Bond et al.
1513
Fig. 5. View of the structure of 3·CH2Cl2 (illustrating only the
major components of the positional disorder of nitro and bromo
substituents and CH2Cl2 solvate molecule).
utilized to manipulate the solid state structures of DTTAs. In
the current paper we examine the structures of these DTTAs,
using the same methodology.
Molecular electrostatic potential maps
Calculations of the molecular electrostatic potential maps
were carried out using semi-empirical methods. We found
that of the methods employed (MNDO, AM1, PM3, and
PM5), several gave similar geometries for the DTTA ring
(Table 1) close to that observed experimentally for 2. Whilst
the twist angle between DTTA and aryl substituent is sus-
ceptible to crystal-packing effects, all semi-empirical meth-
ods gave substantial deviations from coplanarity. We chose
the PM3 method, since this provided the smallest deviations
from planarity and therefore best agreement with the ob-
served structure. The molecular electrostatic potential maps
(MEPs) determined using other methodologies did not differ
substantially, although some variations in partial point
charges were found.
Structure of 3·CH2Cl2
The structures of 1, 2, and the unsolvated form of 3
(above), as well as many other DTTA derivatives contain
half-molecules in the asymmetric unit related through an in-
version centre. In the case of 3·CH2Cl2, the asymmetric unit
comprises half a molecule of 3 and half a molecule of
CH2Cl2, which are located on a mirror plane passing through
the two Br–NO2 functional groups. In the case of 3·CH2Cl2
the Br–NO2 functional groups exhibit positional disorder in
an approximately 3:1 ratio. The terminal group disorder does
not affect the DTTA ring geometry, which is comparable
with other aryl derivatives described above (Table 3). The
DTTA ring is planar within (0.01 Å) and approximately
coplanar with the aryl ring (twist angle 0.4°).
The molecular electrostatic potential is the energy of a
unit of positive charge (“a proton”) as it interacts with a
molecule, i.e., its interaction with the unperturbed molecular
charge distribution, caused by the electrons and positive nu-
clei. The isopotential map represents a contour linking
points with the same interaction energy. The value of the
plot may be positive if the MEP at that point is repulsive, or
negative if the sum of the electrostatic interactions is attrac-
tive (21). The isopotential maps for 1–3 are shown in Fig. 6
with dark grey regions representing a negative, attractive po-
tential towards a positive charge, and light grey regions cor-
responding to positive, repulsive interactions. We feel that
the electrostatic approach to analyse the bonding in thiazyl
rings is particularly appropriate because of the strong polar-
ity of the thiazyl bonds. Since electrostatic interactions ex-
hibit an r–1 dependency, whereas van der Waals forces are a
function of r–6, then contacts beyond the sum of the van der
Waals contacts may exhibit a substantial ionic contribution
to the lattice energy. Current semi-empirical calculations on
the DTTA heterocycle indicate substantial bond polarity
within the heterocyclic ring as well as substantial partial
charges on the nitro functional group. Conversely the Br
atom is electrostatically near neutral. Given the sparsity of
contacts less than the sum of the van der Waals radii in 1–3,
the electrostatic contribution is likely to be significant.
An examination of the in-plane packing motifs of 1, 2,
and 3 (i.e., close to the molecular plane) reveals a recurring
theme of intermolecular contacts to S. In 1, the S···O con-
tacts are 3.292(2) Å (Fig. 1) and geometrically similar inter-
actions also occur in 3 and 3·CH2Cl2 (Figs 4 and 5).
Intermolecular S···Br contacts are observed in 2 and 3. In 2,
the S···Br contact is 3.885(1) Å. In 3 and 3·CH2Cl2 the cor-
responding values are somewhat longer (around 3.997(3) Å
and 4.243(3) Å respectively). In addition, intermolecular
S···N contacts close to the ring plane are observed in 2 and
3. The intermolecular S···N contact between symmetry-
related DTTA rings in the second crystallographically
unique molecule of 2 is 3.596(4) Å. These are also observed
in the structure of unsolvated 3, at 3.663(7) Å.
A comparison of the structures of 3 and 3·CH2Cl2 imme-
diately reveals a disruption of the chain-like motif present in
unsolvated 3. The CH2Cl2 molecule is sited on the mirror
plane and is also orientationally disordered over two sites in
an approximately 1:1 ratio. The CH2Cl2 is located between
molecules of 3 along the chain-forming direction. The disor-
der in both CH2Cl2 and Br–NO2 groups complicates discus-
sion of the intermolecular interactions between these two
groups, although it is tempting to envisage the molecular di-
pole of the CH2Cl2 molecule aligning with the molecular di-
pole of 3. Lateral interactions between molecular chains
comprise S···Br (and S···O2N) interactions (Fig. 5). The
S···Br contact, present in the major component of the disor-
der, is 4.243(3) Å, somewhat longer than the corresponding
S···Br contacts in 2.
Discussion
The molecular structures of the aryl-substituted DTTA
molecules, described herein, are similar to those reported for
other derivatives described in earlier reports (8–10). Their
planar 10ꢃ aromatic structures are consistent with previous
studies, which indicate that folded structures are stabilized
only when there is the possibility of ꢃ-donation into the
thiazyl ring.
Our recent structural studies in dithiadiazolyl radical
chemistry has successfully utilized molecular electrostatic
potential (MEP) maps, coupled with a point charge model,
to analyse the observation of polymorphism in the rigid
dithiadiazolyl radicals, ClCNSSN and HCNSSN (18). To at-
tempt to rationalize the structures of 1–3, we have employed
similar techniques. The focus of these current studies has
been to examine whether structure-directing groups can be
A comparison of the observed structures with their molec-
ular electrostatic potential maps clearly reveals the matching
of electropositive (light grey) and electronegative (dark grey)
© 2002 NRC Canada
Bond, Andrew D.
