Irradiation matrix-isolated radicals

The third mechanism of isomerization, photoinduced rearrangements of radical cations, has been pursued in a variety of systems. Matrix isolated radical cations have been noted to undergo some rigorous reorganizations as well as subtle ones. For example, the ring opening of cyclohexadiene to hexatriene radical cation and the interconversion of its different rotamers have been achieved by irradiation with UV or visible light [173-174]. 

Attempts to establish the existence and structure of chlorine tetraoxide have not led to a unique and undisputed situation. Byberg identified the species in his comprehensive studies on crystalline forms such as irradiated potassium perchlorate and was led to the exchange coupled interpretation [7]. Studies of the species in a neon matrix suggested the C31, form to Grothe and Willner [9]. Somewhat elaborate calculations by Van Huis and Schaefer [8] favor the C21, geometry as an optimized structure of the isolated radical. 

Three possible explanations for these observations are (a) a photo-chemical process, (b) a mobile defect, similar to the (CH) soliton and (c) photo-excited charge-transfer. The first of these can be eliminated since photochemistry even in lOH monomer requires u-v irradiation and the radicals produced in irradiated monomer ( ) and related matrix isolated species ( ) have spectra with strong hyperfine structure. 

Vice versa, siloxycarbenes have been generated as short lived intermediates by irradiation of acylsilanes [27-29]. Irradiation of matrix isolated acylsilanes 14c, e and f resulted both in the a-cleavage to give radical pairs and in the rearrangement to siloxycarbenes 13 [29]. Since the formation of these intermediates is reversible, the radical pairs and carbenes 13 could only be identified by oxygen trapping. 

Ethylene and thioformaldehyde are the products of irradiation of matrix-isolated thietane at lOK. Sulphur-carbon bond homolysis has also been observed on irradiation of the nucleoside membrane transport inhibitor, 6-[ (4-nitrobenzyl) thio] -9- (/8-D-ribofuranosyl) -purine (94), and the oxazolidin-2-one (95) has been converted into the allyl derivative (96) by photochemically induced radical allylation.Efficient conversion of cyclic thioacetals into the corresponding carbonyl compounds under neutral conditions has been achieved by 2,4,6-triphenylpyrylium tetrafluoroborate-sensitised irradiation in moist dichloromethane,and diaryl sulphides and the corresponding sulphoxides and sulphones have been reported to undergo anion-promoted carbon-sulphur bond photocleavageboth processes appear to involve an initial electron transfer. Sulphur-hydrogen bond horaolysis has been reported in t-butanethiol and is also responsible for the photoinitiated thiylation of fluorobromoethylenes and of trialkylethynylsilanes and t-butylacetylene. 

Radical-anions are generated by electrochemical reduction of fluorinated pyridines (Scheme 31348) and other heterocycles.349 Radical-anions have also been produced from polyfluoropyridines by X-ray irradiation and studied by ESR using matrix isolation techniques. It was concluded that crossover of [Pg.64]

Radical cations of saturated hydrocarbons have strong electronic absorptions in the visible and near-infrared region of the spectrum. The strongly colored nature of alkane radical cations is in striking contrast to neutral alkanes that absorb electronically only in the vacuum UV. The electronic absorption of alkane radical cations has been studied in the solid phase by matrix isolation using y-irradiation [1-3] and in the gas phase by ion cyclotron resonance (ICR) photodissociation in either the steady-state or pulsed mode of operation [4]. Both methods have their specific merits and drawbacks. A major concern in matrix isolation spectroscopy is spectral purity (because of the possible presence of other absorbing species) and... 

While very little is known about compounds of these two types, simple representatives have been known for a long time. The very first observed compound of multiply bonded silicon was the diatomic radical Si=N, detected in the gas phase in 1913399 and identified in 1925400. It has been investigated in considerable detail more recently401,402. The isonitrile HNSi was studied in matrix isolation as early as 1966403. It was prepared by UV irradiation of silyl azide. 

The matrix isolation method combined with ionizing radiation (i.e., y-rays, X-rays, etc.) at low temperature is versatile and has been extensively developed for ESR study of radical ions [3-13]. The procedure consists of dissolution of the solute molecule (atom) of interest in an appropriate solvent (matrix), freezing at low temperature (in general below 77 K), irradiation (or illumination), and ESR measurements before and after thermal treatment. The solvent molecules (atoms) are ionized by the irradiation to yield an electron (e ) and a positive hole (h ). The electron is transferred in general to a solute molecule with higher electron affinity than that of the solvent molecule to form a solute molecular radical anion (Fig. 5.2). On the other hand, the positive hole is transferred to a solute molecule with first ionization energy (potential) lower than that of the matrix molecule, resulting in the... 

Investigations on benzyl radical were also undertaken by Misic, Piech and Bally.They used this species to generate the corresponding car-bocation under matrix-isolation conditions. The study extended also to the allyl species (radical and cation), and used X-ray irradiation of the radicals to produce in situ the cations. The radicals were generated by pulsed pyrolysis in the gas phase. 

Fig. 6.20 Difference IR spectra in the O-H and C=0 stretching regions showing the photochemistry of benzylperoxy radical matrix isolated in argon. Bands pointing upward appeared and bands pointing downward disappeared during the irradiation, (a) 10 min irradiation at 365 nm at 3 K. (b) Same matrix as in (a) warmed at 25 K. (c) Reference spectrum of benzaldehyde, matrix isolated in 1 % H20-doped argon. The difference spectra, taken at 3 K and after warming at 30 K, show the formation of the 4-H2O complex, (d) Same matrix as (b) after additional 10 min irradiation at 320 nm [39]...

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