research papers Acta Crystallographica Section C
Structural Chemistry ISSN 2053-2296
Molecular and supramolecular ionic aggregates HxOyz in organic and organometallic crystalline hydrates Ivan Bernala and Steven F. Watkinsb* a
Chemistry Department, Hunter College, City University of New York, New York, NY 10021, USA, and bChemistry Department, Louisiana State University, Baton Rouge, LA 70803, USA Correspondence e-mail: [email protected] Received 12 March 2014 Accepted 1 May 2014
Ionic aggregates of the form HxOyz (z 6¼ 0) have been characterized during an analysis of 245 crystal structures extracted from the Cambridge Structural Database [Allen (2002). Acta Cryst. B58, 380–388]. A systematic nomenclature is proposed for these species. Three modes of hydrogen bonding are described, characterized in part by the distance between contiguous O atoms: normal (NHB; O O = 2.6– ˚ ), charge assisted (CAHB; O O = 2.5 A ˚ ) and molecular 3.0 A ˚ (MHB; O O = 2.4 A). The three modes are consistent with previous reports, our experimental results, and quantumchemical-optimized geometries and energetics. No evidence is presented concerning the possible existence or stability of these aggregates in solution. Rather, emphasis is placed on the necessity in crystal structure analysis to develop thoroughly existing hydrogen-bonded networks, ignorance of which can lead to erroneous crystal structure models and other physicochemical data associated with composition and charge balance. Keywords: molecular ionic aggregates; supramolecular ionic aggregates; organometallic crystalline hydrates; organic crystalline hydrates; systematic nomenclature; hydrogenbonding modes; Cambridge Structural Database search; hydroxone; hydroxonium; hydroxonane; hydroxonate.
1. Introduction Aqueous acid–base equilibria involve ionic aggregates which are more complex than single hydronium and hydroxide ions. These molecular and supramolecular species, when captured in crystalline hydrates, can be characterized by X-ray and neutron diffraction (Bernal, 2006, 2007). Unfortunately, suspected mischaracterizations are found in some X-ray structural reports due to misplaced or misidentified H atoms (Bernal & Watkins, 2013). A common scenario, apparent in a number of publications, is as follows: two or more (hydrate) O atoms are originally
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‘solved’ in isolated positions in the unit cell, but the author fails to consider symmetry-equivalent atomic positions. Contiguity of these O atoms and the resulting hydrogen bonding is thus unrecognized, and the author considers an insufficient number of maxima in the Fourier difference synthesis surrounding and between these O atoms. Thus, maxima representing H atoms in correct locations are not considered. H-atom placement in a crystal structure analysis is greatly aided by the realization that there must be a hydrogen-bond path between two contiguous O atoms with an internuclear ˚ , similar to the geometric constraints distance of less than 3 A on alkyl and aromatic H atoms. This, together with constraints on hydrogen-bond geometry and hydrogen-bond type (see below), can be strong discriminators between Fourier maxima due to H-atom electron density and others due to ‘noise’ or lone pairs on electronegative atoms. Furthermore, missing or misplaced H atoms cannot be recovered or corrected by referees, editors, or readers without access to and model refinement with diffraction data. This paper describes molecular and supramolecular ionic aggregates HxOyz (z 6¼ 0) catalogued during the analysis of crystal structures of organic and organometallic hydrates (see Supporting information). Crystal structure files (CIFs) of these hydrates were extracted from the Cambridge Structural Database (CSD; Allen, 2002; Version 1.15, updated to 2 February 2014), using the following search criteria: (i) substructure search for H3O+ or HO; (ii) title search for ‘hydronium’, ‘oxonium’ or ‘hydroxide’; (iii) R factor 7.0%; (iv) no disorder; (v) no powder diffraction; (vi) no structures containing atoms labeled ‘H?’ or ‘O?’; (vii) no structures with warning comments (e.g. ‘short intermolecular hydrogen contacts of the oxonium cations’) which indicate problems with the refined model; (viii) for structures determined multiple times, select lowest R factor and/or lowest temperature and/or neutron probe over X-ray probe. Each CIF in the resulting subset was then inspected visually by both authors using the molecular graphics programs Mercury (Macrae et al., 2006) and DIAMOND (Brandenburg & Putz, 2004), and was rejected if the H-atom count differed from the reported moiety formulas, or if no H atom was located near a contiguous O O bond path (O—H O angles less than about 120o). The resulting 245 CIFs were accepted as the working set. Finally, a further 76 X-ray structures were withheld from the working set for suspected H-atom misplacement; that is, using the contiguous O O distance as a diagnostic tool, we believe the reported species and/or their hydrogen-bond geometries are inconsistent with the three hydrogen-bond types described herein, and therefore the reported structures may be inaccurate. The remaining 169 CIFs were accepted as credible reports of HxOyz architectures.
doi:10.1107/S2053229614009826
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research papers In order to discuss HxOyz species further, we will use the following systematic nomenclature. For more than two centuries, the suffix ‘–ium’ has been used almost universally to identify metals and their cations, while suffix ’–ate’ is widely used for oxyanions. In addition, suffix ‘–ane’ identifies classes of neutral organic and inorganic compounds (e.g. alkanes, siloxanes). The relatively recent but ambiguous term ‘oxonium’ refers not only to H3O+, but to any trivalent oxycation R3O+. With the foregoing in mind, we propose:
Table 1 Estimated eigenvalues of the geometry-optimized system Hamiltonian. HxFyz
E (a.u.)
HxOyz
E (a.u.)
HF F (HF)2 FHF F2 H2 2 F 2 H H
100.48485750 99.88869321 200.97983857 200.45018330 199.58111621 1.18003403 99.76168096 0.50225698 0.53416365
H2O HO (H2O)2 H5O2+ H3O2 O2 3 O H3O+ H5O3 (H3O)3 H3O3+
76.46451156 75.83092703 152.93672331 153.25665439 152.34087192 150.37948850 75.09091483 76.73680050 228.84090051 229.40704300 229.75965981
hydroxone: the class of molecular and supramolecular species HxOyz which subsumes the following three subclasses: hydroxonium: molecular and supramolecular cations H2n+1On+, H2n+2On2+ etc. Excluding chemically inaccessible ‘H+’, the first two members of this subclass are molecular species H3O+ (hydronium) and H5O2+ (Zundel cation); hydroxonane: neutral molecular and supramolecular aggregates with formula H2nOn. Higher-order members (n > 1) consist of hydrogen-bonded aggregates (H2O)n bound by normal hydrogen bonds; hydroxonate: molecular and supramolecular anions H2n–1On, H2n–2On2 etc. The first two members of this subclass are molecular species HO (hydroxide) and H3O2. All 245 CIFs of the working set were modified to eliminate all atoms except those of the hydroxones; the remaining hydroxone networks, developed in 222 packing diagrams, yielded 430 unique contiguous O O distances (Fig. 1). It is important to note that hydroxone aggregates generally hydrogen bond to other molecules and ions in the crystal (e.g. carboxylates, sulfates, phosphates, nitrates etc); no contiguous O O distances from those interactions are included in Fig. 1. This distribution, and our observations described below, are consistent with earlier suggestions that there are three types of hydrogen bonds (e.g. Gilli et al., 1994; Perrin & Nielson, 1997):
MHB: molecular hydrogen bonds narrowly distributed ˚; about O O = 2.4 A CAHB: charge-assisted hydrogen bonds more broadly ˚; distributed about O O = 2.5 A NHB: normal hydrogen bonds very broadly distributed ˚. about O O = 2.8 A We contend that contiguous O O distances are necessary and diagnostic metrics for correct supramolecular formulation, and that failure to characterize an existing hydrogenbond network can lead to errors in structural analysis. For example, thermodynamic considerations notwithstanding, cationic and anionic hydrate species have been reported to coexist in the same lattice, even as fragments in the same supramolecular aggregate. Our analysis leads us to believe that no credible evidence for such pairings exists.
