Intermolecular H-bonding

Intermolecular H bond Dimeric Polymeric 3600-3500 3400-3200 (s) Rather sharp. Absorptions arising from H bond with polar solvents also appear in this region. Broad... 

Fig. 2.38 sheet forming y-peptides. (A) Crystal structure of the two stranded antiparallel sheet formed by a,j -unsaturated y-dipeptide 152 with a-methyl substituted residues [208], Both intermolecular H-bonds are characterized by a N---0 distance of 2.84 A and an angle (N-H- -O) ofl54.2°. (B) Crystal structure of the infinite parallel sheet arrangement formed by vinylogous dipeptide 153 [208], Intermolecular H-bonds are characterized by a N -O distance of 2.88 A and 3.24 A and an... 

The solubilization of water in lecithin-reversed micelles has been found to be an exothermic process. This finding confirms that water interacts with the zwitterionic head group of lecithin, promoting the formation of strong intermolecular H bonds [104]. 

While SCN usually binds via its N atom to Ni11 (or it adopts a N,S bridging mode), rare S-coordination has also been observed.437 (107) involves both N- and S-coordinated terminal thiocyanate, where the Ni—S—C angle is 98.3(1)°, whereas the Ni—N—C angle is 168.7(2)°. S bonding seems to be stabilized by intermolecular H bonding to the uncoordinated thiocyanato-N atom. 

The most conclusive evidence for the presence of duplexes la la and 2 2 came from 2-D H-NMR [nuclear Overhauser effect spectroscopy (NOESY)] and X-ray crystallography. The NOESY spectrum of lb in CDCI3 contains interstrand NOEs (Fig. 9.2a) between protons c and e, c and i, and c and j, which are consistent with an H bonded dimer. The crystal structures of la and 2 both revealed the expected dimeric stmctures held together by intermolecular H bonds (Fig. 9.2b). 

Analysis of the pyrene-labeled homoduplex 5 5 (Eig. 9.3b) by NMR, mass spectrometry, and TLC suggested that 5 5 had a stability similar to that of 3 4. NOESY spectra revealed cross-strand NOEs consistent with the formation of the self-dimer 5 5 (Zeng et al. 2003). Based on a fluorescence method described in the literature (Sontjens et al. 2000), the dimerization constant of the pyrene-labeled duplex 5 5 was found to be (6.77 + 4.12) x 10 M. The studies on duplexes 3 4 and 5 5 clearly demonstrated that the stabilities of our duplexes are indeed only determined by the number of intermolecular H bonds, and both hetero- and homoduplexes can be easily designed and constructed. 

The NOESY spectrum of 3 6 showed that one end of this pair was locked by intermolecular H bonds, whereas the other end around the mismatched site consisting of the two amide carbonyl groups at that end was open. In contrast, the NOESY study indicated that both ends of 3 7 were locked, but no or very weak NOEs were detected for protons close to the internal mismatched site. These results indicated that, in spite of the presence of mismatched sites, these mutant duplexes still formed. More detailed analysis showed five interstrand H bonds in 3 6 or 3 7, suggesting that these strands were still fully registered. However, the oli-goamide backbones must be twisted near the mismatched sites to alleviate any unfavorable interactions. 

The high fidelity of the H bonded duplexes was again demonstrated by duplexes 8 8 (see Section 8.5), 9 9, and 3 4, with eight, six, and four intermolecular H bonds, by gel permeation chromatography (GPC). When mixed in solution, each of these three duplexes clearly existed independently (Fig. 9.5). When tethered to various structural units, these duplexes can be used in mixtures to sort out and assemble the corresponding units in a highly specific fashion. 

In a study of tautomeric heterocycle systems Zubatyuk et al. [39] concluded that the position of tautomeric equilibrimn is controlled by the aromaticity of the heterocycle. They have also shown a strong correlation between the strength of the a intramolecular hydrogen bond and the aromaticity of a resonant spacer. In DPGUAN03, where two symmetry independent molecules, establish similar hydrogen bonds, the lowest aromaticity corresponds to the strongest intermolecular H-bonds. 

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