The formation of reducing and oxidizing radicals has been investigated in the reaction of hydroxyl radicals with formate, alcohols, and diols in pulse-irradiated aqueous solutions. Reducing radicals (C02~a nd a-alcohol radicals) were identified by their reaction with tetranitromethane, oxidizing alkoxy radicals by their reaction with iodide. The reaction of hydroxyl radicals with formate and ethylene glycol exclusively leads to a radical with reducing properties. The principal reducing radical from methanol, CH2OH, is formed from only 93% of the reacting hydroxyl radicals. The remaining 7% are accounted for by methoxy radicals formed in the reaction OH-+ CH3OH -* H20 + CHsO-. Relative probabilities for hydrogen atom abstraction from the a position, from the OH group, and from other positions of alcohols and diols by OH radicals are derived from the experimental data.

Radiation chemical methods were used to investigate the reactions of glycine anions, H2NCH2CO2 - (Gly-), with •OH, (CH3)2C•OH, and •CH3 radicals. A major and most significant product from all of these processes is CO2. Pulse-radiolysis revealed that the initial step in the •OH-induced mechanism is oxidation of the amino group, producing +H2N•-CH2-CO2 - and HN•-CH2-CO2 - with yields of 63% and 37%, respectively. The amino radical cation, +H2N•-CH2-CO2 -, suffers fast (≤100 ns) fragmentation into CO2 + •CH2NH2. The other primary radical, HN•-CH2-CO2 -, can also be converted into the decarboxylating +H2N•-CH2-CO2 - by reaction with proton donors such as phosphate (H2PO4 -/k = 7.4 × 107 M-1 s-1, and HPO4 2-/k = 2.5 × 105 M-1 s-1) or the glycine zwitterion, Gly± (k = 3.9 × 105 M-1 s-1), but only on a much longer (typically μs to ms) time scale (k ≈ 4 × 105 M-1 s-1). Competitively, the HN•-CH2-CO2 - transforms into a carbon-centered radical H2N-C•H-CO2 - either by an intramolecular 1,2-H-atom shift (k = (1.2 ± 1.0) × 103 s-1) or by bimolecular reaction with Gly- (k = (3.0 ± 0.2) × 104 M-1 s-1). Both C-centered radicals, H2N-C•H-CO2 - and •CH2NH2, are reductants as verified through their reactions with Fe(CN)6 3- and methyl viologen (MV2+) in pulse-radiolysis experiments (k ≈ 4 × 109 M-1 s-1). The eventual complete transformation of all primary radicals into H2N-C•H-CO2 - and •CH2NH2 was further substantiated by γ-radiolytic reduction of Fe(CN)6 3-. In the presence of suitable electron donors, the HN•-CH2-CO2 - radical acts as an oxidant. This was demonstrated through its reaction with hydroquinone (k = (7.4 ± 0.5) × 107 M-1 s-1). Although the C-centered H2N-C•H-CO2 - radical is not generated in a direct H-atom abstraction by •OH, this radical appears to be the exclusive product in the reaction of Gly- with (CH3)2C•OH, •CH2NH2, and •CH3 (k ≈ 102 M-1 s-1). A most significant finding is that H2N-C•H-CO2 - can be converted into the decarboxylating N-centered radical cation +H2N•-CH2-CO2 - by reaction with proton donors such as Gly± (k ≈ 3 × 103 M-1 s-1) or phosphate and thus also becomes a source of CO2. The •CH2NH2-induced route establishes, in fact, a chain mechanism which could be proven through dose rate effect experiments and suppression of the chain upon addition of Fe(CN)6 3- or MV2+ as a scavenger for the reducing precursor radicals. The possible initiation of amino acid decarboxylation by C-centered radicals and the assistance of proton donors at various stages within the overall mechanism are considered to be of general significance and interest in chemical and biological systems.

