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Solvent molecules have a significant impact on the mechanism of the gold(I)‐catalyzed hydration of alkynes as they enable an efficient proton transfer step. As an alternative to such a water‐assisted proton transfer, the counterion can serve as a proton shuttle. However, it seems likely that solvent molecules play a vital role for the overall reaction mechanism in either case.

For the electronic excitations in metallic systems under periodic boundary conditions, momentum conservation and a uniform electron−electron interaction imply a clear distinction of plasmons and single-particle excitations. For finite molecular systems, this distinction is less clear, but excitations formed by a coherent superposition of elementary particle−hole transitions that show a collective oscillation of the transition electron density have nevertheless been identified as plasmons in molecules. To aid this distinction, a scaling approach [

The performance of current density functionals is analyzed in detail for the electric field gradients (EFG) of hydrogen chloride and copper chloride by comparison with ab initio methods and available experimental data. The range of density functionals applied shows good agreement with coupled cluster H and Cl field gradients for HCl, as has been demonstrated previously for other main-group element containing compounds. However, the performance of most density functionals is very poor for the Cu EFG in CuCl (EFG for Cu -0.44 a.u. at the coupled-cluster singles and doubles with perturbative triples [CCSD(T)] level, compared to, e.g., +0.54 a.u. at the B-LYP level). Only the “half-and-half” hybrid functionals give field gradients with the correct sign. The reason for the poor performance of the density functional theory is analyzed in detail comparing density functional with ab initio total electronic densities ρ(r). Due to the conservation of the number of particles, a change in the valence part of the electron density can lead to changes in the core part of the density. Errors in valence electronic properties like the dipole moment and in core properties like the Cu and Cl EFGs may therefore be connected. In fact the errors in both properties show a distinct linear relationship, indicating that if the dipole moment is correctly described by density functionals, the Cu and Cl EFGs may be accurate as well. Furthermore, at the atomic level, electric field gradients are described with reasonable accuracy by current density functionals as calculations for the Cu 2P excited state and the Cu2+ 2D ground state show. A comparison between the different density functionals shows that the incorrect behavior of the electronic density appears to be mainly due to defects in the exchange part of the functional.

For a range of additions to alkynes gold is known to exhibit a much higher catalytic activity than a corresponding platinum compound. In order to approach the origin of this behavior we first investigate the propyne activation by the gold and platinum catalysts AuCl3 and PtCl2(H2O) where both metals possess a d(8) electron configuration and where the catalysts exhibit similar steric effects. Propyne serves as a representative for alkynes. Fully relativistic ab initio calculations of these alkyne-catalyst complexes are presented at the Dirac-Hartree-Fock self-consistent field (DHF-SCF), density functional theory (DFT/B3LYP), and Green's function (GF) level in order to properly account for the large relativistic effects of gold and platinum. For the alkyne/catalyst complexes both the perpendicular and in-plane conformations were studied as these possess very similar ground state energies and may easily transform into each other. Strongly varying orbital populations together with sizable energetic and structural differences of the frontier orbitals are found and can be considered as a major source of the differing catalytic activity. These mainly comprise vanishing LUMO densities at the carbon centers in the platinum complex together with increased LUMO energies making a nucleophilic attack harder than in the gold compound. As Green's function calculations show, DFT/B3LYP seems to overestimate correlation contributions leading to an unphysical energetic lowering of many unoccupied orbitals.

In solid state physics, electronic excitations are often classified as plasmons or single-particle excitations. The former class of states refers to collective oscillations of the electron density. The random-phase approximation allows for a quantum-theoretical treatment and a characterization on a microscopic level as a coherent superposition of a large number of particle-hole transitions with the same momentum transfer. However, small systems such as molecules or small nanoclusters lack the basic properties (momentum conservation and uniform exchange interaction) responsible for the formation of plasmons in the solid-state case. Despite an enhanced interest in plasmon-based technologies and an increasing number of studies regarding plasmons in molecules and small nanoclusters, their definition on a microscopic level of theory remains ambiguous. In this work, we analyze the microscopic properties of molecular plasmons in comparison with the homogeneous electron gas as a model system. Subsequently, the applicability of the derived characteristics is validated by analyzing the electronic excitation vectors with respect to orbital transitions for two linear polyenes within second order versions of the algebraic diagrammatic construction scheme for the polarization propagator.

After the ionization of a valence electron, the created hole can migrate ultrafast from one end of the molecule to another. Because of the advent of attosecond pulse techniques, the measuring and understanding of charge migration has become a central topic in attosecond science. Here, we pose the hitherto unconsidered question whether ionizing a core electron will also lead to charge migration. It is found that the created hole in the core stays put, but in response to this hole interesting electron dynamics takes place which can lead to intense charge migration in the valence shell. This migration is typically faster than that after the ionization of a valence electron and transpires on a shorter time scale than the natural decay of the core hole by the Auger process, making the subject very challenging to attosecond science.

With the concept of scaled-opposite-spin (SOS), a pragmatic semi-empirical approximation has been introduced to the extended algebraic diagrammatic construction scheme of second order (ADC(2)-x) that leads to a significant saving in computational effort. The parameters included were fitted with respect to a benchmark set of electronically excited states in standard organic molecules that include some doubly-excited states, as well. Like the original, unscaled ADC(2)-x scheme it can be used to identify electronically excited states with high double excitation character, however at reduced computational cost. At the same time, it is possible to reduce the overestimation of doubly-excited configurations that is inherent to ADC(2)-x. Additionally, a scheme for the strict variant (ADC(2)-s) was derived directly from SOS-MP2 by application of the intermediate state formalism and compared to an existing version of SOS-ADC(2)-s.