Ansgar Schäfer

Fully optimized contracted Gaussian basis sets for atoms Li to Kr

Various contracted Gaussian basis sets for atoms up to Kr are presented which have been determined by optimizing atomic self‐consistent field ground state energies with respect to all basis set parameters, i.e., orbital exponents and contraction coefficients.

ROVIBRATIONALLY AVERAGED NUCLEAR MAGNETIC SHIELDING TENSORS CALCULATED AT THE COUPLED-CLUSTER LEVEL

Nuclear magnetic shielding tensor functions for H2, HF, N2, CO, and F2 are calculated at the coupled‐cluster singles and doubles level augmented by a perturbative correction for triple excitations [CCSD(T)]. The shielding constants for the lowest rovibrational states of these diatomics are obtained by solving the rovibrational Schrodinger equation with the finite‐element method. For H, C, and F, absolute scales for the nuclear magnetic shielding constants have been obtained by combining computed diamagnetic shieldings with paramagnetic contributions deduced from measured spin‐rotation constants and calculated rovibrational corrections. Since the experimental spin‐rotation constants for N2 and CO are inaccurate, shielding scales for N and O based on coupled cluster calculations are probably the most accurate available.

Zinc Silicates: Very Efficient Heterogeneous Catalysts for the Addition of Primary Alcohols to Alkynes and Allenes

There is an astonishing parallel between the mechanism generally accepted for the addition of water to CO2 catalyzed by the enzyme carbonic anhydrase and the mechanism calculated for the addition of methanol to allene catalyzed by the naturally occurring zinc silicate hemimorphite. The latter reaction was investigated in detail following the observation that hemimorphite as well as an amorphous zinc silicate prepared in situ are excellent heterogeneous catalysts for the addition of primary alcohols to alkynes and allenes [Eq. (1)].

Absolute acidities and site specific properties of zeolite catalysts modelled by advanced computational chemistry technology

Abstract Hartree–Fock structure predictions are made for two crystallographically distinct Bronsted sites, O1H and O3H, of the zeolite catalyst H-faujasite. The structure optimisation adopted very large cluster models without symmetry (163 atoms; 2033 basis function) and applied the parallel TURBOMOLE code on a multi-processor compute server. The predicted acidity strength of the two sites, 1166±10 kJ/mol, is the same within chemical accuracy, but – in agreement with experiment – characteristic differences are found for the OH stretching frequencies (3617 and 3574 cm−1) and the 1 H NMR chemical shifts (4.0 and 4.5 ppm).

Anab initio investigation of Cu2Se and Cu4Se2

SummaryExperimentally known copper selenium clusters show extraordinary geometrical features, especially short Cu-Cu distances. We report the first theoretical investigation of Cu2Se and Cu4Se2. Various quantum chemical methods (SCF, MP2, CPF, CCSD, CCSD(T), LDF) are applied to determine the importance of dynamic electron correlation. We find that inclusion of correlation does not essentially change the electronic structure of the clusters but has a strong influence on geometries. To reduce the computational effort we apply effective core potentials (ECPs) in combination with small, but carefully optimized basis sets. The applicability of simple modellings of correlation energies for approximate inclusion of correlation effects in SCF geometry optimizations is tested.

Electronic excitation energies in copper selenide clusters

We report calculated singlet and triplet excitation energies in bare copper selenide clusters (Cu2Se)n up to n=18 and ligand‐covered clusters (Cu2Se)n(PH3)m up to n=10 (m≤8). Various approaches for the calculation of excitation energies (including coupled cluster methods) are compared for small clusters which suggests the method of configuration interaction restricted to single substitutions (CIS) to give reasonable results with an error in the predicted energies of roughly +1 eV. The lowest CIS excitation energies show a steep increase with the cluster size up to about n=6 and a slow decrease for larger clusters, for which a rationalization is given. Even for the largest of the treated clusters the computed excitation energies are higher than 3.5 eV which suggests that these clusters are still far from showing β‐Cu2Se bulk properties (energy gap 1 eV). The electronic excitations are found to be essentially of charge transfer character (Se→Cu).

Zinksilicate: hochwirksame heterogene Katalysatoren für die Addition primärer Alkohole an Alkine und Allene

Eine erstaunliche Parallele existiert zwischen dem allgemein anerkannten Mechanismus fur die vom Enzym Carboanhydrase katalysierte Addition von Wasser an CO2 und dem Mechanismus, der fur die Addition von Methanol an Allen, katalysiert durch das naturlich auftretende Zinksilicat Hemimorphit, berechnet wurde. Die letztgenannte Reaktion wurde genau untersucht, da sowohl Hemimorphit als auch ein in situ hergestelltes, amorphes Zinksilicat ausgezeichnete heterogene Katalysatoren fur die Addition primarer Alkohole an Alkine und Allene sind [Gl. (1)].