Molecular chains dimerization

AMI calculations on dimers of nitroanilines are of interest in investigating the inter-molecular forces which orientate the individual molecules in crystal chains. Dimerization in solid crystal may be considered responsible for the differences in molecular geometry between solid and gas phases97. 

The intermolecular vibrations are very strongly affected by polymerization, since weak van der Waals interactions are replaced by covalent bonds. In the Raman spectra, new intermolecular modes appear at 97 cm-1 in dimers [71,101] and at 118 cm-1 in molecular chains [8,71], and neutron scattering shows a significant upward shift in the density of vibrational states [102-104]. A simple argument shows that for each direction of polymerization, one third of the original low-fre-... 

Practical realizations of regular antiferromagnetic chains are provided either by regular chains of monomers, with one electron per monomer or p = 1, or by regular chain of dimers, with one electron (one hole) per dimer or p = 3(2). Then the 2kv distortion results in a dimerization of the molecular chains in the first case, and in a tetramerization, or, more correctly, a dimerization of dimers [19] in the second case (see also Fig. 5). 

After the authors of Ref. 47, the motion of charged solitons in dimerized chains requires a relatively low excitation energy, and it could take place according to the following scenario. At low electric field, this motion is hindered by the three-dimensional interactions between molecular chains. The current is then due to a few defects more or less free from these interactions. As the field is increased, more and more charged solitons are driven to motion and by a cooperative effect of these mobile solitons, a critical field th is finally attained at which value the three-dimensional order is lost. The solitons are then able to move in one chain almost independent of the others, and this new degree of freedom is believed to... 

All the results above lead consistently to the conclusion that the negative-resistance effect is a common property of mixed-chain 1 1 charge-transfer complexes and segregated-chain 1 1 charge-transfer salts, the only requirement being the dimerization of the molecular chains in both cases [47]. 

Fig. 6. Stereoscopic views of common molecular chains in three crystal forms of hen egg white lysozyme, (a) Views of monoclinic (A), tetragonal (B), and triclinic (C) cells, illustrating the recurrence of the chain corresponding to the triclinic c axis, (b) The triclinic c-axis chain (A) aligned with one subunit of the dimer in the asymmetric unit of the monoclinic cell (B). (C) The triclinic a-axis array oriented with the other subunit of the monoclinic asymmetric unit. From Salemme et al. (1988).

Krol, M. Comparison of various implicit solvent models in molecular dynamics simulations of immunoglobulin G light chain dimer. J. Comput. Chem. 2003,24,531 6. 

The results of Che spectroscopic studies were interpreted in terms of nonequivalence of adjacent anhydroglucose units in the molecular chains, requiring the basic repeat unit of structure to be taken as the dimeric anhydrocellobiose unit. The difference between cellulose 1 and II was associated with the locus of the nonequivalence. In cellulose II it was thought to be at the glycosidic linkages, while in cellulose I it was taken to be centered at C6 and the adjacent segment of the pyranose rings. 

He is the one-electron Hamiltonian of a molecular chain exhibiting a lattice dimerization, wnose amplitude is specified by the longitudinal displacement Uq of the molecules from their location in die undistorted chain. The electrons are also subjected to a potential, say from the inorganic anion chains and from some mean-field electron correlation, of period twice the regular chain spacing d, acting on the bonds (i.e. between sites) with an amplitude B. ... 

This effect was ascribed to the larger Pockels effect, due to a larger number density of traps in the material. Thus, dimer cahon sites are preferenhally formed along a longer molecular chain. 

A parabolic dependence on hydrophobicity and molecular weight was noticed for the enzyme inhibitory activity in QSAR 10. K[ values used for deriving QSAR 10 were measured by inhibiting recombinant single-chain dimeric HIV protease. The same authors reported another statistically better QSAR model based on ClogP and the CMR term, however, C log P and CMR were highly collinear (r = 0.70). QSAR 11 reported for antiviral activity has less predic-... 

The isotactic form is the only polymer of commercial importance. An unusual feature is that bulk polymer is transparent because the molecular chain is only very slightly birefringent. A second unusual feature is that the crystalline density and the amorphous density are very similar. The transparency is further enhanced by nucleation giving finer texture, and by balancing crystalline and amorphous densities even more closely, by copolymerization. The monomer is obtained by the dimerization of propylene. 

The pseudopolyrotaxane schematized in Fig. 3 has a supramolecular main-chain bond based on the dimerization of carboxyl groups. Philp and Stoddart had earlier suggested [27] that a molecular chain with appropriately spaced TT-rich rings could thread its way through several macrocycles containing n-acceptors (Fig. 9A) and subsequently be capped with large stoppers to form a permanent rotaxane polymer. The structure may be viewed as a molecular abacus with rotaxane units noncovalently linked to the molecular chain. Newer polyro-taxane polymers are described in this volume in Chapter 8. 

Synthetic polymers used to form fibers are often classified on the basis of their mechanism of polymerization--step growth (condensation) or chain growth (addition) polymerization. Step growth polymerization involves multifunctional monomers which undergo successive condensation with a second monomer or with itself to form a dimer, which in turn condenses with another dimer to form a tetramer, etc., usually with loss of a small molecule such as water. Chain growth involves the instantaneous growth of a long molecular chain from unsaturated monomer units, followed by initiation of a second chain, etc. The two methods are outl ined below schematically ... 

Figure A3.10.10 STM image (55 x 55 mn ) of a Si(lOO) surface exposed to molecular bromine at 800 K. The dark areas are etch pits on the terraces, while the bright rows that run perpendicular to the terraces are Si dimer chains. The dimer chains consist of Si atoms released from terraces and step edges during etching [28],...

Rearrangement to an open chain imine (165) provides an intermediate whose acidity toward lithiomethylthiazole (162) is rather pronounced. Proton abstraction by 162 gives the dilithio intermediate (166) and regenerates 2-methylthiazole for further reaction. During the final hydrolysis, 166 affords the dimer (167) that could be isolated by molecular distillation (433). A proof in favor of this mechanism is that when a large excess of butyllithium is added to (161) at -78°C and the solution is allowed to warm to room temperature, the deuterolysis affords only dideuterated thiazole (170), with no evidence of any dimeric product. Under these conditions almost complete dianion formation results (169), and the concentration of nonmetalated thiazole is nil. (Scheme 79). This dimerization bears some similitude with the formation of 2-methylthia-zolium anhydrobase dealt with in Chapter DC. Meyers could confirm the independence of the formation of the benzyl-type (172) and the aryl-type... 

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