Symmetric Ionic Hydrogen Bonds

On one end of the spectrum of H-bonds are those that are very weak, as in the case of numerous C—H---X interactions. On the other end of this continuum are very strong interactions. HX molecules are quite acidic. If paired with a proton acceptor that has a negative charge, the interaction can become very strong indeed. In fact, the H-bond can become so short that the proton loses its association with one molecule versus the other. That is, the proton can be drawn into the midpoint of the X—X axis. Such a H-bond is referred to as centrosymmetric. 

---

Ammonium ions A represent a special guest class because the cation can be formed by proton transfer in acidic solutions or by acidic functionalities such as the OH-groups of the calixarenes. The N H-bond allows formation of ionic hydrogen bonds with H-bond acceptors such as oxygen (e.g., at the narrower rim of calixarenes II or at the wider rim of resorcinarenes IV). In spherical tetraalkylammonium ions B, the positive charge is dispersed over the four alkyl groups. In contrast, the positive charge is non-symmetrically... 

As we discussed in the previous chapter (Sec. 1.18), the unit of such a non-ionic compound, whether solid, liquid, or gas, is the molecule. Because the methane molecule is highly symmetrical, the polarities of the individual carbon-hydrogen bonds cancel out as a result, the molecule itself is non-polar. 

Protic solvents solvate polar functional groups and thereby make them more exposed. The only polar functional groups that can be found on a conventional nonpolar reversed phase are the sUanol groups. If these are made more prominent by solvation, then the undesired ionic interactions with basic analytes come more to the fore and the elution bands become broad and asymmetric. If, however, an aprotic solvent such as acetonitrile is used with these columns, then the lone pair electrons of acetonitrile can form hydrogen bonds with the silanol groups, and these will be shielded from the basic analytes. The elution bands are much narrower and more symmetrical. 

---