Proton ordering

Intermediate between the extended four-coordinate connectivities that dominate the low- T solid phase and the two-coordinate ring/chain connectivities that dominate at higher T are certain //jree-coordinate polyhedral structures that retain a degree of cooperative proton ordering. Two examples of such trigonally coordinated buckyball clusters, a 24-mer and a 28-mer, are shown in Fig. 5.31. The... 

Each vertex of a buckyball cluster is attached by three H-bonds, and hence must have net donor (2D1A) or acceptor (1D2A) character that seems to preclude significant cooperativity. However, by suitably pairing each donor and acceptor monomer, one may produce connected dimers that are each of effective 3D3A pseudo-closed-CT character. Such cooperative dimer units may then be joined in proton-ordered fashion to form closed polyhedra that retain a high degree of cooperative stabilization. 

Both clusters in Fig. 5.31 exhibit considerable cooperative proton-ordering that distinguishes them from many alternative proton-disordered (and destabilized) isomeric forms that could be imagined. For example, the 24-mer (Fig. 5.31 (a)) has two fully cooperative W6C-like caps as well as a fully cooperative 12-ring girdle, ... 

This essential absence of longer-range proton order in tetrahedral H-bond networks is the origin of the famous zero-point entropy of ice L. Pauling, J. Am. Chem. Soc. 57 (1935), 2680 and L. Pauling, note 16, pp. 466-468. 

Adams, St., Ehses, K.-H., and Spilker, J. (1993). Proton ordering in Peierls-distorted hydrogen molybdenum bronze Hq.ssMoOs Structure and physical properties. Acta Cryst. B49, 958-67. 

Figure 13.3 Proton ordering in hydrogen bonding, illustrating favorable (Grotthuss-like) ordering in a chain (above) or cyclic hexamer (a), contrasted with the unfavorable isomer (b), one of many similar that cannot survive thermodynamically. All clusters are ice rule-compliant (cf. Sidebar 5.18).

Figure 13.4 Low-level 18-cluster QCE model (RHF/3-21G level) of the water phase diagram, showing (above) the dominant W24 clathrate-type cluster of the ice-like solid phase, and (below) the overall phase diagram near the triple point (with a triangle marking the actual triple point). Note that numerous other clusters in the W2o-W26 range were included in the mixture, but only that shown (with optimal proton ordering) acquired a significant population.

Experimental Evidence for Proton-Ordered Ring/Chain Structures... 

As a third route to LDA, heating or decompression of the high-pressure polymorph ice VIII can be employed, which is the proton-ordered pendant to ice VII. Most high-pressure forms of ice quench-recovered at 77 K experience a... 

Finally, radiation can be employed to produce LDA. Ice III or its proton-ordered counterpart ice IX are amorphized by particle bombardment at electron doses above 2400 electrons nm 2 [164]. Ice I amorphizes by keV ion-bombardment at 10-80 K [165]. Similarly, after a dose of few eV per mol of UV photons amorphization of ice I is observed [166, 167]. The conversion rates increase as the temperature decreases [168]. Using 700 keV proton irradiation at 13 K, even oscillations between crystalline and amorphous ice can be achieved, whereas above 27 K, the amorphous ice remains [169]. 

---