417 research outputs found
Magic Melters' Have Geometrical Origin
Recent experimental reports bring out extreme size sensitivity in the heat
capacities of Gallium and Aluminum clusters. In the present work we report
results of our extensive {\it ab initio} molecular dynamical simulations on
Ga and Ga, the pair which has shown rather dramatic size
sensitivity. We trace the origin of this size sensitive heat capacities to the
relative order in their respective ground state geometries. Such an effect of
nature of the ground state on the characteristics of heat capacities is also
seen in case of small Gallium and Sodium clusters indicating that the observed
size sensitivity is a generic feature of small clusters.Comment: 4 pages, 6 figure
Finite Temperature Behavior of Small Silicon and Tin Clusters: An Ab Initio Molecular Dynamics Study
The finite temperature behavior of small Silicon (Si, Si, and
Si) and Tin (Sn and Sn) clusters is studied using
isokinetic Born-Oppenheimer molecular dynamics. The lowest equilibrium
structures of all the clusters are built upon a highly stable tricapped
trigonal prism unit which is seen to play a crucial role in the finite
temperature behavior of these clusters. Thermodynamics of small tin clusters
(Sn and Sn) is revisited in light of the recent experiments on
tin clusters of sizes 18-21 [G. A. Breaux et. al. Phys. Rev. B {\bf 71} 073410
(2005)]. We have calculated heat capacities using multiple histogram technique
for Si, Sn and Si clusters. Our calculated specific heat
curves have a main peak around 2300 K and 2200 K for Si and Sn
clusters respectively. However, various other melting indicators such as root
mean square bond length fluctuations, mean square displacements show that
diffusive motion of atoms within the cluster begins around 650 K. The finite
temperature behavior of Si and Sn is dominated by isomerization
and it is rather difficult to discern the temperature range for transition
region. On the other hand, Si does show a liquid like behavior over a
short temperature range followed by the fragmentation observed around 1800 K.
Finite temperature behavior of Si and Sn show that these clusters
do not melt but fragment around 1200 K and 650 K respectively.Comment: 9 figure
Ab initio Molecular Dynamical Investigation of the Finite Temperature Behavior of the Tetrahedral Au and Au Clusters
Density functional molecular dynamics simulations have been carried out to
understand the finite temperature behavior of Au and Au clusters.
Au has been reported to be a unique molecule having tetrahedral
geometry, a large HOMO-LUMO energy gap and an atomic packing similar to that of
the bulk gold (J. Li et al., Science, {\bf 299} 864, 2003). Our results show
that the geometry of Au is exactly identical to that of Au with
one missing corner atom (called as vacancy). Surprisingly, our calculated heat
capacities for this nearly identical pair of gold cluster exhibit dramatic
differences. Au undergoes a clear and distinct solid like to liquid like
transition with a sharp peak in the heat capacity curve around 770 K. On the
other hand, Au has a broad and flat heat capacity curve with continuous
melting transition. This continuous melting transition turns out to be a
consequence of a process involving series of atomic rearrangements along the
surface to fill in the missing corner atom. This results in a restricted
diffusive motion of atoms along the surface of Au between 650 K to 900 K
during which the shape of the ground state geometry is retained. In contrast,
the tetrahedral structure of Au is destroyed around 800 K, and the
cluster is clearly in a liquid like state above 1000 K. Thus, this work clearly
demonstrates that (i) the gold clusters exhibit size sensitive variations in
the heat capacity curves and (ii) the broad and continuous melting transition
in a cluster, a feature which has so far been attributed to the disorder or
absence of symmetry in the system, can also be a consequence of a defect
(absence of a cap atom) in the structure.Comment: 7 figure
Stability of binary complexes of Pb(II), Cd(II) and Hg(II) with maleic acid in TX100-water mixtures
Binary complexes of maleic acid with toxic metal ions such as Pb(II), Cd(II) and Hg(II) have been studied in 0.0-2.5% v/v tritonX-100 (TX100) - water media at 303 K at an ionic strength of 0.16 M. The active forms of the ligand are LH2, LH- and L2-. The derived ‘best fit’ chemical speciation models are based on crystallographic R-factors, χ2 and Skewness and Kurtosis factors. The predominant species formed are of the type ML2, ML2H and ML3. The trend in variation of complex stability constants with change in the mole fraction of the medium is explained on the basis of prevailing electrostatic and non-electrostatic forces. The species distribution as a function of pH at different compositions of TX100-water mixtures and plausible speciation equilibria are presented and discussed. KEY WORDS: Maleic acid, TritonX-100, Toxic metal ions, Mole fraction, Binary complexes Bull. Chem. Soc. Ethiop. 2014, 28(3), 383-391.DOI: http://dx.doi.org/10.4314/bcse.v28i3.
Density functional investigation of the interaction of acetone with small gold clusters
The structural evolution of Aun (n = 2, 3, 5, 7, 9, and 13) clusters and the adsorption of organic molecules such as acetone, acetaldehyde, and diethyl ketone on these clusters are studied using a density functional method. The detailed study of the adsorption of acetone on the Aun clusters reveals two main points. (1) The acetone molecule interacts with one gold atom of the gold clusters via the carbonyl oxygen. (2) This interaction is mediated through back donation mainly from the spd-hybridized orbitals of the interacting gold atom to the oxygen atom of the acetone molecule. In addition, a hydrogen bond is observed between a hydrogen atom of the methyl group and another gold atom (not involved in the bonding with carbonyl oxygen). Interestingly, the authors notice that the geometries of Au9 and Au13 undergo a significant flattening due to the adsorption of an acetone molecule. They have also investigated the role of the alkyl chain attached to the carbonyl group in the adsorption process by analyzing the interaction of Au13 with acetaldehyde and diethyl ketone
Size--sensitive melting characteristics of gallium clusters: Comparison of Experiment and Theory for Ga and Ga
Experiments and simulations have been performed to examine the
finite-temperature behavior of Ga and Ga clusters.