2. Theoretical basis and previous results
A previous paper (Bernal & Watkins, 2014) described supramolecular halide aggregates associated with fluoride, bifluoride (FHF), chloride and bichloride anions. Since HF and H2O are isoelectronic (as are F/HO and FHF/H5O2+/H4O2/H3O2), it is not surprising that there are structural similarities between hydrofluoride and hydroxone species. In an effort to understand these similarities, we have explored the electronic properties of a number of H—F and H—O species using the ab initio quantum chemical program GAUSSIAN09 (Frisch et al., 2009). In all theoretical models mentioned in this report, singlepoint and geometry-optimized total energies were calculated using density functional theory (DFT) (RB3LYP or UB3LYP) with basis set 6-311++g(3df,3pd). The results are shown in Table 1. Energies of some of the gas-phase reactions between species in Table 1 are also experimentally accessible and widely available in text books (e.g. Brown et al., Figure 1 2012). Comparison of calculated and Distribution of contiguous O O distances in crystalline HxOyz aggregates. Acta Cryst. (2014). C70, 566–574
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Table 4
Calculated and experimental reaction energies.
Statistics for geometric parameters observed in H3O+ species.
E†
Reaction HF(g) ! H(g) + F(g) F2(g) ! 2F(g) H2(g) ! 2H(g) H2O(g) ! 2H(g) + O(g) O2(g) ! 2O(g) HF(g) ! 12F2(g) + 12H2(g) H2O(g) ! H2(g) + 12O2(g)
580/567 152/155 461/436 969/926 519/495 274/269 249/241
%error 2.3 1.9 5.7 4.6 4.8 1.9 2.9
E Defined
Observed
Bond Energy Bond Energy Bond Energy Bond Energy 2 Bond Energy ca Hf [HF(g)] ca Hf [H2O(g)]
† Calculated/experimental energy change (kJ mol1)
Table 3 Calculated reaction energies. No.
Reaction
E†
1 2 3 4 5 6 7 8
(HF)2(g) ! 2HF(g) (H2O)2(g) ! H2O(g) + H2O(g) H6O3(g) ! H2O(g) + (H2O)2(g) FHF(g) ! F(g) + HF(g) H5O2+(g) ! H2O(g) + H3O+(g) H3O2(g) ! H2O(g) + OH(g) H7O3+(g) ! H2O(g) + H5O2+(g) H5O3(g) ! H2O(g) + H3O2(g)
27 20 15 201 145 119 101 94
O—H H—O—H
Range
Mean ()
˚ 0.75–1.30 A 74–141
˚ 0.93 (11) A 110 (8)
(and therefore energies) for this ‘charge-assisted’ hydrogen bonding (CAHB) in which neutral hydroxonanes link to one of the four molecular ions as (H3O+)(H2O)n, (H5O2+)(H2O)n, (OH)(H2O)n or (H3O2)(H2O)n. The first three of these charged hydroxone species have all been observed and form the basis for this report, but we have been unable to find in the CSD any hydroxonates of the form (H3O2)(H2O)n. In the following diagrams of hydroxone aggregates, no hydrogen bonds are shown to surrounding species, such as amine or sulfate ligands. But these external hydrogen bonds do exist in most cases and must certainly influence the detailed geometry of the species displayed.
† Calculated energy change (kJ mol1)
3. Catalog of hydroxonium cations observed values (Table 2) reveals a satisfactory degree of accuracy. Note that the calculated energies are consistently higher than the experimental values, a systematic error which could probably be improved by using larger basis sets. The reactions in Table 3 are not easily accessible to experiment, but the calculated energy values should be of the same degree of accuracy as those in Table 2. Thus, reactions 1 through 3 feature ‘normal’ hydrogen bonding (NHB) within neutral supramolecular H—F and H—O assemblies. Reactions 4 through 6 feature three isoelectronic molecular ions, i.e. FHF, H5O2+ and H3O2, all of which contain linear symmetric divalent hydrogen. Bonding in symmetric ‘X—H— X’ is often portrayed as ‘three-center four-electron bonding’ (Pimentel, 1951), and the molecular orbital architecture is consistent with this characterization. However, we use the less mechanistic term ‘molecular’ hydrogen bonding (MHB) which is clearly an order of magnitude stronger than NHB, at least for X = F or O. Reactions 7 and 8 represent hydrogen bonding between neutral and ionic species, and the bond energies fall between those of normal hydrogen bonds and molecular hydrogen bonds. Fig. 1 suggests that there is a distribution of distances
Of the 245 crystal structures analyzed for this report, 208 are reported to contain hydroxonium species. However, on the basis of criteria developed during this analysis, 58 of these structures were suspected to have misplaced or missing H atoms (Bernal & Watkins, 2013). Presented below are exam-
Figure 3 Figure 2 Geometry-optimized model of H3O+.
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(a) Geometry-optimized molecular ion H5O2+, (b) H5O2+ in SUGKAG (Sergienko et al., 1991) and (c) (H2O)(H3O+) in BOVMOP (Attar Gharamaleki et al., 2009). Acta Cryst. (2014). C70, 566–574
research papers Table 5 Molecular versus supramolecular forms of the Zundel ion. 33 with < 0.17
˚) H—O1 (A ˚) H—O2 (A ˚) O1 O2 (A
14 with > 0.23
Range
Mean ()
Range
Mean ()
1.10–1.25 1.18–1.33 2.36–2.48 0.00–0.17
1.20 (3) 1.24 (3) 2.42 (3) 0.03 (5)
0.85–1.16 1.35–1.84 2.43–2.74 0.23–0.74
0.98 (11) 1.54 (15) 2.51 (9) 0.44 (18)
ples of observed isomers of hydroxonium aggregates H3O+, H2O5+, H7O3+, H9O4+, H11O5+, H13O6+, H14O62+, H18O82+, H22O102+, and three-dimensional polymer (H13O6+)n.
bond acceptors. This seems to be the case, for example, in crystal structure SUGKAG (Fig. 3b; Sergienko et al., 1991), ˚ . On the other hand, if an asymmetric with O O = 2.36 A force field surrounds the cation, and/or its exterior H atoms form strong hydrogen bonds to surrounding species, H5O2+ transitions from a molecular to a supramolecular aggregate in which a hydronium cation is bound by a charge-assisted hydrogen bond to a water molecule as in BOVMOP (Fig. 3c; Attar Gharamaleki et al., 2009). We have found no unambiguous measure to distinguish between the molecular and supramolecular forms of H5O2+, but one possible indicator may be the asymmetry of the central H atom, defined by: ¼
3.1. H3O+ (hydronium)
There are 66 credible X-ray and three neutron crystal structures (see Appendix A for CSD refcodes) which contain hydronium ions unbound to other hydroxone species. Fig. 2 shows the calculated geometry of the hydronium ion, while statistics for observed species are given in Table 4. Given the relative imprecision with which H-atom positions are determined in X-ray analysis, together with the range of distorting forces in individual lattices, agreement between the average observed and calculated geometries of H3O+ is remarkable.
2jdðHO1Þ dðHO2Þj : dðHO1Þ þ dðHO2Þ
This measure is imprecise in X-ray crystal structures due to its reliance on H-atom placement, and may be most useful in the extremes; for example, = 0 for SUGKAG and = 0.74 for BOVMOP. Nevertheless, there does seem to be a natural break in our sample of 44 observed X-ray and three neutron structures, between = 0.17 and = 0.23, with O O distances
3.2. Molecular and supramolecular H5O2+ (Zundel cation)
The geometry-optimized model of H5O2+ (Tables 1 and 3, and Fig. 3a) has C2h symmetry and a symmetric divalent H atom forming a molecular hydrogen bond, with O O = ˚ (consistent with Fig. 1). The field-free environment of 2.414 A the calculated model can be approximated in a crystal if the surrounding force field is highly symmetric and/or if the exterior H atoms are not strongly bonded to other hydrogen-
Figure 5 Figure 4 (a) (H2O)(H3O+)(H2O) and (b) (H5O2+)(H2O). Acta Cryst. (2014). C70, 566–574
(a) (H2O)(H3O+)(H2O)2, (b) trans-(H2O)(H5O2+)(H2O) and (c) cis(H2O)(H5O2+)(H2O). Bernal and Watkins
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research papers ˚ and = 0.50 (10). Isomer 2 of H7O3+ O O = 2.47 (4) A (Fig. 4b) is formulated as a Zundel cation with CAHB to one water molecule. Two instances of this form [JOTQOY03 (Calleja et al., 2001; neutron) and SLBZAC10 (Attig & Mootz, ˚ 1976)] have average dimensions O O (MHB) = 2.42 (2) A ˚ and = 0.16 (5), and O O (CAHB) = 2.64 (9) A and = 0.53 (9). 3.4. Three isomers and two conformers of H9O4+ (Eigen cation)
Figure 6 (H3O+)(H2O)3.
