Geometry proton disorder

Although the crystallographic structure of the most common form of ice, phase Ih, has been known for many decades its proton-disordered local structure is still being discussed ". As a demonstration of this fact, we mention that the geometry of water molecules in ice Ih has only recently been determined accurately, with the help of rather sophisticated quantum mechanical calculations . 

All the quantities contributing to the partition function in Eqs [39-41] can be computed once the intermolecular potentials are known. The calculation proceeds as follows. Several proton-disordered hydrate structures with zero net dipole are chosen. The minimum energy of each gas hydrate structure is computed from a geometry optimization, usually using the steepest descent method. The vibrational entropies are from normal mode analysis of lattice dynamics results. The host and guest contributions are determined from the Hessian matrix V (second derivative of the intermolecular potential with respect to the displacements). The Hessian matrix can be divided into four sub-matrices, V, ... 

Another example is the 1 1 1 complex of acetylacetone with the host 74 and a water molecule in which again the enol form was observed. In the case of the 2 2 complex of acetylacetone with (R,R)-(—Hra .s-4,5-bis(hydroxydiphcnylmcthyl)-2,2/-dimethyl-l,3-dioxacyclopentane 84 however, a crystal measured at room temperature showed a disordered enolic proton, i.e. the presence of two enol forms. The same complex measured at 100 K revealed the pure enol form for both symmetrically independent molecules of acetylacetone [83], The geometry of the enolic molecules resembled that obtained by gas-phase electron diffraction studies at room temperature [84],... 

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