Bifurcated bonds

Figure 3.5 Some examples of branched H bonds (a) Ihe bifurcated bond in l,3-dioxanol-5 and trifurcated bonds in (b) W.A/-bis(2-hydroxyethyI)glycinei and (c) the nilriloiriacetaie dianion. ...

As observed above, the solid state differs from the liquid state of solutions, under the usual experimental conditions, but some new ideas for an approach to hydrogen bonding originate from crystal analysis28. An example of this is offered by the fact that the hydrogen bonds observed in crystal structures are rarely linear. Bifurcated bonds (5,... 

Bifurcated bonds Three-centre hydrogen bond... 

About 300 (of the 1500 N—H- 0=C bonds considered) are three-centre bonds77. Three-centre bond (as shown in 15) is considered a better definition than bifurcated bond . The three-centre bond is a situation where the proton interacts with two hydrogen bond acceptor atoms both bonds are shorter than the sum of the van der Waals radii of the atoms involved. The two hydrogen bond acceptors may be different (Y Z in 15). 

It is worth mentioning that the dihydrogen bond in solid complex 9 is elongated to 2.0 A versus 1.82 A in solution. According to the x-ray molecular structure, this effect correlates with the appearance of an additional contact between the NH proton and the [BF4] anion through the F atom due to a bifurcated bonding Ir-H- H(N)- F-B. The related iridium complexes have shown the same effects. 

Examples Gas phase dimers with strong acids/bases Acids Minor components of bifurcated bonds... 

This configuration, 1, was first proposed by Albrecht and Corey, who applied the descriptor bifurcated bond in 1939 to account for the nonhydrogen bond geometry of the NH and C=0 groups in the crystal structure of a-glycine [63]. This suggestion was confirmed both by a very careful X-ray study [64] and by neutron diffraction in 1972 [65], with the dimensions shown in Fig. 2.2. 

There are, however, some examples of this bifurcated bond configuration observed in the nucleosides, but only in conjunction with three-center bonds, and therefore referred to as three-center bifurcated bonds, as in 3 and 4 ... 

Hydrogen-bond configurations which combine the three-center bond with the bifurcated bond are observed in the crystal structures of violuric acid deuterate (Ihble 8.1) and the paraelectric ammonium sulfate as shown below... 

The hydrogen bonding in 2,-deoxycytidine HCl [DOCYTC] centers around the four-coordinated chloride ion (Fig. 17.40). It contains an unusual bifurcated bond... 

Bureiko and coworkers ° have studied hydrogen bonding of intramolecularly bonded phenols (e.g. 2,6-dinitro- and 2,4,6-trinitrophenols) at low temperature in freons. At 90 K the rotation of the OH group is slow on the NMR time scale. Addition of proton acceptors causes a bifurcated bond, as evidenced by a shift to lower frequency of the OH resonance. An example is dioxane (38). 

Grech and coworkers studied 8-hydroxy-A,A-dimethyl-l-naphthylamine and suggested that the high frequency shift in dioxane compared to cyclohexane was due to formation of a bifurcated bond. This is somewhat unexpected. See also how multiple hydrogen bonding may affect equilibria (Section U.K). 

The study of Rozas et al. relies on the HB criteria proposed by Koch and Popelier21 and proves the existence of bifurcated bond paths and distinguishes four different types of interactions based on the degree of symmetry and magnitude of the two pb values. Therefore, looking at the geometry, electron density, and energy results, the nature of these HBs as three-centred interactions has been confirmed. 

Analysis of the HB network based on IS configurations has shown that local PES minima contain both linear bonds (LBs) and bifurcated bonds (BBs) whose fraction is both temperature- and density-dependent. The HB network tends to form a random but nearly tetrahedral network (no bifurcated bonds) on cooling, or on lowering the pressure. 

H-bonds (bifurcated bonds) the number of water bonds increases up to 5. Typical classes of water clusters with various numbers of H-bonds are shown in Fig. 2.7. 

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