˚ (CAHB), respeccentered at 2.42 (3) (MHB) and 2.51 (9) A tively, consistent with the observed distribution of hydrogen bonds (Fig. 1). Statistics are shown in Table 5, and specific refcodes are listed in Appendix B.
Isomer 1 of H9O4+ is observed in crystal structure IVAYIN (Fig. 5a; Stasko et al., 2004) and features a central hydronium ion. The three O O distances show the expected progression ˚ ) to NHB (2.77 A ˚ ). from CAHB (2.50 and 2.58 A + Isomer 2 of H9O4 is found in five crystal structures, and contains a central Zundel cation with water molecules on both ends. There are two conformers observed for this isomer: trans [Fig. 5b; ASUHIF (Rafizadeh et al., 2004), CATCAC (Rafizadeh et al., 2005) and SAZNEN (Rafizadeh & Amani, 2006)]
3.3. Two isomers of H7O3+
Isomer 1 of H7O3+ (Fig. 4a) is formulated as a central hydronium ion donating via CAHB to two acceptor water molecules. This form is found in six structures [CBZSUL01 (Roziere & Williams, 1978; neutron), GIDZEZ (Deacon et al., 2007), HAZCAN (Zhi-Min Jin et al., 2005), SALSUL (Mootz & Fayos, 1970), SOJZOH (Stoyanov et al., 2008) and XUMQIF (Czado & Mu¨ller, 2002)], with average dimensions
Figure 8 Figure 7 (a) (H2O)(H3O+)(H2O)3 (BEXFEQ; Zhong et al., 2004) and (b) (H3O+)(H2O)4 (IYEPEH; Makha et al., 2004).
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(a) (H2O)(H3O+)[(H2O)2]2 (DIJZEB10; Merschenz-Quack & Mootz, 1990), (b) (H2O)2(H5O2+)(H2O)2 (DUWXAV; Liu et al., 2010), and (c) (H5O2+)(H2O)4 (IHEJOU; Choi et al., 2002). Acta Cryst. (2014). C70, 566–574
research papers 3.5. Two isomers of H11O5+
All O O distances in the two isomers of H11O5+ are as expected for charge-assisted and normal hydrogen bonds: BEXFEQ (Fig. 7a; Zhong et al., 2004) and IYEPEH (Fig. 7b; Makha et al., 2004). 3.6. Three isomers of H13O6+
Figure 9 H14O62+ (XEMMEI; Wu et al., 2006).
and cis [Fig. 5c; GETLUN (Chekhlov, 2006) and ZATLAJ01 (Anderson et al., 2011)]. The average O O distances show the expected end-to-end progression of hydrogen bonds: ˚ ] to MHB [2.43 (1) A ˚ ] to CAHB CAHB [2.59 (5) A ˚ ]. [2.60 (4) A Isomer 3 of H9O4+ features a central hydronium ion and three acceptor water molecules. It is found in DIJZEB10 (Fig. 6; Merschenz-Quack & Mootz, 1990), and also in IZAMIG (2; Abramov et al., 2010), JEDYAT (Matczak-Jon et al., 2006), JODJIV (Cole et al., 1991), SOJZEX (Stoyanov et al., 2008), and WEWTUN and WEWVEZ (Cole et al., 1994). ˚. The O O (CAHB) distances average 2.54 (6) A
Isomer 1 of H13O6+ (DIJZEB10; Fig. 8a; Merschenz-Quack & Mootz, 1990), features a central hydronium ion bound to three hydroxonanes (one water monomer and two water dimers) via CAHB. The water dimers display normal hydrogen bonds. Isomers 2 and 3 of H13O6+ [DUWXAV (Fig. 8b; Liu et al., 2010) and IHEJOU (Fig. 8c; Choi et al., 2002)] feature central Zundel ions. The O O distances are as expected for MHB, CAHB and NHB. 3.7. H14O62+
This unique six-membered hydroxonium ring consists of two hydronium ions and four water molecules (Fig. 9; XEMMEI; Wu et al., 2006). 3.8. Two isomers of H18O82+
Isomer 1 is a ring with two Zundel cations and four water molecules (Fig. 10a; FEGTEQ; Hanson, 1987), while isomer 2 consists of two extra annular Zundel cations attached to a ring of four water molecules (Fig. 10b; WIDTEJ; Hubregtse et al., 2007). 3.9. H22O102+
This supramolecular hydroxonium ion (Fig. 11; LOHJOH; Antsyshkina et al., 2000) features two extra-annular water molecules connected to an eight-membered ring consisting of two hydronium cations and six water molecules.
Figure 10 (a) An isomer of H18O82+ (FEGTEQ; Hanson, 1987) and (b) an isomer of H18O82+ (WIDTEJ; Hubregtse et al., 2007). Acta Cryst. (2014). C70, 566–574
Figure 11 H22O102+ (LOHJOH; Antsyshkina et al., 2000). Bernal and Watkins
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Figure 14 (H2O)(OH).
H3O2 in which the hydroxide ion accepts a charge-assisted hydrogen bond from a water molecule (Fig. 14). The average ˚ and = 0.7 (2). dimensions are O O (CAHB) = 2.59 (2) A 4.2. Polymeric forms
Figs. 15 through 19 show various polymeric forms in which hydroxide ions accept either three or four charge-assisted hydrogen bonds from water molecules while forming ribbons and sheets separated by alkylammonium cations.
5. Conclusions Figure 12 The repeating motif [(H5O2+)(H2O)4]n (RABCOI; Dahlems et al., 1996).
3.10. Polymeric H13O6+
The crystal of RABCOI (Fig. 12; Dahlems et al., 1996) contains a three-dimensional polymeric network in which Zundel cations (green O atoms) link successive two-dimensional sheets of fused hexameric rings of water molecules (red O atoms).
In the final stages of crystal structure analysis, it is important to detect and report all existing hydrogen-bonded networks, especially for crystalline hydrates, in order to elucidate and formulate correctly existing hydroxone supramolecular aggregates, but also to avoid mistakes in the placement of H atoms within these aggregates. Thus, if the distance between ˚ , an H atom must contiguous O atoms is less than about 3 A
4. Catalog of hydroxonate anions Only 37 of the 245 crystal structures we have analyzed were reported to contain hydroxonate anions. Of these, we suspect that 16 may misrepresent the supramolecular architecture, most often due to misplaced or missing H atoms (Bernal & Watkins, 2013). Of the 21 remaining crystal structure reports of hydroxonate species, 13 of these (Appendix C) contain hydroxide ions isolated from other hydroxone species. 4.1. Molecular and supramolecular H2O2
We were unable to find in the CSD any credible observations of molecular anion H3O2. The geometry-optimized model (Tables 1 and 3, and Fig. 13) has molecular symmetry C2 ˚ and = 0. with O O (MHB) = 2.414 A There are two reports [DAPSCR10 (Palenik et al., 1976) and EXOQIR (Fox & Bergman, 2004)] of supramolecular
Figure 15 [(OH)(H2O)3]n in SEYLAJ (Mootz & Seidel, 1990).
Figure 16 Figure 13 Geometry-optimized model of molecular ion H3O2.
