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71.
We have investigated rarely observed 77Se J-couplings (spin-spin couplings) in the mixed-metal face-capped octahedral clusters [Re5OsSe8(CN)6]3- and [Re4Os2Se8(CN)6]2- at natural abundance. To the best of our knowledge, these are the first observations of Se-Se spin-spin interactions between mu3-Se sites, important for stereochemical assignments in hexarhenium analogues, Chevrel phase materials, and similar cluster materials. NMR techniques such as COSY, INADEQUATE, and 2D J-resolved spectroscopy have been used in conjunction to study these interactions. The two isomers (cis and trans) of [Re4Os2Se8(CN)6]2- were distinguishable, and selective isotopic labeling of [Re5OsSe8(CN)6]3- with 13CN ligands enabled resonances to be assigned by observing the 2J (Se-M-C) couplings. For [Re5OsSe8(CN)6]3-, two different 2J (Se-M-Se) couplings were measurable on a single cluster, and these are related to one another through spin-spin interactions across a face diagonal or along an edge of the cube of inner selenium ligands. A rigorous analysis based on combinatorial math has been invoked to assign the couplings on the basis of the probability of multiple-spin interactions. The face diagonal association is found to result in a J-coupling interaction larger in magnitude than that from coupling along an edge of the cube-information critical for making stereochemical assignments of selenium sites. 相似文献
72.
Group 2 metal complexes [Ca(SA)2(phen)]n (1), [Sr2(SA)4(phen)4] (2), and [Ba(SA)2(phen)2]n (3) (SA = salicylate) have been obtained by the addition of 1,10-phenanthroline (phen) to the corresponding metal salicylates, while the bipyridine derivatives {[Ca3(SA)6(H2O)4](4bpy)2}n (4), {[Sr(SA)2(H2O)3](4bpy)1.5(H2O)}n (5), and {[Ba(SA)2(H2O)3]-(4bpy)1.5(H2O)}n (6) have been synthesized starting from the respective metal carbonates, salicylic acid (SA-H), and 4,4'-bipyridine (4bpy). The new compounds have been characterized by elemental analysis, pH measurements, thermal analysis, and spectroscopic measurements (IR, NMR, ultraviolet, and fluorescence). Molecular structure determination by single-crystal X-ray diffraction has been carried out for all the compounds. The thermal analysis studies indicate the loss of coordinated and/or lattice water molecules below 200 degrees C in 4-6 and the absence of any coordinated or uncoordinated water molecules in compounds 1-3. Compounds 1 and 3-6 exist as one-dimensional polymers while compound 2 crystallizes as a discrete dimer. Considerable variations have been observed in the molecular structures of 1-6 in terms of the geometry around the metal, the binding mode of salicylate, and the coordination behavior of the pyridine ligand. Calcium ion is hexacoordinated in 1, while in 4 both hexa- and heptacoordinated calcium ions are simultaneously present. Strontium exhibits coordination numbers of nine and eight in 2 and 5, respectively. The barium ions in 3 and 6 assume coordination numbers of eight and seven, respectively. While the OH group of the salicylate ligand does not directly bind the metal in 1-3, it coordinates to the metal ions in complexes in 4-6 in the un-ionized form. The 4bpy molecules show no direct ligation to the metal in 4-6; the phen ligands in 1-3, however, occupy one side of the coordination sphere around the metal. The presence of additional O-H...O, C-H...O, and N-H...O hydrogen bonding and pi-pi stacking in these compounds results in the formation of polymeric structures. The results obtained for the calcium complexes in this study have been compared with the available data in structural calcium chemistry with the aid of a detailed analysis of the Cambridge Structural Database. 相似文献
73.
The role of call admission control (CAC) in high-speed networks is to maintain the network utilization at a high level, while
ensuring that the quality of service (QoS) requirements of the individual calls are met. We use the term static CAC to describe
schemes that always allocate the same bandwidth to a specific group of multiplexed calls, independent of the other traffic
sharing the link. Dynamic CAC, on the other hand, denotes a scheme in which the bandwidth allocation to a group of calls sharing
a queue is influenced by the traffic in other queues destined for the same outgoing link. We propose a generic dynamic call
admission scheme for VBR and ABR traffic whose aim is to reduce the blocking rate for VBR calls at the expense of a higher
blocking rate for ABR calls. Our scheme is generic because it builds up on a pre-existing static scheme, e.g., one based on
a simple notion of effective bandwidth. Our simple approach results in a significant reduction of the blocking rate for VBR
traffic (several orders of magnitude), if the bandwidth requirements of a single call are a reasonably small fraction of the
link capacity. At the same time, the deterioration of service for ABR traffic can be contained.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
74.
