Matrices frozen

This contrasts to the situation for the oxirane radical cation, for which the nature of the species observed experimentally in frozen matrices was long disputed [63], Tomasi and his group [64] first reported calculations on the fate of the oxirane radical cation produced by vertical excitation. Later calculations [65-67] all agree that the C- C ring-opened product 15 is formed from the oxirane radical cation with a low activation energy and that this is the structure observed in frozen matrices. Remarkably, the prototype W.-H. electrocyclic... 

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... 

A crucial methodological step forward was the discovery " that one could observe weU-defined electron spin resonance (ESR) spectra of frozen solutions of triplet species in random orientation. By the early 1960s, spectra of the triplet states of a number of carbenes had been recorded. Thus, when Dowd showed that photolysis of frozen matrices of the diazene (11) or the ketone (12) (Scheme 5.1) gave TMM (4), the spectroscopic tools for the characterization of this key non-Kekule compound lay to hand. Trimethylenemethane was the first non-Kekule molecule to be identified by ESR spectroscopy. 

The bifunctional radical ions 24 + and 26 + are two examples of a family of species, commonly called distonic, which enjoy significant current interest. Much of the research in distonic radical ions is being carried out in the gas phase, typically in tandem mass spectrometers with positive ions as the most frequent targets. However, distonic radical anions have also been studied, and some species are accessible in fluid solution or in frozen matrices. 

Radical cations (94 +) of simple alkynes (e.g., butyne) generated by y-irradia-tion in frozen matrices are stable at 77 K. Upon warming, changes in the ESR spectra support an interesting cycloaddition forming cyclobutadiene radical cations... 

Carbonyl oxides are extremely photolabile even under matrix conditions and irradiation with red light (600 nm) rapidly produces dioxrranes (82). The dioxi-ranes are stable under these conditions but at 400-nm irradiation are converted into esters (83) or lactones. Ketones have been observed as byproducts in the carbene-O2 reactions in frozen matrices. Since the reaction of triplet carbene with O2 is very... 

In fact, rearrangements of singlet alkylnitrenes generated in frozen matrices cannot even be suppressed by developing strain in imine products such as 1-3, although some triplet nitrene is detected by EPR and UV-vis spectroscopy upon low-temperature photolysis of 1-azidonorbornane. ... 

Thallium(I) oxide, T120, is a well-characterized compound and a useful source of Tl+ salts.1 Matrix isolation studies of the solid gave structural data in agreement with spectroscopic results.290,291 The so-called peroxide T102 is obtained electrochemically from aqueous T12S04, but the properties include insolubility in water, alkali and some dilute adds, and treatment with dilute HC1 yields oxygen. The stoichiometry suggests that the compound is in fact a superoxide, and this has been confirmed by spectroscopic studies on frozen matrices.292,293... 

A number of Lewis base adducts of diorganosilylenes have been observed by electronic spectroscopy in frozen matrices (see Section V.A)115. The silylene-donor adducts of ethers, tertiary alcohols, tertiary amines and sulfides were found to revert to silylenes upon warming, but primary and secondary alcohol adducts underwent rearrangement to the H—O formal insertion products (equation 55). 

From the reactions of carbon monoxide in frozen matrices with Me2Si155 and several arylsilylenes Mes(R)Si (R = Mes, 2,6-diisopropylphenoxy, f-Bu)156, adducts were formed (see Section V.A.2). Theoretical calculations led to consideration of both a linear silaketene and a pyramidal n-donor base complex structure for the Me2Si(CO) adduct249. The observation that warming the carbon monoxide adducts of the arylsilylenes led to the formation of disilenes was interpreted as indicating the formation of a nonplanar complex that could dissociate as do other silylene-n-donor base complexes156. 

The mesityl complex [(mes)Os(C2H4)(CH3CN)] results in addition of benzene when irradiated with ultraviolet light in benzene solution (Fig. 19) [103]. Photolysis of the complex in perdeuterotoluene in frozen matrices results in D atom abstraction. Photolysis of Os(VI) dioxo complexes such as [0s(0)2(Bu-salch)] (Scheme 1) with olefins yields epoxides, in one instance with some degree of chiral selectivity [104],... 

The third class of organic donor molecules are a-donors, viz., alkanes and cycloalkanes. These substrates have inherently high ionization and oxidation potentials. Therefore, their radical cations are not readily available by photoinduced electron transfer, but typically require radiolysis and electron impact in the condensed phases or the gas phase, respectively. Thus, radical cations of simple alkanes (methane [206], ethane [207]) or unstrained cycloalkanes (cyclopentane, cyclohexane) [208] were identified and characterized following radiolysis in frozen matrices. In contrast, strained ring compounds have significantly lower oxidation potentials so that the radical cations of appropriate derivatives can be generated by photoinduced electron transfer. 

Scheme 12). Subsequent CO capture by this latter species gives a second mole of product. " It has been independently shown that CpM(CO) H, where M = Ru, Os, species lose CO upon photolysis in frozen matrices (see my Ru, Os paper). 

Terpyridine, N N N ligands (245-249) and their N C N and N N C analogues (250-257) have been successfully coordinated to Pt(II), leading to neutral, mono- or doubly charged species, which in some cases display bright luminescence, both in degassed fluid solutions and frozen matrices. In particular, it has been shown by Che et al. that they can form supramolecular structures, such as nanowires, nanosheets, and polymeric mesophases, with interesting optical properties (214—216,258). 

While no ESR signals can be detected during these photopolymerisation, in frozen matrices containing the reaction components irradiation gives rise to signals which... 

Although the resonance energy of silabenzene is calculated to be about three-fourths of that of benzene, silabenzenes are extremely reactive and, thus, have been examined only in frozen matrices and in the gas phase. By far the simplest route to silabenzenes is pyrolysis of the 1,4-dihydro derivatives. 

Actually, the ESR spectra are more complex than the above simple description based on the exclusive formation of fully extended radical cations. In most frozen matrices, depending on the chain length, radical cations in gauche conformations were also detected [41, 49-53]. In the gauche forms, the spin delocalization is ter-... 

This organometallic radical product could, however, be detected by irradiation in glassy frozen matrices [34]. The alkyl radicals, as very short-lived metal-free products, could be observed and analyzed by FT-ESR with respect to CIDEP (chemically induced dynamic electron polarization) effects [34]. 

In solutions or in frozen matrices the effect of the environment is not negligible any more. The NIR spectra of Cgg were measured in various frozen noble gas matrices [34] and or T)sd distortions were found. The result was the same in the apolar methylcyclohexane matrix, while the distortion was H2h in the polar 2-methyltetrahydrofuran (2-MeTHF) matrix [35]. One might expect the same polarity dependence in solutions, but electrochemically generated Cgg ions showed a... 

NIR Spectrum consistent with Dsd and symmetry both in benzonitrile and in dichloromethane [36], These findings indicate that the symmetry of the environment has to be taken into account besides simple polarity considerations in frozen matrices [37] and in solutions. 

Chemical reactions of alkyltin compounds which occur by way of radical cations are described in Section 5.3.7. The radical cations themselves have been examined in frozen matrices mainly by the technique of ESR spectroscopy, and in the gas phase by mass spectrometry. 

Trifluoromethyl chloroformate, C1C(0)0CF3, exists in the conformation in which the O CF3 bond is synperiplanar with respect to the C=0 bond, with Cs symmetry,30 but neither calculated nor experimental OCOC dihedral angles are given in the paper. Although the anti form can be observed in infra-red spectra of the compound isolated from heated samples into frozen matrices, it is estimated that there is less than 1 % of this form in the gas at room temperature. 

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. 

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