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The «-complex of hexamethylbenzene (HMB) and mercuric trifluoroacetate (HgT2) is converted with high quantum yields to pentamethylbenzyl trifluoroacetate and either mercurous trifluoroacetate or metallic mercury upon irradiation of the charge-transfer absorption band. X-ray crystallography establishes the charge-transfer (CT) excitation to derive from the j)1 2-bonding of the mercuric atom to only two of the aromatic carbons of HMB. The CT photochemistry proceeds via the radical-ion pair [HMB+-,HgT2~•] in accord with Mulliken theory. The high quantum efficiency is ascribed to the rapid ligand dissociation of HgTf• which minimizes the energy wastage due to back electron transfer. The thermal reactions following photoactivation are monitored by following the changes in the CT absorption band and the ESR spectrum of HMB+•. The charge-transfer photochemistry of the [HMB,HgT2] complex shows an unusually high dependency on the solvent-the nature of the mercurycontaining products and the quantum yield for pentamethylbenzyl trifluoroacetate in dichloromethane being distinctively different from that obtained in trifluoroacetic acid. The formation of metallic mercury with quantum yields in trifluoroacetic acid which can be as high as 3 arises from a radical-chain process which sustains itself after the light is turned off. The critical role played by the mercurous trifluoroacetate radical •Hg02CCF3 in the CT photochemistry is described.

SummaryMalonylmethyl radical I [. CH,CH(COOE:t),] and its thioester analogue I1 [. CH,CH(COOEt) (COSEt)] were generated by standard photolytic and thermolytic methods from perester and bromo precursors. The structures of I and I1 were examined by ESR spectroscopy and found to exist in preferred conformations. However, no indication for their rearrangement by 1,2-shift of either an ethoxycarboxyl or (ethylthio)carbonyl group to the corresponding succinyl radicals I11 and IV, respectively, was found at temperatures below -40 "C. At higher temperatures of up to 140 "C, the search for malonylmethyl -+ succinyl rearrangement was examined by thorough-product analysis of the perester decomposition. There is evidence for the rearrangement of the radical I to I11 by photolysis and of the radical I1 to IV by thermolysis at 130 "C in chlorobenzene to only a small extent.

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Bis(pentamethylpheny1)methane shows exceptional reactivity in electron transfer owing to the formation of a novel n-stabilized cation radical, which is mechanistically delineated by time-resolved spectroscopy.We report the unusual properties of bis(pentamethy1-pheny1)methane (decamethyldiphenylmethane; DDM) as an highly active electron donor which is readily converted to the n-stabilized cation radical (DDM'+) merely upon dissolution in trifluoroacetic acid [eqn. (1)l.t The novel structure of DDM'+ is readily deduced from the intense ESR spectrum observed at = 2.0023. Thus, Fig. 1(A) shows the well resolved hyperfine splittings in DDM'+ to consist of triplets (UH = 5.45 G) of tridecets (u12H = 3.34 G) of tridecets (a12H = 1.67 G) (1 G = 10-4 T) from the single methylene, four ortho and four metu groups,$ respectively, as confirmed in the computer simulated spectrum [Fig. l(B)]. The extensively delocalized structure of DDM'+ is reminiscent of the intermolecular analogue,3 in which hexamethylbenzene cation .CHq t The production of cation radicals by the oxidation of neutral donors in acidic media is a well accepted spectral method.' $ The small hyperfine splitting (HFS) from the para methyl groups of less than the spectral resolution (<0.03 G) has precedent in ref. 2.