observed high sensitivity of the vibrations of the NH groups to local
environment,
• for the methyl-substituted compounds, also the splitting and relative broadening
of the bands associated with the bending vibrations of the methyl group, which
points to a partially free rotation of this group even in the matrix media, thus
implying a large amplitude methyl torsional vibration that, in turn, leads to an
increased dispersion of the vibrational levels associated with the bending modes.
In consonance with the spectroscopically suggested appreciable conformational
freedom of the methyl groups, the B3LYP/6-311++G(d,p) calculated methyl
internal rotation barriers for MTI and MTBI amount only to 1.2 and 2.7 kJ mol
−1 ,
respectively [11, 12].
7.2.2 Hydantoins
Hydantoins are heterocyclic compounds that are derived from imidazolidine. They
exhibit relevant physiological activities, for example, as anticonvulsive,
antiepileptic, anti-inflammatory, and anticancer drugs [17–22] and have also been
suggested as potential therapeutic agents for the treatment of HIV-1 [23, 24].
Like mercaptoimidazoles, hydantoins may also exhibit tautomerism, and tautomeric forms of hydantoins bearing either one or two OH moieties may also be
conceivable to exist. Taking the di-keto most stable tautomer as reference, these
tautomers can be viewed as resulting from migration of the lactam hydrogen atoms
to the carbonyl oxygen atoms. However, these tautomers have been shown to be
higher in energy than the most stable tautomer by at least 70 kJ mol
−1 [25] and
have never been observed experimentally for the simplest members of the family.
In our studies on hydantoins [26–30], we have considered both 1-methyl- and
5-methyl-substituted hydantoins (1MH, 5MH) and 5-acetic acid hydantoin (AAH),
besides the parent, unsubstituted hydantoin (H) (Fig. 7.3). Those studies allowed us
to establish the basic structural properties of the hydantoin moiety. It could be
concluded, for example, that the hydantoin ring tends to assume a planar (or
quasi-planar) geometry, sharing common geometrical and electronic features in all
investigated compounds. To mention just the most important ones:
Fig. 7.3 Representation of the studied hydantoin molecules
7 Hydantoins and Mercaptoimidazoles: Vibrational …
205
environment,
• for the methyl-substituted compounds, also the splitting and relative broadening
of the bands associated with the bending vibrations of the methyl group, which
points to a partially free rotation of this group even in the matrix media, thus
implying a large amplitude methyl torsional vibration that, in turn, leads to an
increased dispersion of the vibrational levels associated with the bending modes.
In consonance with the spectroscopically suggested appreciable conformational
freedom of the methyl groups, the B3LYP/6-311++G(d,p) calculated methyl
internal rotation barriers for MTI and MTBI amount only to 1.2 and 2.7 kJ mol
−1 ,
respectively [11, 12].
7.2.2 Hydantoins
Hydantoins are heterocyclic compounds that are derived from imidazolidine. They
exhibit relevant physiological activities, for example, as anticonvulsive,
antiepileptic, anti-inflammatory, and anticancer drugs [17–22] and have also been
suggested as potential therapeutic agents for the treatment of HIV-1 [23, 24].
Like mercaptoimidazoles, hydantoins may also exhibit tautomerism, and tautomeric forms of hydantoins bearing either one or two OH moieties may also be
conceivable to exist. Taking the di-keto most stable tautomer as reference, these
tautomers can be viewed as resulting from migration of the lactam hydrogen atoms
to the carbonyl oxygen atoms. However, these tautomers have been shown to be
higher in energy than the most stable tautomer by at least 70 kJ mol
−1 [25] and
have never been observed experimentally for the simplest members of the family.
In our studies on hydantoins [26–30], we have considered both 1-methyl- and
5-methyl-substituted hydantoins (1MH, 5MH) and 5-acetic acid hydantoin (AAH),
besides the parent, unsubstituted hydantoin (H) (Fig. 7.3). Those studies allowed us
to establish the basic structural properties of the hydantoin moiety. It could be
concluded, for example, that the hydantoin ring tends to assume a planar (or
quasi-planar) geometry, sharing common geometrical and electronic features in all
investigated compounds. To mention just the most important ones:
Fig. 7.3 Representation of the studied hydantoin molecules
7 Hydantoins and Mercaptoimidazoles: Vibrational …
205