Unusual proton transfer patterns in crystalline ammonia hemihydrate (2NH3⋅H2O)

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Abstract
The crystalline hemihydrate of ammonia (2NH3⋅H2O) resembles water–ice in having an H‐bonded network structure. However, the H bonds are all hetero in nature with the result that the hemihydrate offers a marked contrast to ice in its proton transport properties. At low temperatures ice is characterized by a very few highly mobile ion defects with the mobility believed to depend on H3O+ tunneling involving O—H—O linkages. By contrast, the hemihydrate is presumed to have an abundance of ion‐pair defects (NH+4, OH−) which, the present study suggests, are lacking in mobility. As a result proton transport in water–ice at 140 K is comparatively very rapid. The ability to matrix‐isolate D2O intact in crystalline ice by a codeposition at T≊125 K, was basic to the 140 K water–ice proton‐transport study. Similarly, all compositions of the hemihydrate embodied in the formula xNH3⋅(2−x)ND3⋅yH2O⋅(1−y) D2O are now accessible by codeposition methods, and thermally induced proton exchange has been monitored for a number of such crystalline samples. Though little, if any, exchange occurs for the pure hemihydrate at T≊140 K, proton transfer within the ammonia subsystem can be induced by either ammonia or water doping of the hemihydrate. Since ion defects are abundant in the pure hemihydrate, the role of the dopant molecules is considered to be the mobilization of the NH+4 ion defects rather than a modification of the ion‐defect concentrations, the normal dopant role in ice proton transfer. Despite the induced proton transfer within the ammonia subsystem, the water subsystem invariably remained blocked to proton transfer such that, despite reaction times of several hours at 135 K, much less than 5% of the water molecules normally experienced isotopic exchange.