Search PubMed⌕ Search

Biomedical subjects

S N Khanna

Publications and source records attributed to S N Khanna.

11 recordsLinked to original sources
PubMed2006

Multiple valence superatoms.

We recently demonstrated that, in gas phase clusters containing aluminum and iodine atoms, an Al(13) cluster behaves like a halogen atom, whereas an Al(14) cluster exhibits properties analogous to an alkaline earth atom. These observations, together with our findings that Al(13)(-) is inert like a rare gas atom, have reinforced the idea that chosen clusters can exhibit chemical behaviors reminiscent of atoms in the periodic table, offering the exciting prospect of a new dimension of the periodic table formed by cluster elements, called superatoms. As the behavior of clusters can be controlled by size and composition, the superatoms offer the potential to create unique compounds with tailored properties. In this article, we provide evidence of an additional class of superatoms, namely Al(7)(-), that exhibit multiple valences, like some of the elements in the periodic table, and hence have the potential to form stable compounds when combined with other atoms. These findings support the contention that there should be no limitation in finding clusters, which mimic virtually all members of the periodic table.

Journal Article↗
PubMed2006

Structural, electronic, and chemical properties of multiply iodized aluminum clusters.

The electronic structure, stability, and reactivity of iodized aluminum clusters, which have been investigated via reactivity studies, are examined by first-principles gradient corrected density functional calculations. The observed behavior of Al13I(x)- and Al14I(x)- clusters is shown to indicate that for x < or = 8, they consist of compact Al13- and Al14++ cores, respectively, demonstrating that they behave as halogen- or alkaline earth-like superatoms. For x > 8, the Al cores assume a cagelike structure associated with the charging of the cores. The observed mass spectra of the reacted clusters reveal that Al13I(x)- species are more stable for even x while Al14I(x)- exhibit enhanced stability for odd x(x > or = 3). It is shown that these observations are linked to the formation and filling of "active sites," demonstrating a novel chemistry of superatoms.

Journal Article↗
PubMed2006

Structure and stability of Co(n)(pyridine)(m)- clusters: absence of metal inserted structures.

A synergistic approach combining the experimental photoelectron spectroscopy and theoretical electronic structure studies is used to probe the geometrical structure and the spin magnetic moment of Co(n)(pyridine)(m) (-) clusters. It is predicted that the ground state of Co(pyridine)(-) is a structure where the Co atom is inserted in a CH bond. However, the insertion is marked by a barrier of 0.33 eV that is not overcome under the existing experimental conditions resulting in the formation of a structure where Co occupies a site above the pyridine plane. For Co(2)(pyridine)(-), a ground-state structure is predicted in which the Co(2) diametric moiety is inserted in one of the CH bonds, but again because of a barrier, the structure which matches the photoelectron spectrum is a higher-energy isomer in which the Co(2) moiety is bonded directly to nitrogen on the pyridine ring. In all cases, the Co sites have finite magnetic moments suggesting that the complexes may provide ways of making cluster-based magnetic materials.

Journal Article↗
PubMed2005

Interaction of Pd and PdCl2 with cellulose: a theoretical investigation.

The pyrolytic fragmentation of cellulose in the presence of atomic palladium (Pd) and palladium(II) chloride (PdCl2) has been studied with use of hybrid density functional theory and cellobiose as a model for cellulose. The configuration changes in the host, rearrangement of geometries of the products, and the respective reaction energetics for different fragmentation pathways are analyzed. While Pd is found to undergo insertion at the beta-1,4-linkage oxygen (O1)-carbon (C-1) of the rings, Pd(II) chloride is observed to promote the cleavage of the chain as well as rearrangement of the rings. A detailed mechanism for the formation of levoglucosan from one of the fragments following the interaction with PdCl2 is also highlighted.

Cellobiose↗
PubMed2005

Evolution of the electronic structure of Be clusters.

Using a modified symbiotic genetic algorithm approach and many-body interatomic potential derived from first principles, we have calculated equilibrium geometries and binding energies of the ground-state and low-lying isomers of Be clusters containing up to 41 atoms. Molecular-dynamics study was also carried out to study the frequency of occurrence of the various geometrical isomers as these clusters are annealed during the simulation process. For a selected group of these clusters, higher-energy isomers were more often found than their ground-state structures due to large catchment areas. The accuracy of the above ground-state geometries and their corresponding binding energies were verified by carrying out separate ab initio calculations based on molecular-orbital approach and density-functional theory with generalized gradient approximation for exchange and correlation. The atomic orbitals were represented by a Gaussian 6-311G** basis, and the geometry optimization was carried out using the GAUSSIAN 98 code without any symmetry constraint. While the ground-state geometries and their corresponding binding energies obtained from ab initio calculations do not differ much from those obtained using the molecular-dynamics approach, the relative stability of the clusters and the energy gap between the highest occupied and the lowest unoccupied molecular orbitals show significant differences. The energy gaps, calculated using the density-functional theory, show distinct shell closure effects, namely, sharp drops in their values for Be clusters containing 2, 8, 20, 34, and 40 electrons. While these features may suggest that small Be clusters behave free-electron-like and, hence, are metallic, the evolution of the structure, binding energies, coordination numbers, and nearest-neighbor distances do not show any sign of convergence towards the bulk value. We also conclude that molecular-dynamics simulation based on many-body interatomic potentials may not always give the correct picture of the evolution of the structure and energetics of clusters although they may serve as a useful tool for obtaining starting geometries by efficiently searching a large part of the phase space.

