Halides rotator phases

Halide salts of quaternary ammonium and phosphonium cations, and those derived from heterocyclic bases (both aromatic and alicyclic), have long been known (Table 11.14). Most are relatively high melting (and would not be considered ionic liquids), though the structural characteristics of quaternary ammonium halides have been studied by those with interests in plastic crystals and so-called rotator phases [7]. Many deconpose at or near their melting temperatures, a characteristic that would limit interest in them as reaction media [667]. Indeed, it is probably as a result of the view, widely held in the past [668], that such materials may be intrinsically unstable that the onset of the recent massive interest in ionic liquids was delayed. 

Formation of the Sic Phase in Binary Aqueous Alkyltrimethylam-monium Halide Solutions. These considerations are well illustrated by the formation of the Slc phase in aqueous alkyltrimethylammonium halide solutions (17) (Figure 7). This phase is apparently composed of Si micelles—probably on balance prolate—arranged in a primitive, cubic lattice and rotating fairly freely at the lattice points. The lattice is formed by dodecyl- and tetradecyltrimethylammonium chlorides but not by the hexadecyl or octadecyl chlorides nor by any of the corresponding bromides. This may be expressed as follows. 

During the period when theorists were addressing the question as to the possible existence of ion pairs, there was a good deal of experimental inquiry as well. The neutral pair nature of H3N" HC1 was confirmed in the gas phase by its rotational spectrum. The intermole-cular distance derived from the spectral data was noted to be in remarkable agreement with an earlier ab initio computation with MP2 correlation. The following year saw the measurement of the microwave spectrum of H N—HBr, and this complex too had the characteristics of a neutral pair. Nor are any of the complexes pairing a hydrogen halide with HjP of the ion-pair type. 

When the pseudo-spherical ammonium ion is mostly replaced by a truly spherical ion the complex sequence of phase changes found in the pure ammonium halides is suppressed. The mixed potassium ammonium halide salts retain their NaCl cubic structure down to the lowest temperatures. The alkali metal ions support the structure leaving the ammonium ions as free to rotate at 1 K as at 300K [13]. The INS spectrum of this system is quite different from the pure salt and there are no sharp features in any region of the spectrum. We shall analyse the impact that this freedom has on the internal modes about 1400 cm. ... 

This is useful for preparative and catalytic chemistry, for example, a metal halide can be converted to a hydride in the presence of base or under phase-transfer or high-pressure conditions. Reversible proton transfer to pendant basic sites can also occur (Equation (7)) and is facilitated by hydrogen bonding interactions, for example. Equation (8) where the OH and IrH hydrogens scramble via rotation of the H2 ligand. 

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