Solid water radical chemistry

Radiation chemistry of ice, i.e., solid water, is described in two newer book chapters (Kroh 1991 Wypych 1999), but it was also discussed in many former books on radiation chemistry. Here also, as with liquid water fast kinetic spectroscopic techniques were used to identify the early processes. Pulse radiolysis measurements have been carried out mostly with single crystal ice, which is sufficiently transparent. As it is common in solid-state radiation chemistry some of the intermediates remain trapped in the matrix and can be studied by means of standard spectroscopic techniques like optical absorption and EPR spectroscopy using these techniques hydroxyl radicals, hydrogen atoms, and trapped electrons were identified at low temperature. The intermediates disappear upon warming up the soUd sample. The hydrogen atoms formed at —269°C completely disappear when the solid is warmed to —196°C. The hydroxyl radicals produced at the latter temperature decay between -170°G and -140°C. 

There have been a number of studies of the mechanism of thermal decomposition of HNIW (Path and Brill 1993) with a particular emphasis on the role of free radicals (Pace 1991, 1992). Ryzhkov and McBride (1996) compared the reactions at low temperature in the a and ft modifications, and found that the same cavities that contain water in the hemihydrate play an important role in differences in the solid state chemistry between the two modifications. 

Upon contact with peroxide one Ti-O is ruptured with formation of a silanol group. The OH bond of peroxide cleaves in this reaction. One of the oxygen atoms of the peroxide radical is then used for selective oxidation. Water or alcohol is formed as a coproduct. A significant issue in the solid state chemistry of the catalyst is prevention of leaking of Ti from the catalyst. This could happen if subsequent bondcleavage of Ti-O occurs. Enough strain has to be present around the catalytic Ti site so that after reaction the Ti-O bond is repaired. 

Our results are consistent with the radiation chemistry data for most of these systems in the solid phase and with the occurrence of reactions well established in vapor phase radiolysis. In the alcohols (and to some extent in acetone) the formation of the principal radical species seems best explained on the basis of ionization, followed by an ion-molecule reaction and subsequent trapping of the appropriate radicals in the frozen matrix at 77°K. As in the case of water (34), electrons can be stabilized in a non-glassy matrix. 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

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