Specific heats and average collision cross sections have been measured as a
function of temperature, and the results compared to simulations performed
using first principles Density--Functional Molecular--Dynamics. The
experimental results show that while Ga apparently undergoes a
solid--liquid transition without a significant peak in the specific--heat,
Ga melts with a relatively sharp peak. Our analysis of the
computational results indicate a strong correlation between the ground--state
geometry and the finite--temperature behavior of the cluster. If the
ground--state geometry is symmetric and "ordered" the cluster is found to have
a distinct peak in the specific--heat. However, if the ground--state geometry
is amorphous or "disordered" the cluster melts without a peak in the
specific--heat.Comment: 6 figure
Anatomical variation in the formation and course of median nerve: a cadaveric study
Background: Median nerve is one of the terminal branches of brachial plexus. Its formed by the union of medial root and lateral root coming respectively from medial and lateral cords of brachial plexus. Knowledge of anatomical variations of median nerve at origin and course is important in repair of traumatic injuries and surgical correction of brachial plexus injuries. These conditions need dissection of median nerve and knowledge of its variations.Methods: Present study included 53 cadavers and 106 upper limbs from our department of Anatomy. In this study, anatomically embalmed cadavers which were kept for routine dissection for under graduates were included. The present study we studied the anatomical variations in origin and course of median nerve in arm. We also studied the relation of median nerve with axillary and brachial arteries.Results: In this study we found origin of median nerve from 3 roots in 26.41%, 4 roots from 1.88%. Regarding the relation of median nerve with axillary artery we observed in 8.49% cadavers median nerve lies medial to axillary artery and in 0.94% Median nerve is passing along the lateral side of brachial artery without crossing the artery.Conclusions: This study shows high percentage of deviations from normal anatomy in origin of median nerve. Anatomical variation in brachial plexus and adjacent arteries knowledge is important for anatomist, plastic surgeon and vascular surgeons.
DEVELOPMENT OF OSMOTICALLY CONTROLLED ORAL DRUG DELIVERY SYSTEM FOR NATEGLINIDE AN ANTI-DIABETIC DRUG
Objective: The purpose of the present study was to develop an oral push-pull osmotic drug delivery system for the drug Nateglinide which is a bio pharmaceutics classification system (BCS) class II drug. Methods: The tablets were prepared by the wet granulation method using ingredients microcrystalline cellulose (Adsorbent), potassium chloride (Osmotic agent), poly ethylene glycol (4000 and 6000) (Hydrophilic polymer, Plasticizer), starch (Disintegrant), and aerosil. The granules were compacted by double compression method and were coated with eudragit by dipping method. Different batches were prepared to study the effect of the various ingredients and their effect on the release of the drug from the system by varying the concentrations of the ingredients in each batch. Dissolution was assessed using USP dissolution apparatus 2 in phosphate buffer pH 6.8 for 12 h. Results: Certain key findings observed includes a decrease in micro crystalline cellulose content reduced the release of the drug due to the reduction of the hydrophilic content in the tablet which complements the uptake of water from the surroundings, and increase in the ethylene glycol leads to decrease in the release which resulted due to excess swelling and increase in the osmotic agent concentration lead to satisfactory release of the drug and followed zero-order release. Conclusion: To conclude, the push-pull osmotic tablet of Nateglinide was able to deliver the drug in a controlled pattern for a prolonged period of time. This type of formulation can be used in conditions like hyperglycemia where the patient compliance can improve by reducing the dosing frequency and the plasma drug levels can be maintained, the total drug load is also reduced so that the dose related side-effects are also reduced. Keywords: Controlled release, Push-pull osmotic pump, Nateglinid
Melamine formaldehyde-metal organic gel interpenetrating polymer network derived intrinsic Fe-N-doped porous graphitic carbon electrocatalysts for oxygen reduction reaction
Fe, N doped porous graphitic carbon electrocatalyst (Fe-MOG-MF-C), obtained by pyrolysis of an Interpenetrating Polymer Network (IPN) comprised of melamine formaldehyde (MF as hard segment) and Metal-Organic Gel (MOG as soft segment), exhibited significant Oxygen Reduction Reaction (ORR) activity in alkaline medium. BET surface area analysis of Fe-MOG-MF-C showed high surface area (821 m2 g-1), while TEM, Raman and XPS results confirmed Fe and N co-doping. Furthermore, a modulated porous morphology with a higher degree of surface area (950 m2 g-1) has been accomplished for the system (Fe-MOG-MFN-C) when aided by a sublimable porogen, such as naphthalene. XPS results further demonstrated that these systems exhibited a better degree of distribution of graphitic N and an onset potential value of 0.91 V vs. RHE in 0.1 M KOH solution following an efficient four-electron ORR pathway. The electrocatalytic activity of Fe-MOG-MFN-C is superior to that of Fe-MOG-MF-C by virtue of its higher graphitic N content and surface area. Thus, the study presents a new class of IPN derived MF-MOG nanocomposites with the potential to generate extended versions of in situ Fe-N doped porous graphitic carbon structures with superior ORR activity
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