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Ribbons of [(OH)(H2O)4]n in PAZDOJ (Staben et al., 1998) and SEYLEN01 (Mootz & Seidel, 1990). Acta Cryst. (2014). C70, 566–574
research papers ˚ (MHB), probably H2O. If the O O distance is about 2.4 A then the bonded pair represents one of two isoelectronic molecular ions H5O2+ or H3O2. In all cases, the crystallographer should scrutinize the difference Fourier synthesis in the vicinity of contiguous oxygen pairs, keeping in mind the possibility of disorder, to place the correct number of H atoms in the appropriate geometry. Furthermore, differentiation and identification of neutral and ionic hydroxone species is essential to maintain charge balance compatible with the overall formulation of the unit-cell contents.
Figure 17 Double-width ribbons of [(OH)(H2O)4]n in MATLEY (Wiebcke & Felsche, 2000a).
APPENDIX A The following 69 CSD refcodes refer to credible X-ray (or neutron where noted) crystal structures which contain H3O+ ions unattached to other hydroxone species: AFOPEQ, APOGIW, AWORIO, AZAHUE, BOBZAT, BOQYAI, BOTHEY01, BOVJIG, DAPVAT, DAYBEL, DAYJUJ, DEKCAX, DEVMUM, DEWLOG01, DULYEP, DUMKIF, DUPJUT01, ECOZUS, EGOHOY, ESUDAX02, ETHDPH05, FITTOS, GEPGUE, GIQXEJ, GOCJIR, HACWIR, HAGSUE, HAJZAV, HAYZEN, HOEDSO01, HUQDEC, IQAJIU, IZAFEV, IZIQIR, JAGLOS, JEHPAN, JEJLOY, KAPKOC, KARTII, MUYHEU, NUWMUN, OXACDH38, PAHHAJ, QEGLAP, QEQNOP, QONNAJ, RAHNEV, RASYOZ, RIDLUM, SOPNIU01, SSALAD03, TAKGUJ, TANKUQ, TAYPUE, TFMSUL02 (neutron), TFSLAC, TOLSAM01 (neutron), TOLSAM12 (neutron), TPPOBR01, UKESOT, VARKOO, VAZVIB, VIKHON, XANZUJ, XIRSOH01, XIRSUN, YELPAG, YEPVIY, YOZRIP.
Figure 18 [(OH)(H2O)4]n in QIJDES (Wiebcke & Felsche, 2001).
APPENDIX B
Figure 19 [(OH)(H2O)4]n in WOBSAH (Wiebcke & Felsche, 2000b).
exist close to the O O internuclear line. If the O O ˚ (NHB), a reasonable distance is in the range 2.80.2 A working hypothesis is that the two hydrogen-bonded species ˚ are neutral (H2O). If the O O distance is about 2.5 A (CAHB), then one of the bonded species is probably a molecular ion (H3O+, H5O2+, OH, or H3O2) and the other is Acta Cryst. (2014). C70, 566–574
The following 47 CSD refcodes refer to credible X-ray (or neutron where noted) crystal structures which contain Zundel ions [molecular H5O2+ or supramolecular (H3O+)(H2O)] unattached to other hydroxone species: ACAYIM, APANIP, BOVMOP, BOVMUV, CIKLOY, CIKYIF, COLNUM01 (neutron), GARHIR, GOKVUX, HAHFOM, HUHVIP, KEZHIG, KEZHOM, LOHQOP, LUYWUX, NANNOF, NASFIX, NAVWUC, NINDOD01 (neutron), NIZWAU, OZEDIH, QAFMUF, QUQNUL, RAFZEF, RAGWAZ, REVCEB02, REVCEB03, SAJJET, SIHBIV01, SIHBIV05, SODFOG, SOGZEU, SUGKAG, TFMSAD, TFMSAD01, UDADUY, VEXMIU, VIVQUN, VOMFEJ, VOPNAP, VOPNAP02, WASJAD01, WETQOC, XEHMED, YEKLIJ, YOXALT03 (neutron), ZACMEV.
APPENDIX C The following 13 CSD refcodes refer to credible X-ray crystal structures which are reported to contain hydroxide ions unattached to other hydroxone species: AYACEJ, AZEKOF, CEXHAO, DUYCOQ, HIHFAF, HUDPOM, IBAMAB, Bernal and Watkins
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research papers KEKQAS, PYCXMN01, QIXPOD, SUJHOV, TEHFOD, XEHWEN. We thank Professor Lou Massa (Hunter College) for helpful comments concerning the theoretical aspects of these investigations. Supporting information for this paper is available from the IUCr electronic archives (Reference: EG3156) and includes literature citations for all 245 cationic and anionic hydroxone structures found during our search of the CSD up to the most recent updates of Version 1.15 (ca March 2014).
References Abramov, P. A., Aldoshin, S. A., Peresypkina, E. V., Sokolov, M. N. & Fedin, V. P. (2010). Zh. Strukt. Khim. 51, 731–736. Allen, F. H. (2002). Acta Cryst. B58, 380–388. Anderson, K. M., Goeta, A. E., Martin, J. E., Mason, S. A., McIntyre, G. J., Sansam, B. C. R., Wilkinson, C. & Steed, J. W. (2011). Cryst. Growth Des. 11, 4904–4919. Antsyshkina, A. S., Sadikov, G. G., Poznyak, A. L. & Sergienko, V. S. (2000). Zh. Neorg. Khim. (Russ. J. Inorg. Chem.), 45, 65–70. Attar Gharamaleki, J., Aghabozorg, H., Derikvand, Z. & Yousefi, M. (2009). Acta Cryst. E65, m824–m825. Attig, R. & Mootz, D. (1976). Acta Cryst. B32, 435–439. Bernal, I. (2006). C. R. Chim. 9, 1454–1466. Bernal, I. (2007). C. R. Chim. 10, 1209–1215. Bernal, I. & Watkins, S. F. (2013). Acta Cryst. C69, 808–810. Bernal, I. & Watkins, S. F. (2014). C. R. Chim. Online. http://dx.doi.org/10.1016/ j.crci.2013.10.001. Brandenburg, K. & Putz, H. (2004). DIAMOND. University of Bonn, Germany. Brown, T. L., LeMay, H. E. Jr, Bursten, B. E., Murphy, C. J. & Woodward, P. M. (2012). Chemistry, the Central Science, 12th ed. Upper Saddle River, New Jersey: Prentice Hall. Calleja, M., Mason, S. A., Prince, P. D., Steed, J. W. & Wilkinson, C. (2001). New J. Chem. 25, 1475–1478. Chekhlov, A. N. (2006). Zh. Neorg. Khim. 51, 621–625. Choi, J.-H., Suzuki, T. & Kaizaki, S. (2002). Acta Cryst. C58, m539–m541. Cole, E., Copley, R. C. B., Howard, J. A. K., Parker, D., Ferguson, G., Gallagher, J. F., Kaitner, B., Harrison, A. & Royle, L. (1994). J. Chem. Soc. Dalton Trans. pp. 1619-1629. Cole, E., Parker, D., Ferguson, G., Gallagher, J. F. & Kaitner, B. (1991). Chem. Commun. pp. 1473-1475. Czado, W. & Mu¨ller, U. (2002). Z. Kristallogr. New Cryst. Struct. 217, 339– 340.
