Sequential bonding

Ionophores such as A-23187 and X-14885A are flexible, so despite the need for conformational change, established for A-23187 by a 1H and 13C NMR solution study (556), their complex formation reactions can take place quite quickly as they can change their conformations rapidly as required for sequential bonding to the cation, and thus proceed in a series of energetically not-too-demanding steps. Formation rate constants for the Ca2+ complexes of A-23187 and X-14885A are 6 x 105 and... 

As an extension of the Heck reaction, Pd-catalyzed hydroarylation of alkynes and alkenes continnes to attract high level of research interest in simple couphng processes and in cyclization reactions. The use of this type of transformation as part of a domino reaction will be of increasing interest. The research in the field of domino reactions is attracting considerable attention in synthetic organic chemistry since it enables the rapid assembly of complex molecirles in one-pot processes. Very elegant examples of palladium-catalyzed cascade processes where a single catalytic cycle entails several sequential bond transformations have been recently reported [la, b, 2a, b, c]. 

In the first expression, the summation is extended to the quadruples of sequentially bonded atoms ABCD and the energy constants Vn are specific for the quadruples of the types of the atoms involved. The torsion angle is that between the planes ABC and BCD. The n = 1 term describes a rotation which is periodic by 360°, the n = 2 term is periodic by 180°, the n = 3 term is periodic by 120° and so on. The Vn constants determine the contribution of atoms A and D to the barrier of rotation around the... 

Sequential Bond Dissociation Energies, D, and Divalent State Stabilization Energies, DSSE, of Monosubstituted Silanes and Methanes ... 

Table 4.5. The non-additivity of hydrogen-bond energies in sequentially bonded H20 polymers [103]. Fixed geometry O-H = 0.9915 A H-O-H = 100.053°. Calculations at HF/STO-3G level...

For example, the thermal reactions of Os6(CO),8 with CO (56, 425) and P(OMe)3 (56, 98, 392) lead to the open raft species Osg(CO)2i and [Os6(CO)2i ) P(OMe)3 J (n = 1-6), respectively, probably by sequential bond hssion in the bicapped tetrahedral cluster. As indicated in Scheme 21, under slightly different reaction conditions, elimination of an osmium vertex occurs, resulting in the formation of clusters containing a trigonal-bipyrami-dal metal framework. These react further with nucleophiles to give open Osj clusters and finally substituted triosmium species. Cluster unfolding has also beeen observed in the reactions of Os5(CO),jH2 with a variety of nucleophiles (Scheme 22). 

The three variations of the retrosynthetic analysis of quadrone discussed in the preceding section were based upon one general concept the sequential bond-after-bond disconnection of the core of the target molecule into a set of smaller fragments. Logistically it is an absolutely reliable approach a consistent... 

Select the strategic bond. This recommendation is especially important in those numerous cases in which the initial analysis does not identify an efficient strategic reaction and, hence, a sequential bond-by-bond dismantling of the structure becomes obligatory. The first bond in this sequence (the strategic bond) defines all subsequent steps in the retrosynthetic analysis. This approach was illustrated in the syntheses of quadrone (Section 3.2.5). 

Schultz RH, CreUin KC, Armentrout PB (1991) Sequential bond-energies of Fe(CO) (X=l-5) - systematic effects on collision-induced dissociation measurements. J Am Chem Soc 113 8590... 

III. Generation of the methyl group of thymidylate from methy-lene-tetrahydrofolate involves four sequential bond breaking and forming steps at the stereospecifically labeled one-carbon unit the results show that each of these steps occurs in a highly stereospecific manner. However, because of the multitude of steps involved, the result does not yet allow us to describe the steric course of each individual step. 

IV. BIVALENT SILICON, tt-BONDED SPECIES AND SEQUENTIAL BOND... 

Csaszar, A.G., Handy, N.C. Exact quantum-mechanical vibrational kinetic-energy operator of sequentially bonded molecules in valence internal coordinates, J. Chem. Phys. 1995,102, 3962-7. 

The translation step in the flow of genetic information results in the synthesis of proteins. The synthesis takes place when properly coded mRNA forms a complex with the component of a ribosome. Transfer RNA molecules carrying amino acids align themselves along the mRNA in an order representing the correct primary structure of the protein. The order is determined by the matching of complementary codons on the mRNA to anticodons on the tRNA. The amino acids sequentially bond together to form the protein, which then spontaneously forms characteristic secondary and tertiary structures. 

Another approach in which the bond length alternation is taken into account is based on the sum of squared differences of the sequential bond lengths. The index called BAG (bond alternation coefficient) reads ... 

The chain properties illustrated above are often described with terms such as stiffness and flexibility . It is important to clarify that stiffness and structural rigidity may not necessarily be alternative to flexibility and structural disorder, since two different concepts enter into the above definitions one is concerned with the number of different accessible conformations, the other with the (average) direction of the sequential bonds, i.e., with the chain topology. 

Other integrated approaches rely on sequential bonding and patterning of multiple bulk layers rather than film deposition and patterning. One popular approach involves patterning all microchannels and microfluidic structures for one layer using, e.g., deep reactive ion... 

Periodic Trends Ligand Effects Sequential Bonds to Fe ... 

Here cj) and Ni are the volume fraction and number of united atom groups, z is the lattice coordination number, and = n + 22 12 is the blend exchange energy. The parameter NJj expresses the number of dissimilar sets of i sequential bonds in... 

The acid dissociation kinetics for doubly deprotonated palladium(ii) triglycine complexes, [Pd(H 2GGG)] (5), takes place wa four sequential bond-dissociation steps... 

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