Dual-phase oscillations are observed in Belousov–Zhabotinsky system with 4-hydroxy-3-methoxybenzaldehyde (vanillin) as the substrate and manganous sulphate or ammonium Ce(IV) sulphate as the catalyst. The nonoscillatory period of time between the two phases decreases with increase in the concentration of the catalyst and the substrate. Under uncatalyzed and ferroin catalyzed conditions the system exhibits single-phase oscillations. The first-phase oscillations are due to vanillin whereas the second-phase oscillations are brought about by 4-hydroxy-3-methoxybenzoic acid (vanillic acid) formed during the course of the first-phase reactions. The reactions are explained with relevant steps of the FKN mechanism. © 1998 John Wiley & Sons, Inc. Int J. Chem. Kinet 30: 201–206, 1998. 相似文献
75.
76.
Ramaswamy Murugavel Prof. Subramaniam Kuppuswamy Nayanmoni Gogoi Alexander Steiner Dr. John Bacsa Dr. Ramamoorthy Boomishankar Dr. K. G. Suresh Prof. 《化学:亚洲杂志》2009,4(1):143-153
Tetranuclear manganese(II) phosphates [Mn(dipp)(bpy)]4?4 H2O ( 1 ) and [Mn4(dmpp)2(dmppH)4(bpy)4(H2O)2]?H2O ( 2 ) have been prepared from Mn(OAc)2?4 H2O and 2,6‐diisopropylphenyl phosphate (dippH2) or 2,6‐dimethylphenyl phosphate (dmppH2) in the presence of 2,2′‐bipyridine (bpy). In contrast, the reaction between [Mn(bpy)2(OAc)(ClO4)]?H2O and dippH2 affords [Mn(bpy)2(dippH)]2?2 ClO4?2 CH3OH ( 3 ). The reactions of Mn(OAc)2?4 H2O, dippH2, and pyridine (py) or 3,5‐dimethylpyrazole (dmpz) in CH3CN under reflux afford hexanuclear complexes [Mn6(dipp)6(py)8]?2CH3CN ( 4 ) and [Mn6(dipp)6(dmpz)6(AcOH)2]?2 H2O ( 5 ), respectively. Although compounds 1 and 2 are tetrameric, the former is a closed cubane‐like structure resembling the D4R secondary building unit of zeolites, whereas the latter exists in a staircase structure with fused Mn2O4P2 rings. The core structure of 3 contains a Mn2O4P2 eight‐membered ring that resembles the S4R building block of zeolites. Single‐crystal X‐ray diffraction studies reveal that compounds 4 and 5 have a similar core structure and differ from each other by the neutral ligands coordinated to manganese ions. All six phosphate ligands exist in a doubly deprotonated [(RO)PO32?] form and exhibit two types of binding modes [5.222] and [3.111]. An interesting feature of compounds 1 – 5 is that although they are oligonuclear complexes, there is an absence of oxido bridges. The magnetic properties of compounds 1 – 5 have been investigated in the temperature range 5–298 K, and it was found that all the compounds obey the Curie law. 相似文献
77.
siRNA. A guide for RNA silencing 总被引:19,自引:0,他引:19
RNAi is routinely used to eliminate gene activity for experimental purposes. However, the precise molecular mechanism of RNAi is unknown. Recent papers partially illuminate this mechanism in human cells, advancing the potential application of RNAi toward the treatment of human disease. 相似文献
78.
Ramaswamy Murugavel Mayank Pratap Singh M. Nethaji 《Journal of chemical crystallography》2012,42(1):12-17
Abstract
Condensation reaction involving substituted aminobenzoic acids (2-aminobenzoic acid and 4-aminobenzoic acid) and acetylacetone results in the formation of ketoimines [CH3C(= O)CH2C(CH3)(= NAr)] (Ar = C6H4COOH-4; 1 and C6H4COOH-2 2). Compounds 1 and 2 have been characterized by spectroscopic techniques and by single crystal X-ray diffraction studies. The absorption, emission and life-time measurement studies have also been performed for the new compounds. While compound 1 forms a linear chain type of aggregation though intermolecular hydrogen bonding, compound 2 forms a discrete dimer in the solid state. 相似文献79.
Solving Large Airline Crew Scheduling Problems: Random Pairing Generation and Strong Branching 总被引:1,自引:0,他引:1
Diego Klabjan Ellis L. Johnson George L. Nemhauser Eric Gelman Srini Ramaswamy 《Computational Optimization and Applications》2001,20(1):73-91
The airline crew scheduling problem is the problem of assigning crew itineraries to flights. We develop a new approach for solving the problem that is based on enumerating hundreds of millions random pairings. The linear programming relaxation is solved first and then millions of columns with best reduced cost are selected for the integer program. The number of columns is further reduced by a linear programming based heuristic. Finally an integer solution is obtained with a commercial integer programming solver. The branching rule of the solver is enhanced with a combination of strong branching and a specialized branching rule. The algorithm produces solutions that are significantly better than ones found by current practice. 相似文献
80.