Vibrational ground state properties of H(5)(+) and its isotopomers from diffusion Monte Carlo calculations

Diffusion Monte Carlo computations, with and without importance sampling, of the zero-point properties of H(5)(+) and its isotopomers using a recent high accuracy global potential energy surface are presented. The global minimum of the potential possesses C(2v) symmetry, but the calculations predict a D(2d) geometry for zero-point averaged structure of H(5)(+) with one H atom "in the middle" between two HH diatoms. The predicted zero-point geometries of the deuterated forms have H in the middle preferred over D in the middle and for a nonsymmetric arrangement of D atoms the preferred arrangement is one which maximizes the number of D as the triatomic ion. We speculate on the consequences of these preferences in scattering of H(2)+H(3)(+) and isotopomers at low energies, such as those in the interstellar medium.     

Oxygen island formation on Pt(111) studied by dynamic Monte Carlo simulation

The formation of oxygen islands on the Pt(111) surface has been studied as a function of temperature by low energy electron diffraction (LEED) experiments and dynamic Monte Carlo (DMC) simulations. By raising the temperature, the (2 x 2) LEED spot intensity increases gradually and decays after a peak at around 255 K (T(p)) with full width of half maximum of 160 K. This behavior is interpreted by DMC simulations with the kinematical LEED analysis. In the DMC simulation, an oxygen atom hops to the neighboring site via the activation barrier of the saddle point. The potential energies at initial, saddle, and final points are changed at each hopping event depending on the surrounding oxygen atoms. By comparing the observed T(p) with the simulated one, the interaction energy E of oxygen atoms on Pt(111) was determined to be 25+/-3 meV at 2a(0). The DMC simulations visualize how the oxygen islands are formed and collapse on Pt(111) with increase of the temperature and well reproduce the surface configurations observed by scanning tunneling microscopy.     

Full-dimensional quantum calculations of ground-state tunneling splitting of malonaldehyde using an accurate ab initio potential energy surface

Quantum calculations of the ground vibrational state tunneling splitting of H-atom and D-atom transfer in malonaldehyde are performed on a full-dimensional ab initio potential energy surface (PES). The PES is a fit to 11 147 near basis-set-limit frozen-core CCSD(T) electronic energies. This surface properly describes the invariance of the potential with respect to all permutations of identical atoms. The saddle-point barrier for the H-atom transfer on the PES is 4.1 kcalmol, in excellent agreement with the reported ab initio value. Model one-dimensional and "exact" full-dimensional calculations of the splitting for H- and D-atom transfer are done using this PES. The tunneling splittings in full dimensionality are calculated using the unbiased "fixed-node" diffusion Monte Carlo (DMC) method in Cartesian and saddle-point normal coordinates. The ground-state tunneling splitting is found to be 21.6 cm(-1) in Cartesian coordinates and 22.6 cm(-1) in normal coordinates, with an uncertainty of 2-3 cm(-1). This splitting is also calculated based on a model which makes use of the exact single-well zero-point energy (ZPE) obtained with the MULTIMODE code and DMC ZPE and this calculation gives a tunneling splitting of 21-22 cm(-1). The corresponding computed splittings for the D-atom transfer are 3.0, 3.1, and 2-3 cm(-1). These calculated tunneling splittings agree with each other to within less than the standard uncertainties obtained with the DMC method used, which are between 2 and 3 cm(-1), and agree well with the experimental values of 21.6 and 2.9 cm(-1) for the H and D transfer, respectively.     

