Core bond

Mini Femte Slider Maonirwd R iis Wire Wound Coils Glass Bonded Core LXWKH 4X9.2X0.86 mm 55 mg rnass... 

Recall from Chapter 8 that valence electrons are accessible for bonding. Core electrons are too close to the nucleus and too tightly bound to be shared with other atoms. 

The chemistry of octahedral metal clusters culminates in the center of the Periodic Table with the heavy transition metals Nb, Ta, Mo, W, and Re. There is a plethora of clusters where the M-M bonded core is surrounded (and shielded) by non-metal ligands. When moving to the left of the Periodic Table the decrease in valence electron concentration calls for a stabilization through incorporation of interstitial atoms into the cluster core. Actually, the stabilization of the cluster occurs... 

In the first case, a typical staircase-like motif of the H-bonded core was observed showing a close similarity with 28 30 and 29 30. Some H-bonds were longer resulting in a partially formed staircase motif with an S -shaped motif (Figure 35). Thus, the complementarity between the partners is affected by increasing the steric congestion and the stacking properties of the substituents in a trans- 1,2-diol scaffold. 

Figure 35 (a) H-bonded assembly of the adduct 20 33 (top view down the b axis), (b) Side view of the assembly (H-bonds represented by thin lines), (c) Simplified representation of the view in (b) showing the S -shaped motif on the H-bonded core, (d) Single strand of partially H-bonded diamine and diol units. The arrangement of the units in combination with the direction of the H-bonds (thin fines) imparts the right-handed helical shape of the strand producing a pseudo-triheficate structure. 

Figure 45 (a) ORTEP view of the molecular adduct 39 35 (H-bonds are represented by thin lines), (b) ORTEP view of the inclusion complex between benzene and adduct 39 35. (c) Side view of the H-bonding network of adduct 39 35. (d) Simplified representation of the view in (c) showing the right-handed helical motif of the ribbon like H-bonded core of the assembly, (e) Single strand for H-bonded units extracted from the triple-stranded heli-cate structure in 39 35 showing left-handed helicity. (f) Stereoview of the inclusion complex between benzene and adduct 39 35 [60],... 

Figure 48 (a) H-bonded assembly of the carbamate 41. (b) Interlinking H-bonds with contiguous cell units, (c) Side view of the assembly, (d) Pleated-sheet staircase-like H-bonded core with bifurcated H-bonds. (e) Single strand of diamine 29 and CO2 separated from the triple-stranded helicate for viewing purposes. Note right-handed helicity [51]. 

Assuming the pleated sheet-like motif and the ribbon-like motif as reference H-bonded core structures, a further increase in the steric demand of the residues, as for example the presence of aromatic rings as substituents on the diol or diamine moiety, or the absence of the required trans configuration, lead to a loss of efficiency in the recognition process with formation of incomplete ribbon-like or staircase-like structures. In the latter cases, competition experiments show that matched complexes with the pleated sheet-motif or with the ribbon-like motif of the core crystallize more easily than those characterized by incomplete or low organized core structures. Thus, when the complex 29 38 was melted with 1 equiv. of (i ,i )-tra x-l,2-cyclohexanediol (31) and the mixture was crystallized from benzene, again 29 31 was obtained as the only crystalline product (Scheme 22) [60]. 

Treat the metal cluster [RunHlCOh 2. with known metal core geometry shown below (a) as a cluster that obeys the counting rules for main-group clusters (b) as a fused cluster structure and (c) as a radial- and surface-bonded core cluster with external cluster fragments bound by radial bonds only. Which provides a satisfactory explanation at the level of electron counting ... 

For an overall view of transition metal systems one has to confront a number of problems besides correlation, or sometimes as a part of it many electrons in open shells producing large number of close-lying electronic states, different spin multiplicities, magnetism, metal-metal bonds, core or " semicore electrons that occupy the same part of the space as the valence shell, fast electrons requiring relativistic corrections etc. In addition, many of transition metal systems of practical interest are highly complex and often incompletely characterized, so that careful modeling is to be added to the list of difficulties. 

FIGURE 61.16 Cylinder block casting made with resin-silicate bonded core. 

The two major categories of polymer composite used in aerospace are laminates and sandwich stmctures, which require some differences in NDT inspection methods. The inspection of choice at the time of manufacture may be different from the inspection of choice for in-service assessments. The form of the stmcture determines the types of features of concern and the NDT techniques that may be apphed to detect and measure them. Table 15.1 summarizes some general thoughts and considerations on NDT of polymer composite stmctures. American Society for Testing and Materials (ASTM) also provides guidance on NDT for polymer composites [12]. The primary concerns in laminates are voids and porosity, inclusions, delamination, the fiber-to-resin ratio, and wrinkles (wavy or out-of-plane ply). Ultrasound is the predominate method for composite laminate inspection. In sandwich structures, the core material-to-facesheet bond, core condition, and fluid ingress are the usual concerns. 

---