Journal Article↗
PubMed2005

Al cluster superatoms as halogens in polyhalides and as alkaline earths in iodide salts.

Two classes of gas-phase aluminum-iodine clusters have been identified whose stability and reactivity can be understood in terms of the spherical shell jellium model. Experimental reactivity studies show that the Al13I-x clusters exhibit pronounced stability for even numbers of I atoms. Theoretical investigations reveal that the enhanced stability is associated with complementary pairs of I atoms occupying the on-top sites on the opposing Al atoms of the Al13- core. We also report the existence of another series, Al14I-x, that exhibits stability for odd numbers of I atoms. This series can be described as consisting of an Al14I-3 core upon which the I atoms occupy on-top locations around the Al atoms. The potential synthetic utility of superatom chemistry built upon these motifs is addressed.

Journal Article↗
PubMed2004

Formation and properties of halogenated aluminum clusters.

The fast-flow tube reaction apparatus was employed to study the halogenation of aluminum clusters. For reactions with HX (X=Cl, Br, and I), acid-etching pathways are evident, and we present findings for several reactions, whereby Al(n)X(-) generation is energetically favorable. Tandem reaction experiments allowed us to establish that for Al(n)Cl(-), Al(n)I(-), and Al(n)I(2) (-), species with n=6, 7, and 15 are particularly resistant to attack by oxygen. Further, trends in reactivity suggest that, in general, iodine incorporation leaves the aluminum clusters' electronic properties largely unperturbed. Ab initio calculations were performed to better interpret reaction mechanisms and elucidate the characteristics of the products. Lowest energy structures for Al(13)X(-) were found to feature icosahedral Al(13) units with the halogen atom located at the on-top site. The charge density of the highest occupied molecular orbital in these clusters is heavily dependent on the identity of X. The dependence of reactivity on the clusters' charge state is also discussed. In addition, we address the enhanced stability of Al(13)I(-) and Al(13)I(2) (-), arguing that the superhalogen behavior of Al(13) in these clusters can provide unique opportunities for the synthesis of novel materials with saltlike structures.

Journal Article↗
PubMed2004

Self-stimulated NO reduction and CO oxidation by iron oxide clusters.

It is shown that a bare Fe2O3 cluster can oxidize CO to form CO2 and reduce NO to form N2 by undergoing compositional changes between Fe2O2 and Fe2O3 states. Investigations based on density functional theory reveal that the above reactions occur through an interesting sequence. An initial CO or NO adsorbed on the Fe2O3 weakens one of the O-Fe bonds to create a loosely attached O site. A subsequent CO gets oxidized by this O and transforms the cluster to a reduced Fe2O2 that now reduces NO via multiple oxidation and reduction steps that return the cluster to the oxidized Fe2O3 state. It is shown that the small size allows geometrical rearrangements that eliminate reaction barriers, allowing energetics and not barriers to be the primary motor for catalysis. Detailed reaction paths and the corresponding energetics are presented to illustrate the viability of the proposed mechanisms.

Adsorption↗
PubMed2002

Magic numbers in metallo-inorganic clusters: chromium encapsulated in silicon cages.

A systematic theoretical study of the equilibrium geometries and total energies of Cr encapsulated in Si clusters reveals that Cr@Si(12) is more stable than its neighbors. The origin of this enhanced stability is consistent with the 18-electron sum rule commonly used in the synthesis of stable chemical complexes, and may provide a criterion for a systematic search of magic numbers in metalloinorganic clusters. The 6 mu(B) magnetic moment of the caged Cr atom, the largest among the 3d transition metal atoms, is completely quenched. This effect of caging on the properties of transition metal atoms may lead to the synthesis of novel cluster based materials.

Journal Article↗
PubMed1994

Biatrial drainage of right superior vena cava with anomalous right pulmonary venous connection.

A case of right superior vena cava draining to both atria, predominantly to the left atrium, with anomalous right pulmonary venous connection to the lower right superior vena cava is reported. The haemodynamic significance of these anomalies is discussed, and the technique of surgical repair is described. The literature on this rare but interesting clinical entity is briefly reviewed.

Cardiac Surgical Procedures↗