Symmetrical and Bifurcated Hydrogen Bonds

It is very difficult to locate hydrogen atoms in large molecules by crystallographic methods. Nevertheless, proximity observations of heavy atoms strongly suggest the existence of bifurcated hydrogen bonds of the type shown below  

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The 1 1 1 complex [54] is obtained by reaction of the cryptand 15 with 2,3-naphtha-lenediol in the presence of water. The water molecule is bound by bifurcated hydrogen bonds to four oxygen atoms of the cryptand, one binds symmetrically (HO = 2.33 and 2,56 A) while the other bind assymmetrically (2.03 and 2.56 A). The 2,3-naph-thalenediol forms one intramolecular hydrogen bond while the second diol is coordinated to the water molecule (H- O = 1.79 A). 

Figure 10.3. Orbital interaction diagrams for a) symmetrical three-center hydrogen bonding and b) bifurcated hydrogen bonding.

The first step of the catalytic process is the hydrogen bond directed assembly and orientation of the reactants. In this example, the azlactone and methanol form a ternary starting complex with the organocatalyst (Fig. 1) [39]. The pseudo-Lewis acidic thiourea forms two bifurcated, nearly symmetric hydrogen bonds (2.147 A, < (0,H,N) = 155.5° and 2.146A, <(0,H,N) = 155.8°) to the carbonyl oxygen atom of the azlactone. 

Fig. 12 Three types of hydrogen bond (a) short or symmetric, (b) normal, and (c) long and bifurcated. Typical O... O distances are shown...

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