Dimerization crystals

Moreover, since the repeating unit in a-type linear polymers is about 7.5 A, according to a molecular model evaluation, the stacks in the starting crystals prefer to have repeating units of a similar length (L). However, L (5.932 A) in 2 OPr-a-dimer crystals is apparently shorter than the favourable distance (—7.5 A). On the other hand, in the 2 OPr-a-dimer-PrOH complex, reactant dimer molecules are separated from each other by a more suitable distance (L = 6.736 A). 

Perreault, D., Drouin, M., Michel, A., Miskowski, V.M., Schaefer, W.P. and Harvey, P.D. (1992) Silver and Gold Dimers. Crystal and Molecular Structures of Ag2(dmpm)2Br2 and [Au2(dmpm)2] (PFe)2 and Relation between Metal-Metal Force Constants and Metal-Metal Separations. Inorganic Chemistry, 31, 695-702. 

Both thermodynamic and kinetic factors need to be considered. Take, for instance, acetic acid. The liquid contains mostly dimer but the crystal contains the catemer and no (polymorphic) dimer crystal has ever been obtained. Various computations (R. S. Payne, R. J. Roberts, R. C. Rowe, R. Docherty, Generation of crystal structures of acetic acid and its halogenated analogs , J. Comput. Chem, 1998, 19,1-20 W. T. M. Mooij, B. P. van Eijck, S. L. Price, P. Verwer, J. Kroon, Crystal structure predictions for acetic acid , J. Comput. Chem., 1998, 19, 459-474) show the relative stability of the dimer. Perhaps the dimer is not formed in the crystal because it is 0-dimensional and as such, not able to propagate so easily to the bulk crystal as say, the 1-dimensional catemer. 

The above data indicate that the association energy for dimerization of stacked dimeric species G is —26.3 (—29.2) kcalmoU (energies at the 6-31G and 3-21G level of theory). The energies for the polymerization of the species C and F are —40.4 (—47.0) kcalmoU and —40.6 (—46.1) kcalmoU, respectively. This calculated energy of aggregation suggests the formation of a polymeric ladder as more favorable than the stacked dimer. In the experiment, it is seen that the stacked dimer crystallizes only on rapid cooling... 

Two-Proton Transfer in Carboxylic Acid Dimer Crystals... 

The rhenium derivative (CH3)3GeRe(COb, can be converted into the carbene complex (CH3)2(jeRe(CO)4C< G )CH ), which in solution is in equilibrium with its dimer. The dimer crystallizes preferentially, and incorporates an eight-membered GeReCO]2 heterocycle (181). 

The solvent-free phenyl derivatives of the heavier alkali metals have not been structurally characterized. Phenyl sodium dimers crystallize in the presence of donors such as, PMDTA and TMEDA. Using the bulky mesityl-substituted derivatives, a solvent-free arylsodium compound has been characterized [Na(C6H3Mes2-2,6)]2. ... 

Topotactic forces affect the ordinary reaction in some other ways. There is the case of a small molecule partidpating in crystalline-state photodimerization (see Sect, m.a.), e.g. small molecules such as water or n-hexane in the atmosphere are involved in the reaction29,30. These small molecules, sometimes even the moisture in the air, not only greatly affect the rate and conversion of the reaction but are also included in the crystal of the photoproduct as a crystal molecule at the end of the reaction. In such dimer crystals, a small molecule may be useful for filling up the cave in the crystal, thus stabilizing the... 

FIGURE 3-18 IR Spectra of formic acid gaseous monomer, gaseous dimer, crystal, and glass. [From Millikan, Ph.D. Thesis, University of California (1956).]... 

The mixed valence dinuclear species U2(OBu )g was obtained from unstable K[U2(OBu )g]. The dimer crystallizes as a face-sharing bioctahedron Theoretical studies have been carried out to understand the lack of metal-metal bonding in these dinuclear uranium alkoxide structures. ... 

Observation 1 When acetoin produced by fermentation and then recovered from the fermentation broth by distillation was left standing for a day, the rotation first decreased. When it reached a minimum at point K, dimer crystals started precipitating, and the rotation started to increase. 

On one hand, there is an equilibrium between keto form molecules and oxide form molecules, and on the other hand there is an equilibrium between oxide form molecules and dimer molecules. A superposition of these two equilibria means that from a liquid monomer mixture of keto form and oxide form molecules obeying the first equilibrium, some solid dimer crystals must form to satisfy the second equilibrium. However, this dimer formation... 

The dimer crystal, recrysta11ized from ethanol, shows essentially the same crystal diffraction pattern as the crystal as-prepared, but upon photoirradiation it gives a higher molecular weight polymer (ilinh 0.36 dL/g->ri inh 0.64 dL/g). According to X-ray diffraction analysis, the photoproduct at the inital stage (after the irradiation for 5 h) is crystalline and then becomes progressively amorphous upon further polymerization. 

The Streptomyces PLD consists of two highly interacting components of similar topology [13]. Overall, the structure is very similar to die Nuc dimer. Each component consists of a /1-sheet of 8-9 strands surrounded by nine a-helices. The catalytic center is also very similar to that of the Nuc dimer. Crystallization with a phosphate buffer indicates that the phosphate head group of the substrate lies in contact with His, Lys and Asn residues contributed from both HKD domains [13]. 

Such kaleidoscopic topochemical behaviour of 2 OPr is exemplified in Scheme 6 (Hasegawa, 1992). 2 OPr crystallizes with a-type packing [2 OPr(a)] from 1-propanol solution but into the jS-type [2 OPr(j3)] from a mixture of ethanol and water. On photoirradiation using a filter (>410 nm), the a-type homo-adduct dimer (2 OPr-a-dimer) and the jS-type heteroadduct dimer (2 OPr- -dimer) are produced from 2 OPr(a) and 2 OPr(j3), respectively, both in nearly quantitative yield. The dimer crystal 2 OPr-a-dimer as-prepared is entirely stable on photoirradiation whereas, on further photoirradiation without a filter, 2 OPr-)8-dimer is converted nearly quantitatively into a highly strained [2.2] paracyclophane (2 OPr-cyclo). The photostable 2 OPr- i -dimer crystal is, however, transformed into a highly photoreactive crystal complex (2 OPr-a-dimer-PrOH) if 2 OPr-a-dimer is recrystallized from 1-propanol solution. The crystal (2 OPr-a -dimer-PrOH) photopolymerizes into a crystalline a-type homo-adduct linear polymer. Judging from their X-ray diffraction patterns, recrystallized l3-type dimer (2 OPr-)8-dimer) has a different crystal structure from that of the crystal... 

X-ray structural studies showed that benzoic acid dimers crystallize in a monodin-ic cell with the hydrogens aligned close to the crystallographic c-axis [22]. Time of flight neutron diffraction measurements were carried out by Brougham et al. [23] and these yielded the crystal structure of a deuterated benzoic acid. The stmcture of the monomer units is assumed to be unchanged upon complexation. 

Figure 14. Stereoviews of the defensin dimer crystal structure [coordinates courtesy of D. Eisenberg (160)7. (A) Perspective that shows the basket shape of the molecule and the van der Waals surfaces of the charged side chains. In some cases, side-chain atom positions are not defined and have been placed in a low-energy conformation. (B) Perspective that shows the beta-sheets within each dimer and the subunit interaction.

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