1 Introduction

Recently, Bernal et al. [1] announced the discovery of a hydronium di-cation of composition H₁₄O₆²⁺ that had been trapped in a crystalline lattice by a cucurbituril molecule. The cucurbituril molecule has an upper and a lower crown of six carbonyl moieties which act as a perfect trap for the H₁₄O₆²⁺ cation—one of each trapped on top and bottom of the molecule. In view of the fact that such a development was totally un-precedented, we decided to search for the existing literature in order to ascertain whether or not there were additional examples of cyclic hydronium cations which had also gone un-noticed. In fact, the search produced an identical cation of composition H₁₄O₆²⁺, which appears in the CSD compilation¹ as BIKVIA10 [2]. For more details, see a review of the stereochemistry of hydronium ions [3].

2 Discussion

A more recent search has revealed the presence of another cyclic hydronium di-cation of composition H₁₈O₈²⁺, which has been in the literature since 1987 [4] and has also gone un-noticed till now. This remarkable new species can be viewed as a ring of eight waters which have been protonated twice at positions trans to each other, forming a ring not too different in geometry from that in cyclooctane. Fig. 1 shows the di-cation and its bond lengths. Note that it is located at an inversion center of the space group P(1¯; No. 2).

As in the case of the previous cyclic species (H₁₄O₆²⁺) the new cation is trapped in the lattice by an organic species, in this case bearing sulfonates and hydroxyl functional groups. The cation is trapped by hydrogen bonds, as shown in Fig. 2. An interesting additional comment is that the same organic species can also trap a classically known hydronium mono-cation; namely H₉O₄¹⁺, which is the monomeric species of the, formally, dimeric species we describe herein. However, as shown in Fig. 3, H₉O₄¹⁺ discovered by Hanson, and described in the same paper [4], is a branched, star-like species which cannot be dimerized into the (H₁₈O₈²⁺) di-cation without a drastic rearrangement. The fact that both of these species can be trapped by the same organic molecule is, in itself, a noteworthy observation, even though the conditions for their crystallization differ² [4].

Dr. A.W. Hanson was notified by us of our discovery of his paper and was asked to join us in this report as a co-author. He graciously declined to join us; but has a copy of the manuscript. He may be contacted at his e-mail address AAlHanson@aol.com. A search in SciFinder Scholar reveals the existence of this hydronium di-cation only in Ref. [4]. See footnote 2 also.