C—H⋯O contacts in the crystal structure of 1,3-dithiane 1,1,3,3-tetraoxide

In the title compound, the molecules stack parallel with the a axis with multiple H⋯O contacts involving the axial H and O atoms. Many more H⋯O contacts between the stacks exist, which mostly involve the equatorial hydrogen and oxygen atoms. The highly polarized hydrogen atoms of the –SO2—CH2—SO2– moiety make no exceptionally short H⋯O contacts but clearly play a leading role in the formation of the stacks.

The crystal structure of 1,3-dithiane 1,1,3,3-tetraoxide, C 4 H 8 O 4 S 2 , has been determined to examine the intermolecular C-HÁ Á ÁO hydrogen bonds in a small molecule with highly polarized hydrogen atoms. The crystals are monoclinic, space group Pn, with a = 4.9472 (5), b = 9.9021 (10), c = 7.1002 (7) Å and = 91.464 (3) with Z = 2. The molecules form two stacks parallel to the a axis with the molecules being one a translation distance from each other. This stacking involves axial hydrogen atoms on one molecule and the axial oxygen atoms on the adjacent molecule in the stack. None of these C-HÁ Á ÁO contacts is particularly short (all are > 2.4 Å ). The many C-HÁ Á ÁO contacts between the two stacks involve at least one equatorial hydrogen or oxygen atom. Again, no unusually short contacts are found. The whole crystal structure basically consists of a complex network of C-HÁ Á ÁO contacts with no single, linear C-HÁ Á ÁO contacts, only contacts that involve two (bifurcated), and mostly three or four neighbors.

Chemical context
This is the second in a series looking at the C-HÁ Á ÁO contacts in cyclic organic structures containing multiple sulfone groups [see Harlow et al. (2019) for the first structure in the series: 1,4dithiane 1,1,4,4-tetraoxide]. Any methylene group adjacent to a sulfone has polarized hydrogen atoms. Methylene groups bonded to two sulfones are polarized to such an extent that they may form a C-HÁ Á ÁX (X = N, O) hydrogen bond (Harlow et al., 1984) as illustrated.
The structure of the unsubstituted 1,3-disulfone, however, has not been previously reported and was of interest particularly because of its high melting point (583 K) and decomposition temperatures (ca 623 K), which are suggestive of potentially strong C-HÁ Á ÁO contacts. The 1 H NMR spectrum of the compound dissolved in DMSO shows a singlet, 2H, at = 5.238 ppm (highly polarized hydrogen atoms on C2 between the two SO 2 groups); a triplet, 4H, at 3.370 ppm (moderately polarized hydrogen atoms on C4 and C6 with one adjacent SO 2 group); and a pentet, 2H, at 2.260 ppm (relatively unpolarized hydrogen atoms on C5). See Li & Sammes (1983) ISSN 2056-9890 for further details of 1 H NMR spectra and hydrogen-atom polarity in disulfones. Fig. 1 is an ORTEP drawing of the 1,3-dithiane 1,1,3,3-tetraoxide molecule with atom labels using the suffix 'a 0 for axial and 'e 0 for equatorial atoms. The bond distances and angles are very similar to those reported for the 1,4-disulfone taking into consideration the small amount of distortion related to the different positions of the two sulfonyl groups, i.e. 1,3-vs 1,4-sites in the ring.

Structural commentary
The hydrogen atoms were fully refined with isotropic displacement parameters as there seemed to be a correlation between the 'strength' of the C-H polarization and the displacement parameters of the hydrogen atoms: more polarization seems to yield a smaller radius. Any further comments on this correlation from a structural standpoint would require a diffraction study using neutrons instead of X-rays to better define both the positions and the displacement parameters of the hydrogen atoms.

Supramolecular features
Obviously, it is the packing of the molecules that is especially fascinating given that the molecules stack parallel to the a axis at a distance of one translation on a, i.e. the length of the a axis, ca 4.9472 (5) Å at 120 K. A general packing diagram is shown in Fig. 2. The n-glide, the only symmetry operator in space group Pn, curiously preserves the polarity of the stacks and creates a polar crystal with, for example, all of the axial oxygen atoms in the stacks pointing in the same a-axis direction (up in Fig. 3) and most of the axial hydrogen atoms pointing down. The stacking is directly stabilized by C-HÁ Á ÁO contacts between neighboring molecules in the stack and only involves the axial oxygen and hydrogen atoms (see Fig. 4 for the details). In Table 1, these axial C-HÁ Á ÁO contacts are designated with symmetry 'i 0 .
In addition, there are multiple OÁ Á ÁH contacts between the stacks, all of which involve at least one equatorial atom. Some Packing diagram showing the stacking of the molecules parallel to the a axis.

