Vlasta Bonačić-Koutecký

Density functional study of structural and electronic properties of bimetallic silver–gold clusters: Comparison with pure gold and silver clusters

Bimetallic silver–gold clusters offer an excellent opportunity to study changes in metallic versus “ionic” properties involving charge transfer as a function of the size and the composition, particularly when compared to pure silver and gold clusters. We have determined structures, ionization potentials, and vertical detachment energies for neutral and charged bimetallic AgmAun [3⩽(m+n)⩽5] clusters. Calculated VDE values compare well with available experimental data. In the stable structures of these clusters Au atoms assume positions which favor the charge transfer from Ag atoms. Heteronuclear bonding is usually preferred to homonuclear bonding in clusters with equal numbers of hetero atoms. In fact, stable structures of neutral Ag2Au2, Ag3Au3, and Ag4Au4 clusters are characterized by the maximum number of hetero bonds and peripheral positions of Au atoms. Bimetallic tetramer as well as hexamer are planar and have common structural properties with corresponding one-component systems, while Ag4Au4 and Ag8...

An accurate relativistic effective core potential for excited states of Ag atom: An application for studying the absorption spectra of Agn and Agn+ clusters

A new 11-electron relativistic effective core potential (11e-RECP) for Ag atom based on correlated level of theory and the associated atomic orbital (AO) basis set have been derived which allows for an accurate determination of excited states. This has been verified by comparing the calculated excited states of the dimer with experimental data. Therefore, we applied the new 11e-RECP in the framework of the linear response equation-of-motion coupled-cluster (EOM-CC) method to determine absorption spectra of small Agn=2−4 and Agn=2−4+ clusters. The correlation treatment of 11 electrons per atom and calculations of transition energies and oscillator strengths in a large energy interval allowed us to investigate the influence of d-electrons on the spectroscopic patterns. We have found that d-electrons play a crucial role for accurate predictions of absorption spectra in spite of the fact that they are not always directly involved in the leading excitations contributing to the intense transitions. The calculat...

Influence of Charge State on the Mechanism of CO Oxidation on Gold Clusters

We present results from our joint experimental and theoretical study of the reactivity of anionic and cationic gold oxide clusters toward CO, focusing on the role of atomic oxygen, different charge states, and mechanisms for oxidation. We show that anionic clusters react by an Eley-Rideal-like mechanism involving the preferential attack of CO on oxygen rather than gold. In contrast, the oxidation of CO on cationic gold oxide clusters can occur by both an Eley-Rideal-like and a Langmuir-Hinshelwood-like mechanism at multiple collision conditions as a result of the high adsorption energy of two CO molecules. This large energy of CO adsorption on cationic gold oxide clusters is the driving force for the CO oxidation. Therefore, in the presence of cationic gold species at high pressures of CO, the oxidation reaction is self-promoting (i.e., the oxidation of one CO molecule is promoted by the binding of a second CO). Our findings provide new insight into the role of charge state in gold-cluster-based nanocatalysis.

Stoichiometric Zirconium Oxide Cations as Potential Building Blocks for Cluster Assembled Catalysts

Employing guided-ion-beam mass spectrometry, we identified a series of positively charged stoichiometric zirconium oxide clusters that exhibit enhanced activity and selectivity for three oxidation reactions of widespread chemical importance. Density functional theory calculations reveal that these clusters all contain the same active site consisting of a radical oxygen center with an elongated zirconium-oxygen bond. Calculated energy profiles demonstrate that each oxidation reaction is highly favorable energetically and involves easily surmountable barriers. Furthermore, the active stoichiometric clusters may be regenerated by reacting oxygen-deficient clusters with a strong oxidizer. This indicates that these species may promote multiple cycles of oxidation reactions and, therefore, exhibit true catalytic behavior. The stoichiometric clusters, having structures that resemble specific sites in bulk zirconia, are promising candidates for potential incorporation into a cluster assembled catalyst material.

Potential energy and dipole moment surfaces for simultaneous torsion and pyramidalization of ethylene in its lowest‐lying singlet excited states: A CI study of the sudden polarization effect

Ab initio CI calculations have been carried out in a double‐zeta AO basis for the simultaneous torsion and pyramidalization energy and dipole moment surfaces of the two lowest (V and Z) singlet excited states of ethylene. In nonpyramidalized geometries these two electronic states are found to undergo a potential crossing for a twisting angle of ϑ=82 °, with the Z(1A1) species being the more stable for the perpendicular D2d conformation. The dipole moments of these states are found to increase quite rapidly with pyramidalization for the entire range of twisting angle from ϑ=75 ° to ϑ=90 °, but this effect is found to reach a definite maximum in the neighborhood of the 82 ° crossing region for the unpyramidalized species and it is argued that these two phenomena are in fact closely related to one another. The CI results are found to be strongly dependent on the choice of one‐electron basis in the significant portion of key structural regions and it is concluded that the use of natural orbitals optimized for...

