Isolated water molecules in mineral lattices

The role of water in lattices is of great interest to the mineralogist and over 500 minerals contain water in more or less important bonding scenarios. The view of water as a static element within a mineral was challenged by several spectroscopic techniques including INS [10]. Especially in weakly bonding environments the water molecules are disordered and diffraction experiments are unable to determine the hydrogen positions, INS techniques can provide useful information in these circumstances. 

Zeolites form an important subset of minerals that, in their protonated form, are used as acid catalysts. One open question has been the exact nature of the water on these catalysts because the acidity of the Bronsted 

In the absence of water, typical vibrational frequencies were found for the protonated zeolite but an unanticipated degree of heterogeneity was also discovered. With the addition of one water molecule per Bronsted site, single-load, the ZSM-5 spectrum was considerably modified and strongly resembled earlier work on mordenite. The interpretation of this spectrum was, however, quite different fi-om the previous work and lead to its reassessment. A difference spectrum showing only the INS of water was compared with the results of two calculated spectra based on either a H-bonded water molecule or a HsO. The comparison clearly favours the H-bonded water molecule model and the presence of hydroxonium ions was discounted. 

Independently vibrating water molecules were used to explain the INS from a series of A-type zeolites. Sharp transitions appeared at low frequencies in the lithium, 63 cm , and sodium, 29 cm , zeolites but the potassium and calcium zeolites showed no bands at all in this region [14]. The relatively sharp features of the INS spectrum of ZSM-5 with low water content gave way at higher water content to broader, less structured spectra. The spectra resembled ice Ih in form, see below, but had distinctly different librational band frequencies. Whereas, in ice these bands are at about 600 cm , in the ZSM-5-water system they appear about 500 cm and about 400 cm in leucite-water [15]. A similar frequency drop is also seen in the INS of water, ca 3%, on silica gel [16]. This is related to the earlier observation that the more open the structure of bulk water in a material then the lower will be the librational band frequencies [17]. The structural aspects of water in confined geometries has been reviewed recently [18]. 

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