Dose packing

When Li metal is cold-worked it transforms from body-centred cubic to cubic close-packed in which each atom is surrounded by 12 others in twinned cuboctahedral coordination below 78 K the stable crystalline modification is hexagonal dose-packed in which each lithium atom has 12 nearest neighbours in the form of a cuboctahedron. This very high coordination... 

Figure 3. Schematic drawing of the crystal structure of f-MnOj. The manganese atoms are randomly distributed in the octahedral voids of the hexagonal dose packing of oxygen atoms (adapted from [47]).

Anti neoplastic dosages vary widely depending on type of tumor psoriasis 10—50 mg/wk IV, IM, PO RA dose pack directed... 

Calculate the radius of the cavity formed by three circular disks of radius r that lie in a dose-packed planar arrangement. Check your answer experimentally by using three compact disks and measuring the radius of the cavity that they form. 

Brookite is also based upon a double hexagonal close-packed arrangement of the oxygen ions. The dimensions of the unit of structure differ so much on account of distortion from those for the ideal arrangement, however, that the existence of dose-packing was recognized only after the structure had been determined. 

It may be pointed out that in some structures easily derivable with the coordination theory, such as the rutile structure, the anion arrangement approximates no type of dose-packing whatever. 

The most important metals for catalysis are those of the groups VIII and I-B of the periodic system. Three crystal structures are important, face-centered cubic (fee Ni, Cu, Rh, Pd, Ag, Ir, Pt, Au), hexagonally dose-packed (hep Co, Ru, Os) and body-centered cubic (bcc Fe). Figure 5.1 shows the unit cell for each of these structures. Note that the unit cells contain 4, 2, and 6 atoms for the fee, bcc, and hep structure, respectively. Many other structures, however, exist when considering more complex materials such as oxides, sulfides etc, which we shall not treat here. Before discussing the surfaces that the metals expose, we mention a few general properties. 

The difference between the fee and hep structure is best seen if one considers the sequence of dose-packed layers. For fee lattices this is the (111) plane (see Figs. 5.1 and 5.3), for hep lattices the (001) plane. The geometry of the atoms in these planes is exactly the same. Both lattices can now be built up by stacking dose-packed layers on top of each other. If one places the atoms of the third layer directly above those of... 

The effect is huge for the dose packed Fe(llO) surface, but it should be considered that the sticking here is remote so that the effect of the potassium is basically to equalize the sticking on the three surfaces. 

The reactivity of the basal planes varies substantially and is largest for the more open Fe(lll) surface, which basically can be considered as being a very rough surface. With the more stable dosed packed surfaces the reactivity drops considerably and there are many indications here (R.C. Egeberg, S. Dahl, A. Logadottir, J.H. Ear-... 

Draw the simple (111), (100), and (110) surfaces of face-centered cubic metals, the (110) and (100) surfaces of body-centered cubic metals, and the (001) surface of an hexagonally-dosed packed surface. 

Murray, C.B., Kagan, C.R. and Bawendi, M.G. (2000) Synthesis and characterization of monodisperse nanocrystals and dose-packed nanocrystal assemblies. Annual Review of Materials Science, 30 (1), 545-610. 

Fig. 29 Cartoon demonstrating arrangement of dendrons in hexagonally dose-packed cylinder morphology (left) or in lamellar morphology (right). From [93]. Copyright 2002 American Chemical Society...

Our strategy is to incorporate a prolate redox molecule into the hydrocarbon phase of the organized monolayer. Steric restraints imposed by a dose-packed monolayer would presumably force the redox molecule to adopt an orientation parallel to the hydrocarbon tails. Spacing can then be controlled by a short hydrocarbon chain between the redox center and the metal. A class of molecules fitting these requirements are the assymetric 4,4 -bipyridiniums ... 

Fig. 6-1. TVo-dimensional atomic structure on the (100) plane of platinum crystals (1x1) = cubic close-packed surface plane identical with the (100) plane (5 x 20) = hexagonal dose-packed surface plane reconstructed finm the original (100) plane. [From Kolb, 1993.]...

Weekly therapy may be instituted with the Rheumatrex Dose Packs, which are designed to provide doses over a range of 5 to 15 mg administered as a single weekly dose. The dose packs are not recommended for weekly doses greater than 15 mg. Tailor schedules to the individual patient. An initial test dose may be given prior to the regular dosing schedule to detect any extreme sensitivity to adverse effects. Maximal myelosuppression usually occurs in 7 to 10 days. 

Weekly dose Physicians and pharmacists should emphasize that the dose is taken weekly. Mistaken daily use has led to fatal toxicity. Encourage patients to read the Patient Instructions in the Dose Pack. Do not write or refill prescriptions on a PRN basis. 

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Methefrexate (Rheumatrex Dose Pack, Trexall) [Antineeplastic/ Antirheumatic (DMARDs), Immunosuppressant/ Antimetabolite] warning Administration only by experienced healthcare provider do not use in women of childbearing age unless absolutely necessary (teratogenic) impaired elimination w/ impaired renal Fxn, ascites, pleural effusion severe myelosuppression if used w/ NSAIDs hepatotox can induce lung Dz ... 

Fig. 6.8. Hexagonal dose-packed surface, (a) Real. space, (b) Reciprocal space. The length of the primitive reciprocal lattice vector is b = (4Tr/y 3 a).

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Properties. Thallium is grayish white, heavy, and soft. When freshly cut, it has a metallic luster that quickly dulls to a bluish gray tinge like that of lead. A heavy oxide cmst forms on the metal surface when in contact with air for several days. The metal has a dose-packed hexagonal lattice below 230°C, at which point it is transformed to a body-centered cubic lattice. At high pressures, thallium transforms to a face-centered cubic form. The triple point between the three phases is at 110°C and 3000 MPa (30 kbar). The physical properties of thallium are summarized in Table 1. 

Some thiophenolate monolayers also have been investigated. Thiophenolate, C6H5S—, forms ordered monolayers on Ag(lll) with a (/7 x v/3j88°)R40.9°, and benzene rings closely packed in face-to-face stacked columns (294). Benzylthiolate (295), >-pyridinethiolate (296), and o-pyridinethiolate (296), also form ordered monolayers on Ag(lll), but with fewer dose-packed aromatic rings. 

The ftir studies reveal that the alkyl chains in SAMs of thiolates on Au(lll) usually are tilted 26-28° from the surface normal, and display 52 55° rotation about the molecular axis. This tilt is a result of the chains reestablishing VDW contact in an assembly with 0.5 nm S—S distance, larger than the distance of -0.46 nm, usually quoted for perpendicular alkyl chains in a dose-packed layer. On the other hand, thiolate monolayers on Ag(l 11) are more densely packed owing to the shorter S—S distance. There were a number of different reports on chain tilt in SAMs on Ag(l 11), probably owing to different amounts of oxide, formed on the clean metallic surface (229,230,296,297). In carefully prepared SAMs of alkanethiolates on a clean Ag(l 11) surface, the alkyl chains are practically perpendicular to the surface. 

Fig. 7.77. Three crystallographic arrangements as seen for the dose-packed cubic lattice of silver.

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