Dimers mixed heterodimers

Organic nitriles associate to dimers by dipole-dipole interaction of their nitrile groups (Sect. 2.1) water molecules, on the other hand, associate by formation of hydrogen bonds (preceding section). However, if water and nitrile (in particular acetonitrile ) are mixed, heterodimers and -trimers are formed ... 

Mirbt and coworkers [42] proposed that the underlying mechanism behind the formation of a mixed heterodimer is the huge difference in atomic radii of the atomic species involved (indium and gallium). They found, for cation-rich conditions, that the stress energy of reconstructions involving interconnected cation dimers (e.g., Ga-Ga or In-In) increases with increasing anion atomic radius. The reduction of the TE by the dimer formation A could therefore be smaller for symmetric dimers (e.g., Ga-Ga or In-In) than that for mixed dimers. The covalent radii of In and P differ by 0.034 nm, while for Ga and As, the difference amounts to 0.006 run (Table 13.1). This would also explain why such structures are not formed on GaAs(OOl). 

Subunit Mixing in Eukaryotic Regulatory Proteins Several families of eukaryotic transcription factors have been defined based on close structural similarities. Within each family, dimers can sometimes form between two identical proteins (a homodimer) or between two different members of the family (a heterodimer). A hypothetical family of four different leucine-zipper proteins could thus form up to ten different dimeric species. In many cases, the different combinations appear to have distinct regulatory and functional properties. 

Whilst Rebek used the inclusion of a guest as secondary proof of the capsules, Bohmer used the formation of heterodimers to show that the NMR spectra seen were not solely due to the monomeric species. In DMSO-d6, only a mixture of the two species was seen. However, the use of a non-polar solvent showed sets of peaks corresponding to all the possible dimers (AA,BB and AB) in the expected statistical mix. 

An Interesting example of the role of the exclmer as Intermediate comes from the system l-(2,6-dlchlorophenyl)-4-phenylbuta-dlene. Irradiation of a mixed crystal of this compound together with the corresponding 4-thlenyl derivative gives dimers by a (2n + 2tr) photocycloaddltlon reaction. The products are homo-dlmers of the phenyl and of the thienyl derivatives, and hetero-dimers. The two heterodimers formed are enantiomeric and, because the crystal Is of chiral structure, they are formed In different yields (54). Thus Irradiation of a single crystal has been found (55) to lead to a 70% enantiomeric excess. 

Heterodimers 70 and 71 are the first examples of what is theoretically an enormous family of possible (hetero)fullerene heterodimers. It is expected that fullerene heterodimers will exhibit interesting electronic (optical, electron accepting) properties. Nuber and Hirsch recognized the fact that they had obtained a mixture of three aza[70] fullerene dimers upon treatment of a mixture of two isomeric bisaza[70]fulleroids. One of their products is most likely a mixed dimer [8]. This is a hetero -dimer in a sense that differs from 70 and 71 ... 

Dissociation and dimerization of C-nitroso componnds was studied by Vancik et al. under different solvent-free conditions, inclnding mechanochemical [69], Reaction rates and kinetic isotope effects were obtained from experiments carried out with labeled C-nitroso compound (p-bromonitrosobenzene) as shown for homodimers 376 and 377 (Scheme 3.104). Heterodimers 380 and 381 were prepared by mixing equimolar amounts of p-Cl, P-NO2 p-l nitrosobenzene derivatives, and isotope labeled... 

A similar situation is found for the GaP(001)(2 x 4) surface reconstruction, where a model containing a mixed Ga-P dimer was proposed [43]. Heterodimers also seem to play a role for the III antimonides, AlSb and GaSb. In these two cases, the covalent radii of the involved atoms differ by 0.022 nm and 0.014 nm, respectively (both values are higher than that for GaAs). For the (4 x 3) reconstructions of AlSb(OOl) and GaSb(OOl), surface models containing Ill-Sb dimers were proposed [44]. 

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