol structure, i.e., the ratio of the perfluoromethylene oxide (C1) to perfluoroethylene oxide (C2) monomer units in the backbone. The influence of the C1/C2 ratio on the intrinsic mobility of the Zdol polymer was also investigated by computing the energetic barriers to internal rotation about the C–O and C–C bonds in model compounds by both ab initio and molecular mechanics methods. The calculations indicate that increasing the C1/C2 ratio increases the relative flexibility of the Zdol polymer. The kinetic results demonstrate that the rate at which submonolayer Zdol films bond to CHx is non‐classical (time‐dependent) regardless of the Zdol chain stiffness. The Zdol bonding rate can best be described by a kinetic equation of the form, dB/dt=k(t)A, where the rate coefficient, k(t) can be expressed as a power function in time: k(t)= kB t -h. The values of the initial bonding rate constant, k B, and the functional form of the time dependence, t -h, are both strongly dependent on the Zdol backbone flexibility. The magnitude of the initial bonding rate constants generally increase with increasing Zdol chain mobility. A discontinuous change in both the magnitude of k B and the functional form of the time dependence is, however, observed at 64°C when the C1/C2 ratio is increased from 0.97 to 1.08. The bonding rate coefficient scales as t -0.5 for the relatively rigid Zdol backbone structures with C1/C2 < 1, while a t -1.0 time‐dependent bonding rate is observed for the more flexible Zdol backbones with C1/C2 < 1. The initial rate constant, k B, also changes abruptly near C1/C2 ≈ 1, with k B of the flexible Zdol chains (samples with C1/C2) being approximately an order of magnitude greater than the more rigid chains (C1/C2 < 1). These results indicate that the physical state of the confined Zdol film can be either liquidlike or solidlike depending upon the molecular stiffness of the backbone employed. The t -0.5 time‐dependent bonding rate is shown to be consistent with a one‐dimensional, diffusion‐limited reaction from a solidlike Zdol structure, whereas the t -1.0 bonding rate results when bonding occurs from a liquidlike Zdol film structure. The temperature dependence of the Zdol 4000 bonding rate coefficient for the Zdol backbone characterized by C1/C2 = 0.97 (solidlike at T = 64°C) was found to undergo a transition from a t -0.5 time dependence for T < 150°C, to a t -1.0 time dependence at T > 180. This transition occurs over relatively narrow temperature range (150 < T < 180°C) and is attributed to a 2D melting of the confined Zdol film. 相似文献