Protons polarizing other atoms

This chapter considers a number of other types of interactions that are somewhere near the boundaries of a true H-bond. We discuss the details of these interactions and the magnitudes of some of the indicators. One issue discussed is the proton-accepting ability of an electronegative atom when involved in a bond to another electronegative atom, leaving the bond of low or zero polarity. Hydrogen atoms bonded to carbon are typically of low acidity, so their ability to participate in H-bonds is questionable as well. 

The most important point to remember in classifying into sources and sinks is to stay flexible. Almost all electron sinks have lone pairs that can serve as sources. Lone pairs on sinks can complex with Lewis acids or be protonated. A polarized multiple bond can serve as an electron-withdrawing group, making protons on adjacent atoms acidic. Looking at both reactive partners can help you decide. If one partner is clearly a sink, like sulfuric acid, then the other partner must serve as a source, even though it may be a polarized multiple bond sink like a carbonyl protonate the carbonyl lone pair as a source. 

Spectroscopic methods can work with the chiral selector associated with the ligand either in solid state or in solution. The chiroptical spectroscopies, circular dichroism, and optical rotatory dispersion, represent an important means for evaluating structural properties of selector-ligand adducts [14]. NMR can specifically investigate proton or carbon atom positions and differentiate one from the other. X-ray crystallography is a powerful technique to investigate the absolute configuration of diastereoisomeric complexes but in the solid state only. Fluorescence anisotropy is a polarization-based technique that is a measure, in solution, of the rotational motion of a fluorescent molecule or a molecule + selector complex [15]. 

Hyperpolarized noble gas atoms, like xenon, can interact with protons and other nuclei via dipolar cross-relaxation, a mechanism which is supported by stochastic processes of motion. By means of this interaction, hyperpolarized xenon can transfer its polarization to protons and other nuclei of interest. An efficient way to make use of this SPINOE mechanism is to freeze hyperpolarized xenon onto the surface of the sample. During the defrosting process large amounts of xenon can penetrate into the liquid sample, resulting in a large SPINOE enhancement. 

---