Radicals matrix photochemistry

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]...

As discussed by Bally [33], matrix photochemistry has obtained excellent results, provided that back reaction is not overwhelming. This is a problem with radicals, much less for carbenes and nitrenes when formed by elimination reactions from diazo compounds and respectively azides. In the latter case, N2 is eliminated with no significant alteration of the matrix. There are exceptions, however, as is the case of parent carbene, the identification of which has long been hindered by the efficient recombination occurring, and finally demonstrated by exchange [42]. 

If the photolysis takes place in an inert gas matrix, both the homolytic splitting of the metal-metal bond and the breaking of a metal-nitrogen bond will be followed by a fast backreaction to the parent compound. The radicals formed by homolysis of the metal-metal bond can not diffuse from the matrix site and will recombine to the parent compound. Moreover, the photoproduct obtained by breaking of a metal nitrogen bond, will not be stabilized by a coordinating solvent molecule and therefore react back to the parent compound. Because of this the photochemistry of some of these complexes has also been studied in a Cl-k-matrix at 10K and for comparison in a PVC film, which is a less rigid medium than the matrix especially at room temperature. 

The only claim of direct evidence for the intermediates proposed above come from ESR measurements subsequent to photolysis of Mn2(CO)j0 in tetrahydrofuran at room temperature.112) A long-lived, ESR detectable, radical was found and proposed to be Mn(CO)5 THF. The intermediate disappears upon addition of I2 and the formation of Mn(CO)sI is observed. These data seem to be wholly consistent with the photochemistry outlined above, but the interpretation of the ESR signal as that due to an Mn(CO)s moiety seems untenable because it is too long-lived. The Re(CO)s species proposed as an intermediate in the photolysis of Re2(CO)10 has recently been synthesized by atom/ligand co-condensation synthesis and infrared data in the matrix at low temperature support a square-pyramidal structure.113) An ESR signal was also observed from a species thought to be Mn(CO)s formed by subliming Mn2(CO)i0 on to a cold tip.114) The ESR detectable species is now believed to be OOMn(CO)s.115)... 

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. 

Decarboxylation of (175) occurs on its irradiation in an argon matrix at 10 K using 254 nm light. Spectroscopic analysis of the resulting matrix indicates the presence of a complex between carbon dioxide and the carbene (176). Tiaprofenic acid (177) undergoes facile photochemical decarboxylation, and this is reported to take place from an upper triplet excited state." A study of the transient photochemistry of 5-(p-toluyl)-l-methyl-2-pyrrolylacetic acid has been reported. Decarboxylation results in the formation of a carbanion in its triplet state. A laser-flash study using irradiation at 266 nm of the xanthene-9-carboxylate (178) has shown that the radical (179) is formed. This study used NaY zeolites and studied the oxidation of the radical within the cage structure. Calculations have indicated that decarboxylation of (180) and (181) and deprotonation of cycloheptatriene and cyclopentadiene affords the same anions (182) and (183), respectively. ... 

Recent work on the role of solvated electrons in intra-DOM reduction processes has demonstrated the importance of trapped e in reactions with species adsorbed on the DOM matrix [98-100]. Modeling of DOM mediated photoreactions indicated the importance of sorption of molecules to DOM for reaction to occur [98, 99]. This is consistent with the lifetime of e" precluding escape from the aqueous DOM matrix into bulk solution. Since many important reactions with environmental implications involve binding or adsorption to DOM - see, for example, [3,101,102] - the role of matrix effects and the caged electron could be very significant. Some workers have suggested that since e remains primarily trapped within the DOM matrix, Oj must be formed by direct electron transfer from the excited triplet state of DOM to O2 [14]. However, it is equally if not more plausible that Oj may be produced by the reduction of Oj by radicals or radical ions produced by intramolecular electron transfer reactions from irradiated DOM [25]. The participation of radicals in the production of carbonyl sulfide and carbon monoxide from irradiated DOM in South Florida coastal waters was recently demonstrated by Zika and co-workers [81-83] and potential pathways for the formation of free radicals from irradiated DOM were discussed. Clearly, the relative contribution of e q and associated transients to the photochemistry of DOM has not been unequivocally resolved in the literature. 

The photochemistry of the thus formed benzylperoxy radical was then studied in the matrix. Irradiation at 365 nm caused a formal 1,3 hydrogen migration followed by cleavage of the peroxyl bond and prolonged irradiation finally yielded phenyl radical 31, CO, and water. This result shows that the benzyl radical is transformed via a series of exothermic steps into even more reactive radicals, such as OH and phenyl radicals [39] (see Scheme 6.13, Fig. 6.20). 

The photochemistry of HN3 in the argon and nitrogen matrices was studied for the first time by J. Pimentel et al. and the IR spectra of triplet nitrene NH and radical NH2 were recorded. A series of studies of matrix isolated NH (in ground triplet and excited singlet A states) and its deutero-substitute analogue (ND) were later performed using UV and luminescence spectroscopy. " The spectroscopy and relaxation of the lowest excited singlet state of NH / ND ( A) were studied in detail in Ne, Ar, Kr and Xe matrices... 

Related topics that He outside the scope of this chapter include thermal reactions in matrices between alkenes and reactive species such as F atoms and NH, even when these have been generated photochem-icaUy, the photochemistry of alkene radical cations, and matrix studies of azaalkenes, silaalkenes, and other alkene analogues. 

Matrix IR Spectra of Radicals Photochemistry of Matrix-Isolated Radicals Carbenes and Their Reactions Cyclopentadienylidene and Related CarbenesAryl Carbenes ... 

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