Anti-structure

The syn isomer can achieve a conjugated system with angles of up to 35° between adjacent p orbitals. The anti isomer is much more twisted. An X-ray crystal structure of the syn isomer shows C—C bond lengths between 1.368 and 1.418 A for the conjugated system (Fig. 9.3). ° The spectroscopic properties of the syn isomer are consistent with considering it to be a delocalized annulene. B3LYP calculations indicate that both the syn and anti structures are stabilized by delocalization, the syn (17.6kcal/mol) more so than the anti (8.1 kcal). ... 

Although the results eu e equal as far as utility is concerned, we shedl continue to use our symbolism, for reasons which will become clearer later on. The following Table is a comparison of defect symbolism, as used by prior Authors. Note that our symbolism most resembles that of Kroeger, but not in aU aspects. These prior authors also considered other intrinsic defects that we have not touched, namely interstices and the so-called "anti-structure" occupation. 

Anti-Frenkel Anti-Structure Vacancy- AntiStructure Xi Vx + Mx Vm -h Mx... 

Anti-Structure Interstitial Anti-Structure Vacancy + Ml Xm + Vx... 

We have now introduced into our nomenclature a distinction between structure and anti-structure defects. What this means is that stacking faults can sometimes result in the formation of and Mx defects. These... 

Schottky Frenkel Anti-Frenkel Anti-Structure Vacancy-Structure. 

The structures of ladder polysilanes were determined by X-ray crystallography.41,42 In Fig. 6, molecular structures of the ladder polysilanes with the anti structure are shown. A remarkable feature of the anti ladder polysilanes is a helical structure. These molecules can be regarded as silicon double helices, in which two polysilane main chains twist in one way and are bridged by Si-Si bonds. The twist angles between the terminal rungs are... 

We feel that it is very important not to draw conclusions about bonding just from the observation of the occurrence of a particular structure. It is well known that the same structure can serve for crystals of presumably different bond types Mg2Sn and Li20, CuZn and CsCl are examples of pairs of compounds with the same well-known structures. Less well-known perhaps but more important in the present context is the wide occurrence of anti-structures in which cation and anion positions are interchanged. In Table 1 a number of structure-antistructure pairs is given, mainly to emphasize how wide-spread the phenomenon is, and that it encompasses both simple and complex structures. 

Poly(8-methyladenylic acid) can form a regular, single-strand helix, the first such structure found to exist in aqueous solution. In this polymer, the glycosidic conformation is a regularly alternating syn-anti structure. It is interesting that the alternation of syn and anti conformations may be correlated with Z-DNA formation and that the bulky 8-methyl group is expelled to the outside of the stack s array of bases 171). 

Fig. 14. Complex 39 contains a linear, two-coordinate germanium atom with conformations about the Mn—Ge bonds such that the molecule adopts a centrosymmetric, anti structure. Work by Herrmann (50) describes the C5Me5...

HfaS and HfaSe crystallize in the anti-structure, cations and anions being interchanged. 

In contrast to the predictions of INDO calculations, however, ab initio calculations232 (3-21 G basis set) on CH2=CH2-, CH CHF-, CH2=CF2 and CF2=CF2- have predicted that planar structures are not stable minima for these radical anions. The planar structures relax to the anti structures upon minimization, and the stabilization due to non-planar distortions, arising from a mixing of the 7r - and the high lying cr -orbitals, increases upon fiuorination. For these four molecules the 90° twisted structures are saddle points, and the CH2CF2 radical anion was predicted to adopt the chair structure. 

A stepwise mechanism for the formation of Binor-S was offered by Katz, a strong proponent for nonconcerted processes 9). Katz based his position on doubts regarding the actual structure of Binor-S stemming from the apparent absence of some ir bands in the Raman spectrum. He therefore suggested the anti structure 57. Such a structure would, of course, preclude the more symmetrical involvement of metal centers proposed by... 

In contrast to this equal propensity toward formation of a H-bond, there is a definite preference for a proton to locate on the syn side when coming much closer and forming a covalent bond. That is, the syn conformer of RCOOH is more stable than the anti structure by about 5 kcal/mol in the gas phase. If this is the case, why then is there so little apparent pK difference between the syn and anti structures The answer to this question resides in solvation phenomena. The presence of the polarizable medium preferentially stabilizes the anti conformer of the carboxylic acid due to its higher dipole moment. 

By analogy with the normal layer structures (large polarizable anions) we should expect the anti structures to occur for the large polarizable cations such as Ag" ", Cs", Tl" , and Pb ". However, the situation is more complicated because not all these compounds are normal valence compounds there is clearly some delocalization of electrons in a number of them. It is interesting to compare the oxides of Cs ... 

Kuhn, T. S., Robert Boyle anti Structural Chemistry in the Seventeenth Century) Isis, 43 (1952), pp. 1-23. 

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