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Dahlems, T., Mootz, D. & Schilling, M. (1996). Z. Naturforsch. Teil B, 51, 536– 544. Deacon, G. B., Harika, R., Junk, P. C., Skelton, B. W. & White, A. H. (2007). New J. Chem. 31, 634–645. Fox, D. J. & Bergman, R. G. (2004). Organometallics, 23, 1656–1670. Frisch, M. J., et al. (2009). GAUSSIAN09. Gaussian Inc., Wallingford, CT, USA. Gilli, P., Bertolasi, V., Ferretti, V. & Gilli, G. (1994). J. Am. Chem. Soc. 116, 909–915. Hanson, A. W. (1987). Acta Cryst. C43, 296–300. Hubregtse, T., Kooijman, H., Spek, A. L., Maschmeyer, T., Sheldon, R. A., Arends, I. W. C. E. & Hanefeld, U. (2007). J. Inorg. Biochem. 101, 900–908. Jin, Z.-M., He, L., Chen, L.-Z., Li, L.-S., Lai, Z.-F. & Li, L. (2005). Acta Cryst. E61, o4325–o4327. Liu, J., Hu, Y., Lin, R., Sun, W., Liu, X. & Yao, W. (2010). J. Coord. Chem. 63, 1369–1378. Macrae, C. F., Edgington, P. R., McCabe, P., Pidcock, E., Shields, G. P., Taylor, R., Towler, M. & van de Streek, J. (2006). J. Appl. Cryst. 39, 453–457. Makha, M., Raston, C. L., Sobolev, A. N. & White, A. H. (2004). Chem. Commun. pp. 1066–1067. Matczak-Jon, E., Slepokura, K. & Kafarski, P. (2006). J. Mol. Struct. 782, 81– 93. Merschenz-Quack, A. & Mootz, D. (1990). Acta Cryst. C46, 1478–1480. Mootz, D. & Fayos, J. (1970). Acta Cryst. B26, 2046–2054. Mootz, D. & Seidel, R. (1990). J. Inclusion Phenom. Mol. Recognit. Chem. 8, 139–157. Palenik, G. J., Wester, D. W., Rychlewska, U. & Palenik, R. C. (1976). Inorg. Chem. 15, 1814–1819. Perrin, C. L. & Nielson, J. B. (1997). Annu. Rev. Phys. Chem. 48, 511–544. Pimentel, G. C. (1951). J. Chem. Phys., 19, 446–448. Rafizadeh, M. & Amani, V. (2006). Acta Cryst. E62, m90–m91. Rafizadeh, M., Ranjbar, M. & Amani, V. (2004). Acta Cryst. E60, m479–m481. Rafizadeh, M., Ranjbar, M. & Amani, V. (2005). Anal. Sci. X-ray Struct. Anal. Online, 21, x113–x114. Roziere, J. & Williams, J. M. (1978). J. Chem. Phys. 68, 2896–2901. Sergienko, V. S., Ilyukhin, A. B., Luzikova, A. V. & Porai-Koshits, M. A. (1991). Koord. Khim. 17, 1489–1496. Staben, D., Dahlems, T. & Mootz, D. (1998). Z. Kristallogr. New Cryst. Struct. 213, 199–200. Stasko, D. J., Perzynski, K. J. & Wasil, M. A. (2004). Chem. Commun. pp. 708– 709. Stoyanov, E. S., Stoyanova, I. V., Tham, F. S. & Reed, C. A. (2008). J. Am. Chem. Soc. 130, 12128–12138. Wiebcke, M. & Felsche, J. (2000a). Acta Cryst. C56, 901–902. Wiebcke, M. & Felsche, J. (2000b). Acta Cryst. C56, 1050–1052. Wiebcke, M. & Felsche, J. (2001). Acta Cryst. C57, 306–308. Wu, R., Zhang, T., Qiao, X., Zhang, J., Yang, L., Yu, W., Qiao, X. & Huaxue, J. (2006). Chin. J. Struct. Chem. 25, 849–853. Zhong, W., Parkinson, J. A., Parsons, S., Oswald, I. D. H., Coxall, R. A. & Sadler, P. J. (2004). Inorg. Chem. 43, 3561–3572.
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supplementary materials
supplementary materials
sup-1
Structural Chemistry ISSN 2053-2296
Molecular and supramolecular ionic aggregates HxOyz in organic and organometallic crystalline hydrates Ivan Bernala and Steven F. Watkinsb* a
Chemistry Department, Hunter College, City University of New York, New York, NY 10021, USA, and bChemistry Department, Louisiana State University, Baton Rouge, LA 70803, USA Correspondence e-mail: [email protected] Received 12 March 2014 Accepted 1 May 2014
Ionic aggregates of the form HxOyz (z 6¼ 0) have been characterized during an analysis of 245 crystal structures extracted from the Cambridge Structural Database [Allen (2002). Acta Cryst. B58, 380–388]. A systematic nomenclature is proposed for these species. Three modes of hydrogen bonding are described, characterized in part by the distance between contiguous O atoms: normal (NHB; O O = 2.6– ˚ ), charge assisted (CAHB; O O = 2.5 A ˚ ) and molecular 3.0 A ˚ (MHB; O O = 2.4 A). The three modes are consistent with previous reports, our experimental results, and quantumchemical-optimized geometries and energetics. No evidence is presented concerning the possible existence or stability of these aggregates in solution. Rather, emphasis is placed on the necessity in crystal structure analysis to develop thoroughly existing hydrogen-bonded networks, ignorance of which can lead to erroneous crystal structure models and other physicochemical data associated with composition and charge balance. Keywords: molecular ionic aggregates; supramolecular ionic aggregates; organometallic crystalline hydrates; organic crystalline hydrates; systematic nomenclature; hydrogenbonding modes; Cambridge Structural Database search; hydroxone; hydroxonium; hydroxonane; hydroxonate.
1. Introduction Aqueous acid–base equilibria involve ionic aggregates which are more complex than single hydronium and hydroxide ions. These molecular and supramolecular species, when captured in crystalline hydrates, can be characterized by X-ray and neutron diffraction (Bernal, 2006, 2007). Unfortunately, suspected mischaracterizations are found in some X-ray structural reports due to misplaced or misidentified H atoms (Bernal & Watkins, 2013). A common scenario, apparent in a number of publications, is as follows: two or more (hydrate) O atoms are originally
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‘solved’ in isolated positions in the unit cell, but the author fails to consider symmetry-equivalent atomic positions. Contiguity of these O atoms and the resulting hydrogen bonding is thus unrecognized, and the author considers an insufficient number of maxima in the Fourier difference synthesis surrounding and between these O atoms. Thus, maxima representing H atoms in correct locations are not considered. H-atom placement in a crystal structure analysis is greatly aided by the realization that there must be a hydrogen-bond path between two contiguous O atoms with an internuclear ˚ , similar to the geometric constraints distance of less than 3 A on alkyl and aromatic H atoms. This, together with constraints on hydrogen-bond geometry and hydrogen-bond type (see below), can be strong discriminators between Fourier maxima due to H-atom electron density and others due to ‘noise’ or lone pairs on electronegative atoms. Furthermore, missing or misplaced H atoms cannot be recovered or corrected by referees, editors, or readers without access to and model refinement with diffraction data. This paper describes molecular and supramolecular ionic aggregates HxOyz (z 6¼ 0) catalogued during the analysis of crystal structures of organic and organometallic hydrates (see Supporting information). Crystal structure files (CIFs) of these hydrates were extracted from the Cambridge Structural Database (CSD; Allen, 2002; Version 1.15, updated to 2 February 2014), using the following search criteria: (i) substructure search for H3O+ or HO; (ii) title search for ‘hydronium’, ‘oxonium’ or ‘hydroxide’; (iii) R factor 7.0%; (iv) no disorder; (v) no powder diffraction; (vi) no structures containing atoms labeled ‘H?’ or ‘O?’; (vii) no structures with warning comments (e.g. ‘short intermolecular hydrogen contacts of the oxonium cations’) which indicate problems with the refined model; (viii) for structures determined multiple times, select lowest R factor and/or lowest temperature and/or neutron probe over X-ray probe. Each CIF in the resulting subset was then inspected visually by both authors using the molecular graphics programs Mercury (Macrae et al., 2006) and DIAMOND (Brandenburg & Putz, 2004), and was rejected if the H-atom count differed from the reported moiety formulas, or if no H atom was located near a contiguous O O bond path (O—H O angles less than about 120o). The resulting 245 CIFs were accepted as the working set. Finally, a further 76 X-ray structures were withheld from the working set for suspected H-atom misplacement; that is, using the contiguous O O distance as a diagnostic tool, we believe the reported species and/or their hydrogen-bond geometries are inconsistent with the three hydrogen-bond types described herein, and therefore the reported structures may be inaccurate. The remaining 169 CIFs were accepted as credible reports of HxOyz architectures.
doi:10.1107/S2053229614009826
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research papers In order to discuss HxOyz species further, we will use the following systematic nomenclature. For more than two centuries, the suffix ‘–ium’ has been used almost universally to identify metals and their cations, while suffix ’–ate’ is widely used for oxyanions. In addition, suffix ‘–ane’ identifies classes of neutral organic and inorganic compounds (e.g. alkanes, siloxanes). The relatively recent but ambiguous term ‘oxonium’ refers not only to H3O+, but to any trivalent oxycation R3O+. With the foregoing in mind, we propose:
Table 1 Estimated eigenvalues of the geometry-optimized system Hamiltonian. HxFyz
E (a.u.)
