Towards a rational design of ruthenium CO2 hydrogenation catalysts by Ab initio metadynamics

Complete reaction pathways relevant to CO2 hydrogenation by using a homogeneous ruthenium dihydride catalyst ([Ru(dmpe)2H2], dmpe=Me2PCH2CH2PMe2) have been investigated by ab initio metadynamics. This approach has allowed reaction intermediates to be identified and free-energy profiles to be calculated, which provide new insights into the experimentally observed reaction pathway. Our simulations indicate that CO2 insertion, which leads to the formation of formate complexes, proceeds by a concerted insertion mechanism. It is a rapid and direct process with a relatively low activation barrier, which is in agreement with experimental observations. Subsequent H2 insertion into the formate--Ru complex, which leads to the formation of formic acid, instead occurs via an intermediate [Ru(eta2-H2)] complex in which the molecular hydrogen coordinates to the ruthenium center and interacts weakly with the formate group. This step has been identified as the rate-limiting step. The reaction completes by hydrogen transfer from the [Ru(eta2-H2)] complex to the formate oxygen atom, which forms a dihydrogen-bonded Ru--HHO(CHO) complex. The activation energy for the H2 insertion step is lower for the trans isomer than for the cis isomer. A simple measure of the catalytic activity was proposed based on the structure of the transition state of the identified rate-limiting step. From this measure, the relationship between catalysts with different ligands and their experimental catalytic activities can be explained.     

Direct energy functional minimization under orthogonality constraints

The direct energy functional minimization problem in electronic structure theory, where the single-particle orbitals are optimized under the constraint of orthogonality, is explored. We present an orbital transformation based on an efficient expansion of the inverse factorization of the overlap matrix that keeps orbitals orthonormal. The orbital transformation maps the orthogonality constrained energy functional to an approximate unconstrained functional, which is correct to some order in a neighborhood of an orthogonal but approximate solution. A conjugate gradient scheme can then be used to find the ground state orbitals from the minimization of a sequence of transformed unconstrained electronic energy functionals. The technique provides an efficient, robust, and numerically stable approach to direct total energy minimization in first principles electronic structure theory based on tight-binding, Hartree-Fock, or density functional theory. For sparse problems, where both the orbitals and the effective single-particle Hamiltonians have sparse matrix representations, the effort scales linearly with the number of basis functions N in each iteration. For problems where only the overlap and Hamiltonian matrices are sparse the computational cost scales as O(M2N), where M is the number of occupied orbitals. We report a single point density functional energy calculation of a DNA decamer hydrated with 4003 water molecules under periodic boundary conditions. The DNA fragment containing a cis-syn thymine dimer is composed of 634 atoms and the whole system contains a total of 12,661 atoms and 103,333 spherical Gaussian basis functions.     

Investigation of polymer thin films by use of Bi-cluster-ion-supported time of flight secondary ion mass spectrometry

The investigation and analysis of polymer thin films with Bi _n ⁺, n = 1–7 cluster ions has been demonstrated by means of static secondary ion mass spectrometry (SIMS). The highly specific signal enhancement of these primary ions combined with the individual fragmentation pattern of poly(4-vinylphenol) and poly(methyl methacrylate) is the basic principle for a modified approach of data reduction derived from the well-established g-SIMS procedure. Based on mass spectra, which correspond to different cluster ion sizes, not only a clear distinction between the two polymers is feasible but also a further simplification of the data can be demonstrated. It has been successfully proven that characteristic polymer-relevant species can be refined out of the large amount of unspecific and highly fragmented secondary ions, which are usually present in SIMS spectra. Therefore, a more precise and direct interpretation of complex organic fragments becomes feasible, which consequently enables the investigation of even more sophisticated samples.     

Endocyclic cleavage in glycosides with 2,3-trans cyclic protecting groups

An endocyclic pathway is proposed as a reaction mechanism for the anomerization from the β (1,2-trans) to the α (1,2-cis) configuration observed in glycosides carrying 2,3-trans cyclic protecting groups. This reaction occurs in the presence of a weak Lewis or Br?nsted acid, while endocyclic cleavage (endocleavage) in typical glycosides was observed only when mediated by protic media or strong Lewis acids. To rationalize the behavior of this class of compounds, the reaction mechanism and the promoting factors of the endocleavage are investigated using quantum-mechanical (QM) calculations and experimental studies. We examine anomerization reactions of thioglycosides carrying 2,3-trans cyclic protecting groups, employing boron trifluoride etherate (BF(3)·OEt(2)) as a Lewis acid. The estimated theoretical reactivity, based on a simple model to predict transition state (TS) energies from the strain caused by the fused rings, is very close to the TS energies calculated by the TS search along the C1-C2 bond rotation after the endo C-O bond breaking. Excellent agreement is found between the predicted TS energies and the experimental reactivity ranking. The series of calculations and experiments strongly supports the predominance of the endocyclic rather than the exocyclic mechanism. Furthermore, these investigations suggest that the inner strain is the primary factor enhancing the endocleavage reaction. The effect of the cyclic protecting group in restricting the pyranoside ring to a (4)C(1) conformation, extensively discussed in conjunction with the stereoelectronic effect theory, is shown to be a secondary factor.     

SAM investigations of temperature programmed surface segregation of impurities in α-iron

The time development of the surface concentration of impurities such as sulphur and nitrogen in high-purity polycrystalline α-iron samples has been investigated by means of Auger Electron Spectroscopy (AES) and Scanning Auger Microscopy (SAM) during linearly increasing the temperature from about 100 to 850°C. The subsequent segregation of N and S revealed strong differences in the segregation kinetics of these elements — especially with respect to the contribution of the dominating transport mechanism (grain boundary diffusion and bulk diffusion). Calculating the effective diffusion coefficient of S from two subsequent segregation runs, strongly differing values were found. An interpretation of the change in the diffusion properties of sulphur is given based on SIMS (Secondary Ion Mass Spectroscopy) investigations performed before and after the thermal treatment.     

A comparative study of O2, CO,and NO binding to iron–porphyrin

Minimum-energy structures of O₂, CO, and NO iron–porphyrin (FeP) complexes, computed with the Car–Parrinello molecular dynamics, agree well with the available experimental data for synthetic heme models. The diatomic molecule induces a 0.3–0.4 Å displacement of the Fe atom out of the porphyrin nitrogen (N_p) plane and a doming of the overall porphyrin ring. The energy of the iron–diatomic bond increases in the order Fe(SINGLE BOND)O₂ (9 kcal/mol) < Fe(SINGLE BOND)CO (26 kcal/mol) < Fe(SINGLE BOND)NO (35 kcal/mol). The presence of an imidazole axial ligand increases the strength of the Fe(SINGLE BOND)O₂ and Fe(SINGLE BOND)CO bonds (15 and 35 kcal/mol, respectively), with few structural changes with respect to the FeP(CO) and FeP(O₂) complexes. In contrast, the imidazole ligand does not affect the energy of the Fe(SINGLE BOND)NO bond, but induces significant structural changes with respect to the FeP(NO) complex. Similar variations in the iron–imidazole bond with respect to the addition of CO, O₂, and NO are also discussed. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 31–35, 1998