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Principles Of Fluorescence Spectroscopy

The structural properties of free nanoclusters are reviewed. Special attention is paid to the interplay of energetic, thermodynamic, and kinetic factors in the explanation of cluster structures that are actually observed in experiments. The review starts with a brief summary of the experimental methods for the production of free nanoclusters and then considers theoretical and simulation issues, always discussed in close connection with the experimental results. The energetic properties are treated first, along with methods for modeling elementary constituent interactions and for global optimization on the cluster potential-energy surface. After that, a section on cluster thermodynamics follows. The discussion includes the analysis of solid-solid structural transitions and of melting, with its size dependence. The last section is devoted to the growth kinetics of free nanoclusters and treats the growth of isolated clusters and their coalescence. Several specific systems are analyzed.

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Structural properties of nanoclusters: Energetic, thermodynamic, and kinetic effects

Size dependence of nanostructures: Impact of bond order deficiency

Metals and organic molecules are at the opposite sides in chemistry. Thus one may think that chemical bonding models, such as aromaticity, developed in organic chemistry will not be applicable in systems involving metallic elements. Yet in recent years there have been significant advances in extending the aromaticity and antiaromaticity concepts into the realm of metal clusters and alloys. These new developments have shown that aromaticity and antiaromaticity in metal systems have frequently multiple nature, being o-aromatic/antiaromatic and n-aromatic/antiaromatic, which is not found in organic aromatic/antiaromatic molecules.

http://casey.brown.edu/chemistry/research/LSWang/publications/209.pdf

All-Metal Aromaticity and Antiaromaticity

The herb Artemisia annua has been used for many centuries in Chinese traditional medicine as a treatment for fever and malaria. In 1971, Chinese chemists isolated from the leafy portions of the plant the substance responsible for its reputed medicinal action. This compound, called qinghaosu (QHS, artemisinin), is a sesquiterpene lactone that bears a peroxide grouping and, unlike most other antimalarials, lacks a nitrogen-containing heterocyclic ring system. The compound has been used successfully in several thousand malaria patients in China, including those with both chloroquine-sensitive and chloroquine-resistant strains of Plasmodium falciparum. Derivatives of QHS, such as dihydroqinghaosu, artemether, and the water-soluble sodium artesunate, appear to be more potent than QHS itself. Sodium artesunate acts rapidly in restoring to consciousness comatose patients with cerebral malaria. Thus QHS and its derivatives offer promise as a totally new class of antimalarials.

Quinghaosu (artemisin): an antimalarial drug from china

Density functional molecular dynamical simulations have been performed on Ga 1 7 and Ga 1 3 clusters to understand the recently observed higher-than-bulk melting temperatures in small gallium clusters [G. A. Breaux et al., Phys. Rev. Lett 91, 215508 (2003)]. The specific-heat curve, calculated with the multiple-histogram technique, shows the melting temperature to be well above the bulk melting point of 303 K, viz., around 650 and 1400 K for Ga 1 7 and Ga 1 3 , respectively. The higher-than-bulk melting temperatures are attributed mainly to the covalent bonding in these clusters, in contrast with the covalent-metallic bonding in the bulk.

/pdf/why-do-gallium-clusters-have-a-higher-melting-point-than-the-3ivdx2wa2z.pdf

Why do gallium clusters have a higher melting point than the bulk

Artemisinin constitutes the frontline treatment to aid rapid clearance of parasitaemia and quick resolution of malarial symptoms. However, the widespread promiscuity about its mechanism of action is baffling. There is no consensus about the biochemical target of artemisinin but recent studies implicate haem and PfATP6 (a calcium pump). We investigated the role of iron and artemisinin on PfATP6, in search of a plausible mechanism of action, via density functional theory calculations, docking and molecular dynamics simulations. Results suggest that artemisinin gets activated by iron which in turn inhibits PfATP6 by closing the phosphorylation, nucleotide binding and actuator domains leading to loss of function of PfATP6 of the parasite and its death. The mechanism elucidated here should help in the design of novel antimalarials.

/pdf/a-plausible-mechanism-for-the-antimalarial-activity-of-53567pkxtd.pdf

A plausible mechanism for the antimalarial activity of artemisinin: A computational approach

A systematic and detailed investigation of the finite-temperature behavior of small sodium clusters, Nan, in the size range of n=8–50 are carried out. The simulations are performed using density-functional molecular dynamics with ultrasoft pseudopotentials. A number of thermodynamic indicators such as specific heat, caloric curve, root-mean-square bond-length fluctuation, deviation energy, etc., are calculated for each of the clusters. Size dependence of these indicators reveals several interesting features. The smallest clusters with n=8 and 10 do not show any signature of melting transition. With the increase in size, broad peak in the specific heat is developed, which alternately for larger clusters evolves into a sharper one, indicating a solidlike to liquidlike transition. The melting temperatures show an irregular pattern similar to the experimentally observed one for larger clusters [Schmidt et al., Nature (London) 393, 238 (1998)]. The present calculations also reveal a remarkable size-sensitive e...

First-principles investigation of finite-temperature behavior in small sodium clusters.

We report spin-polarized density functional calculations of ferromagnetic properties for a series of ZnO clusters and ZnO solid containing one or two substitutional carbon impurities. We analyze the eigenvalue spectra, spin densities, molecular orbitals, and induced magnetic moments for ZnC, Zn2C, Zn2OC, carbon-substituted ZnnOn (n = 3−10, 12) clusters and the bulk ZnO. The results show that the doping induces magnetic moment of ∼2 μB in all the cases. All systems with two carbon impurities show ferromagnetic interaction, except when carbon atoms share the same zinc atom as the nearest neighbor. This ferromagnetic interaction is predominantly mediated via π-bonds in the ring structures and through π- and σ-bonds in the three-dimensional structure. The calculations also show that the interaction is significantly enhanced in the solid, bringing out the role of dimensionality of the Zn−O network connecting two carbon atoms.

Ferromagnetism in carbon-doped zinc oxide systems.

Density-functional simulations have been performed on Na 55 , Na 92 , and Na 142 clusters in order to understand the experimentally observed melting properties [M. Schmidt et al., Nature (London) 393, 238 (1998)]. The calculated melting temperatures are in excellent agreement with the experimental ones. The calculations reveal a rather subtle interplay between geometric and electronic shell effects, and bring out the fact that the quantum mechanical description of the metallic bonding is crucial for understanding quantitatively the variation in melting temperatures observed experimentally.

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Sajeev Chacko

Papers

Why do gallium clusters have a higher melting point than the bulk

A plausible mechanism for the antimalarial activity of artemisinin: A computational approach

First-principles investigation of finite-temperature behavior in small sodium clusters.

Ferromagnetism in carbon-doped zinc oxide systems.

First principles calculations of melting temperatures for free Na clusters