HxOyz
E (a.u.)
HF F (HF)2 FHF F2 H2 2 F 2 H H
100.48485750 99.88869321 200.97983857 200.45018330 199.58111621 1.18003403 99.76168096 0.50225698 0.53416365
H2O HO (H2O)2 H5O2+ H3O2 O2 3 O H3O+ H5O3 (H3O)3 H3O3+
76.46451156 75.83092703 152.93672331 153.25665439 152.34087192 150.37948850 75.09091483 76.73680050 228.84090051 229.40704300 229.75965981
hydroxone: the class of molecular and supramolecular species HxOyz which subsumes the following three subclasses: hydroxonium: molecular and supramolecular cations H2n+1On+, H2n+2On2+ etc. Excluding chemically inaccessible ‘H+’, the first two members of this subclass are molecular species H3O+ (hydronium) and H5O2+ (Zundel cation); hydroxonane: neutral molecular and supramolecular aggregates with formula H2nOn. Higher-order members (n > 1) consist of hydrogen-bonded aggregates (H2O)n bound by normal hydrogen bonds; hydroxonate: molecular and supramolecular anions H2n–1On, H2n–2On2 etc. The first two members of this subclass are molecular species HO (hydroxide) and H3O2. All 245 CIFs of the working set were modified to eliminate all atoms except those of the hydroxones; the remaining hydroxone networks, developed in 222 packing diagrams, yielded 430 unique contiguous O O distances (Fig. 1). It is important to note that hydroxone aggregates generally hydrogen bond to other molecules and ions in the crystal (e.g. carboxylates, sulfates, phosphates, nitrates etc); no contiguous O O distances from those interactions are included in Fig. 1. This distribution, and our observations described below, are consistent with earlier suggestions that there are three types of hydrogen bonds (e.g. Gilli et al., 1994; Perrin & Nielson, 1997):
MHB: molecular hydrogen bonds narrowly distributed ˚; about O O = 2.4 A CAHB: charge-assisted hydrogen bonds more broadly ˚; distributed about O O = 2.5 A NHB: normal hydrogen bonds very broadly distributed ˚. about O O = 2.8 A We contend that contiguous O O distances are necessary and diagnostic metrics for correct supramolecular formulation, and that failure to characterize an existing hydrogenbond network can lead to errors in structural analysis. For example, thermodynamic considerations notwithstanding, cationic and anionic hydrate species have been reported to coexist in the same lattice, even as fragments in the same supramolecular aggregate. Our analysis leads us to believe that no credible evidence for such pairings exists.
2. Theoretical basis and previous results
A previous paper (Bernal & Watkins, 2014) described supramolecular halide aggregates associated with fluoride, bifluoride (FHF), chloride and bichloride anions. Since HF and H2O are isoelectronic (as are F/HO and FHF/H5O2+/H4O2/H3O2), it is not surprising that there are structural similarities between hydrofluoride and hydroxone species. In an effort to understand these similarities, we have explored the electronic properties of a number of H—F and H—O species using the ab initio quantum chemical program GAUSSIAN09 (Frisch et al., 2009). In all theoretical models mentioned in this report, singlepoint and geometry-optimized total energies were calculated using density functional theory (DFT) (RB3LYP or UB3LYP) with basis set 6-311++g(3df,3pd). The results are shown in Table 1. Energies of some of the gas-phase reactions between species in Table 1 are also experimentally accessible and widely available in text books (e.g. Brown et al., Figure 1 2012). Comparison of calculated and Distribution of contiguous O O distances in crystalline HxOyz aggregates. Acta Cryst. (2014). C70, 566–574
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Table 4
Calculated and experimental reaction energies.
Statistics for geometric parameters observed in H3O+ species.
E†
Reaction HF(g) ! H(g) + F(g) F2(g) ! 2F(g) H2(g) ! 2H(g) H2O(g) ! 2H(g) + O(g) O2(g) ! 2O(g) HF(g) ! 12F2(g) + 12H2(g) H2O(g) ! H2(g) + 12O2(g)
580/567 152/155 461/436 969/926 519/495 274/269 249/241
%error 2.3 1.9 5.7 4.6 4.8 1.9 2.9
E Defined
Observed
Bond Energy Bond Energy Bond Energy Bond Energy 2 Bond Energy ca Hf [HF(g)] ca Hf [H2O(g)]
† Calculated/experimental energy change (kJ mol1)
Table 3 Calculated reaction energies. No.
Reaction
E†
1 2 3 4 5 6 7 8
(HF)2(g) ! 2HF(g) (H2O)2(g) ! H2O(g) + H2O(g) H6O3(g) ! H2O(g) + (H2O)2(g) FHF(g) ! F(g) + HF(g) H5O2+(g) ! H2O(g) + H3O+(g) H3O2(g) ! H2O(g) + OH(g) H7O3+(g) ! H2O(g) + H5O2+(g) H5O3(g) ! H2O(g) + H3O2(g)
27 20 15 201 145 119 101 94
O—H H—O—H
Range
Mean ()
˚ 0.75–1.30 A 74–141
˚ 0.93 (11) A 110 (8)
(and therefore energies) for this ‘charge-assisted’ hydrogen bonding (CAHB) in which neutral hydroxonanes link to one of the four molecular ions as (H3O+)(H2O)n, (H5O2+)(H2O)n, (OH)(H2O)n or (H3O2)(H2O)n. The first three of these charged hydroxone species have all been observed and form the basis for this report, but we have been unable to find in the CSD any hydroxonates of the form (H3O2)(H2O)n. In the following diagrams of hydroxone aggregates, no hydrogen bonds are shown to surrounding species, such as amine or sulfate ligands. But these external hydrogen bonds do exist in most cases and must certainly influence the detailed geometry of the species displayed.
† Calculated energy change (kJ mol1)
3. Catalog of hydroxonium cations observed values (Table 2) reveals a satisfactory degree of accuracy. Note that the calculated energies are consistently higher than the experimental values, a systematic error which could probably be improved by using larger basis sets. The reactions in Table 3 are not easily accessible to experiment, but the calculated energy values should be of the same degree of accuracy as those in Table 2. Thus, reactions 1 through 3 feature ‘normal’ hydrogen bonding (NHB) within neutral supramolecular H—F and H—O assemblies. Reactions 4 through 6 feature three isoelectronic molecular ions, i.e. FHF, H5O2+ and H3O2, all of which contain linear symmetric divalent hydrogen. Bonding in symmetric ‘X—H— X’ is often portrayed as ‘three-center four-electron bonding’ (Pimentel, 1951), and the molecular orbital architecture is consistent with this characterization. However, we use the less mechanistic term ‘molecular’ hydrogen bonding (MHB) which is clearly an order of magnitude stronger than NHB, at least for X = F or O. Reactions 7 and 8 represent hydrogen bonding between neutral and ionic species, and the bond energies fall between those of normal hydrogen bonds and molecular hydrogen bonds. Fig. 1 suggests that there is a distribution of distances
Of the 245 crystal structures analyzed for this report, 208 are reported to contain hydroxonium species. However, on the basis of criteria developed during this analysis, 58 of these structures were suspected to have misplaced or missing H atoms (Bernal & Watkins, 2013). Presented below are exam-
Figure 3 Figure 2 Geometry-optimized model of H3O+.
