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Figure 7
This figure was drawn using the coordinates obtained from the species listed in the CSD (Allen, 2002BB2) as SLBZAC10 (Attig & Mootz, 1976BB6). It shows the ambiguity that may occur in describing the nature of crystalline H7O3+ cations, as discussed in a recent review (Bernal & Watkins, 2014BB7). On the right, there is a water with a long (1.763 Å) hydrogen bond to an H5O2+ hydroxonium cation; that is one model. A second model is one in which, realising that H5O2+ is itself somewhat asymmetric, the description needed is that of a terminal H3O+ cation on the left with a H2O–H2O fragment attached to it. Again, as discussed in Bernal & Watkins (2014BB7), the only cases in which the description is completely unambiguous is when the hydroxonium cation lies at a special position such as on a twofold axis, a center of inversion etc.

Journal logoSTRUCTURAL
CHEMISTRY
ISSN: 2053-2296
Volume 71| Part 1| January 2015| Pages 32-38
doi:10.1107/S2053229614025637
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