Structure and stability of Ne+He(n): experiment and diffusion quantum Monte Carlo theory with "on the fly" electronic structure

New data are reported for the mass-spectrometry fragmentation patterns of helium clusters, either pure or containing a Ne or an Ar atom. The patterns for He(n)+ and Ar+He(n) show clear evidence of structure, while those of Ne+He(n) do not. To better understand the surprising result for the Ne+He(n) fragments, diffusion quantum Monte Carlo (DMC) calculations of the energies and structural properties of these ions were performed using a diatomics-in-molecule (DIM) parametrization of the potential energy. Using DIM for electronic energy evaluation allows us to sample 10(9) configurations even for a cluster as large as Ne+He14. The results of the DMC calculation are very surprising. For n > 7, the DMC random walkers rarely venture within 100 cm(-1) of the minimum potential energy. Analysis of the resulting particle density distributions shows that the zero-point energy does more than spread the wave function around the potential-energy minima, resulting in very diffuse wave functions. For some of the clusters the quantum effects nearly exclude the region of the potential minimum from the overall wave function. An important result of this effect is that the incremental bonding energy of the nth helium atom varies quite smoothly with n, for n > 5. This eliminates the expected shell structure and explains the lack of magic-number-type features in the data.     

H-ZSM-5催化甲苯与碳酸二甲酯和甲醇甲基化的反应机理

对二甲苯是重要的石油化工产品之一, 可以通过甲苯甲基化生产. 本文采用“our own-N-layeredintegrated molecular orbital+molecular mechanics”(ONIOM)和密度泛函理论(DFT)结合的方法, 计算了H-ZSM-5催化甲苯与碳酸二甲酯(DMC)和甲醇甲基化反应机理. 考察了反应物吸附和产物脱附. 描述了主要的中间物种和过渡态的结构. 用计算的速率常数来估计甲苯甲基化反应的动力学活性. H-ZSM-5 催化的甲苯与DMC和甲醇甲基化的机理不同. 甲苯和DMC甲基化包括DMC完全解离, 接着甲基化生成二甲苯异构体. 相比而言, 在甲苯甲基化反应中, 甲醇作为甲基化试剂的活性比DMC更好. 甲苯和甲醇甲基化的分步反应路径和联合反应路径的本征活化能相似. 在773 K, 分步反应路径的速率常数比联合反应路径更高. 在甲苯和这两种试剂甲基化的反应中, 生成对二甲苯为动力学优先, 而间二甲苯为能量最低产物. 我们的计算结果和实验观察到的现象一致.     

Full-dimensional vibrational calculations for H5O2+ using an ab initio potential energy surface

We report quantum diffusion Monte Carlo (DMC) and variational calculations in full dimensionality for selected vibrational states of H(5)O(2) (+) using a new ab initio potential energy surface [X. Huang, B. Braams, and J. M. Bowman, J. Chem. Phys. 122, 044308 (2005)]. The energy and properties of the zero-point state are focused on in the rigorous DMC calculations. OH-stretch fundamentals are also calculated using "fixed-node" DMC calculations and variationally using two versions of the code MULTIMODE. These results are compared with infrared multiphoton dissociation measurements of Yeh et al. [L. I. Yeh, M. Okumura, J. D. Myers, J. M. Price, and Y. T. Lee, J. Chem. Phys. 91, 7319 (1989)]. Some preliminary results for the energies of several modes of the shared hydrogen are also reported.     

Heavy-Atom Tunneling in Semibullvalenes: How Driving Force,Substituents, and Environment Influence the Tunneling Rates

The Cope rearrangement of selectively deuterated isotopomers of 1,5-dimethylsemibullvalene 2 a and 3,7-dicyano-1,5-dimethylsemibullvalene 2 b were studied in cryogenic matrices. In both semibullvalenes the Cope rearrangement is governed by heavy-atom tunneling. The driving force for the rearrangements is the small difference in the zero-point vibrational energies of the isotopomers. To evaluate the effect of the driving force on the tunneling probability in 2 a and 2 b , two different pairs of isotopomers were studied for each of the semibullvalenes. The reaction rates for the rearrangement of 2 b in cryogenic matrices were found to be smaller than the ones of 2 a under similar conditions, whereas differences in the driving force do not influence the rates. Small curvature tunneling (SCT) calculations suggest that the reduced tunneling rate of 2 b compared to that of 2 a results from a change in the shape of the potential energy barrier. The tunneling probability of the semibullvalenes strongly depends on the matrix environment; however, for 2 a in a qualitatively different way than for 2 b .     

