Proton tunnelling and deuteration-induced phase transitions in hydrogen-bonded crystals

Experimental facts about appearance of new crystalline phases at low temper ature related to deuterium substitution are collected and mechanisms of the isotope effect discussed. Compounds considered are (i) tri-alkali hydrogen selenates and sulphates, (ii) ammonium hexachlorometallates, (iii) bromo- and iodo-hydroxyphenalenone and (iv) chromium hydrogen dioxide. In (i), (ii i) and (iv), linear O-H-O hydrogen bonds are involved. Proton tunnelling is the Likely mechanism of the peculiar phase behaviour of(i). Definitely the tunnelling plays an essential role in (iii) but not in the extreme situati on in (iv). In (ii), rotational tunnelling of an ammonium ion is the likely ingredient of the deuteration-induced phase transitions. The term proton c loud is introduced to describe the quantum mechanical distribution of the n ucleus in the hydrogen bond and in the multi-valley rotational potential fo r an ammonium ion.