7.4 Neat Condensed Phases—Polymorphism and Phase
Transitions
7.4.1 Mercaptoimidazoles: The Case of the Thioimidazole
Dimer
Thioimidazole undergoes easy dimerization upon crystallization from tetrahydrofuran (THF) solution, which results in production of the disulfide derivative {(2[(1H-imidazol-2-yl) disulfanyl]-1H-imidazole]; IDI}. Suitable crystals for single
crystal XRD experiments were collected, and their structure was solved. Very
interestingly, the material was found to constitute a new polymorph of the compound, which, as described below, presents unique structural features.
The structure of a crystalline phase of IDI had been reported previously by
Bazargani et al. [36]. That polymorph (I) crystallizes in the monoclinic space group
C2/c, with a = 14.083(3) Å, b = 6.3928(13) Å, c = 9.922(2) Å and b = 122.29(3)°,
with four molecules per unit cell (Z = 4). The asymmetric unit of the crystals of
polymorph I contains one half-molecule (Z′ = ½). The S–S bond distance is 2.0713
(14) Å, the planar imidazole rings form an angle between them of 21.83(19)°, and
the C–S–S–C torsion angle is 83.62(17)°, with the two hydrogen atoms connected
Fig. 7.8 B3LYP/6-311++G
(d,p) calculated (scaled)
infrared spectra (2320–
965 cm
−1 region) for
isocyanic acid, CO and
methanimine (top; I), infrared
difference spectrum
(irradiated matrix) minus
(as-deposited) (middle, II),
and spectrum of the
as-deposited 5MH in argon
matrix (bottom; III).
Reprinted with permission
from [30], copyright (2017)
American Chemical Society
212
R. Fausto et al.
Transitions
7.4.1 Mercaptoimidazoles: The Case of the Thioimidazole
Dimer
Thioimidazole undergoes easy dimerization upon crystallization from tetrahydrofuran (THF) solution, which results in production of the disulfide derivative {(2[(1H-imidazol-2-yl) disulfanyl]-1H-imidazole]; IDI}. Suitable crystals for single
crystal XRD experiments were collected, and their structure was solved. Very
interestingly, the material was found to constitute a new polymorph of the compound, which, as described below, presents unique structural features.
The structure of a crystalline phase of IDI had been reported previously by
Bazargani et al. [36]. That polymorph (I) crystallizes in the monoclinic space group
C2/c, with a = 14.083(3) Å, b = 6.3928(13) Å, c = 9.922(2) Å and b = 122.29(3)°,
with four molecules per unit cell (Z = 4). The asymmetric unit of the crystals of
polymorph I contains one half-molecule (Z′ = ½). The S–S bond distance is 2.0713
(14) Å, the planar imidazole rings form an angle between them of 21.83(19)°, and
the C–S–S–C torsion angle is 83.62(17)°, with the two hydrogen atoms connected
Fig. 7.8 B3LYP/6-311++G
(d,p) calculated (scaled)
infrared spectra (2320–
965 cm
−1 region) for
isocyanic acid, CO and
methanimine (top; I), infrared
difference spectrum
(irradiated matrix) minus
(as-deposited) (middle, II),
and spectrum of the
as-deposited 5MH in argon
matrix (bottom; III).
Reprinted with permission
from [30], copyright (2017)
American Chemical Society
212
R. Fausto et al.