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(a) Geometry-optimized molecular ion H5O2+, (b) H5O2+ in SUGKAG (Sergienko et al., 1991) and (c) (H2O)(H3O+) in BOVMOP (Attar Gharamaleki et al., 2009). Acta Cryst. (2014). C70, 566–574
research papers Table 5 Molecular versus supramolecular forms of the Zundel ion. 33 with < 0.17
˚) H—O1 (A ˚) H—O2 (A ˚) O1 O2 (A
14 with > 0.23
Range
Mean ()
Range
Mean ()
1.10–1.25 1.18–1.33 2.36–2.48 0.00–0.17
1.20 (3) 1.24 (3) 2.42 (3) 0.03 (5)
0.85–1.16 1.35–1.84 2.43–2.74 0.23–0.74
0.98 (11) 1.54 (15) 2.51 (9) 0.44 (18)
ples of observed isomers of hydroxonium aggregates H3O+, H2O5+, H7O3+, H9O4+, H11O5+, H13O6+, H14O62+, H18O82+, H22O102+, and three-dimensional polymer (H13O6+)n.
bond acceptors. This seems to be the case, for example, in crystal structure SUGKAG (Fig. 3b; Sergienko et al., 1991), ˚ . On the other hand, if an asymmetric with O O = 2.36 A force field surrounds the cation, and/or its exterior H atoms form strong hydrogen bonds to surrounding species, H5O2+ transitions from a molecular to a supramolecular aggregate in which a hydronium cation is bound by a charge-assisted hydrogen bond to a water molecule as in BOVMOP (Fig. 3c; Attar Gharamaleki et al., 2009). We have found no unambiguous measure to distinguish between the molecular and supramolecular forms of H5O2+, but one possible indicator may be the asymmetry of the central H atom, defined by: ¼
3.1. H3O+ (hydronium)
There are 66 credible X-ray and three neutron crystal structures (see Appendix A for CSD refcodes) which contain hydronium ions unbound to other hydroxone species. Fig. 2 shows the calculated geometry of the hydronium ion, while statistics for observed species are given in Table 4. Given the relative imprecision with which H-atom positions are determined in X-ray analysis, together with the range of distorting forces in individual lattices, agreement between the average observed and calculated geometries of H3O+ is remarkable.
2jdðHO1Þ dðHO2Þj : dðHO1Þ þ dðHO2Þ
This measure is imprecise in X-ray crystal structures due to its reliance on H-atom placement, and may be most useful in the extremes; for example, = 0 for SUGKAG and = 0.74 for BOVMOP. Nevertheless, there does seem to be a natural break in our sample of 44 observed X-ray and three neutron structures, between = 0.17 and = 0.23, with O O distances
3.2. Molecular and supramolecular H5O2+ (Zundel cation)
The geometry-optimized model of H5O2+ (Tables 1 and 3, and Fig. 3a) has C2h symmetry and a symmetric divalent H atom forming a molecular hydrogen bond, with O O = ˚ (consistent with Fig. 1). The field-free environment of 2.414 A the calculated model can be approximated in a crystal if the surrounding force field is highly symmetric and/or if the exterior H atoms are not strongly bonded to other hydrogen-
Figure 5 Figure 4 (a) (H2O)(H3O+)(H2O) and (b) (H5O2+)(H2O). Acta Cryst. (2014). C70, 566–574
(a) (H2O)(H3O+)(H2O)2, (b) trans-(H2O)(H5O2+)(H2O) and (c) cis(H2O)(H5O2+)(H2O). Bernal and Watkins
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research papers ˚ and = 0.50 (10). Isomer 2 of H7O3+ O O = 2.47 (4) A (Fig. 4b) is formulated as a Zundel cation with CAHB to one water molecule. Two instances of this form [JOTQOY03 (Calleja et al., 2001; neutron) and SLBZAC10 (Attig & Mootz, ˚ 1976)] have average dimensions O O (MHB) = 2.42 (2) A ˚ and = 0.16 (5), and O O (CAHB) = 2.64 (9) A and = 0.53 (9). 3.4. Three isomers and two conformers of H9O4+ (Eigen cation)
Figure 6 (H3O+)(H2O)3.
˚ (CAHB), respeccentered at 2.42 (3) (MHB) and 2.51 (9) A tively, consistent with the observed distribution of hydrogen bonds (Fig. 1). Statistics are shown in Table 5, and specific refcodes are listed in Appendix B.
Isomer 1 of H9O4+ is observed in crystal structure IVAYIN (Fig. 5a; Stasko et al., 2004) and features a central hydronium ion. The three O O distances show the expected progression ˚ ) to NHB (2.77 A ˚ ). from CAHB (2.50 and 2.58 A + Isomer 2 of H9O4 is found in five crystal structures, and contains a central Zundel cation with water molecules on both ends. There are two conformers observed for this isomer: trans [Fig. 5b; ASUHIF (Rafizadeh et al., 2004), CATCAC (Rafizadeh et al., 2005) and SAZNEN (Rafizadeh & Amani, 2006)]
3.3. Two isomers of H7O3+
Isomer 1 of H7O3+ (Fig. 4a) is formulated as a central hydronium ion donating via CAHB to two acceptor water molecules. This form is found in six structures [CBZSUL01 (Roziere & Williams, 1978; neutron), GIDZEZ (Deacon et al., 2007), HAZCAN (Zhi-Min Jin et al., 2005), SALSUL (Mootz & Fayos, 1970), SOJZOH (Stoyanov et al., 2008) and XUMQIF (Czado & Mu¨ller, 2002)], with average dimensions
Figure 8 Figure 7 (a) (H2O)(H3O+)(H2O)3 (BEXFEQ; Zhong et al., 2004) and (b) (H3O+)(H2O)4 (IYEPEH; Makha et al., 2004).
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(a) (H2O)(H3O+)[(H2O)2]2 (DIJZEB10; Merschenz-Quack & Mootz, 1990), (b) (H2O)2(H5O2+)(H2O)2 (DUWXAV; Liu et al., 2010), and (c) (H5O2+)(H2O)4 (IHEJOU; Choi et al., 2002). Acta Cryst. (2014). C70, 566–574
research papers 3.5. Two isomers of H11O5+
All O O distances in the two isomers of H11O5+ are as expected for charge-assisted and normal hydrogen bonds: BEXFEQ (Fig. 7a; Zhong et al., 2004) and IYEPEH (Fig. 7b; Makha et al., 2004). 3.6. Three isomers of H13O6+
Figure 9 H14O62+ (XEMMEI; Wu et al., 2006).
and cis [Fig. 5c; GETLUN (Chekhlov, 2006) and ZATLAJ01 (Anderson et al., 2011)]. The average O O distances show the expected end-to-end progression of hydrogen bonds: ˚ ] to MHB [2.43 (1) A ˚ ] to CAHB CAHB [2.59 (5) A ˚ ]. [2.60 (4) A Isomer 3 of H9O4+ features a central hydronium ion and three acceptor water molecules. It is found in DIJZEB10 (Fig. 6; Merschenz-Quack & Mootz, 1990), and also in IZAMIG (2; Abramov et al., 2010), JEDYAT (Matczak-Jon et al., 2006), JODJIV (Cole et al., 1991), SOJZEX (Stoyanov et al., 2008), and WEWTUN and WEWVEZ (Cole et al., 1994). ˚. The O O (CAHB) distances average 2.54 (6) A
Isomer 1 of H13O6+ (DIJZEB10; Fig. 8a; Merschenz-Quack & Mootz, 1990), features a central hydronium ion bound to three hydroxonanes (one water monomer and two water dimers) via CAHB. The water dimers display normal hydrogen bonds. Isomers 2 and 3 of H13O6+ [DUWXAV (Fig. 8b; Liu et al., 2010) and IHEJOU (Fig. 8c; Choi et al., 2002)] feature central Zundel ions. The O O distances are as expected for MHB, CAHB and NHB. 3.7. H14O62+
This unique six-membered hydroxonium ring consists of two hydronium ions and four water molecules (Fig. 9; XEMMEI; Wu et al., 2006). 3.8. Two isomers of H18O82+
Isomer 1 is a ring with two Zundel cations and four water molecules (Fig. 10a; FEGTEQ; Hanson, 1987), while isomer 2 consists of two extra annular Zundel cations attached to a ring of four water molecules (Fig. 10b; WIDTEJ; Hubregtse et al., 2007). 3.9. H22O102+
This supramolecular hydroxonium ion (Fig. 11; LOHJOH; Antsyshkina et al., 2000) features two extra-annular water molecules connected to an eight-membered ring consisting of two hydronium cations and six water molecules.
