Macroscopic and structural effects of hydrogen-bond transformations

Abstract

A brief account of transformations of internal structure of hydrogen bonds, and transformations involving the breaking and formation of hydrogen bonds is highlighted for a selected group of crystals. The main stress is laid on the short bistable –OH…O= hydrogen bonds, illustrating several general properties, often blurred for weaker hydrogen bonds. A set of parameters relating the electronic structure of the oxygen atoms and geometry of hydrogen bonds is described. This allows one to combine the hydrogen bond dimensions with changes in the electronic structure of the oxygen atoms, accompanying any transformations of hydrogen bonds. Several resultant effects are described: coupling of displacements of molecules or ions with the H-atom transfer, the isotope effect of deuteration on T _c in ferroelectric crystals, a skew trajectory of a disordered H-atom with respect to the O···O direction, or shortening of the O···O distance when the H-atom becomes disordered. It is shown that hydrogen-bond patterns, molecular arrangements and thermodynamic properties of crystals are interdependent. Several examples and possible applications of the presented model are given.

Keywords:

• Hydrogen bond
• Phase transition
• Disordered structure
• Ferroelectric crystals
• Superionic state
• Hydrogen-bonded crystals
• Critical temperature
• Isotope effect
• Hydrogen-bond transformation

Acknowledgement

This work was supported by the Polish Committee of Scientific Research, Grant No 2-0662-91-01.

Notes

^a Averaged dimensions of two symmetry-independent hydrogen bonds in squaric acid at 298 K used for the calculations.

^b Experimental lengths of d _d and d _a measured at 394 K.

Symmetry codes: (′) x, 1 − y, z + 0.5; (′′) , y, 0.5 − z.