Michał Hapka

Optical absorption spectra of gold clusters Aun (n = 4, 6, 8,12, 20) from long-range corrected functionals with optimal tuning

Absorption UV spectra of gold clusters Au(n) (n = 4, 6, 8, 12, 20) are investigated using the time-dependent density functional theory (TDDFT). The calculations employ several long-range corrected xc functionals: ωB97X, LC-ωPBEh, CAM-B3LYP∗ (where ∗ denotes a variant with corrected asymptote of CAM-B3LYP), and LC-ωPBE. The latter two are subject to first-principle tuning according to a prescription of Stein et al. [Phys. Rev. Lett. 105, 266802 (2010)] by varying the range separation parameter. TDDFT results are validated for Au(4) and Au(8) against the equation-of-motion coupled cluster singles and doubles results and the experiment. Both long-range correction and the inclusion of a fixed portion of the exact exchange in the short-range are essential for the proper description of the optical spectra of gold. The ωB97X functional performs well across all studied cluster sizes. LC-ωPBEh, with parameters recommended by Rohrdanz et al. [J. Chem. Phys. 130, 054112 (2009)], affords the best performance for clusters of n > 4. The optimally tuned CAM-B3LYP∗ features the range separation parameter of 0.33 for Au(4) and 0.25 for all the larger clusters. For LC-ωPBE the tuning procedure resulted in incorrect transition energies and oscillator strengths despite the fact that the optimized functional showed the accurate linear dependence on fractional electron numbers. Au(n) (n = 4, 6, 8) feature optical gaps above of 3 eV and Au(20) of ∼2.9 eV. In Au(12) this gap narrows to ∼2.1 eV. The calculated spectrum for Au(20) involves intensity being concentrated in only a few transitions with the absorption maximum at 3.5 eV. The intense 3.5 eV absorption is present in all cluster sizes of n > 4. The calculated HOMO-LUMO gaps for all cluster sizes are within 0.5 eV of the difference between the vertical ionization potential and electron affinity. The reasons for this and for the failure of conventional xc functionals for optical spectra of gold are discussed.

Dynamics of gas phase Ne(*) + NH3 and Ne(*) + ND3 Penning ionisation at low temperatures.

Two isotopic chemical reactions, Ne(*) + NH3, and Ne(*) + ND3, have been studied at low collision energies by means of a merged beams technique. Partial cross sections have been recorded for the two reactive channels, namely, Ne(*) + NH3 → Ne + NH3(+) + e(-), and Ne(*) + NH3 → Ne + NH2(+) + H + e(-), by detecting the NH3(+) and NH2(+) product ions, respectively. The cross sections for both reactions were found to increase with decreasing collision energy, Ecoll, in the range 8 μeV < Ecoll < 20 meV. The measured rate constant exhibits a curvature in a log(k)-log(Ecoll) plot from which it is concluded that the Langevin capture model does not properly describe the Ne(*) + NH3 reaction in the entire range of collision energies covered here. Calculations based on multichannel quantum defect theory were performed to reproduce and interpret the experimental results. Good agreement was obtained by including long range van der Waals interactions combined with a 6-12 Lennard-Jones potential. The branching ratio between the two reactive channels, Γ = [NH2(+)]/[NH2(+)] + [NH3(+)], is relatively constant, Γ ≈ 0.3, in the entire collision energy range studied here. Possible reasons for this observation are discussed and rationalized in terms of relative time scales of the reactant approach and the molecular rotation. Isotopic differences between the Ne(*) + NH3 and Ne(*) + ND3 reactions are small, as suggested by nearly equal branching ratios and cross sections for the two reactions.

Observation of orbiting resonances in He(3S1) + NH3 Penning ionization

Resonances are among the clearest quantum mechanical signatures of scattering processes. Previously, shape resonances and Feshbach resonances have been observed in inelastic and reactive collisions involving atoms or diatomic molecules. Structure in the integral cross section has been observed in a handful of elastic collisions involving polyatomic molecules. The present paper presents the observation of shape resonances in the reactive scattering of a polyatomic molecule, NH3. A merged-beam study of the gas phase He((3)S1) + NH3 Penning ionization reaction dynamics is described in the collision energy range 3.3 μeV < Ecoll < 10 meV. In this energy range, the reaction rate is governed by long-range attraction. Peaks in the integral cross section are observed at collision energies of 1.8 meV and 7.3 meV and are assigned to ℓ = 15,16 and ℓ = 20,21 partial wave resonances, respectively. The experimental results are well reproduced by theoretical calculations with the short-range reaction probability Psr = 0.035. No clear signature of the orbiting resonances is visible in the branching ratio between NH3 (+) and NH2 (+) formation.