Figure 10 (a) An isomer of H18O82+ (FEGTEQ; Hanson, 1987) and (b) an isomer of H18O82+ (WIDTEJ; Hubregtse et al., 2007). Acta Cryst. (2014). C70, 566–574
Figure 11 H22O102+ (LOHJOH; Antsyshkina et al., 2000). Bernal and Watkins
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Figure 14 (H2O)(OH).
H3O2 in which the hydroxide ion accepts a charge-assisted hydrogen bond from a water molecule (Fig. 14). The average ˚ and = 0.7 (2). dimensions are O O (CAHB) = 2.59 (2) A 4.2. Polymeric forms
Figs. 15 through 19 show various polymeric forms in which hydroxide ions accept either three or four charge-assisted hydrogen bonds from water molecules while forming ribbons and sheets separated by alkylammonium cations.
5. Conclusions Figure 12 The repeating motif [(H5O2+)(H2O)4]n (RABCOI; Dahlems et al., 1996).
3.10. Polymeric H13O6+
The crystal of RABCOI (Fig. 12; Dahlems et al., 1996) contains a three-dimensional polymeric network in which Zundel cations (green O atoms) link successive two-dimensional sheets of fused hexameric rings of water molecules (red O atoms).
In the final stages of crystal structure analysis, it is important to detect and report all existing hydrogen-bonded networks, especially for crystalline hydrates, in order to elucidate and formulate correctly existing hydroxone supramolecular aggregates, but also to avoid mistakes in the placement of H atoms within these aggregates. Thus, if the distance between ˚ , an H atom must contiguous O atoms is less than about 3 A
4. Catalog of hydroxonate anions Only 37 of the 245 crystal structures we have analyzed were reported to contain hydroxonate anions. Of these, we suspect that 16 may misrepresent the supramolecular architecture, most often due to misplaced or missing H atoms (Bernal & Watkins, 2013). Of the 21 remaining crystal structure reports of hydroxonate species, 13 of these (Appendix C) contain hydroxide ions isolated from other hydroxone species. 4.1. Molecular and supramolecular H2O2
We were unable to find in the CSD any credible observations of molecular anion H3O2. The geometry-optimized model (Tables 1 and 3, and Fig. 13) has molecular symmetry C2 ˚ and = 0. with O O (MHB) = 2.414 A There are two reports [DAPSCR10 (Palenik et al., 1976) and EXOQIR (Fox & Bergman, 2004)] of supramolecular
Figure 15 [(OH)(H2O)3]n in SEYLAJ (Mootz & Seidel, 1990).
Figure 16 Figure 13 Geometry-optimized model of molecular ion H3O2.
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Ribbons of [(OH)(H2O)4]n in PAZDOJ (Staben et al., 1998) and SEYLEN01 (Mootz & Seidel, 1990). Acta Cryst. (2014). C70, 566–574
research papers ˚ (MHB), probably H2O. If the O O distance is about 2.4 A then the bonded pair represents one of two isoelectronic molecular ions H5O2+ or H3O2. In all cases, the crystallographer should scrutinize the difference Fourier synthesis in the vicinity of contiguous oxygen pairs, keeping in mind the possibility of disorder, to place the correct number of H atoms in the appropriate geometry. Furthermore, differentiation and identification of neutral and ionic hydroxone species is essential to maintain charge balance compatible with the overall formulation of the unit-cell contents.
Figure 17 Double-width ribbons of [(OH)(H2O)4]n in MATLEY (Wiebcke & Felsche, 2000a).
APPENDIX A The following 69 CSD refcodes refer to credible X-ray (or neutron where noted) crystal structures which contain H3O+ ions unattached to other hydroxone species: AFOPEQ, APOGIW, AWORIO, AZAHUE, BOBZAT, BOQYAI, BOTHEY01, BOVJIG, DAPVAT, DAYBEL, DAYJUJ, DEKCAX, DEVMUM, DEWLOG01, DULYEP, DUMKIF, DUPJUT01, ECOZUS, EGOHOY, ESUDAX02, ETHDPH05, FITTOS, GEPGUE, GIQXEJ, GOCJIR, HACWIR, HAGSUE, HAJZAV, HAYZEN, HOEDSO01, HUQDEC, IQAJIU, IZAFEV, IZIQIR, JAGLOS, JEHPAN, JEJLOY, KAPKOC, KARTII, MUYHEU, NUWMUN, OXACDH38, PAHHAJ, QEGLAP, QEQNOP, QONNAJ, RAHNEV, RASYOZ, RIDLUM, SOPNIU01, SSALAD03, TAKGUJ, TANKUQ, TAYPUE, TFMSUL02 (neutron), TFSLAC, TOLSAM01 (neutron), TOLSAM12 (neutron), TPPOBR01, UKESOT, VARKOO, VAZVIB, VIKHON, XANZUJ, XIRSOH01, XIRSUN, YELPAG, YEPVIY, YOZRIP.
Figure 18 [(OH)(H2O)4]n in QIJDES (Wiebcke & Felsche, 2001).
APPENDIX B
Figure 19 [(OH)(H2O)4]n in WOBSAH (Wiebcke & Felsche, 2000b).
exist close to the O O internuclear line. If the O O ˚ (NHB), a reasonable distance is in the range 2.80.2 A working hypothesis is that the two hydrogen-bonded species ˚ are neutral (H2O). If the O O distance is about 2.5 A (CAHB), then one of the bonded species is probably a molecular ion (H3O+, H5O2+, OH, or H3O2) and the other is Acta Cryst. (2014). C70, 566–574
The following 47 CSD refcodes refer to credible X-ray (or neutron where noted) crystal structures which contain Zundel ions [molecular H5O2+ or supramolecular (H3O+)(H2O)] unattached to other hydroxone species: ACAYIM, APANIP, BOVMOP, BOVMUV, CIKLOY, CIKYIF, COLNUM01 (neutron), GARHIR, GOKVUX, HAHFOM, HUHVIP, KEZHIG, KEZHOM, LOHQOP, LUYWUX, NANNOF, NASFIX, NAVWUC, NINDOD01 (neutron), NIZWAU, OZEDIH, QAFMUF, QUQNUL, RAFZEF, RAGWAZ, REVCEB02, REVCEB03, SAJJET, SIHBIV01, SIHBIV05, SODFOG, SOGZEU, SUGKAG, TFMSAD, TFMSAD01, UDADUY, VEXMIU, VIVQUN, VOMFEJ, VOPNAP, VOPNAP02, WASJAD01, WETQOC, XEHMED, YEKLIJ, YOXALT03 (neutron), ZACMEV.
APPENDIX C The following 13 CSD refcodes refer to credible X-ray crystal structures which are reported to contain hydroxide ions unattached to other hydroxone species: AYACEJ, AZEKOF, CEXHAO, DUYCOQ, HIHFAF, HUDPOM, IBAMAB, Bernal and Watkins
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research papers KEKQAS, PYCXMN01, QIXPOD, SUJHOV, TEHFOD, XEHWEN. We thank Professor Lou Massa (Hunter College) for helpful comments concerning the theoretical aspects of these investigations. Supporting information for this paper is available from the IUCr electronic archives (Reference: EG3156) and includes literature citations for all 245 cationic and anionic hydroxone structures found during our search of the CSD up to the most recent updates of Version 1.15 (ca March 2014).
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