Crystal Structure and Solid-State Packing of 4-Chloro-5H-1,2,3-dithiazol-5-one and 4-Chloro-5H-1,2,3-dithiazole-5-thione

The crystal structure and solid-state packing of 4-chloro-5H-1,2,3-dithiazol-5-one and two polymorphs of 4-chloro-5H-1,2,3-dithiazole-5-thione were analyzed and compared to structural data of similar systems. These five-membered S,N-rich heterocycles are planar with considerable bond localization. All three structures demonstrate tight solid-state packing without voids which is attributed to a rich network of short intermolecular electrostatic contacts. These include Sδ+…Nδ−, Sδ+…Oδ−, Sδ+…Clδ− and Sδ+…Sδ− interactions that are well within the sum of their van der Waals radii (∑VDW). B3LYP, BLYP, M06, mPW1PW, PBE and MP2 were employed to calculate their intramolecular geometrical parameters, the Fukui condensed functions to probe their reactivity, the bond order, Bird Index and NICS(1) to establish their aromaticity.


Intramolecular Geometry
Crystals of the dithiazolone 2b were grown by sublimation under a static vacuum (1.6 Pa) at 30 • C. Dithiazolethione 2c demonstrated polymorphism: polymorphs 2c-α and 2c-β were obtained by slow evaporation of concentrated solutions in pentane and benzene, respectively. Suitable single crystals of dithiazoles 2b, 2c-α and 2c-β were then loaded on a goniometer and their crystal structure and solid-state packing were determined at 100 K by single-crystal X-ray diffractometry (Table S1 in Supplementary Information). Below, IUPAC numbering (not a crystallographic one) is used to assist the comparison between the new 5H-1,2,3-dithiazoles reported herein and those reported in the literature.
The C5-O bond length 1.208(2) Å in dithiazolone 2b is similar to that reported for dithiazolone 3 [37] and is typical of a C=O double bond (1.21 Å) [41]. The angles around C5 (Table 1) support a sp 2 -hybridized C of a carbonyl group [41]. The endocyclic C4-C5-S1 angle of 108.7(1) • is narrower and accounts for the five-membered ring strain. The other two C4-C5-O 126.6(2) • and S1-C5-O 124.7(1) • are wider with the one next to the Cl being slightly larger possibly due the steric interactions between the lone pairs of Cl and O atoms.