Symmetry-adapted perturbation theory based on unrestricted Kohn-Sham orbitals for high-spin open-shell van der Waals complexes.

Two open-shell formulations of the symmetry-adapted perturbation theory are presented. They are based on the spin-unrestricted Kohn-Sham (SAPT(UKS)) and unrestricted Hartree-Fock (SAPT(UHF)) descriptions of the monomers, respectively. The key reason behind development of SAPT(UKS) is that it is more compatible with density functional theory (DFT) compared to the previous formulation of open-shell SAPT based on spin-restricted Kohn-Sham method of Żuchowski et al. [J. Chem. Phys. 129, 084101 (2008)]. The performance of SAPT(UKS) and SAPT(UHF) is tested for the following open-shell van der Waals complexes: He···NH, H(2)O···HO(2), He···OH, Ar···OH, Ar···NO. The results show an excellent agreement between SAPT(UKS) and SAPT(ROKS). Furthermore, for the first time SAPT based on DFT is shown to be suitable for the treatment of interactions involving Π-state radicals (He···OH, Ar···OH, Ar···NO). In the interactions of transition metal dimers ((3)Σ(u)(+))Au(2) and ((13)Σ(g)(+))Cr(2) we show that SAPT is incompatible with the use of effective core potentials. The interaction energies of both systems expressed instead as supermolecular UHF interaction plus dispersion from SAPT(UKS) result in reasonably accurate potential curves.

Differential action of methylselenocysteine in control and alloxan-diabetic rabbits

Antidiabetic action of inorganic selenium compounds is commonly accepted. Since in diet selenium mainly exists as selenoamino acids, potential hypoglycemic properties of methylselenocysteine (MSC) were investigated in four groups of rabbits: untreated and MSC-treated control animals as well as alloxan-diabetic and MSC-treated diabetic rabbits. MSC (at a dose of 1mg/kg body weight) was administered daily for 3 weeks via intraperitoneal injection. The data show, that in MSC-treated control animals plasma glucose concentration was diminished, while plasma urea and creatinine levels as well as urine albumin content were elevated and necrotic changes occurred in kidney-cortex. Decreased GSH/GSSG ratios in blood, liver and kidney-cortex were accompanied by increased glutathione peroxidase and glutathione reductase activities and a diminished renal gamma-glutamylcysteine synthetase activity. Death of 50% of control animals was preceded by a dramatic decline in blood glucose concentration. Surprisingly, in MSC-treated diabetic rabbits, plasma glucose levels were either normalized or significantly decreased. Blood and liver GSH/GSSG ratios were increased and renal functions were markedly improved, as indicated by a diminished albuminuria and attenuated histological changes characteristic of diabetes. However, after administration of MSC to diabetic rabbits plasma urea and creatinine levels as well as renal GSH/GSSG ratios were not altered. In view of MSC-induced marked accumulation of selenium in kidneys and liver of control rabbits, accompanied by a decline in blood glucose level, disturbance of glutathione homeostasis and kidney-injury, application of MSC in chemotherapy needs a careful evaluation. On the contrary, MSC supplementation might be beneficial for diabetes therapy due to an improvement of both glycemia and renal function.

Theoretical Studies of Potential Energy Surface and Bound States of the Strongly Bound He(1S)–BeO (1Σ+) Complex

We perform electronic structure calculations of the potential energy surface of the He···BeO((1)Σ(+)) complex. We use several different methods to characterize this unusual interaction. We apply coupled cluster singles, doubles, and noniterative triples [CCSD(T)] and the multireference configuration interaction [MRCI] levels of theory. The nature of the interaction is studied with symmetry-adapted perturbation theory (SAPT) based on DFT and CCSD description of the intramonomer electron densities. Our best estimate of the well depth is 1876.5 cm(-1) at the CCSD(T) level, while the dissociation energy, corrected for the zero-point energy, is equal to 1446.7 cm(-1). The global minimum is located for the collinear He···Be-O geometry at Re = 4.45a0. The rotational constant of the He-BeO complex in its ground state is 0.863 cm(-1). We also calculate bound states of the He···BeO complex for J = 0 and J = 1 (total angular momentum).

First-principle interaction potentials for metastable He(3S) and Ne(3P) with closed-shell molecules: application to Penning-ionizing systems.