On a possible mechanism of the multiple fluorescence of p-N, N-dimethylaminobenzonitrile and related compounds

Abstract Ab initio and CNDO/s calculations support a mechanism which involves a geometrical relaxation (i.e., the twist of the bond between the ring and NR2 group) for the appearance of the long wavelength fluorescence (Fa) observed in some p-substituted dimethylanilines. For the twisted geometries of the dimethylamino compounds a high charge transfer occurs in a low lying excited 1A state for which this geometrical relaxation seems to be energetically favourable. The polarized nature of such a 1A state might be responsible for the strong solvent dependence of the Fa fluorescence.

Prediction of structural and environmental effects on the S1S0 energy gap and jump probability in double-bond cis—trans photoisomeriz

Abstract Upon a twist of 180°, the ground and an excited state of a double bond undergo an avoided touching. The degree to which the touching is avoided is of

Theoretical exploration of femtosecond multi-state nuclear dynamics of small clusters

We investigate ultrafast multi-state nuclear dynamics in a triatomic cluster. In particular, we explore how the intracluster nuclear dynamics of the Ag3−/Ag3/Ag3+ system is reflected in the femtosecond pump-probe negative ion-to neutral-to positive ion (NENEPO) signals. The nuclear dynamics is based on classical trajectories on the ground electronic adiabatic state potential hypersurfaces obtained from accurate ab initio quantum chemistry calculations. The nuclear dynamics of Ag3 initiated from the linear transition state involves distinct sequential processes of configurational relaxation to the triangular configuration, intracluster collisions, and the onset of IVR, resonant, and dissipative IVR, and vibrational equilibration. We determined the timescales for these processes and discussed their dependence on the initial cluster temperature. The Wigner representation of the density matrix was utilized to simulate the NENEPO-zero kinetic energy (NENEPO-ZEKE) signal and the total (integrated over the photo...

Ab initio study of structural and optical response properties of excess-electron lithium-hydride and sodium-fluoride clusters

Abstract Structural and optical response properties of stoichiometric small Li n H n and Na n F n and non-stoichiometric Li n H m and Na n F m ( n − m = 1, 2, 3, 8) clusters containing single and multiple excess electrons are studied using ab initio methods accounting for electron correlation. We show that calculated absorption patterns are excellent fingerprints of structural and bonding properties. The optical response of Na n F n −1 ( n = 2–6) clusters is characterized by the common feature that is the appearance of a dominant intense transition in the infrared regime, independently whether the excess electron is localized at the cuboid corner vacancy or at the external atom attached to the filled cuboid. In contrast, Li n H n −1 ( n = 2–6) clusters exhibit substantially different spectroscopic patterns with respect to halides also for nuclearities with analogous structures. Na n F n −2 ( n = 3–6) clusters containing two excess electrons can be divided according to their optical and structural properties into cuboid “lattice” defect species (Na 4 F 2 , Na 6 F 4 ) and segregated metallic-ionic systems (Na 3 F, Na 5 F 3 ). For the former, intense transitions are found in the infrared-visible region, and for the latter only in the visible regime. Li n H n −2 ( n = 3–6) clusters exhibit common optical response features with dominant transitions in the visible regime which is characteristic for segregated metallic-ionic systems. Ground and excited state properties of above systems are in good agreement with available experimental data. Comparison with properties found for pure alkali-metal clusters allow one to investigate metallization and localization processes in finite systems.

Stereochemical effects in the quenching of Na*(3 2P) by CO: Crossed beam experiment and ab initio CI potential energy surfaces

Electronic to vibration‐rotational‐translational energy transfer in the quenching of Na(3 2P3/2) by CO has been studied with state of the art crossed atomic, molecular, and laser beam techniques at 0.16 eV initial kinetic energy, and by ab initio CI calculations for the potential energy surfaces involved in the process. Double differential quenching cross sections are measured as a function of scattering angle and energy transferred to the molecule. A pronounced structure in the energy transfer spectra as well as a partial backward scattering is attributed to two different mechanisms, a ’’direct’’ one and one which proceeds through ’’complex’’ formation. The observations are explained by the calculated potential energy surfaces (PES) for the first excited states A 2A′(A′ 2A″) and the ground state X 2A′ which exhibit two crossing seams below the 2.1 eV excitation energy: (i) one for colinear approach of Na* on the carbon side of CO with its lowest energy 1.06 eV at Rc(Na–CO)= 5.5a0, rc(C–O) = 2.35a0 respo...