Difference generation

Fig. 18.6. A flat-faced pressure window. The pressure difference generates tensile stresses in the low-pressure face.

In a boiler, with the continued application of heat, steam under pressure is produced via a combination of steam bubble formation (nucleate boiling) and direct evaporation at the steam-water interface (convective boiling), as shown in the sketch of different generated steam flow forms in Figure 1.1. 

Synthetic pathways have been deliberately aimed at producing megamers. For example, poly(amido amine) dendrimers of different generations have been combined to give well-defined core-shell megamers in which a central, large core dendrimer is surrounded by a well-defined number of smaller dendrimers. 

PAMAM dendrimers are synthesized in a multistep process. Starting from a multifunctional amine (for example ammonia, ethylenediamine, or tris(2-amino-ethyl)amine) repeated Michael addition of methylacrylate and reaction of the product with ethylenediamine leads to dendrimers of different generation numbers [1,9]. Two methylacrylate monomers are added to each bifunctional ethylenediamine generating a branch at each cycle. Unreacted ethylenediamine has to be completely removed at each step to prevent the initiation of additional dendrimers of lower generation number. Excess methylacrylate has also to be removed. Bridging between two branches of the same or of two different dendrimers by ethylenediamine can also be a problem, and has to be avoided by choosing appropriate reaction conditions. 

Often, one needs to compare different 3D structures or conformations of a molecule. That is done internally by the 3D stmcture generation program to weed out too similar conformations of fragments. Another aspect is the need of the computational chemist to compare different generated or experimental structures. A well-established measure is the so-called root mean square (RMS) value of all atom-atom distances between two 3D structures. The RMS value needed here is a minimum value achieved by superimposing the two 3D structures optimally. Before calculating the RMS, the sum of interatomic distances is minimized by optimizing the superimposition in 3D. 

Tadjeddine A. 2000. Spectroscopic investigation of surfaces and interfaces by using infrared-visible sum and difference generation. Surf Rev Lett 7 423-436. 

In a way related are the complexes formed by Hg salts and multicrown dendrimers of different generations (dendrimers with a polypropylene amine interior of different volume and benzo[15]-crown-5 ether periphery), studied by extraction methods using radioactive 203Hg2+.210 Up to 12 Hg2+ ions were found to be bound per dendrimer molecule, obviously in the amine-dominated interior, not in the crown-ether periphery. 

Of course, in free-convection mass transfer the transition time is dependent on the density difference generated at the electrode. The dimensionless time variable of the transient process is... 

Recently, the Okada group described a new class of polymerization systems [43] oligoglycopeptide-type sugar-balls were obtained by a radial growth polymerization (RGP) of a-amino acid AT-carboxyanhydrides with PAMAM dendrimers of different generations. 

The latter effect has been demonstrated by Meijer et al., who attached chiral aminoalcohols to the peripheral NH2-groups of polypropylene imine) dendrimers of different generations [100]. In the enantioselective addition of diethyl-zinc to benzaldehyde (mediated by these aminoalcohol appendages) both the yields and the enantioselectivities decreased with increasing size of the dendrimer (Fig. 28). The catalyst obtained from the 5th-generation dendrimer carrying 64 aminoalcohol groups at its periphery showed almost no preference for one enantiomer over the other. This behavior coincides with the absence of measurable optical rotation as mentioned in Sect. 3 above. The loss of activity and selectivity was ascribed to multiple interactions on the surface which were... 

PAMAM dendrimers of different generations have been loaded with DOTA-type129,130 or DTPA-type131-133 chelates, most often using the p-NCS-benzyl linker. For these types of... 

Jacobsen et al. reported enhanced catalytic activity by cooperative effects in the asymmetric ring opening (ARO) of epoxides.[38] Chiral Co-salen complexes (Figure 4.27) were used, which were bound to different generations of commercial PAMAM dendrimers. As a direct consequence of the second-order kinetic dependence on the [Co(salen)] complex concentration of the hydrolytic kinetic resolution (HKR), reduction of the catalyst loading using monomeric catalyst leads to a sharp decrease in overall reaction rate. 

Size and core parameters of globules of different generations... 

The Lipari-Szabo approach was used to understand the cause of the pH-dependent relaxivities of PAMAM-type dendrimeric Gdm complexes (74). Three different generations (5,7,9) of PAMAM dendrimers loaded with the [Gd(EPTPA)(H20)]2 chelate via a benzyl-thiourea linkage have been investigated (Scheme 8). The relaxivities show a strong and reversible pH dependency for all three dendrimer complexes,... 

In many cases the octahedron based structures appear with a truncation R = 75%. In that case they can be described as platelets. On the other hand the fact that the growth rate of the various crystal faces might be different generates irregular shaped platelets. A particulary common shape are the triangular plates (shown in Fig. 8). These are the result of truncating a single tetrahedron with ill faces. 

They used different generations of intact dendrimers to transfect plasmid DNA in a variety of cells (Table 18.2) using luciferase, CAT (chloramphenicol acetyltransferase) and /i-galactosidasc reporter genes to quantify transfection efficiency. 

The research group of Van Leeuwen has focused on catalysis at the core of a carbosilane dendrimer in an effort to be able to control stereoselectivity [10]. To this end, a ferrocenyl diphosphine backbone was functionalized with different generations of carbosilane dendrons producing a series of dendrimer phosphine ligands with an increasing steric demand (see 7 for an example, Scheme 6). In situ... 

A number of groups have reported the preparation and in situ application of several types of dendrimers with chiral auxiliaries at their periphery in asymmetric catalysis. These chiral dendrimer ligands can be subdivided into three different classes based on the specific position of the chiral auxiliary in the dendrimer structure. The chiral positions may be located at, (1) the periphery, (2) the dendritic core (in the case of a dendron), or (3) throughout the structure. An example of the first class was reported by Meijer et al. [22] who prepared different generations of polypropylene imine) dendrimers which were substituted at the periphery of the dendrimer with chiral aminoalcohols. These surface functionalities act as chiral ligand sites from which chiral alkylzinc aminoalcoholate catalysts can be generated in situ at the dendrimer periphery. These dendrimer systems were tested as catalyst precursors in the catalytic 1,2-addition of diethylzinc to benzaldehyde (see e.g. 13, Scheme 14). 

Simple procedures to isolate or purify the different generations are required. 

All PAMAM dendrimers (both NH3 and EDA core G = 0-12) were provided by Dendritic Sciences, Inc., Mt. Pleasant, Michigan. Efforts were made to utilize several different generational lot samples for the mass spectral data. 

Fig. 24. Bead density profiles in an EV dendrimer with different generation numbers. The curves for gn=6> 7, and 8 are shifted up by 0.1,0.2 and 0.3 in the vertical axis. Reprinted with permission from [230]. Copyright (1996) American Chemical Society...

---