We present new interaction potential curves, calculated from first-principles, for the He((3)S, 1s(1)2s(1))···H2 and He((3)S)···Ar systems, relevant in recent Penning ionization experiments of Henson et al. [Science 338, 234 (2012)]. Two different approaches were applied: supermolecular using coupled cluster (CC) theory and perturbational within symmetry-adapted perturbation theory (SAPT). Both methods gave consistent results, and the potentials were used to study the elastic scattering and determine the positions of shape resonances for low kinetic energy (up to 1 meV). We found a good agreement with the experiment. In addition, we investigated two other dimers composed of metastable Ne ((3)P, 2p(5)3s(1)) and ground state He and Ar atoms. For the Ne((3)P)···He system, a good agreement between CC and SAPT approaches was obtained. The Ne((3)P)···Ar dimer was described only with SAPT, as CC gave divergent results. Ne* systems exhibit extremely small electronic orbital angular momentum anisotropy of the potentials. We attribute this effect to screening of an open 2p shell by a singly occupied 3s shell.

Density functional theory approach to gold-ligand interactions: Separating true effects from artifacts

Donor-acceptor interactions are notoriously difficult and unpredictable for conventional density functional theory (DFT) methodologies. This work presents a reliable computational treatment of gold-ligand interactions of the donor-acceptor type within DFT. These interactions require a proper account of the ionization potential of the electron donor and electron affinity of the electron acceptor. This is accomplished in the Generalized Kohn Sham framework that allows one to relate these properties to the frontier orbitals in DFT via the tuning of range-separated functionals. A donor and an acceptor typically require different tuning schemes. This poses a problem when the binding energies are calculated using the supermolecular method. A two-parameter tuning for the monomer properties ensures that a common functional, optimal for both the donor and the acceptor, is found. A reliable DFT approach for these interactions also takes into account the dispersion contribution. The approach is validated using the water dimer and the (HAuPH3)2 aurophilic complex. Binding energies are computed for Au4 interacting with the following ligands: SCN(-), benzenethiol, benzenethiolate anion, pyridine, and trimethylphosphine. The results agree for the right reasons with coupled-cluster reference values.

Interaction of Boron–Nitrogen Doped Benzene Isomers with Water

The interaction of 1,2-dihydro-1,2-, 1,3-dihydro-1,3- and 1,4-dihydro-1,4-azaborine isomers with one and two water molecules has been studied using a variety of supermolecular (Møller-Plesset = MP, and coupled cluster = CC) as well as perturbational (symmetry-adapted perturbation theory = SAPT) electron-correlation methods in the complete basis-set limit. It has been found that the water molecule binds to azaborine isomers through O-H···π, π-H···O, and dihydrogen bonding linkages. The SAPT interaction energy decomposition shows that these complexes are mostly stabilized by dispersion followed closely by induction contributions. Pauli repulsion hinders water molecule to be polarized by azaborine in the O-H···π type of complexes. According to the interacting-quantum-atoms analysis, the structures with a primary binding of the O-H···π type benefit from an additional stabilization factor resulting from the interaction of the oxygen and the second hydrogen atom of water, i.e., the one which does not point toward the ring, while the interaction of hydrogens from water with azaborines plays a destabilizing role for the π-H···O type. The same method states that the intermolecular bindings between azaborines and the water molecule have a multicenter character with a small bond polarization, and they are classified as closed-shell (noncovalent) by quantum theory of atoms-in-molecules analysis at bond critical points. The complexes of azaborines with two water molecules tend to arrange in a circular fashion with a recognizable water dimer attached to the azaborine molecule. A comparison with the CCSD(T) benchmarks shows that the nonadditive contribution to the interaction energy of the trimers is negative and with a good accuracy can be accounted for by the MP2 method. A good agreement between Hartree-Fock (HF) and MP2 nonadditive energy, as well as the decomposition of HF nonadditive interaction energies divulge the importance of nonadditive induction energy in the trimers. The interaction energies for the azaborine with one water calculated with the SAPT(DFT), MP2, SCS-MP2, and MP2C methods are in satisfactory agreement with each other. Finally, it has been found that the population analysis from the electron localization function offers the most comprehensive explanation of the orientational preferences of the water molecule in the complex.

Tuned range-separated hybrid functionals in the symmetry-adapted perturbation theory

The aim of this study is to present a performance test of optimally tuned long-range corrected (LRC) functionals applied to the symmetry-adapted perturbation theory (SAPT). In the present variant, the second-order energy components are evaluated at the coupled level of theory. We demonstrate that the generalized Kohn-Sham (GKS) description of monomers with optimally tuned LRC functionals may be essential for the quality of SAPT interaction energy components. This is connected to the minimization of a many-electron self-interaction error and exemplified by two model systems: polyacetylenes of increasing length and stretching of He 3 (+). Next we provide a comparison of SAPT approaches based on Kohn-Sham and GKS description of the monomers. We show that LRC leads to results better or comparable with the hitherto prevailing asymptotically corrected functionals. Finally, we discuss the advantages and possible limitations of SAPT based on LRC functionals.