Freons radiolysis

The ion-molecule reaction between thiirane and its radical cation to form a thiirane sulfide radical cation and ethylene has been studied by Qin, Meng and WiUiams [134]. ESR studies using a low-temperature sohd-state Freon radiolysis technique provided compeUing evidence that the hemibonded dimer radical cation of thiirane is an intermediate in this so-called sulfur-transfer reaction see Scheme 2. 

Recently, the radical cation of PBN has been characterized by matrix spectroscopy and its reactivity has been studied by fast spectroscopic methods (Zubarev and Brede, 1994), and found to conform to the behaviour deduced from the OsCU and TBPA + studies. y-Radiolysis of PBN in a glassy matrix of isobutyl chloride or Freon-113 (CF2C1CFC12) at 77 K produced an intensely green glass containing PBN +, the epr spectrum of which had an anisotropic nitrogen coupling constant Ay = 2.75 mT and gy = 2.0037. Tlie mechanism of the radiolysis reaction is well established (Neta, 1976) and involves the formation of solvated electrons (e ), which add to the matrix species and produce chloride ion, and positive holes (h+) which eventually come to rest at the matrix component of lowest Ip (Symons, 1997), in this case PBN (see reactions (30) and (31)). 

Actually, the earliest derivative of a vinylcyclopropane radical cation was a serendipitous discovery. It was formed by an unusual hydrogen shift upon photo-induced electron transfer oxidation of tricyclo[4.1.0.0 ]heptane (26). This result has been questioned on the grounds that the same rearrangement was not observed in a Freon matrix. However, there is no basis for the assumption that radical cation reactions in frozen matrices at cryogenic temperatures should follow the same course as those at room temperature in fluid solution and in the presence of a radical anion, which is potentially a strong base. In several cases, matrix reactions have taken a decidedly different course from those in solution. For example, radiolysis of 8 in a Freon matrix generated the bicyclo[3.2.0]hepta-2,6-diene radical cation (27 ), or caused retro-Diels-Alder cleavage yet, the... 

Radiolysis of the diacetylene hexa-l,5-diyne (50) generates the hexa-1,2,4,5-tetraene radical cation (51 +) via a Cope rearrangement in the Freon matrix. ... 

The vinylcyclopropane radical cation, generated at 77 K by X-irradiation of (139) in a Freon-113 matrix, was shown to rearrange at 105-110 K to afford two ring-opened distonic radical cationic species.300 The rearrangement reactions of the radical cations of 1,3- and 1,4-pentadiene and cyclopentene and the formation of spin adducts with 2.4.6-tri-/-butylnitrosobenzene (BNB) are discussed. The pulse radiolysis of 1,1 -binaphthyl-2,2,-diyl hydrogenphosphate (BiNPCUH) (140) in deaerated f-butanol at... 

Electron transfer from c-CeH was studied by many groups as a convenient and simple target for pulse radiolysis [37, 90-93]. Radiolysis of c-C Hu in Freon-113... 

Electron-transfer reactions of higher cycloalkanes were also studied. Electron transfer from C-C7H14 to unstable holes generated by radiolysis in Freon-113 gave rise to a stable radical cation, c-Ci A f its spectrum was interpreted in terms of a twisted chair form with C2 symmetry [37]. Finally, radiolysis of c-CgHie in a Freon-113 matrix generated a Jahn-Teller-active radical cation, c-CgHie, with three sets of non-equivalent protons [37]. A detailed discussion of these species exceeds the scope of this review. 

Radiolysis of 96 resulted in an interesting, temperature-dependent spectrum. At 4 K, the species is Jahn-Teller-active and exhibits a static distortion from D3/, to C2v symmetry. In contrast to the bicyclo[2.2.1]heptane radical cation, the SOMO of 96 + involves four endo-C-H bonds, an = 3.8 mT. At 77 K in perfluorocyclohexane or Freon-113, the radical cation is dynamically averaged, with splitting from 12 equivalent protons [235]. 

The electronic absorption spectrum of the cation-radical of thiophene itself has been observed following low-temperature y-radiolysis of the heterocycle in a Freon matrix.The radical has also been implicated in the oxidation of thiophene by dibenzoyl peroxide it is believed to be formed at the contact of certain transition metal layer-silicates with thiophene.The anodic oxidation of 2,5-dimethylthiophene has been studied by Japanese workers who found strong evidence for the formation of the cation-radical as the primary oxidation product.In the presence of strong nucleophiles such as cyanide ion, the cation-radical undergoes nucleophilic attack before further oxidation. In the presence of more basic species such as acetate ion, the cation-radical is deprotonated to give a thienylmethyl radical which undergoes further reaction. The results were compared with similar observations for the oxidation of 2,5-dimethylfuran. Czech workers have also studied the anodic oxidation of substituted thiophenes. This work has focused on the preparative value of anodic oxidations in acidified methanol. Cation-radical formation is implied for the primary step, but the value of the method lies in the fact that sulfur is ultimately eliminated from the substrate and functionalized y-dicarbonyl compounds result. 

The very rapid oxidation of phenols by solvent radical cations can be expected to yield phenol radical cations as the first products. These species are short-lived, except in highly acidic solutions, and were not observed in the microsecond pnlse radiolysis experiments described above. They were detected, however, in frozen matrices and with nanosecond pulse radiolysis Gamma irradiation of phenols in w-butyl chloride or in l,l,2-trichloro-l,2,2-trifluoroethane (Freon 113) at 77 K produced phenol radical cations, which were detected by their optical absorption and ESR spectra . Annealing to 133 K resulted in deprotonation of the radical cations to yield phenoxyl radicals. Pulse radiolysis of p-methoxyphenol and its 2,6-di-fert-butyl derivative in w-butyl chloride at room temperature produced both the phenol radical cations and the phenoxyl radicals. The phenol radical cations were formed very rapidly k = 1.5 x 10 ° M s ) and decayed in a first-order process k = 2.2 x 10 s ) to yield the phenoxyl radicals. The phenoxyl radicals were partially formed in this slower process and partially in a fast process. The fast process of phenoxyl formation probably involves proton transfer to the solvent along with the electron transfer. When the p-methoxy group was replaced with alkyl or H, the stability of the phenol radical cation was lower and the species observed at short times were more predominantly phenoxyl radicals. 

The radical cations of thiophene 113a, and its derivatives have been made in Freon by y-radiolysis [243,244] and in solution by UV irradiation [245]. 

The optical absorption spectra of the high mobility solvent holes resemble those for the radical cations isolated in freon matrices [20,22-25]. All of these spectra are bell-shaped featureless curves with maxima in the visible and/or near IR regions. In pulse radiolysis studies, the absorption signal from the solvent hole always overlaps with the signals from the fragment (and/or secondary) radical cations ("satellite ions"), even at the earliest observation times [22-25,57]. Therefore, complex deconvolutions are needed to extract the spectra of the solvent holes. This leaves large uncertainty as for the exact shape of the absorption spectra and the extinction coefficients. 

Similarly, radiolytically produced radical cations can be stabilized in zeohtes and related materials. This possibility was exploited by spectroscopists to study the EPR of radical cations and some neutral radicals even before the development of inert matrices such as rare gases and freons for radical cation stabilization. Recently, work in our laboratory has developed the use of inert zeolites as microreactors to control radical cation reactions and to study radiation chemistry in heterogeneous systems. In the case of active catalysts, radiolysis can potentially produce radical cations of products as weU as starting material. Thus, like the spontaneous oxidation process described above, radiolysis combined with EPR permits a method of post-reaction analysis of products by in situ spectroscopy. 

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