Crystal Packing and Short Contacts
1,2,3-Dithiazol-5-ones/thiones 2b and 2c-α crystal pack in the highly symmetrical Pbca space group with eight symmetry operators in operation, primarily a series of 2-fold screw axis and glide planes (Table S3 in Supplementary Information). The second polymorph of dithiazolethione 2c-β is of lower symmetry (P-1) with only two symmetry operators in effect (identity and inversion).
There is a rich network of structure-directing intermolecular interactions in the crystal packing of dithiazoles 2b, 2c-α and 2c-β (Figures 1-3). These mainly electrostatic interactions optimize contacts between electronegative and electropositive regions in neighboring molecules. Inside these five-membered rings the S-N and C-N bonds are polar due to the difference in electronegativity of their atoms; 2.58 for S and 3.04 for N and 2.55 for C. The S-N and C-N bonds should therefore be considered polarized in the sense of S δ+ . . . N δ− and C δ+ . . . N δ− . It is expected that the location of two electropositive S atoms next to each other will create a strong electropositive region near the S atoms ( Table 2). The presence of lone pairs on the N, Cl and on O and S atoms of the C=O carbonyl and C=S thione groups create pockets of electronegative regions. To better understand the electrostatic contribution to bonding we calculated the molecular electrostatic potential maps (MEP) for dithiazol-5-ones/thiones 2b and 2c at the B3LYP/def2-TZVPD level of theory (Table 2); red color corresponds to a maximum negative charge value of t, i.e., electronegative character, while blue color corresponds to a maximum positive charge value of 3.0 × 10 −2 esu, i.e., electropositive character.
The MEP for 2b and 2c are as expected blue near the endocyclic electropositive S atoms and red in the vicinity of N, Cl, O, S where the lone pairs of these atoms are located, and a build-up of partial negative charge is expected. Consequently, close intermolecular contacts between the endocyclic S and the rest of the electronegative atoms (N, Cl, O and exocyclic S) should be electrostatically favorable. It should be noted that the exocyclic S atom in 2c has areas that are red, i.e., negatively charged, where the lone pairs are expected to reside and an area in the center of the atom along the C=S bond axis that is green. Our calculations on Fukui functions (Section 2.3.2) predict an ambivalent chemical behavior which shows the thione S atom to be a site for both nucleophilic and electrophilic chemistry.  . Each carbon atom of the C-C bond has a significant partial positive charge stemming from the polarization of the C-Cl and C=O bonds due to the difference in electronegativity (C δ+ -Cl δ− , C δ+ =O δ− ). The location of the oxygen atom is, therefore, ideal to create a bifurcated set of C δ+ … O δ− contacts ( Figure S1a in Supplementary Information). These C … O contacts are within the VDW [3.25 Å (minor), 3.67 Å (major)] [44].
The network of intermolecular contacts for dithiazolone 2b is concluded with a S … S contact 3.4771(7) Å [∠C-S … S, 149.75(6)°] between two endocyclic S atoms from neighboring molecules. This represents a short contact due to the proximity of the S atoms. Both atoms are expected to have a partial positive charge S δ+ but since the electron cloud around S is highly polarizable this could also be an electrostatic interaction.
Molecules of the dithiazolone 2b stack along the c-axis ( Figure S1a    The crystal packing of S,N-rich heterocycles is dominated by the presence of short S … N intermolecular contacts. S-N bonds are strongly polar (S δ+ … N δ− ) and therefore electrostatically favored.
The planar geometry of this molecule results in a short intramolecular S … N contact inated by S … Cl, S … N and S … S contacts. Inside the supramolecular triangles, molecules of 2c-β are arranged around a non-crystallographic three-fold axis and are connected by a series of in-plane contacts ( Figure 3). The supramolecular triangles pack next to each other to form a planar infinite 2D sheet ( Figure S3a in Supplementary Information). The contacts inside and between the triangles are not of equal length due to the low symmetry of the crystal. While each triangle has the same number and type of contacts, their length varies. . While this type of interaction is also seen in 2c-a, the highly polarizable nature of sulfur allows for the formation of interactions between the exocyclic thione S atom and the more electronegative N and Cl atoms. This type of interaction originates from the positive σ hole of Intermolecular interactions are usually considered to be contacts considerably shorter than the sum of the van der Waals radii (∑ VDW ) of the atoms participating in these contacts. They are usually 8-20% shorter than the sum of the equilibrium radii [42]. For some atoms van der Waals radii exhibit significant anisotropy. N and O atoms have almost spherical shapes but for S and Cl the ellipticity and therefore the anisotropy increases [43]. For these anisotropic atoms the minor radii (minor axis ca. 0 • or 180 • ) correspond to contacts close to the plane of the molecule and the major radii (major axis ca. 90 • ) for contacts perpendicular to the molecular plane [43]. The sum of the minor and major van der Waals radii (∑ VDW ) of the intermolecular contacts present in the crystal packing of 2b, 2c-α and 2c-β are 3.20, 3.14, 3.18 and 3.20 Å (minor) and 3.63, 3.57, 3.81 and 4.06 Å (major) for S δ+ . . . N δ− , S δ+ . . . O δ− , S δ+ . . . Cl δ− and S δ+ . . . S δ− (endocyclic S to exocyclic thione S), respectively. For the discussion below, we provide both the sum of the minor and major van der Waals radii (∑ VDW ) since the angle of the intermolecular atom approach is somewhere along the 0-180 • range.
Dithiazolone 2b crystallized in the Pbca space group with one molecule in the asymmetric unit cell. Figure Table 2. B3LYP/def2-TZVPD potential energy maps (MEP) of the dithiazolone 2b and of the thione 2c; the isovalue for both surfaces is 0.001.

Perspective 2b 2c
Above the plane of the ring (translucent) Waals radii (∑VDW) since the angle of the intermolecular atom approach is somewhere along the 0-180° range. Table 2. B3LYP/def2-TZVPD potential energy maps (MEP) of the dithiazolone 2b and of the thione 2c; the isovalue for both surfaces is 0.001.

