NMR studies of molecular motion in tetramethylammonium hydroxide pentahydrate

Crystal engineering and characterization of a structure-H ionic clathrate hydrate

Ionic hydrates are known to form numerous clathrate structures in which either the cations or anions sit in cages and the counterions are incorporated into the water framework. Due to the inclusion of the ionic species, such ionic clathrate hydrates not only show many peculiar features such as metal ion encagement and superoxide ion generation, but also exhibit notable physicochemical properties such as outstanding ionic conductivity and thermal stability. Thus, the ionic clathrate hydrates are considered for their potential applicability in various fields, including those that involve solid electrolytes, gas sensors, and energy storage. In this study, we report the design, synthesis, and characterization of the first ionic clathrate hydrate of the hexagonal structure-H (Str.H) crystal type. Diethyl-dimethyl-ammonium hydroxide hydrate was synthesized with CH4 and Xe as help gases, and the crystal structure was identified by powder X-ray diffraction analysis. Further confirmation of the formation of Str.H was obtained from Raman spectroscopy and 13C, 129Xe, and 2H solid-state NMR spectroscopy. From 13C NMR and ab initio calculations, it was shown that the quaternary ion occupies the large cage of Str.H with a conformation different from that in solution, due to constraints imposed by the dimensions of the cage. The H deficiency introduced by substitution of OH– for a water molecule appears, from 129Xe NMR, to be disordered over the framework, and, from 2H NMR, to substantially increase the rate of reorientational mobility of the D atoms in the framework, over that observed for a Str.I hydrate and for ice. The Str.H hydrates are commonly more stable than other structures, thus the present findings on the ionic Str.H clathrate hydrate may offer a new approach for improving the stability of ionic clathrate hydrates for their practical application.

Unusual Properties of Water

Water has been known for its unusual properties from antiquity when, e.g. was found that hot water freezes faster than cold water. Presently, on the web page 'water' Martin Chaplin [1] lists sixty seven properties of water which may be considered 'anomalous' when comparing to 'normal' chemical substances. Much of this can be attributed to the spatial structure of hydrogen bonding in condensed phases of water. Hydrogen bonding constitutes about 2/3 of cohesion energy of water. However, the remaining 1/3 is definitely not negligible. Combination of the two leads to properties of water in the systems where it plays a role. The very comprehensive range of such systems and common presence of water make the enormous variety of structures and properties of water-containing compounds. In the present paper the non-hydrophilic component of properties of water will be emphasized in combination with the structural aspects of supramolecular bonding of water molecules.