Inverse hydrogen bonds

In the normal hydrogen bond X-H- Y, the H atom plays the role of electron acceptor, while the Y atom is the electron donor. The interaction is of the kind 

Some examples of inverse hydrogen bonds are presented below  

The concept of inverse hydrogen bond is a relatively recent development, and additional varieties of this novel type of interaction may be uncovered in the future. 

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An inverse hydrogen bond is formed when a hydrogen atom can give electrons while another, nonhydrogen atom is accepting them ... 

For example, the hydrogen atoms of the strongly polarized bonds in hydrides LiH and BeH2 or BH4 can be electron donors, and the electron-deficient atoms Li, Be, or B can accept electrons to form inverse hydrogen-bonded complexes Li-H Li-H, H-Be-H Li-H, and others [3]. Similar to classical hydrogen bonds, the electronic distribution in these inverse hydrogen bonds, analyzed in the framework of AIM theory, shows that the hydrogen atom is bound to both the electron donor and the electron acceptor by closed-shell interactions. In addition, the bond critical points correspond to all the characteristics associated... 

I. Rozas et al., Inverse hydrogen-bonded complexes. J. Phys. Chem. A101,4236 -244 (1997)... 

The so-called lithium hydrogen bond, Li-H- Li-H, occurs in the hypothetical linear (LiH)2 dimer. The inner Li atom is electron-deficient, and the inner H atom is sufficiently electron-rich to act as a donor in the formation of an inverse hydrogen bond. The calculated bond lengths (in pm) and electron donor-acceptor relationship are illustrated below ... 

In addition to this broad class of conventional hydrogen bonds, several non-conventional variants also exist in nature [2-7]. For example, hydrogen bonds occur in systems with non-conventional hydrogen acceptors, such as isonitriles [8, 9], carbanions [10, 11], carbenes and silynes [12], n-systems [8, 9, 13]. Non-conventional hydrogen bonds also include those known as hydridic, also termed inverse hydrogen bonds since they are characterised by an inverted polarity [9], that is and a newly characterised... 

It can easily be shown that the value of K" is inversely proportional to the value of K and that K is dependent on both the cation and the anion of the ionic liquid. Eience, it is entirely consistent with this model that the difference made by changing the anion should depend on the hydrogen bond acidity of the cation. 

When Shallenberger proposed that sweetness varies inversely with the extent of intramolecular hydrogen-bonding, he pointed out that while this... 

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