Perspective 2b 2c
Above the plane of the ring (translucent) Above the plane of the ring In plane, across the N-Cl-X atom axis (X= O and S for 2b and 2c, respectively) In plane, into the S-S bond Dithiazolone 2b crystallized in the Pbca space group with one molecule in the asymmetric unit cell. Figure 1 shows all the intermolecular interactions expanded for the central molecule in the asymmetric unit cell. This crystal packing is dominated by S … O and S … Cl Waals radii (∑VDW) since the angle of the intermolecular atom approach is somewhere along the 0-180° range. Table 2. B3LYP/def2-TZVPD potential energy maps (MEP) of the dithiazolone 2b and of the thione 2c; the isovalue for both surfaces is 0.001.

Perspective 2b 2c
Above the plane of the ring (translucent) Above the plane of the ring In plane, across the N-Cl-X atom axis (X= O and S for 2b and 2c, respectively) In plane, into the S-S bond Dithiazolone 2b crystallized in the Pbca space group with one molecule in the asymmetric unit cell. Waals radii (∑VDW) since the angle of the intermolecular atom approach is somewhere along the 0-180° range. Table 2. B3LYP/def2-TZVPD potential energy maps (MEP) of the dithiazolone 2b and of the thione 2c; the isovalue for both surfaces is 0.001.

Perspective 2b 2c
Above the plane of the ring (translucent) Above the plane of the ring In plane, across the N-Cl-X atom axis (X= O and S for 2b and 2c, respectively) In plane, into the S-S bond Dithiazolone 2b crystallized in the Pbca space group with one molecule in the asymmetric unit cell. Figure 1 shows all the intermolecular interactions expanded for the central molecule in the asymmetric unit cell. This crystal packing is dominated by S … O and S … Cl Waals radii (∑VDW) since the angle of the intermolecular atom approach is somewhere along the 0-180° range. Table 2. B3LYP/def2-TZVPD potential energy maps (MEP) of the dithiazolone 2b and of the thione 2c; the isovalue for both surfaces is 0.001.

Perspective 2b 2c
Above the plane of the ring (translucent) Above the plane of the ring In plane, across the N-Cl-X atom axis (X= O and S for 2b and 2c, respectively) In plane, into the S-S bond Dithiazolone 2b crystallized in the Pbca space group with one molecule in the asymmetric unit cell. Figure 1 shows all the intermolecular interactions expanded for the central molecule in the asymmetric unit cell. This crystal packing is dominated by S … O and S … Cl In plane, across the N-Cl-X atom axis (X= O and S for 2b and 2c, respectively) Waals radii (∑VDW) since the angle of the intermolecular atom approach is somewhere along the 0-180° range. Table 2. B3LYP/def2-TZVPD potential energy maps (MEP) of the dithiazolone 2b and of the thione 2c; the isovalue for both surfaces is 0.001.

Perspective 2b 2c
Above the plane of the ring (translucent) Above the plane of the ring In plane, across the N-Cl-X atom axis (X= O and S for 2b and 2c, respectively) In plane, into the S-S bond Dithiazolone 2b crystallized in the Pbca space group with one molecule in the asymmetric unit cell. Figure 1 shows all the intermolecular interactions expanded for the central molecule in the asymmetric unit cell. This crystal packing is dominated by S … O and S … Cl Waals radii (∑VDW) since the angle of the intermolecular atom approach is somewhere along the 0-180° range. Table 2. B3LYP/def2-TZVPD potential energy maps (MEP) of the dithiazolone 2b and of the thione 2c; the isovalue for both surfaces is 0.001.

Perspective 2b 2c
Above the plane of the ring (translucent) Above the plane of the ring In plane, across the N-Cl-X atom axis (X= O and S for 2b and 2c, respectively) In plane, into the S-S bond Dithiazolone 2b crystallized in the Pbca space group with one molecule in the asymmetric unit cell. Figure 1 shows all the intermolecular interactions expanded for the central molecule in the asymmetric unit cell. This crystal packing is dominated by S … O and S … Cl In plane, into the S-S bond Waals radii (∑VDW) since the angle of the intermolecular atom approach is somewhere along the 0-180° range. Table 2. B3LYP/def2-TZVPD potential energy maps (MEP) of the dithiazolone 2b and of the thione 2c; the isovalue for both surfaces is 0.001.

