Columnar hydrogen bonds

Figure 4.12. Crystal structure of less soluble (1R, 2S)-7 (R)-2-phenylpropanoic acid PEO. (a) Columnar hydrogen-bond network. (b) Viewed down the 2i-axis of the hydrogen-bond column ( -axis). The solid square shows the unit cell.

Thus, in the cases of these diastereomeric salts which could not be efficiently separated by crystallization, the columnar hydrogen-bond network is not reinforced by additional hydrogen bonds formed by the hydroxy group as that in the less-soluble salts in success. This makes the difference in stability between the pair of diastereomeric salts small, resulting in low efficiency of resolution for this combination. 

Of these crystal structures, 1-phenylethylammonium 2-phenylpropionate (NMACEP, PMACEP, Figure 4.38)115 and 2-phenylbutyrate (PEAPEA, PBUPEA)115,116 are the earliest examples reported by Brianso. The less soluble salt is the combination of the 5-acid with the R-amine (PMACEP and PEAPEA) in each case. The crystal structures of both pairs of the diastereomeric salts (less and more soluble salts) have been determined to show that these two systems have quite similar crystalline characteristics. In both less and more soluble salts a columnar hydrogen-bond network, formed by the carboxylate and ammonium groups, is commonly built around a two fold screw axis two molecules of the carboxylate part and two molecules of the ammonium part form a unit, which is piled up along a 2i-axis to form a supramolecular hydrogen-bond column. 

The crystal structures of the diastereomeric salts of 1-phenylethylamine with chiral carboxylic acids are subsequently reported by other groups (Table 4.7). The comparison of the crystal structure of JAGHEE with that of JAGHII also indicates that columnar hydrogen-bond networks are formed in these crystals, respectively, and that the difference between them can be found in the relative orientation of the substituent on the chiral carbon of the carboxylate moiety in the supramolecular column. 

It is noteworthy that there is high tendency to form a common columnar hydrogen-bond network especially in the less soluble salts (Figure 4.39). This... 

Figure 4.39. ORTEP drawings of the columnar hydrogen-bond networks viewed down the columnar axis. The dotted lines represent hydrogen bonds. The capitals are the CSD reference codes.

Figure 4.40. ORTEP drawings of the columnar hydrogen-bond networks formed in (a) BELKEI, (b) DUYPAO, and (c) PBEPST. The upper and lower figures show the side view and down view of the columns, respectively. The dotted lines represent hydrogen bonds. The capitals are the CSD reference codes.

These authors found that those salts that form conglomerates contain a characteristic columnar hydrogen bond network, in which the ammonium cations and the carboxylate anions are aligned around a 2-fold screw axis, constituting a 2i column (Fig. 9) [81],... 

Figure 7.21 Columnar hydrogen bonding In 17 (a) side view and (b) top view...

Fig. 21. The hydrogen-bonded hexameric unit that builds up the columnar structure of PhaSifOH, with hydrogen atoms omitted for clarity (304).

Fig. 10 The molecules involved in the formation of hydrogen-bonded nematic and columnar phases (upper part) and a representation of the arrangement of the molecules in the mesophases (lower part)...

Helical columns of bifunctional ureidotriazines have also been created in water.40 In this solvent the aromatic cores of compound 39 stack and create a hydrophobic environment that favors the formation of intermolecular hydrogen bonds. The chiral side chains can express their chirality within the columnar polymer because of the helicity generated by the backbone. In contrast, for monofunctional 68 water interferes with the hydrogen bonding and 68 does not stack to form a column. As a consequence the chiral side chain does not express its chirality in the aromatic system. For 39, the bifunctional nature allows for a high local concentration of stacking units. A comparison might be made here to the individual DNA bases that also do not dimerize and stack in water, unless they are connected to a polymer backbone. 

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