Ice and Other Structures with Disordered Hydrogen Bonds

1 Ice and Other Structures with Disordered Hydrogen Bonds 

Neutron and X-ray diffraction studies have revealed that in the hydrogen-bonded triad system X-H. .. A, where atoms X and A are chemically different, the proton is bound to only one of the two atoms. In systems where the atoms X and A are the same but belong to different groupings, the situation is less clear-cut. Proton disorder may occur. Its extent depends on various factors, such as the X... A distance, the environment about the hydrogen bond, the ionic states of X and A, the molecular conformation, and the temperature. The disorder may be localized within a molecule or involve a pair, a chain, a layer, or a three-dimensional ensemble of molecules. 

Such three-dimensional networks of perfectly disordered hydrogen bonds are rare, but they are also found in the low-pressure cubic form of ice and in some of the high pressure forms [74], Proton disorder also occurs in some of the hydrate inclusion compounds in which the water molecules form regular three-dimensional arrangements. These structures have been reviewed by Jeffrey and Saenger [10]. 

Proton disorder may, in principle, occur in hydrogen-bonded chains of carboxylic acids, interlinked as in the chiral crystal structure of 2-d-methyloctodecanoic acid [79], as shown in 15. But the structure determination, based only on two-dimensional X-ray diffraction data, was not sufficiently precise to yield H positions. 

2 Order-Disorder (0/D) of the Carboxyl Dimer and Proton TYansfer 

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