Perspective 2b 2c
Above the plane of the ring (translucent) Above the plane of the ring In plane, across the N-Cl-X atom axis (X= O and S for 2b and 2c, respectively) In plane, into the S-S bond Dithiazolone 2b crystallized in the Pbca space group with one molecule in the asymmetric unit cell. Figure 1 shows all the intermolecular interactions expanded for the central molecule in the asymmetric unit cell. This crystal packing is dominated by S … O and S … Cl Waals radii (∑VDW) since the angle of the intermolecular atom approach is somewhere along the 0-180° range. Table 2. B3LYP/def2-TZVPD potential energy maps (MEP) of the dithiazolone 2b and of the thione 2c; the isovalue for both surfaces is 0.001.

Perspective 2b 2c
Above the plane of the ring (translucent) Above the plane of the ring In plane, across the N-Cl-X atom axis (X= O and S for 2b and 2c, respectively) In plane, into the S-S bond Dithiazolone 2b crystallized in the Pbca space group with one molecule in the asymmetric unit cell. Figure 1 shows all the intermolecular interactions expanded for the central molecule in the asymmetric unit cell. This crystal packing is dominated by S … O and S … Cl The network of intermolecular contacts for dithiazolone 2b is concluded with a S . . . S contact 3.4771(7) Å [∠C-S . . . S, 149.75(6) • ] between two endocyclic S atoms from neighboring molecules. This represents a short contact due to the proximity of the S atoms. Both atoms are expected to have a partial positive charge S δ+ but since the electron cloud around S is highly polarizable this could also be an electrostatic interaction.
Molecules of the dithiazolone 2b stack along the c-axis ( Figure S1a  . This is a near triangular interaction of the same covalently bound Cl to two S atoms of neighboring dithiazoles (Figure 2). The distances of the S . . . Cl contacts in 2c-α are similar to those in dithiazolone 2b. It should be noted that while dithiazolethione 4 has a terminal C-Cl bond, no S . . . Cl contacts appear in its crystal packing [38]. Instead, there is a highly symmetrical bifurcated S . . . S contact of 3.3894(7) and 3.3143(8) Å between the exocyclic C=S sulfur atom and the endocyclic S-S atoms [38]. The thione C=S bond is weakly polar and easily polarizable as sulfur is slightly more electronegative than carbon (2.58 for S vs. 2.55 for C) and is therefore expected to bear partial negative charge. The

4-Chloro-5H-1,2,3-dithiazole-5-thione (2c-β)
The β-phase of dithiazolethione 2c-β crystallized in the P-1 space group with six molecules in the asymmetric unit cell and three crystallographically independent trimers. Figure 3 shows the formation of supramolecular triangles and all the intermolecular interactions expanded for the molecules in the asymmetric unit cell. This crystal packing is dominated by S . . . Cl, S . . . N and S . . . S contacts. Inside the supramolecular triangles, molecules of 2c-β are arranged around a non-crystallographic three-fold axis and are connected by a series of in-plane contacts (Figure 3). The supramolecular triangles pack next to each other to form a planar infinite 2D sheet ( Figure S3a in Supplementary Information). The contacts inside and between the triangles are not of equal length due to the low symmetry of the crystal. While each triangle has the same number and type of contacts, their length varies.
The thione S atoms in 2c-β form bifurcated contacts with the S-S atoms of the dithiazole ring (Figure 3). These S . . . S contacts are in the range of 3.071(4)-3.370(4) Å [∠C-S . . . S, 96.7(4)-136.6(4) • ] and are well within the ∑ VDW [3.20 Å (minor), 4.06 Å (major)]. While this type of interaction is also seen in 2c-a, the highly polarizable nature of sulfur allows for the formation of interactions between the exocyclic thione S atom and the more electronegative N and Cl atoms. This type of interaction originates from the positive σ hole of the S atom. In the C=S bond, some of the electronic charge of the S atom is polarized toward the bond region, leading to a redistribution of electronic density from its outer region (along the extension of the bond) on its equatorial sides [45]. Therefore, negative electrostatic potential is developed around the sites of the S atom while its outer portion along the C=S bond becomes more positive (σ hole). This is evident from the MEP of dithiazolethione 2c (Table 2) Table 3 provides experimental, B3LYP, BLYP, M06, mPW1PW, PBE and MP2 calculated bond lengths for the dithiazolone 2b. Referring to the bond length data, both DFT and MP2 bond lengths compare well with the experimental values. For example, the B3LYP and BLYP C5-S1 bond length has the greatest difference from experimental value where B3LYP is short by 0.038 Å or 2.1% and BLYP is long by 0.079 Å or 4.5% (Table S4 in Supplementary  Information). M06 shows a mild improvement as the C5-S1 bond length is longer by 0.036 Å than the experimental one or overall 2.0%. The results are similar for mPW1PW, PBE and MP2. The S1-S2, N3-C4 and C5-O bond lengths are accurately predicted by PBE, M06 and BLYP, respectively, as these methods provide calculated bond lengths within the experimental range.

