Search results for " clusters"
showing 10 items of 1091 documents
Orbiting Orbitals: Visualization of Vi-Bronic Motion at a Conical Intersection
2013
The Jahn-Teller (JT) active unpaired electron of single metalloporphyrin radical anions is imaged through scanning tunneling microscopy. It is demonstrated that the electron is delocalized over the porphyrin macrocycle and its topographic image is determined by vibronic motion: the orbital of the electron adiabatically follows the zero-point pseudorotation of skeletal deformations. Transformation of the polar graphs of the observed images allows visualization of the adiabatic vibrational density to which the electron is coupled. The vibronic potential at the conical intersection is visualized and the half-integer angular momentum characteristic of the Berry phase is revealed in the radial f…
Self-trapped exciton luminescence in crystalline α-quartz under two-photon laser excitation
2003
Abstract The luminescence of pure crystalline α-quartz is studied under pulsed ArF laser excitation. The luminescence parameters obtained correspond well with those of self-trapped excitons (STEs) in α-quartz, indicating that the excitation process is two-photon. The efficiency of two-photon excitation is of the same order of magnitude as the one-photon excitation of sodium salicylate. The STE luminescence decay kinetics and their temperature dependence under photoexcitation were recorded with higher accuracy than previously. Changes in the decay kinetics with temperature are explained by the splitting of the STE triplet state in zero magnetic field and are analyzed with the assumption of t…
Photochemical window mechanism for controlled atom release in carborane endohedral boxes: Theoretical evidence
2006
Abstract On the basis of high-level quantum chemical calculations, closed-cage carboranes are shown to be prospective nanoscale mass selective conveyors via a photochemical switch. A mechanism is evidenced in which stable carborane boxes with lithium as endohedral atom, Li@CB 11 H 12 , can be photoinduced to sequentially evolve opening the cage, releasing the internal atom through a window in the box structure, and closing the cage toward either ionic or neutral channels where the charge or spin of the fragments may be controlled.
Theoretical spectroscopic parameters of the alkali monofluorides LiF, NaF and KF
1992
Abstract Multireference configuration interaction and second-order perturbation theory are used to determine accurate spectroscopic parameters for the ground state of the alkali monofluorides from LiF to KF. Systematic saturation of the spdf subspaces of the atomic basis sets for the electron affinity of fluorine and the ionization potential of the metals have been shown to be more efficient than for the approximate dissociation energies in the molecule. the calculated rotational and vibrational constants and transitions for the three systems show excellent agreement with the expeirmental data.
An MO study of neutral C8 high-symmetry clusters
1997
Abstract Highly symmetric octacarbon clusters, i.e. a cube (O h ), a twisted cube (T d ), a tricyclic cage (C 3v ) and a planar (D 4h ) structure, were fully optimized at the HF, B3LYP and MP2(Full) levels of theory. These stationary points were shown to be local minima on the potential energy surface by frequency calculations. The stability of these clusters compared to the cyclic planar (C 4h ) ground state was found to be strongly dependent on the level of theory. MP2(Full) calculations suggest that cubic C 8 is a local minimum, its energy being 273 kJ/mol higher than the ground state. Structural parameters and fundamental vibrations are presented.
Structure, magnetizability, and nuclear magnetic shielding tensors of bis-heteropentalenes. IV. Dihydrophospholophosphole isomers
2005
The geometry of the heteropentalenes formed by two phosphole units has been determined at the DFT level. The magnetic susceptibility and the nuclear magnetic shielding at the nuclei of these systems have also been calculated using gauge-including atomic orbitals and a large Gaussian basis set to achieve near Hartree-Fock estimates. A comparative study of the various isomers, of their flattened analogs, and of the parent phosphole molecule, shows that the [3,4-c] isomer is the most aromatic system in the set considered, assuming diatropicity and degree of planarity as indicators, even if it is the less stable in terms of total molecular energy. Plots of magnetic field-induced current densiti…
Effect of hydrogen bonds on polarizability of a water molecule in (H2O)(N) (N = 6, 10, 20) isomers.
2010
Abstract: Polarizabilities of the low-lying isomers of (H2O)N (N = 6, 10, 20) clusters were computed by using Density Functional Theory. The global polarizabilities of the water isomers were found to depend mainly on the total number of water molecules rather than their cluster structures. We show that this result hides in fact a strong heterogeneity of the molecular polarizability within the different isomers. The global polarizability of a cluster was divided into a sum of molecular contributions by using the Hirshfeld partitioning scheme. We reveal that the value of the local polarizability of a molecule in the cluster is correlated with the number and type of the hydrogen bonds (HB) the…
ChemInform Abstract: Getting Discriminant Functions of Antibacterial Activity from Physicochemical and Topological Parameters.
2010
Linear discriminant analysis has been demonstrated to be a very useful tool in the selection and design of new drugs. Up to now we have used it through the search of a topological pattern of activity. In this work our goal is to calculate a complete set of physicochemical parameters using semiempirical (quantum chemical) calculations as well as topological indices (TIs) and try to find out any discriminant function for antibacterial activity through the combined use of both types of descriptors. The physicochemical parameters, such as heat of formation, HOMO, LUMO, dipole moment, polarizability, hyperpolarizability, PM3 generated IR vibrational frequencies, etc., were calculated using PM3 H…
DFT study of polarizabilities and dipole moments of water clusters
2005
Density functional theory (DFT) calculations with different exchange- correlation functionals, Becke's three-parameter exchange functional and the gradient- corrected functional of Lee, Yang, and Paar (B3LYP) and Becke's three-parameter functional with Perdew-Wang correlational functional (B3PW91), are performed to study the dielectric properties of small and medium-sized water clusters. For these H- bonded systems, we optimize the geometries and compute the dipole moments and polarizabilities using a supermolecule approach. The corresponding properties of the individual water molecules in the clusters are extracted from the molecular properties using the Hirshfeld expansion of the electron…
Atomic and electronic structure of both perfect and nanostructured Ni(111) surfaces: First-principles calculations
2011
article i nfo In this study, we perform first principles simulations on both atomically smooth and nanostructured Ni(111) slabs. The latter contains periodically distributed nickel nanoclusters atop a thin metal film gradually growing from adatoms and serving as a promising catalyst. Applying the generalized gradient approximation within the formalism of the density functional theory we compare the atomic and electronic structures of Ni bulk, as well as both perfect and nanostructured (111) surfaces obtained using two different ab initio approaches: (i) the linear combination of atomic orbitals and (ii) the projector augmented plane waves. The most essential inter-atomic forces between the …