Hydrogen bonding unsymmetrical, molecular

Therefore both symmetrical configurations of the hydroxy functions in the host molecule allow the helical tubuland structure. The unsymmetrical epimer anti-2,, syn-7-dihydroxy-2,7-dimethyltricyclo[4.3.1.13-8]undecane (10), the hybrid of 2 and 8, does not possess a molecular twofold axis (or pseudo twofold axis) or the conformation of C—O bonds of Fig. 5, and would not be expected to fit into the helical tubuland structure framework. Its crystal structure is indeed different, with infinite zig-zag sequences of hydrogen bonds constructing a non-including lattice 14). 

Polytopic macrocyclic receptors 1, 2 (Figure 10.1) are able to complex the zwitterionic form of the amino acids by a double non-covalent charge interaction [28,29]. The unsymmetrical benzocrown sulfonamide derivative, 2 which contains benzo-18-crown-6 and benzo-15-crown-5 moieties was used as a ditopic receptor for multiple molecular recognition of the amino acids, by combining two non-covalent interactions ammonium-crown hydrogen bonding and carboxylate- complexed Na+-benzo-15-crown-5 charge interactions [28,33]. 

These three-center bonds are invariably unsymmetrical with one strong bond (H 0, 1.9 to 2.4 A N-ft A, 145-170°) and one weak bond (H 0, 2.4 to 2.8 A N-ft A, 105-135 °). These unsymmetrical chelated structures are more a property of the molecular configuration and crystal packing than of the hydrogen bonds. In the absence of crystal packing forces, i.e., in solution, we doubt whether such chelation would be energetically favorable. 

FIGURE 6-7 Molecular Orbitals for Unsymmetrical Hydrogen Bonding. 

Unsymmetrical U-shaped molecules with two different hydrogen-bonding groups that give homodimers have also been reported. Specifically, a molecular cleft known as Rebek s imide has been shown to self-assemble in the solid state via two N-H -O and two O-H -O hydrogen bonds of two imide-carboxylic acid synthons (Fig. 4).11... 

Temperature-dependent chemical shifts arise when an atom in a molecule is involved in an unsymmetrical intra-molecular re-orientation, e.g. the population of the methyl groups in methyl nitrite in cis and trans forms varies with temperature (Phillips, 1958), or when there is an intermolecular exchange process between distinct sites on two molecules, e.g. the —OH signal in a mixture of hydrogen-bonding molecules, such as methanol and water. A consideration of the time scales involved in observation of temperature-dependent chemical shifts will be delayed until later. 

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