Computational Bond Length Analysis
In addition to a basic comparison of experimental bond lengths and bond angles to the measured X-ray crystallography values we also provide a comparison by estimated standard deviation. In this approach we determine whether two bonds are significantly different. The convention is that measured values are said to be 'significantly different' if the difference in their lengths is greater than three times the weighted standard deviation (WSD).  Table S5 in Supplementary Information provides an assessment of the difference between the experiment and computational approach performance in terms of a multiple of the weighted standard deviation (WSD). A value > 3 or <−3 indicates a significant difference between the computed bond length and the experimental one. Table S5 shows that the C5-S1 is consistently the most challenging for all the computational methods as these values range from 39.5 times the WSD using BLYP to 9.5 times the WSD for PBE and MP2. The calculated S1-S2 bond has a large range where the worst case is 75.3 times the WSD using BLYP but only −0.4 times the weighted standard deviation for PBE. Table 4 provides a comparison between the experimental bond angles and the DFT and MP2 bond angles of the dithiazolone 2b. The difference between the experimental bond angles and computed bond angles are mostly < 1 • except for the S2-N3-C4 bond angle. This latter bond angle ranges from −1.6% error for B3LYP and M06 to −0.2% for MP2. On average the differences of either the DFT or MP2 are < 1% (Table S6 in Supplementary Information).  Table S7 in Supplementary Information provides the same assessment of the bond angles as was provided in Table S5 for the bond lengths.
As with the bond lengths, a value > 3 or <−3 indicates a significant difference between the computed bond angle and the experimental value. The selected computational methods perform well for many of the bond angles except the S1-C5-O where the multiple of the WSD ranges from −15.0 for BLYP to a value of −7.9 for MP2. The C5-S1-S2 bond angle also has a large range of WSD from −19.2 for BLYP to 3.3 for MP2. The biggest discrepancy for all methods is observed for the S2-N3-C4 bond angle, whereas the calculated C4-C5-S1 bond angles are all within 3 times the WSD for every computational method used. Table 5 provides experimental and computed bond lengths of the dithiazolethione 2c and Table S8 in Supplementary Information provides the percent differences. Replacement of the O in the dithiazolone 2b by S in thione 2c does little to the relative differences between our crystallographic bond lengths and the computed bond lengths. Once again B3LYP and BLYP obtain a C5-S1 bond length only slightly longer than the experimental value, by 0.009 and 0.032 Å, respectively. The largest relative difference for B3LYP and BLYP is the S1-S2 bond where B3LYP is 0.029 Å shorter or 1.4% and BLYP is 0.064 Å shorter or 3.1%. This is not true for the other functionals or for MP2. Rather the largest difference between experiment and calculated mPW1PW and PBE is in the S2-N3 bond length but these are all only 1% shorter. MP2 provides the most significant deviation from the experimental bond lengths in the N3-C4 bond where the computational difference is longer by 0.026 Å or 2.0%. Arguably, there are more calculated bond lengths within the experimental range for 2c rather than 2b. In terms of the WSD, the S1-S2 is well above 3 for B3LYP, BLYP and M06, below −3 for MP2 and is 0.1 for mPW1PW and −1.2 for PBE (Table S9 in Supplementary Information). Arguably, mPW1PW and PBE do quite well at determining the bond lengths where only the WSD of S2-N3 is below −3 or −4.3 for mPW1PW and −4.4 for PBE and the WSD of C4-Cl is −3.7 for mPW1PW and −4.1 for the PBE. Table 6 gives the experimental and the DFT and MP2 calculated bond angles of the dithiazolethione 2c. The difference between experimental bond angles and computed bond angles are, with a few exceptions, nearly 1 • of experiment (Table S10 in Supplementary  Information). The notable exception is the S1-C5-S angle where the difference between experiment and calculation is 2.1 • , 2.8 • , 2.1 • , 1.7 • , 1.7 • and 1.6 • smaller for the B3LYP, BLYP, M06, mPW1PW, PBE and MP2, respectively. On average the percentage difference between the experimental and computational bond angles for each method are <0.1 • (Table S10).  Table S11 in Supplementary Information provides the WSD of the computed bond angles for of the dithiazolethione 2c. MP2 performs closest to 3 × WSD of angle S1-C5-S being −7.9 up to 4.1 for S1-S2-N3. Of the DFT methods, PBE compares favorably with MP2 in that this DFT method produces angles with WSD that range from −8.6 for S1-C5-S to 5.1 for the N3-C4-Cl.
While crystal packing can influence the ring geometries, our gas phase calculations reproduce the experimental bond lengths and angles within a few percent. Thus, the gas phase computations provide a useful analysis of the electronic properties of the rings.

