Adsorption of Ethylene on Neutral, Anionic and Cationic Gold Clusters

The adsorption of ethylene molecule on neutral, anionic and cationic gold clusters consisting of up to 10 atoms has been investigated using density-functional theory. It is demonstrated that C2H4 can be adsorbed on small gold clusters in two different configurations, corresponding to the pi- and di-sigma-bonded species. Adsorption in the pi-bonded mode dominates over the di-sigma mode over all considered cluster sizes n, with the exception of the neutral C2H4-Au5 system. A striking difference is found in the size-dependence of the adsorption energy of C2H4 bonded to the neutral gold clusters in the pi and di-sigma configurations. The important role of the electronic shell effects in the di-sigma mode of ethylene adsorption on neutral gold clusters is demonstrated. It is shown that the interaction of C2H4 with small gold clusters strongly depends on their charge. The typical shift in the vibrational frequencies of C2H4 adsorbed in the pi- and the di-sigma configurations gives a guidance to experimentally distinguish between the two modes of adsorption.Comment: 30 pages, 10 figure

An Ab Initio Study of the Reaction Mechanism of Co++NH3

To investigate the mechanism for N–H bond activation by a transition metal, the reactions of Co+(3F,5F) with NH3 have been studied with complete active space self-consistent field (CASSCF), multireference configuration interaction (MR-SDCI), and multireference many body perturbation theory (MRMP) wave functions, using both effective core potential and all-electron methods. Upon their initial approach, the reactants yield an ion–molecule complex, CoNH+3(3E,5A2,5A1), with retention of C3ν symmetry. The Co+=NH3 binding energies are estimated to be 49 (triplet) and 45 (quintet) kcal/mol. Subsequently, the N–H bond is activated, leading to an intermediate complex H–Co–NH+2 (C2ν symmetry), through a three-center transition state with an energy barrier of 56–60 (triplet) and 70–73 (quintet) kcal/mol. The energy of H–Co–NH+2, relative to that of CoNH+3, is estimated to be 60 to 61 (triplet) and 44 (quintet) kcal/mol. However, the highest levels of theory employed here (including dynamic correlation corrections) suggest that the triplet intermediate HCoNH+2 may not exist as a minimum on the potential energy surface. Following Co–N or H–Co bond cleavage, the complexH–Co–NH+2 leads to HCo++NH2 or H+CoNH+2. Both channels (triplet and quintet) are found to be endothermic by 54–64 kcal/mol

Dynamic Reaction Path Analysis Based on an Intrinsic Reaction Coordinate

We propose two methods that may be used to describe the dynamic reaction path (DRP) based on an intrinsic reaction coordinate (IRC) or minimum energy path, to examine how the actual dynamics proceeds relative to the IRC path. In the first of these, any point on the DRP is expressed in terms of the IRC and the distance from the IRC path. In the second method, any DRP point is expressed in terms of the IRC, the curvature coordinate, and the distance from a two‐dimensional ‘‘reaction plane’’ determined by the IRC path tangent and curvature vectors. The latter representation is based on the fact that the 3N−8 dimensional space orthogonal to the reaction plane is independent of an internal centrifugal force caused by the motion along the IRC path. To analyze the relation between geometrical features of the IRC path and the dynamics, we introduce a function that estimates the variation of the reaction plane along the IRC path. As demonstrations, the methods are applied to the dissociationreaction of thiofolmaldehyde (H2CS→H2+CS)

Dynamic Reaction Path Study of SiH4 + H- → SiH5- and the Berry Pseudorotation Mechanism

Recently, we proposed a dynamic reaction path (DRP) analysis with vibration mapping and examined symmetry-conserved processes in the reaction SilL + H- - SiHs- (side and front attack). A key feature of this study was an analysis of the Berry pseudorotation mechanism in SiH5- from the viewpoint of vibrationvibration interactions. In the present study, we extend this methodology to deal with symmetry-relaxed attack by using a function that estimates the distance between any geometry and a reference geometry in configuration space, in order to examine the relation between the incident path of H- and the pseudorotation. When approaching SilL from the side or back, the H- changes its path toward front attack, due to the high energy requirements for the side and back attack paths, resulting in a vibrationally active SiH5- with accompanying Berry pseudorotation. On the other hand, in a frontal or nearly frontal attack, SiH5- seems stable with no pseudorotation

Dynamic Reaction Coordinate Analysis: An Application to SiH4 + H- → SiH5-

An ab initio classical trajectory method, the dynamic reaction coordinate (DRC) method based on ab initio electronic structure calculations, is applied to a study of the chemical reaction Si~ + H- - SiHs-. Both side attack (C2v symmetry) and front attack (C3v symmetry) of H- on SiH4 are examined. To analyze the nature of the intramolecular vibrational energy transfer, the DRC and its corresponding momentum are mapped onto normal modes of both reactant and product systems. These analyses show that Berry pseudorotation occurs repeatedly in the SiH5- produced by the side attack, whereas the SN2 reaction H- + Si~ - Si~ + H- often occurs upon front attack depending on the initial relative velocity

Reaction Path Hamiltonian Based on a Reaction Coordinate and a Curvature Coordinate

We propose a reaction path‐based Hamiltonian in terms of the reaction coordinate, the curvature coordinate, the remaining 3N−8 transverse normal coordinates (whose directions are orthogonal to the path tangent and curvature vectors), and their conjugate momenta, for an Natomic reaction system. The 3N−8 transverse vibrational modes are independent of the motion along the reaction path, although they have coupling terms with the curvature direction in the harmonic approximation. A two‐dimensional plane determined by the path tangent and curvature vectors is termed the ‘‘reaction plane.’’ We introduce a function that estimates changes of the reaction plane along the reaction path, and analyze the reaction path based on this function for an abstraction reaction, CH3+H2→CH4+H. The scheme proposed here should be effective when a reaction path has a sharply curved region

Interfacing Electronic Structure Theory with Dynamics

This paper illustrates the utility of combining high-quality electronic structure calculations, methods for determining reaction paths, and direct dynamics methods for studying ab initiotrajectories on the fly to develop an understanding of complex chemical reactions. The combined methods are applied to the pseudorotation in SiH5-, competing dissociation paths for N2O2, the dissociation of FN3 into NF + N2, and the potential energy surfaces for AlH2. These DRP results may be thought of as samples of what dynamical processes can be encountered on each potential energy surface

Ab Initio Molecular Dynamics Study of H2 Formation Inside POSS Compounds

The mechanism and dynamics of the formation of a hydrogen molecule by incorporating two hydrogen atoms in a stepwise manner into the cavity of some POSS (polyhedral oligomeric silsesquioxanes) compounds has been investigated by ab initio molecular orbital and ab initio molecular dynamics (AIMD) methods. The host molecules in the present reactions are two types of POSS, T8 ([HSiO1.5]8) and T12(D2d) ([HSiO1.5]12). AIMD simulations were performed at the CASSCF level of theory, in which two electrons and two orbitals of the colliding hydrogen atoms are included in the active space. The trajectories were started by inserting the second hydrogen atom into the hydrogen atom-encapsulated-POSS (H + H@Tn → H2@Tn; n = 8 and 12). In many cases, the gradual formation of a hydrogen molecule has been observed after frequent collisions of two hydrogen atoms within the cages. The effect of the introduction of an argon atom in T12 is discussed as well