Force field development for poly(dimethylsilylenemethylene) with the aid of ab initio calculations

The molecular geometries, conformational energies, and zero-point energies of di(trimethylsilylene)methylene have been determined from high-level quantum chemistry calculations. The results are further used in the parametrization of a classical potential energy function suitable for performing simulations of the corresponding polymer, namely, poly(dimethylsilylenemethylene). Di(trimethylsilylene)methylene geometrical parameter optimizations for a proper location of the global minimum and other local minima, constrained at certain dihedral and bond angles, were performed at both the B3LYP/6-311G and MP2(full)/6-311G levels of theory. The global minimum configuration is slightly displaced from a perfectly staggered geometry, approximately by 16.0 degrees, at both levels of theory. Molecular mechanics and Monte Carlo calculations for isolated polymer chains together with molecular dynamics runs for the modeled dimer provide very good results in terms of conformational and thermodynamic properties.     

General methodology in two dimensions for classical simulation of reactive and nonreactive events on ab initio potential energy surfaces

A completely general two-dimensional (2D) methodology for the classical simulation of reactive and nonreactive events on ab initio potential energy surfaces is introduced and tested. The methodology requires the minimum amount of information given a priori—geometries and energies at these geometries. From a list of ab initio geometries and energies, simulations may be executed and a distribution of outcomes obtained. The method introduced attempts a local approach at simulating the dynamics of the system, rather than a global analytic fit to the potential energy surface. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1431–1444, 1998     

A computational study of the dihydrogen bonded complexes HBeH ... HArF and HBeH ... HKrF

We report an ab initio computational study of the properties of two linear dihydrogen-bonded complexes of HBeH with the recently discovered rare gas compounds HArF and HKrF at the MP2(full)/6-311++G(2d,2p) level of theory. The HBeH ... HArF and HBeH ... HKrF complexes were found to have zero-point energy corrected binding energies of 27 and 12 kJ mol(-1), respectively. Large red shifts of the H-Rg vibrational stretching frequency in both complexes were also predicted. The electron density rearrangement of the rare gas compounds on complexation was also examined. We also consider the relative stabilities of D-containing isotopomers of the complexes by comparison of their computed zero-point vibrational energies.     

DFT Study of Methanol Adsorption on Defect-Free CeO2 Low-Index Surfaces

Methanol decomposition is a promising method for hydrogen production. However, the performance of current catalysts for this process is not sufficient for commercial applications. In this work, methanol adsorption on the CeO₂ low-index surfaces is studied by density functional theory (DFT). The results show that methanol always dissociates spontaneously on the (100) surface, whereas dissociation on the (110) surface is site-selective; dissociation does not occur at all on the (111) surface, where only weak physisorption is found. The results confirm that surfaces with higher energies are more catalytically active. Analysis of the surface geometries shows that the dominant factors for the dissociation of methanol are the degree of undercoordination and the charges of the surface ions. The adsorption energy of each methanol molecule decreases with increasing coverage and there is a transition threshold between dissociative and associative adsorption. The present work indicates that a strategy to design catalysts with high activity is to maximize exposure of surfaces on which the ions have a high degree of undercoordination and a strong tendency to donate/accept electrons. The results demonstrate the importance of appropriately selecting and controlling exposed facets and particle morphology for optimizing catalyst performance.     

Ab initio potential energy surface and bound states of the Xe-CO complex

The first two-dimensional potential energy surface for the Xe-CO van der Waals interaction is calculated by the single and double excitation coupled-cluster theory with noniterative treatment of triple excitations. Mixed basis sets, aug-cc-pVQZ for the C and O atoms, and aug-cc-pVQZ-PP for the Xe atom, with an additional (3s3p2d2f1g) set of midbond functions, are used. Our potential energy surface has a single, nearly T-shaped minimum of -131.87 cm(-1) at R(e)=7.80a(0) and theta(e)=102.5 degrees. Based on the potential, the bound state energies are calculated for seven isotopomers of the Xe-(12)C(16)O complex, seven isotopomers of the Xe-(13)C(16)O complex, and three isotopomers of the Xe-(13)C(18)O complex. Compared with available experimental data, the predicted transition frequencies and spectroscopic constants are in good agreement with the experimental results.     