Fukui Condensed Functions
We calculated the condensed electrophilic (f + ), nucleophilic (f − ) and radicalary attack (f 0 ) Fukui functions based on NBO population analysis [46] with the B3LYP, BLYP, M06, mPW1PW and PBE1PBE functionals for both dithiazolone 2b (Tables S12-S14 in Supplementary Information) and the dithiazolethione 2c (Tables S15-S17 in Supplementary  Information). Fukui functions act as reactivity indices that give information about which atoms in a molecule can either loose or accept an electron [47]. The information allows the determination of atoms more prone to undergo nucleophilic or electrophilic attack, e.g., the atom with the largest f + value indicates where the onset of a nucleophilic reaction will take place. The calculated Fukui functions of each atom in the ring are consistent and of similar value among the various methods we used (Tables S12-S17). Each condensed Fukui function clearly identified a preferred site of nucleophilic or electrophilic attack.
For the dithiazolone 2b, the site of highest electrophilicity is the S2 atom (f + 0.305-0.353, Table S12) followed by the S1 atom (f + 0.209-0.223). This is not surprising as sulfur is easily polarizable and S2 is involved in the weakest bonds (S-S and S-N; 430.03 and 467 kJ mol −1 , respectively [48]) of the heterocycle. Interestingly, the S2 atom is also most prone to radicalary attack (f 0 0.249-0.268, Table S14). The LUMO of dithiazolone 2b has significant orbital density on both S2 and S1 atoms ( Figure 4). These two S atoms show considerable blue coloration (positive electrostatic potential) in the MEP (Table 2) further supporting their electrophilic nature. Reactions of dithiazolone 2b with nucleophiles are expected to proceed via attack at either S2 or S1 as these atoms have similar f + values. These reactions will proceed with probable fragmentation of the S-S or S-N bonds, i.e., cleavage of the heterocycle. The high electrophilic Fukui functions (f + ) on S2 and S1 support the observed intermolecular interactions. As discussed above (Section 2.2.1), the crystal packing of 2b is dominated by S . . . O and S . . . Cl intermolecular contacts. A closer look at Figure 1 and Figure S1 in SI show that both S atoms act as acceptors of electron density from the lone pairs of O and Cl. In particular, the S2 atom is participating in both S . . . O and S . . . Cl contacts while S1 only in S . . . O. In reactions with electrophiles, dithiazolone 2b will have the possibility to react from either S atoms (f − 0.181-0.192 for S1 and f − 0.182-0.193 for S2) or from N3 (f − 0.160-0.172) since these three atoms have near equal nucleophilicities.
In contrast to dithiazolone 2b, the dithiazolethione 2c appears to be most prone to either nucleophilic, electrophilic and radical attack at the exocyclic thione S atom with f + , f − and f 0 of~0.3-0.4 (Tables S15-S17). In support, both the HOMO and LUMO of 2c have significant orbital density on the thione S atom (Figure 4). The MEP ( Table 2) shows both red and green coloration for the thione moiety, a consequence of a potential σ hole formation, indicating the ambivalent behavior of this exocyclic sulfur atom [49]. This is further evident in the intermolecular contacts of polymorphs 2c-α and 2c-β. In 2c-α the exocyclic thione S atom behaves as a nucleophile donating its lone pairs to form contacts with the endocyclic electron poor S-S atoms. While this type of interaction is also present in the crystal packing of 2c-β, the ambivalent character of the thione S atom is evident by the formation of S . . . Cl and S . . . N contacts wherein the thione S atom behaves as an electrophile through its σ hole, accepting contacts from the lone pairs of Cl and N.
We also calculated the condensed Fukui functions for Appel's salt 1 (Tables S18-S20 in Supplementary Information) and found that S2 is the site of highest electrophilicity. This agrees with the prominent positive charge for S2 recently reported by Bartashevich and coworkers [50]. Our Fukui calculations indicate that S1 is the site of highest activity towards radical attack and the two Cl atoms are the sides of high nucleophilicity.
ing of 2b is dominated by S … O and S … Cl intermolecular contacts. A closer look at Figures  1 and S1 in SI show that both S atoms act as acceptors of electron density from the lone pairs of O and Cl. In particular, the S2 atom is participating in both S … O and S … Cl contacts while S1 only in S … O. In reactions with electrophiles, dithiazolone 2b will have the possibility to react from either S atoms (f − 0.181-0.192 for S1 and f − 0.182-0.193 for S2) or from N3 (f − 0.160-0.172) since these three atoms have near equal nucleophilicities.

