In practical terms, the heavy-atom skeleton of the two tautomers of all studied
molecules is planar. In the case of the thione tautomers, the heavy-atom skeleton
planar structures correspond to true minima on the investigated potential energy
surfaces. For thiol forms, the planar geometry is also a minimum energy structure
for TBI, while for the methyl-substituted molecules (MTI, MTBI), the calculations
predict two slightly nonplanar equivalent-by-symmetry minima, with N=C–S–H
dihedral angles of a few degrees. However, these forms are separated from each
other by a very low energy barrier (below 0.1 kJ mol
−1 ), which stays below the
zero-point vibrational level of these structures. Under these circumstances, the
heavy-atom skeleton planar geometry corresponds also to the most probable
structure for the two molecules in the ground vibrational state, thus being the one
relevant in practical terms.
An interesting distinct structural feature in the two methyl-substituted molecules
is the orientation of the methyl group (Fig. 7.1). In MTI, the methyl group is
oriented in such a way that the methyl hydrogen atom placed in the plane of the
heavy-atom skeleton of the molecule points away from the sulfur substituent, in
order to minimize the methyl/S steric repulsions. This is true for both thione and
thiol tautomers. On the other hand, in MTBI, the in-plane methyl hydrogen atom
points toward the sulfur substituent in both tautomeric forms, because in this case,
the presence of the benzo substituent introduces a stronger steric/electrostatic
repulsion between the methyl group and the closest located phenyl hydrogen atom.
Such interaction can in fact be expected to attain its maximum importance for an
orientation of the methyl group identical to that found in MTI.
More important is the conclusion, extracted from the structural data obtained for
all molecules, that no intramolecular hydrogen bond exists in any of the tautomers.
In fact, the ∡N–H⋯S and ∡S–H⋯N angles in the thione and thiol tautomers,
respectively, are in the range 67–82°, i.e., they are considerably smaller than the
commonly accepted minimum value for a ∡D–H⋯A (D, donor; A, acceptor) angle
allowing the establishment of a hydrogen bond (*110° [13]). As it will be shown
in Sect. 7.3.1, this structural feature is of extreme importance in determining the
mechanism of unimolecular photo tautomerization in these molecules.
The infrared spectra of the matrix-isolated compounds are shown in Fig. 7.2,
where they can be compared with the theoretically predicted spectra for the two
tautomers. The results are clear regarding the sole presence in the matrices of the
thione tautomer. This result allows us also to conclude that no tautomerization takes
place upon sublimation of the compounds, and that the thione form, which was
initially present in the crystals [14–16], was kept upon the phase transition.
The excellent agreement between the calculated and experimental data facilitated
the assignment of the bands, which can be found elsewhere [11, 12]. The two
striking observations are:
• the extensive site splitting and broadening observed for the bands assigned to
vibrations with dominant contributions of the NH modes (in particular the
bending and rocking modes), which demonstrates the involvement of the NH
moieties in specific interactions with the host matrix atoms and reflects in the
7 Hydantoins and Mercaptoimidazoles: Vibrational …
203
molecules is planar. In the case of the thione tautomers, the heavy-atom skeleton
planar structures correspond to true minima on the investigated potential energy
surfaces. For thiol forms, the planar geometry is also a minimum energy structure
for TBI, while for the methyl-substituted molecules (MTI, MTBI), the calculations
predict two slightly nonplanar equivalent-by-symmetry minima, with N=C–S–H
dihedral angles of a few degrees. However, these forms are separated from each
other by a very low energy barrier (below 0.1 kJ mol
−1 ), which stays below the
zero-point vibrational level of these structures. Under these circumstances, the
heavy-atom skeleton planar geometry corresponds also to the most probable
structure for the two molecules in the ground vibrational state, thus being the one
relevant in practical terms.
An interesting distinct structural feature in the two methyl-substituted molecules
is the orientation of the methyl group (Fig. 7.1). In MTI, the methyl group is
oriented in such a way that the methyl hydrogen atom placed in the plane of the
heavy-atom skeleton of the molecule points away from the sulfur substituent, in
order to minimize the methyl/S steric repulsions. This is true for both thione and
thiol tautomers. On the other hand, in MTBI, the in-plane methyl hydrogen atom
points toward the sulfur substituent in both tautomeric forms, because in this case,
the presence of the benzo substituent introduces a stronger steric/electrostatic
repulsion between the methyl group and the closest located phenyl hydrogen atom.
Such interaction can in fact be expected to attain its maximum importance for an
orientation of the methyl group identical to that found in MTI.
More important is the conclusion, extracted from the structural data obtained for
all molecules, that no intramolecular hydrogen bond exists in any of the tautomers.
In fact, the ∡N–H⋯S and ∡S–H⋯N angles in the thione and thiol tautomers,
respectively, are in the range 67–82°, i.e., they are considerably smaller than the
commonly accepted minimum value for a ∡D–H⋯A (D, donor; A, acceptor) angle
allowing the establishment of a hydrogen bond (*110° [13]). As it will be shown
in Sect. 7.3.1, this structural feature is of extreme importance in determining the
mechanism of unimolecular photo tautomerization in these molecules.
The infrared spectra of the matrix-isolated compounds are shown in Fig. 7.2,
where they can be compared with the theoretically predicted spectra for the two
tautomers. The results are clear regarding the sole presence in the matrices of the
thione tautomer. This result allows us also to conclude that no tautomerization takes
place upon sublimation of the compounds, and that the thione form, which was
initially present in the crystals [14–16], was kept upon the phase transition.
The excellent agreement between the calculated and experimental data facilitated
the assignment of the bands, which can be found elsewhere [11, 12]. The two
striking observations are:
• the extensive site splitting and broadening observed for the bands assigned to
vibrations with dominant contributions of the NH modes (in particular the
bending and rocking modes), which demonstrates the involvement of the NH
moieties in specific interactions with the host matrix atoms and reflects in the
7 Hydantoins and Mercaptoimidazoles: Vibrational …
203