Hydrogen-bonded molecular aggregates symmetry

Molecular modeling studies reveal that a trans antiparallel orientation of the substituents on the ureas is energetically preferred over the parallel arrangement (Fig. 20). The one-dimensional aggregates formed by an antiparallel orientation lead to extensive hydrogen bonds with translation symmetry whereas the parallel orientation requires a screw axis to form one-dimensional aggregates [681. The crystal structure of 35 shows a parallel orientation of the urea substituents around the benzene ring. The authors have conducted... 

The dodecahydrododecaborate anion, B H 2-, is termed unique with considerable justification. This ion and its perhalo derivatives, e.g., Bi2C1i22-, are the most symmetrical molecular aggregates known. The boron atoms occupy the vertices of a regular icosahedron and each is bonded terminally to a hydrogen atom all boron atoms are environmentally equivalent.7,8 This anion is the only known example of the 7 symmetry group.8 General spectral, physical, and chemical properties of Bx2Hi22-are detailed in a paper by Muetterties et al.9... 

R = 4-pyridyl, retain the threefold molecular symmetry and assemble their respective molecules via N-H... N hydrogen bonds [29a, b]. The 3-pyridyl or 4-pyridyl functionalities in these derivatives interfere in amide-to-amide hydrogen bonds (Figure 7.23). The N-H... N hydrogen bond aggregation generates a honeycomb network with channels. These channels in 20 were shown to be porous and display desorption of MeOH and absorption of water at room temperature. Later, it was found that the 3-pyridyl derivative 20 indeed crystallizes in three crystalline forms (A, B and C) from the same solvent (MeOH) [29c]. The crystallization process was shown to be dependent on the concentration and evaporation rates. Highly concentrated solution resulted in form A, moderately concentrated solution resulted in form B and dilute solutions resulted in form C. 

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