二氧化碳和甲醇合成碳酸二甲酯研究进展

本文综述了以二氧化碳和甲醇为起始原料合成碳酸二甲酯（DMC）的研究状况。提出以CO₂和甲醇直接合成DMC,不仅对合成化学,碳资源利用和环境保护具有重大意义,而且可使生产成本显著降低,它是发展DMC生产的一个具有很大潜力的方面。     

Ar(n)HF van der Waals clusters revisited: II. Energetics and HF vibrational frequency shifts from diffusion Monte Carlo calculations on additive and nonadditive potential-energy surfaces for n=1-12

The ground-state energies and HF vibrational frequency shifts of Ar(n)HF clusters have been calculated on the nonadditive potential-energy surfaces (PESs) for n=2-7 and on the pairwise-additive PESs for the clusters with n=1-12, using the diffusion Monte Carlo (DMC) method. For n>3, the calculations have been performed for the lowest-energy isomer and several higher-lying isomers which are the closest in energy. They provide information about the isomer dependence of the HF redshift, and enable direct comparison with the experimental data recently obtained in helium nanodroplets. The agreement between theory and experiment is excellent, in particular, for the nonadditive DMC redshifts. The relative, incremental redshifts are reproduced accurately even at the lower level of theory, i.e., the DMC and quantum five-dimensional (rigid Ar(n)) calculations on the pairwise-additive PESs. The nonadditive interactions make a significant contribution to the frequency shift, on the order of 10%-12%, and have to be included in the PESs in order for the theory to yield accurate magnitude of the HF redshift. The energy gaps between the DMC ground states of the cluster isomers are very different from the energy separation of their respective minima on the PES, due to the considerable variations in the intermolecular zero-point energy of different Ar(n)HF isomers.     

Structure and stability of solvation complexes of trimethyl phosphate and dimethyl methyl phosphonate

The presently used electrolytes in Lithium ion batteries, dimethyl carbonate (DMC), and ethylene carbonate are flammable. Trimethyl phosphate (TMP) and dimethyl methyl phosphonate (DMMP) have been shown to be potential nonflammable electrolytes. Density functional theory is used to calculate the structure and stability of the solvation complexes of TMP and DMMP. The calculations indicate that TMP and DMMP can form a solvation complex of the form Li⁺(X)₄ where X is the TMP or DMMP molecule. Calculations of the solvation energy and bond dissociation energies to remove one TMP and DMMP from the solvation complexes are compared with the same calculations on DMC. The results indicate that TMP and DMMP are considerably more stable than DMC.     

Kinetics of Dimethyl Carbonate Synthesis From Ethylene Carbonate and Methanol Using Alkali-Metal Compounds as Catalysts

Dimethyl carbonate (DMC) was synthesized by the ester exchange reaction of ethylene carbonate and methanol. K₂ CO₃ , KOH, LiOH and NaOH were used as catalysts for DMC synthesis and their catalytic abilities were compared in terms of kinetics. LiOH showed the best reaction rate among the catalysts. The rate constant was estimated to be 0.02538 (dm 3 /mol) 0.21 /min at 298 K.     

