Proton ordering, hydrogen bonds quantum mechanics

The second class comprises the hydrogen-bonded crystals, such as KH2PO4 and KH2ASO4. In these materials the proton can occupy one of two positions equally at elevated temperatures, but at low temperatures the proton system condenses into an ordered configuration. In this sense the transition can be viewed as of the order-disorder type, but is discussed from a quantum mechanical tunneling point of view. ... 

The case when hcj kT is very frequent, and some of the bonds exhibit also a purely quantum-mechanical behavior. The latter is especially typical of covalent bonds of hydrogen atoms which usually have frequencies of the order of 15 kT/h. Thus, a proton transfer reaction is virtually always quantum-mechanical. 

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