Bond Order, Bird Index and NICS(1)
The Wiberg bond orders [51] are calculated within the NBO analysis. This approach to computing bond orders results in values usually close to formal bond order for most chemical systems. Table 7 provides the B3LYP/def2-TZVPD bond orders for the Appel's salt 1, dithiazolone 2b and the dithiazolethione 2c. The bond orders of the Appel's salt 1 reflect the delocalization around the ring where the S1-S2 has a bond order of 1.08 and is the least delocalized of all of the bonds. The bond orders of the dithiazolone 2b and the dithiazolethione 2c show significantly less delocalization. In dithiazolone 2b the N3-C4 bond order is 1.68 and in dithiazolethione 2c the analogous bond order is 1.64. In dithiazolone 2b the C5-O bond order is 1.77 and in dithiazolethione 2c the analogous bond order albeit the C5-S is 1.66. These bond orders suggest significant double bond character. By contrast, the analogous bonds in the Appel's salt 1 have bond orders of 1.48 for the N3-C4 and 1.22 for the C5-Cl2, i.e., less double bond character. The S2-N3 of the dithiazolone 2b has a bond order of 1.12 and the same bond order in the dithiazolethione 2c is 1.13 suggesting a single bond. The same is true for the C5-S1 bond orders in both the dithiazolone 2b and dithiazolethione 2c where the values are 1.06 and 1.15, respectively. The latter bond orders are larger in the Appel's salt 1 being 1.29 and 1.36, respectively. According to the bond orders, Appel's salt 1 is more delocalized than either the dithiazolone 2b or dithiazolethione 2c. The experimental and calculated bond lengths and bond angles of Appel's salt 1 can be found in Tables S21 and S22 (Supplementary Information). Our calculated NBO bond orders for Appel's salt 1 compared favorably with the recently reported QTAIM-based bond orders [50]. The latter were calculated using experimental electron density. The NBO and QTAIM-based bond orders for the endocyclic bonds C5-S1 (1.  To further explore the delocalization, we computed the Bird Index (I n ) [52] along with the nucleus-independent chemical shifts (NICS) [53,54]. The Bird Index, I n evaluates the aromaticity of a ring in terms of the statistical evaluation of the deviations in peripheral bond orders. NICS(1) evaluates the aromaticity of cyclic systems and is defined as the negative value of the absolute shielding computed at 1 Å above the ring centroid. From the experimental bond lengths our computed I n for dithiazolone 2b is 36.1, for the dithiazolethione 2c is 39.9 and for the Appel's salt 1 is 62.2 (I 5 = 38 for oxazole and I 5 = 62 for 1,2-dithiolium [52]). Using the B3LYP bond orders as shown in Table 7, the resulting Bird Indices are only slightly larger or 37.5 for dithiazolone 2b 44.3 for dithiazolethione 2c and 63.6 for Appel's salt 1. Table 8 provides the NICS(1) for the Appel's salt 1, dithiazolone 2b and dithiazolethione 2c. Comparison of the Appel's salt 1 with dithiazolone 2b and dithiazolethione 2c suggests the Appel's salt cation to be more aromatic than either dithiazolone 2b or dithiazolethione 2c. Using the NICS(1) as an index of the aromaticity, the relative ordering of the compounds in terms of aromaticity of the π electrons is Appel's salt 1 > dithiazolone 2b > dithiazolethione 2c. Table 8. Nucleus-independent chemical shifts (NICS) taken 1 Å over the center of the ring of Appel's salt 1, dithiazolone 2b and dithiazolethione 2c rings.

