Hydrogen-bonding sites

This arrangement leaves free hydrogen-bonding sites at the terminal molecules which do not link the tetramers, presumably because of steric hindrance by the a-Me groups on the rings (311). 

Other studies conducted on mixed protonated clusters of ammonia bound with TMA showed that the ion intensity distributions of (NH3)n(TMA)mH+191 display local maxima at (n,m) = (1,4), (2,3), (2,6), (3,2), and (3,8). Observation that the maximum ion intensity occurs at (n,m) = (1,4), (2,3), and (3,2) indicates that a solvation shell is formed around the NHJ ion with four ligands of any combination of ammonia and TMA molecules. In the situation where the maximum of the ion intensity occurs at (n,m) = (2,6) and (3,8), the experimental results suggest that another solvation shell forms which contains the core ions [H3N-H-NH3]+ (with six available hydrogen-bonding sites) and [H3N-H(NH2)H-NH3]+ (with eight available hydrogen-bonding sites). The observed metastable unimolecular decomposition processes support the above solvation model. 

Optimal bromination of a DNA probe is in the range of 30-35 bases per 1,000 bases, a level which can be controlled by the amount of N-bromosuccinimide added. Over labeling can prevent specific interactions with target DNA, even if the point of initial modification is not a hydrogen bonding site. 

The phosphonium salt 21 having a multiple hydrogen-bonding site which would interact with the substrate anion was applied to the phase transfer catalyzed asymmetric benzylation of the p-keto ester 20,[18 191 giving the benzylated P-keto ester 22 in 44% yield with 50% ee, shown in Scheme 7 Although the chemical yield and enantiomeric excess remain to be improved, the method will suggest a new approach to the design of chiral non-racemic phase transfer catalysts. 

K. Manabe, Asymmetric Phase-Transfer Alkylation Catalyzed by a Chiral Quaternary Phosphonium Salt with a Multiple Hydrogen-Bonding Site , Tetrahedron Lett. 1998, 39, 5807-5810. 

K. Manabe, Synthesis of Nobel Chiral Quaternary Phosphonium Salts with a Multiple Hydrogen-Bonding Site, and Their Application to Asymmetric Phase-Transfer Alkylation , Tetrahedron 1998, 54, 14465-14476. 

Figure 9. Top Two tetraurea calixarene monomers connected by a rigid spacer at their bottom rim display diverging hydrogen-bonding sites ideally suited for polymerization. The polymer (53) bears capsules of ca. 1.6 nm x 2.2 nm dimensions like beads on a string. Bottom Photomicrographs of typical Schlieren textures of 53 in chloroform (top row, left) and p-difluorobenzene (top row, middle) as viewed...

Fig. 12.7 An unusually long range (10-bond) secondary D isotope shift IE on an [19F] NMR. The H/D chemical shift isotope effect due to substitution at an OH — N hydrogen bond sited ten bonds away from a para-F is 11 ppb (Hansen, P. E. et al. Acta Chim. Scandanavia 51,881 (1997))...

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