Ab initio potential energy surface and bound states for the Kr-OCS complex

The first ab initio potential energy surface of the Kr-OCS complex is developed using the coupled-cluster singles and doubles with noniterative inclusion of connected triples [CCSD(T)]. The mixed basis sets, aug-cc-pVTZ for the O, C, and S atom, and aug-cc-pVQZ-PP for the Kr atom, with an additional (3s3p2d1f) set of midbond functions are used. A potential model is represented by an analytical function whose parameters are fitted numerically to the single point energies computed at 228 configurations. The potential has a T-shaped global minimum and a local linear minimum. The global minimum occurs at R = 7.146 a(0), θ = 105.0° with energy of -270.73 cm(-1). Bound state energies up to J = 9 are calculated for three isotopomers (82)Kr-OCS, (84)Kr-OCS, and (86)Kr-OCS. Analysis of the vibrational wavefunctions and energies suggests the complex can exist in two isomeric forms: T-shaped and quasi-linear. The calculated transition frequencies and spectroscopic constants of the three isotopomers are in good agreement with the experimental values.     

Diffusion monte carlo study of the hydrogen molecules adsorbed on C4H3Li

Diffusion Monte Carlo (DMC) simulations were used to calculate the binding energies for hydrogen molecules adsorbed on the lithium metal–organic complex C₄H₃Li. The calculations use all‐electron DMC techniques where every electron is explicitly included in the simulation. Also we have systematically studied it using density functional theory (DFT) methods, revealing that each C₄H₃Li can hold up to four H₂ molecules and the adsorption distance is about 2.2 Å. The DMC binding energies are in the range of 0.055–0.143 eV and are compared with those obtained with DFT using various exchange‐correlation functionals, with values ranging from 0.029 to 0.504 eV. These results indicate that caution is required applying DFT methods to weakly bound systems such as hydrogen storage materials based on lithium‐doped metal–organic frameworks. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Static investigation of adsorption and hetero‐diffusion of copper,silver, and gold adatoms on the (111) surface

In this work, we have used the static molecular simulations combined with an interatomic potential derived from the embedded‐atom method to study the adsorption and hetero‐diffusion on the (111) surface of Cu, Ag, and Au adatoms by using LAMMPS code. The investigation is performed for six heterogeneous systems such as Ag/Au(111), Ag/Cu(111), Au/Ag(111), Au/Cu(111), Cu/Ag(111), and Cu/Au(111). First, we have investigated the relaxation trends and the bond lengths of the atoms in the systems. The calculation results show that, the top layer spacing between the first and second layers of the Au(111), Ag(111), and Cu(111) substrates is contracted. This contraction is found to be more important in the Au(111) substrate. On the other hand, the strong reduction of the binding length is found in Au/Cu(111) for the different adsorption sites. In addition, the binding, adsorption, and static activation energies for all studied systems were examined. The results indicated that the binding and adsorption energies reached their maximum values in the Au/Cu(111) and Au/Ag(111) systems, respectively. Moreover, the static activation barriers for hopping diffusion on the (111) surfaces are found to be low compared with those found in the (100) and (110) surfaces. Therefore, our calculations showed that the difference in energy between the hcp and fcc sites on the (111) surfaces is very small. Copyright © 2017 John Wiley & Sons, Ltd.     

用于甲醇直接气相氧化羰基化的负载铜催化剂

 采用浸渍法制备了负载型铜基催化剂,并用其催化甲醇直接气相氧化羰基化合成碳酸二甲酯(DMC), 考察了浸渍溶剂、载体、助催化剂和铜含量的影响. 结果表明,以CuCl为活性组分原料、浓氨水为浸渍溶剂和活性炭为载体制得的负载铜催化剂显示出很高的催化活性,在特定的反应条件下,该催化剂上甲醇的转化率可达27.7%, DMC选择性可达95%. 分子筛负载的铜催化剂上甲醇的转化率低于1%, 但是生成DMC的选择性高达100%. 催化剂活性随着Cu负载量的增加而增大,但负载量过高可引起甲醇的过度氧化反应,导致DMC选择性下降. 催化剂中添加KOH或钯化合物,有利于提高以CuCl2为活性组分原料制得的铜催化剂的活性,但同时也促进了副反应的发生. 随着反应时间的延长,催化剂的活性组分流失,活性下降,但是生成DMC的选择性维持在95%左右.