Nanqin Wang

Cluster modeling of metal oxides: how to cut out a cluster?

Abstract Three principles have been proposed for the cluster modeling of metal oxides, i.e. the neutrality principle, stoichiometry principle and coordination principle. A stoichiometric cluster model is recommended, for it possesses the correct atomic ratio of bulk solid and it automatically meets the requirement of neutrality. Particular attention has been focused on how to cut out a stoichiometric cluster which has as few dangling bonds as possible so as to fulfill the requirement of the coordination principle. A case study for the applications of these three principles to the modeling of ZnO solid has shown their efficiency in setting up a better cluster model of a given size.

Chemisorption of acetonitrile, pyridine and pyrazine on the Si(100)-2x1 surface: theoretical predictions

The chemisorption of acetonitrile (CH3CN), pyridine (C5H5N) and pyrazine (C4H4N2) on the Si(100)-2 × 1 surface has been investigated by means of first-principles density functional cluster model calculations. For acetonitrile, an N-end-on adsorption state and a side-on adsorption state were found, together with a transition state that connects the two adsorption states. The predicted energetics suggests that the side-on adsorption state can be readily formed at rather low temperature via the end-on precursor state. For both pyridine and pyrazine, an N-end-on adsorption state and two side-on adsorption states were revealed. In the pyridine/Si(100) chemisorption system, the primary adspecies would be the N-end-on adsorbed pyridine as the N-end-on adsorption is the most favorable and barrierless. For the pyrazine case, the N-end-on adsorbed pyrazine would be the primary adspecies at low temperature, while at elevated temperatures the primary adspecies would be the side-on adsorbed pyrazine, which is di-σ bonded onto the surface dimer through the 2 and 5 carbon atoms. In particular, the finding that the N-end-on adsorption of pyridine on the Si(100)-2 × 1 surface is substantial enlightens us as to the possibility of constructing a pyridine-based, conductive (or semiconductive) polymer film on the Si surface.

A quantum chemical study of the NO/MgO chemisorption system: hybrid B3LYP calculations on NO/(MgO)(n) (n = 4, 6, 8) model systems

Ab initio B3LYP cluster model calculations have been performed to explore the adsorptive behavior of NO on MgO solid. Two adsorption modes, namely the chain mode and the bridge mode, have been found for NO adsorption at low-coordinate Mg–O pair sites. NO adsorbed in the chain mode is highly activated and the resultant NO22− complex would be an intermediate in the O-exchange reaction between NO and MgO. Two possible configurations of N2O32− surface species have been found in the NO/MgO system. Our calculated IR frequencies of N2O32− surface complexes account well for the temperature dependence of the experimental IR spectra.

CHARGE-CONSISTENCY MODELING OF CO/NIO(100) CHEMISORPTION SYSTEM

Abstract The bonding of CO to the NiO(100) surface has been investigated by means of the DV-Xα method. Emphasis is put on how to get a realistic description of the NiO bulk as well as the bonding mechanism in chemisorption. The calculation results demonstrate that it is very important to maintain the charge consistency so as to reproduce the experimental results of bulk NiO with an embedded cluster model. When compared with the bonding mechanism of CO in a surface electric field, the embedded cluster calculations with BSSE correction show that the interaction between CO and NiO(100) is mainly electrostatic.

CASSCF study of bonding in NiCO and FeCO

A series of CASSCF calculations were performed on the ground states of NiCO and FeCO. The contributions of the srp interactions are checked by examining the validity of the CASSC calculation to describe the molecule with a particular choice of the active space. The calculation results substantiate that the stability of MCO is determined by a balance between p donation from the metal 3d to the CO 2p and p repulsion between the metal s electrons and the CO 5s lone pair and, at the same time, emphasizes the importance of the synergistic srp interactions between the metal and the CO group. The relative importance of srp interactions depends on the nature of the metal. In the case of NiCO, it is the p donation from Ni 3d to CO 2p that makes the p largest contribution to the formation of the Ni—CO bond, while in the case of FeCO, it is the correlation of s electrons that holds the metal and CO together. Q 1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 221)231, 1999

Studies on cluster-surface analogy: ab initio calculations for the CO/Cu chemisorption system

Abstract The significance of employing the concept of a “metallic atom” in the cluster approach of chemisorption on metals is tested further with the CO/Cu chemisorption system. The results of metallic cluster Cu-CO calculations with ζm agree well with the CO/Cu chemisorption system. Comparison between Cu-CO and Ni-CO helps to understand the difference between CO/Cu and CO/Ni.

DV-Xα study of ethylene adsorbed on Ni(100)surface and nickel—graphite intercalation compound

Abstract Nickel-graphite intercalation compound (NiGIC), Ni (100) surface and the interactions between ethylene and NiGIC or Ni (100) are investigated by the DV- X α calculations. Geometric effects and electronic effects are examined by comparison of the three adsorption systems C 2 H 4 NiGIC, C 2 H 4 Ni(100) (π) and C 2 H 4 Ni(100) (di-σ). Owing to the electronic effects, the π * CC ←Niσ-backdonation is stronger than the π CC →Niσ-donation in the C 2 H 4 NiGIC; nevertheless the π CC →Niσ-donation is stronger than the π * CC ←Ni π-backdonation in C 2 H 4 -Ni(100) (π), although there are similar geometric effects in these two π-coordination systems. C 2 H 4  Ni(100)(π) and C 2 H 4 Ni(100) (di-σ), which are similar in electronic effects but different in geometry, have similar static properties, and there maybe marked effects with respect to the kinetics of adsorption.

Studies on cluster-surf ace analogy: ab initio calculations for the CO/Ni chemisorption system

After exploring the underlying physics of the cluster-surface analogy, we introduce the concept of “metallic atom”. Case studies of the Ni-CO cluster as a model of CO/Ni chemisorption are carried out with UHF/STO-3G of the so-called atomic ζa and metallic ζm, where ζa is a basis function optimized from the ground state of a Ni atom, while ζm is a modifier of ζa based the free-electron theory in solid state physics. The calculation results of ζa and ζm are elucidated in the light of more rigorous cluster calculations in the literature.

Studies on the “cluster-surface analogy”: metallic slater basis sets for ab initio calculations of chemisorption

Abstract In this article, we explore the underlying physics of the cluster-surface analogy. The emphasis is on the importance of the concept of a “metallic atom”. Based on the free electron theory in solid-state physics, a set of metallic Slater basis functions for ab initio calculations of chemisorption are presented. The results of the case studies of the M-CO (M = Cr, Fe, Co, Ni or Cu) clusters as models of CO/M chemisorption systems are encouraging, which suggests that a small “metallic“ cluster model may be a reliable, convenient and practical theoretical approach to the study of chemisorption.

Studies on cluster-surface analogy : cluster model calculations for L/Ni (L=CN, CO, NO) chemisorption systems

Abstract The minimal cluster models Ni-L (L = CN, CO, NO) have been employed to study the L/Ni chemisorption systems. Emphasis is on the effects of the choice of basis set. With the metallic Slater basis function ξm, which is a modification of the atomic Slater basis function ξa based on the free electron theory in solid state physics, the results of Ni-L cluster calculations agree well with the UV photoelectron spectroscopy data of L/Ni chemisorption systems. By means of Mulliken population analysis, the interactions between L and Ni are investigated.