vibrational transitions as well as subtractive combination tones are suppressed,
since excited vibrational states are depopulated at the low work temperatures typical
of the matrix isolation experiments, introducing an additional simplification in the
spectra and contributing to their increased resolution.
Another interesting feature of matrix isolation, this time particularly useful for
reactivity studies (in particular in photochemistry) , results from the fact that isolated
molecules in matrices are cage confined. Thus, if fragmentation of a matrix-isolated
species takes place, for example, in result of photoexcitation, most of the times the
obtained fragments stay in the matrix cage where they were formed. Thus, no subsequent cross-reactions involving species resulting from fragmentation of different
reactant molecules can occur, strongly reducing the number of possible products in
comparison with gas phase or solution studies and open the gate for a detailed study of
the mechanisms of unimolecular photochemically induced processes.
In addition to the above-mentioned advantages, standard matrix isolation setups
can be also used, without any modification, for preparation of samples of a solid
pristine compound, by the sole condensation of its vapor. This method of sample
preparation in general leads to producing of amorphous solids that can then be
submitted to a controlled temperature program to induce in situ crystallization,
which can be spectroscopically followed. The temperature program can be chosen
to mimic a temperature program used, for example, in a DSC experiment, so that
spectroscopic data can be directly compared with thermodynamic information.
In the studies described in the present chapter, matrix isolation infrared spectroscopy has been used to investigate the structures of the isolated molecules of the
targeted compounds and their photochemistry. The neat condensed phases and
phase transitions have been investigated by infrared and Raman spectroscopies,
supplemented by DSC, polarized-light thermomicroscopy (PLTM), and XRD.
Computational studies, including the use of several approaches for detailed analysis
of the electron density of the investigated molecules, were used to help in interpreting the experimental data. On the whole, it is our aim to highlight here the
power of vibrational spectroscopy as central technique to investigate the structure
and reactivity of two relevant families of nitrogen-containing heterocyclic molecules: hydantoins and mercaptoimidazoles.
7.2 Structures and Infrared Spectra of the Isolated
Molecules
7.2.1 Mercaptoimidazoles
Mercaptoimidazoles constitute an important family of nitrogen-containing heterocyclic molecules that have been shown to present antioxidant properties toward
several oxygen-derived toxic species. The mercaptoimidazole moiety is present in
natural antioxidant compounds such as ergothioneine or ovothiol [3–6].
Mercaptoimidazoles are also similar compounds to thiouracil, a relevant antithyroid
7 Hydantoins and Mercaptoimidazoles: Vibrational …
201
since excited vibrational states are depopulated at the low work temperatures typical
of the matrix isolation experiments, introducing an additional simplification in the
spectra and contributing to their increased resolution.
Another interesting feature of matrix isolation, this time particularly useful for
reactivity studies (in particular in photochemistry) , results from the fact that isolated
molecules in matrices are cage confined. Thus, if fragmentation of a matrix-isolated
species takes place, for example, in result of photoexcitation, most of the times the
obtained fragments stay in the matrix cage where they were formed. Thus, no subsequent cross-reactions involving species resulting from fragmentation of different
reactant molecules can occur, strongly reducing the number of possible products in
comparison with gas phase or solution studies and open the gate for a detailed study of
the mechanisms of unimolecular photochemically induced processes.
In addition to the above-mentioned advantages, standard matrix isolation setups
can be also used, without any modification, for preparation of samples of a solid
pristine compound, by the sole condensation of its vapor. This method of sample
preparation in general leads to producing of amorphous solids that can then be
submitted to a controlled temperature program to induce in situ crystallization,
which can be spectroscopically followed. The temperature program can be chosen
to mimic a temperature program used, for example, in a DSC experiment, so that
spectroscopic data can be directly compared with thermodynamic information.
In the studies described in the present chapter, matrix isolation infrared spectroscopy has been used to investigate the structures of the isolated molecules of the
targeted compounds and their photochemistry. The neat condensed phases and
phase transitions have been investigated by infrared and Raman spectroscopies,
supplemented by DSC, polarized-light thermomicroscopy (PLTM), and XRD.
Computational studies, including the use of several approaches for detailed analysis
of the electron density of the investigated molecules, were used to help in interpreting the experimental data. On the whole, it is our aim to highlight here the
power of vibrational spectroscopy as central technique to investigate the structure
and reactivity of two relevant families of nitrogen-containing heterocyclic molecules: hydantoins and mercaptoimidazoles.
7.2 Structures and Infrared Spectra of the Isolated
Molecules
7.2.1 Mercaptoimidazoles
Mercaptoimidazoles constitute an important family of nitrogen-containing heterocyclic molecules that have been shown to present antioxidant properties toward
several oxygen-derived toxic species. The mercaptoimidazole moiety is present in
natural antioxidant compounds such as ergothioneine or ovothiol [3–6].
Mercaptoimidazoles are also similar compounds to thiouracil, a relevant antithyroid
7 Hydantoins and Mercaptoimidazoles: Vibrational …
201