Time-resolved and steady-state techniques have been performed to investigate the photophysical properties of C 60 [C(COOEt) 2 ] (Ia), e-C 60 [COOEt) (IIc), and e,e,e-C 60 [C(COOEt) 2 ] 3 (III) (e ) equatorial). Picosecond-resolved energy transfer to the fullerene core results in the rapid formation of the excited singlet state with remarkably blue-shifted singlet-singlet (S* 1 f S* n ) transitions (868 nm (III)) relative to pristine C 60 (920 nm). Intersystem crossing to the energetically lower lying excited triplet state exhibits a deceleration with increasing number of functionalizing addends. The corresponding triplet-triplet (T* 1 f T* n ) absorption energies also show a significant dependence on the degree and site of functionalization, spreading over a range of 100 nm (750 nm for 3 C 60 to 650 nm for e,e,e-( 3 C 60 )[C(COOEt) 2 ] 3 (III)). Energy transfer from radiolytically excited biphenyl ( 3 BP) to the fullerene's ground state corroborates the photolytic data. *0 f 0 Emissions from the lowest level of the excited singlet state (fluorescence) are mirror images of the reversed 0 f *0 absorption transitions with minor Stokes shift. Red shifts of fluorescence-and phosphorescence-related emission, relative to pristine C 60 , again sensitively reflect the perturbation of the fullerene's π-system as a function of the degree and site of functionalization. Cyclic voltammetry and reductive quenching of excited triplet fullerenes demonstrate that functionalization of C 60 obstructs the ease of reduction in the ground and excited triplet state. An increasing number of functional groups results in a cathodic shift of the redox potential (ground state -0.54 to -0.86 V; excited singlet state 1.44 to 0.91 V; excited triplet state +1.01 to +0.64 V Versus SCE for C 60 and e,e,e-( 3 C 60 )[C(COOEt) 2 ] 3 (III), respectively).

If this interpretation is correct, the variable transition-state theory seems to be valid also for dehalogenation reactions. With meso-1,2-dibromo-1,2-diphenylethane as the substrate, the transition-state structure is central when the nucleophile is a halide ion and shifts toward the Elcb-like side with Ph3P. It is possible that with the latter base the incipient positive charge on the phosphorus atom in the transition state favors the buildup of negative charge a t Cg, thus promoting a transition state with carbanion character.The factors affecting base reactivity in dehalogenation reactions have been primarily investigated using the reactions of erythro-l-chloro-2-iodo-1,2-diphenylethane (eq 1, X = I, Y = Cl).61 The base attacks the iodine atom in this case, and elimination of IC1 takes place. The main conclusion of this study is that polarizability and desolvation energy are by far more important factors than basicity in determinating the reactivity of a base in a dehalogenation process. In particular, the role of polarizability is clearly shown by the reactivity order I-> Br-> C1-observed in dipolar aprotic solvent in both the reactions of erythro-lchloro-2-iodo-1,2-diphenylethane and meso-1,2-dibromo-1,2-diphenylethane. Concluding RemarksThe above discussion has shown that, despite the fact that quantitative work in this area has started only (61) E. Baciocchi and C. Lillocci, J . Chem. Soc., Perkin Trans. 2, 802 (1975). recently, some important conclusions regarding the influence of the base on the E2 transition state have been reached. This appears to be particularly true when the effects of basicity are considered. Little doubt now exists that, at least in anti eliminations, the stronger base generally produces a transition state with higher carbanion character. Confirmation of this conclusion in the case of syn eliminations is highly desirable.62Additional work on the effects of base associations appears to be necessary in light of the somewhat contradictory results obtained so far. Moreover, the effects of the steric requirements of the base should be further investigated. Also of great interest would be studies aimed at determining (through a-carbon and leaving group isotope effects) the influence of the base nature on the extent to which the bond between a-carbon and leaving group is weakened in the transition state. Such information would complement that given by p and kH/kD values, thus allowing a complete picture of the transition-state structure to be obtained. Z wish to acknowledge support by the Italian National Research Council (C.N.R.) and the contribution of m y co-workers whose names appear in the cited literature. I wish also to thank Professors R. A. Bartsch and D. J . McLennan for helpful comments. (62) Unpublished results from this laboratory have however shown that in syn eliminations from 12 promoted by phenoxides in Mez §O change in base strength does not significantly influence the carbanion character of the transition state.Organic sulfur compounds, among them sulfides, are kno...