Figure 1
Labeled ORTEP drawing (50% probability) of the 1,3-dithiane 1,1,3,3tetraoxide molecule. The lower case suffixes 'a 0 and 'e 0 are used to distinguish whether the atoms are in the axial or equatorial position on the ring.

Figure 3
Packing diagram rotated to a view approximately parallel to the b axis to show that the stacks created by the n-glide are displaced by x + 1 2 . The figure also shows that both stacks are crystallographically polar with, for example, all the axial oxygen atoms pointing upward and most of the axial hydrogen atoms pointing downward except those on C5. of these also serve to bridge adjacent molecules within a stack further cementing the molecules in the stack. Examination of the OÁ Á ÁH contacts in Table 1 quickly shows that there are no short HÁ Á ÁO contacts (disappointing) but simply a plethora of contacts that hold the molecules in this crystal structure together. When the environment of each oxygen atom is surveyed in detail, it is found that they all interact with four hydrogen atoms, which generally form a distorted quadrilateral with OÁ Á ÁH contact distances that vary from 2.44 to 3.09 Å . This is very similar to what was found for the 1,4disulfone structure where example figures can be found (Harlow et al., 2019). The main difference is that the OÁ Á ÁH distances in the 1,4-disulfone structure were relatively uniform and, in this structure, they are not.
One mystery that remains is why a small molecule with mirror symmetry crystallizes in a non-centrosymmetric, polar space group?

Database survey
A Cambridge Crystallographic Database survey of the 1,3disulfone moiety reveals 22 structures with that motif (CSD version 5.41 + three updates; Groom et al., 2016). Four of the structures were authored by Harlow and Sammes and served as an impetus for the present study. A paper entitled in part 'Study of the Interaction of Silver(I) with -Disulfone in Aqueous Alkaline Media' was of particular interest because it suggested that metal salts could be made with our title compound, i.e. one of the hydrogen atoms on C2 was acidic Two adjacent molecules of a stack with the OÁ Á ÁH contacts detailed. The discrepancy in the H2aÁ Á ÁO bonds is caused by the molecules in the stacks being slightly tilted as evidenced by S1 and S3 not having the same x coordinate: 0.568 vs 0.546 (even though the molecules are related by a translation along a). This leads to a difference in the S1Á Á ÁS3 i and S3Á Á ÁS1 i distances, for example, of 5.646 vs 5.898 Å . The longer SÁ Á ÁS distance is associated with the longer H2aÁ Á ÁO distance. Table 1 Hydrogen-bond geometry (Å , ).  (5) 135 (3) Symmetry codes: (i) x À 1; y; z; (ii) x À 1 2 ; Ày þ 1; z À 1 2 ; (iii) x þ 1 2 ; Ày þ 1; z À 1 2 ; (iv) x À 1 2 ; Ày þ 1; z þ 1 2 ; (v) x þ 1 2 ; Ày þ 1; z þ 1 2 ; (vi) x À 1 2 ; Ày; z þ 1 2 ; (vii) x; y; z þ 1; (viii) x þ 1 2 ; Ày; z þ 1 2 . enough to be easily removed (DeMember et al., 1983) in a solution of KOH.

Synthesis and crystallization
To a stirred solution of 1,3-dithiane (1.05 g, 5.70 mmol, Alfa-Aeser) in glacial acetic acid (25 mL) was added a solution of 30% H 2 O 2 (10 mL) in glacial acetic acid (25 mL) and the mixture was heated to 323 K overnight. The white precipitate was separated on a Buchner funnel and washed with water (3 Â 25 mL) and dried in air. Colorless, rod-like crystals of the compound were harvested from an evaporated KOH (

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Non-hydrogen atoms were refined with anisotropic displacement parameters and all hydrogen atoms were located from a difference-Fourier map and refined freely. Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.