Matrix-isolated molecules, free radical

Shielding and Stabilization. Inclusion compounds may be used as sources and reservoirs of unstable species. The inner phases of inclusion compounds uniquely constrain guest movements, provide a medium for reactions, and shelter molecules that self-destmct in the bulk phase or transform and react under atmospheric conditions. Clathrate hosts have been shown to stabiLhe molecules in unusual conformations that can only be obtained in the host lattice (138) and to stabiLhe free radicals (139) and other reactive species (1) similar to the use of matrix isolation techniques. Inclusion compounds do, however, have the great advantage that they can be used over a relatively wide temperature range. Cyclobutadiene, pursued for over a century has been generated photochemicaHy inside a carcerand container (see (17) Fig. 5) where it is protected from dimerization and from reactants by its surrounding shell (140). 

The area in which matrix isolation is perhaps of greatest value is the stabilization of transient species such as free radicals and high-temperature vapors. Until quite recently, infrared spectroscopy was utilized almost exclusively for the vibrational studies of matrix-isolated species. With the introduction of laser sources and the development of more sensitive, electronic, light detection systems, Raman matrix-isolation studies are now feasible and have recently been applied to a limited number of unstable inorganic fluoride species including the molecules OF (5) and C1F2 (6). Both of these species were formed for Raman study by a novel technique that utilizes the... 

Very soon after the publication of the work of Rice and Freamo there appeared independently the suggestion that the stabilization of free radicals produced would be facilitated by the addition of some inert material which would dilute the substrate passing through the furnace. Many papers have been published on this technique, which is commonly known as the matrix isolation method (16, 33). However, the substances commonly used to form the matrix are molecular solids, so that the forces between the molecules are very weak and consequently radicals can be preserved only at temperatures near the boiling point of liquid nitrogen or even lower. If one could incorporate radicals into a diamond-type lattice, it might be possible to stabilize radicals sufficiently to keep them at room temperature. 

This table lists the fundamental vibrational frequencies of selected three-, four-, and five-atom molecules. Both stable molecules and transient free radicals are included. The data have been taken from evaluated sources. In general, the selected values are based on gas-phase infrared, Raman, or ultraviolet spectra when these were not available, liquid-phase or matrix-isolation spectra were used. 

Resembling the use of the matrix isolation technique, inclusion compounds have been recognized as sources and reservoirs of unstable species, mainly of free radical-type As mentioned before, molecules being blocked in an unusual conformation is a second facility... 

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