Crystal packing effects

The crystal packing effect sometimes plays a crucial role to determine the structure in crystals. The structure of the bromine adduct of selenanthrene... 

The TBP structures of 9 I2 and 11 2I2 are noteworthy, since iodine adducts of selenides are predicted to be MC based on the general rule (x(Se) (=2.48) > xCO (=2.21)). This must be the reflection of the high ability of the imidazoline ring to donate electrons to Se by the formation of the stable cyclic 6ji system. It will decrease xeff of Se in 9 and 11 to give TBP with iodine. Since the electronic conditions in 10 must not be so different from those in 9 and 11, they should not be responsible for the different structures of the adducts. This working hypothesis is supported by DFT calculations.38 Therefore, the crystal packing effect must play a crucial role in determining the structures of the iodine adducts of 9-11. The reactions are followed by spectroscopic and conductometric methods.37... 

If allowance is made to neglect the occurrence of crystal packing effects, this would mean that the direct C-C bond between the two ferrocenyl subunits allows a high electronic interaction between the two iron centres. 

To this point, all the examples presented have been ones in which the origin of the asymmetric induction has been unimolecular in nature, that is, the molecules adopt homochiral conformations in the solid state that favor the formation of one enantiomer over the other, usually through the close intramolecular approach of reactive centers bimolecular crystal packing effects appear to play little or no role in governing the stereochemical outcome of such reactions. This raises the interesting question of whether the soUd-state ionic chiral auxiUary approach to asymmetric synthesis could be made to work for conformationally unbiased reactants, i.e., those possessing symmetrical, conformationally locked structures. Two such cases are presented and discussed below. 

There are two apparent exceptions to the rule of Hooker et al. 1,170 These are c(-Ala-Ala-) and c[-(Me)Ala-(Me)Ala-]. The X-ray structure of c(-Ala-Ala-) 171 172 gives a positive p, but the NV couplet is observed to be negative. However, molecular mechanics calculations 173174 predict a negative p for c(-Ala-Ala-) and its A-methyl analogue, in agreement with the CD assignment, so crystal packing effects are responsible for the apparent exception. 

If steric effects in solid-state cluster compounds could be eliminated by the isolation of discrete molecules with the same cluster units as Chevrel phases, a part of the preceding problem would be solved. The structures of the cluster molecules in the isolated state free from the crystal packing effects are most suitable for such discussion. The geometry of free molecules in vacuum cannot be known from X-ray structure determination, and usually it is difficult to measure crystal packing effects in molecular crystals. In the case of metal complexes with sym-... 

Asymmetry parameters <5(X) have been obtained for a number of substituents8. Several factors limit the reliability of such derived parameters, e.g. the validity of additivity and other assumptions, the influence of conformation, electronic and steric interactions of substituents, additional strain in polycyclic systems, crystal packing effects and size and quality of the data sample. 

Refinement papers should also mention any evidence that the protein is affected by crystallization. Packing effects may be evident in the model itself. For example, packing may induce slight differences between what are otherwise... 

Another difference between the ternary and the binary complex is a bent conformation of the L2 finger in the complex with monastrol. Furthermore, the pointed tip of the core structure (jS6—L10-/17) has a variable conformation in the structure with monastrol. In the two molecules of the asymmetric unit, the tip bends and moves by about 7 A. The conformation observed without monastrol is intermediate between the two conformations of the ternary complex. The double conformation of L10 is clearly a crystal packing effect. However, it shows that the tip of the core domain is rather flexible. 

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