Hydrogen bond cyclic

Pyrazoles with free NH groups form hydrogen-bonded cyclic dimers (195) and trimers (196) as well as linear polymers, depending on the substituents at positions 3 and 5. For R = H, Me or Et, the oligomers are preferred, but for R = Ph, the cyclic dimer and the linear polymers exist. The cyclic trimer (196 R = Ph) is) is not formed because of steric hindrance (B-76MI40402). 

It is postulated that hydrogen-bonded cyclic transition states such as 62 or the analogous one involving H0CH2CH20 will be found to increase relative reactivity adjacent to the azine-nitrogen in aprotic solvents cf. also Sections II,E,2,e and II,F. 

A hydrogen-bonded cyclic transition state can be postulated for a nucleophile like ethanolamine or ethylene glycol anion whose hydrogen bonding to an azine-nitrogen in aprotic solvents can facilitate reaction via a cyclic transition state such as 78, cf. Section II, F. Ethanolamine is uniquely reactive with 2-chloronitrobenzene by virtue of a cyclic solvate (17) of the leaving group, a postulate in line with kinetic evidence. 

FIGURE 2.20 Schematic presentation of the hydrogen-bonded cyclic dimers of enantiomeric antipodes of 2-phenylpropionic acid, Ibuprofen, and Naproxen (the latter two compounds are drugs from the group of profens). 

AS values for the formation of the hydrogen bonded cyclic forms. The bridging hydrogen atom is included in the calculation of ring size... 

J. M. Lopez, F. Mannle, I. Wawer, G. Buntkowsky, and H. H. Limbach, NMR studies of double proton transfer in hydrogen bonded cyclic N,N diarylformamidine dimers Conformational control, kinetic HH/HD/DD isotope effects and tunneling. Phys. Chem. Chem. Phys. 9, 4498 4513 (2007). 

Benzamide (43,51) crystallizes from ethanol in space group Pljc. The main features of the crystal structure (52) (Figure 5) are similar to those found in cinnamide. Hydrogen-bonded cyclic dimers are interlinked along the 5.0-A b axis to form ribbons. The ribbons are stacked along the 5.6-A a axis, in an... 

FIGURE 10. Hydrogen-bonded cyclic dimers in the crystal of 2,2,3,3-tetramethylcyclopropanecarboxylic acid (59), stereoview. Reproduced by permission of the International Union of Crystallography from Reference 110... 

Fig. 3. (a) Chemical structure of a synthetic cyclic peptide composed of an alternating sequence of D- and L-amino acids. The side chains of the amino acids have been chosen such that the peripheral functional groups of the flat rings are hydrophobic and allow insertion into lipid bilayers, (b) Proposed structure of a self-assembled transmembrane pore comprised of hydrogen bonded cyclic peptides. The channel is stabilized by hydrogen bonds between the peptide backbones of the individual molecules. These synthetic pores have been demonstrated to form ion channels in lipid bilayers (71). 

Figure 34. Hydrogen-bonded cyclic hexamers I ("rosettes") or infinite tapes II by self-assembly of melamines and barbitures/cyanurates.

In the naphthoic acid crystal (Table 1.1.7), the by far most dominant structure determinant is, as expected, the hydrogen bonded cyclic dimer, A, described by the Pixel approach as a strongly coulombic interaction, while the polarization component supposedly represents the stabilization deriving from the dynamic rearrangement of the electron density - the partial covalent bond character. Next, at a considerable energetic distance, comes a determinant in which the dispersive stacking contribution dominates. The hydrogen bonded dimer is complemented... 

More complicated catemer patterns are frequently encountered in organic crystals. Particularly interesting examples are the first two carbonic acids, formic acid and acetic acid. Both, as with most other carbonic acids, form doubly hydrogen-bonded, cyclic dimers in the vapor phase, but chains with one hydrogen bond linking the monomers in the molecular crystal [34-40]. The structures are sketched in Figure 8.11. Propionic acid [316] and others... 

A concerted mechanism with a hydrogen-bonded cyclic transition state (42) was proposed for the aminolysis of thioaryl At-allylthiocarbamates, CH2=CHCH2-NHCO SC6H4Z, with X-benzylamines in MeCN based on the negative cross-interaction constant, failure of the reactivity-selectivity principle, kinetic isotope effects greater than unity, and relatively low activation enthalpies and large negative activation entropies. ... 

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