Conclusions
We examined the intramolecular crystal structure and solid-state packing of two neutral monocyclic 4-chloro-5H-1,2,3-dithiazoles, the 5-one 2b and the two polymorphs of 5-thione 2c (2c-α and 2c-β). Both molecules are planar and the intramolecular geometrical parameters support localization of bonds inside the ring. Their crystal packing is dominated by S . . . N, S . . . O, S . . . Cl and S . . . S short intermolecular interactions of primarily electrostatic nature. Condensed Fukui functions indicate electrophilic sites at S2 and at a lesser degree on S1 for the dithiazolone 2b and the thione exocyclic S for the dithiazolethione 2c. The nucleophilic sites for dithiazolone 2b are S1, S2 and N3 on par and the exocyclic S thione for 2c. The ambivalent character of the S thione in 2c is evident by the rich intermolecular contacts it participates with both positive (endocyclic S-S atoms) in polymorph 2c-α and negative (O, N, Cl) atoms in 2c-β. This can be best explained by the formation of a σ hole which polarizes the S atom in regions of negative and positive electron density. Bond orders, Bird Indices and NICS(1) calculations showed that dithiazolone 2b and dithiazolethione 2c do not have an extensive delocalization and are, therefore, less aromatic that Appel's salt 1.

Computational Methodology
Our crystallographic data provided the initial geometry for the pair of structures. Each structure was energy optimized by density functional theory. The combinations of exchange and correlation functionals used in this work include B3LYP [58][59][60][61][62], BLYP [59,63], mPW1PW [64], PBE1PBE [65] and M06 [66] as implemented in Gaussian09 [67]. The methods used in this work are proven to represent similar heterocyclic compounds. B3LYP is a standard of DFT for gas phase molecules and is a well proven compromise between computational cost and accuracy [68,69]. BLYP and PBE are also common functionals applied to gas phase molecules and yield comparable results where the difference is that BLYP and PBE are gradient corrected functionals and B3LYP is a hybrid function. These approaches have been shown to provide good results for magnetic, vibrational, and electronic properties of molecules as compared to DFT functionals that include extensive parameterization [70]. The mPW1PW fits the exact exchange energy of isolated atoms and the differential exchange of ideal gas dimers significantly improving the long-range behavior of the exchange functional [71]. The M06 functional introduces empirically optimized parameters into the exchange-correlation functional and provides good geometry, energy and property data [66]. MP2 is a standard ab initio computational method that includes correlation and is free from the spurious self-interaction of electrons and naturally includes dispersion. The basis set utilized in this work consisted of a single def2-TZVPD. This basis is optimized for properties and computational cost specifically for density functional theory [72]. The calculated nucleophilic (f − ), electrophilic (f + ) and radical attack (f 0 ) characterize the electron reorganization that results in site electrophilic or nucleophilic activation/deactivation [47]. Calculation of the Fukui reactivity-indices was performed for each DFT from the NBO [46] populations using the UCA-FUKUI software [73].