to the ring nitrogen atoms pointing to opposite directions. Intermolecular N–H⋯S
hydrogen bonds result in the formation of linear chains along the c-direction;
further c g ⋯c g interactions between the imidazole rings of adjacent chains in the adirection (c g ⋯c g distance: 3.4466(19) Å) define the supramolecular structure in the
crystal.
The structure of the newly synthesized polymorph (II) was found to be considerably more complex. The crystals of polymorph II belong to the noncentrosymmetric Ia monoclinic space group, with Z = 16 and cell parameters:
a = 7.45910(10) Å, b = 44.1680(8) Å, c = 11.3522(2) Å, and b = 103.0240(10)°.
The crystals belong to the rare category of crystals having an asymmetric unit with
Z′ > 1 (only about 9% of the ca. 800,000 structures contained in the Cambridge
Structural Database share this feature [37]). In the present case, the unit cell contains four symmetry independent molecules. However, the most fascinating structural feature of this new polymorph is that the symmetry independent molecules
exhaust the whole set of conformers of the molecule: the abundant lowest energy
conformer and two high-energy forms not present in solution before crystallization.
The four IDI independent molecules present in the crystal of polymorph II
(extracted from the XRD data; A-D) together with the calculated conformers of the
molecule (1-3) are shown in Fig. 7.9. The calculated relative energies for the different conformers as isolated species, obtained at the B3LYP/6-311++G(d,p) level
of theory, are also given in the figure, as well as the C–S–S–C dihedral angles of all
represented structures. In the crystal, two molecules (C, D) are similar to conformer
1, and the remaining two, A and B, correspond to conformers 3 and 2, respectively.
Taking into account the relative energies of the IDI conformers (Fig. 7.9), the
isolated IDI molecule exists almost exclusively in conformer 1, both in gas phase
and solution (the energies shown in Fig. 7.9 correspond to gas phase data; in THF,
the calculated relative energies for conformers 2 and 3 are 9.7 and 9.3 kJ mol
−1 ,
respectively). The most stable IDI conformer exhibits an intramolecular H-bond
interaction established between the unprotonated nitrogen atom of one of the
imidazole rings and the NH fragment of the second imidazole ring of the molecule.
Because of this interaction, in conformer 1 the number of positions readily available
for establishing intermolecular H-bonds is reduced to half, when compared with the
higher energy conformers. Indeed, to fulfill the four H-bond valences with intermolecular H-bonds in the crystal, the most stable IDI conformer has to break the
intramolecular H-bond. This fact makes the energy balance associated with the
establishment of the intermolecular H-bonds less favorable for conformer 1 than for
both conformers 2 and 3, which require only minor structural rearrangements to
establish intermolecular H-bonds.
It can then be concluded that the simultaneous observation of the three conformers of IDI in polymorph II results from a compromise between the greater
intrinsic stability of the isolated conformer 1, and the easier and more efficient in
energetic terms packing achieved by the higher energy conformers 2 and 3. In
polymorph I, the IDI molecules assume geometry similar to that of conformer 2.
The Raman spectrum (500–50 cm
−1 region) of polymorph II is shown in
Fig. 7.10, where it is compared with the simulated spectrum built based on the
7 Hydantoins and Mercaptoimidazoles: Vibrational …
213
hydrogen bonds result in the formation of linear chains along the c-direction;
further c g ⋯c g interactions between the imidazole rings of adjacent chains in the adirection (c g ⋯c g distance: 3.4466(19) Å) define the supramolecular structure in the
crystal.
The structure of the newly synthesized polymorph (II) was found to be considerably more complex. The crystals of polymorph II belong to the noncentrosymmetric Ia monoclinic space group, with Z = 16 and cell parameters:
a = 7.45910(10) Å, b = 44.1680(8) Å, c = 11.3522(2) Å, and b = 103.0240(10)°.
The crystals belong to the rare category of crystals having an asymmetric unit with
Z′ > 1 (only about 9% of the ca. 800,000 structures contained in the Cambridge
Structural Database share this feature [37]). In the present case, the unit cell contains four symmetry independent molecules. However, the most fascinating structural feature of this new polymorph is that the symmetry independent molecules
exhaust the whole set of conformers of the molecule: the abundant lowest energy
conformer and two high-energy forms not present in solution before crystallization.
The four IDI independent molecules present in the crystal of polymorph II
(extracted from the XRD data; A-D) together with the calculated conformers of the
molecule (1-3) are shown in Fig. 7.9. The calculated relative energies for the different conformers as isolated species, obtained at the B3LYP/6-311++G(d,p) level
of theory, are also given in the figure, as well as the C–S–S–C dihedral angles of all
represented structures. In the crystal, two molecules (C, D) are similar to conformer
1, and the remaining two, A and B, correspond to conformers 3 and 2, respectively.
Taking into account the relative energies of the IDI conformers (Fig. 7.9), the
isolated IDI molecule exists almost exclusively in conformer 1, both in gas phase
and solution (the energies shown in Fig. 7.9 correspond to gas phase data; in THF,
the calculated relative energies for conformers 2 and 3 are 9.7 and 9.3 kJ mol
−1 ,
respectively). The most stable IDI conformer exhibits an intramolecular H-bond
interaction established between the unprotonated nitrogen atom of one of the
imidazole rings and the NH fragment of the second imidazole ring of the molecule.
Because of this interaction, in conformer 1 the number of positions readily available
for establishing intermolecular H-bonds is reduced to half, when compared with the
higher energy conformers. Indeed, to fulfill the four H-bond valences with intermolecular H-bonds in the crystal, the most stable IDI conformer has to break the
intramolecular H-bond. This fact makes the energy balance associated with the
establishment of the intermolecular H-bonds less favorable for conformer 1 than for
both conformers 2 and 3, which require only minor structural rearrangements to
establish intermolecular H-bonds.
It can then be concluded that the simultaneous observation of the three conformers of IDI in polymorph II results from a compromise between the greater
intrinsic stability of the isolated conformer 1, and the easier and more efficient in
energetic terms packing achieved by the higher energy conformers 2 and 3. In
polymorph I, the IDI molecules assume geometry similar to that of conformer 2.
The Raman spectrum (500–50 cm
−1 region) of polymorph II is shown in
Fig. 7.10, where it is compared with the simulated spectrum built based on the
7 Hydantoins and Mercaptoimidazoles: Vibrational …
213