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In the pulse radiolysis of concentrated phosphoric acid solutions, the direct action of radiation on phosphoric acid gives rise to the phosphate radical. The yields of the phosphate radical and the hydroxyl radical are proportional, respectively, to the electron fractions of phosphoric acid and water, and can be quantified as G('0H + H2PO',) = 0.30fw + 0.33, pmol J-'. The forward and reverse rate constants of the equilibrium 'OH + H, PO, e H 2 0 + H2PO', were evaluated as 4.2 x lo4 and 2.5 x lo3 dm3 mol-' s-' respectively, from which E0(H2P0',, H+/H,PO,) = 2.65 V was derived. The decay kinetics and the reactions of the phosphate radical have also been studied.Few studies have been carried out on the pulse radiolysis of phosphoric acid in aqueous solutions.'*2 Grabner et al. have reported that 'OH and H' are converted into the phosphate radical on reaction with H,PO,, whereas Black and Hayon failed to observe the phosphate radical in the pulse radiolysis of 1 mol dm-3 H,P0,.3 In the pulse radiolysis of H2P0, in aqueous solutions, the phosphate radical has been shown to result from the direct action of radiation on the solute.'*4 Recently, we have studied the pulse radiolysis of concentrated nitric and sulfuric acid in order to understand the direct action of radiation on solutes and the reactions of the hydroxyl radical with the acids. This work extends the previous studies to phosphoric acid, in order to examine the direct action of radiation on phosphoric acid, the reaction of the hydroxyl radical with phosphoric acid and the reactions of the phosphate radical.

The formation of nitrous acid has been studied in the 'OC0-y radiolysis of concentrated nitric acid solutions. The yields of nitrous acid are proportional to the electron fraction of nitric acid, on the basis of which, an additional reaction path, NO; O(3P) + NO; with a primary yield of 0.16 pmol J -' , has been verified for the direct action of radiation on nitric acid.

Short-lived pyridinyl radicals (4-carbamylpyridinyl; neutral and cation forms) produced on one-electron reduction of isonicotinamide by e aq -, H • or (CH 3 ) 2 C • OH in aqueous solution have been probed by resonance Raman and ESR spectroscopies. It has been shown that the radical cation formation by direct addition of H • atom to N-protonated isonicotinamide is less efficient (29% yield) than the sequential additions of e aq -and H + . The neutral carbamylpyridinyl radical (1-hydro-4-pyridine amide) protonates in mildly acidic solutions (pK a ∼ 2), in contrast to closed-shell amides which protonate only in highly concentrated aqueous acid solutions. This anomalous property of the amide group conjugated with semireduced pyridine has been explained in terms of ab initio molecular structures which are consistent with resonance Raman frequencies of the radicals and their deuterated isotopomers, and with ESR hyperfine constants. As with stable amides, the proton binds with the oxygen, and not the nitrogen atom, in the radical structure. However, the CO bond is ∼0.02 Å longer in the neutral radical than in isonicotinamide, and there is substantial negative charge on the oxygen atom, which makes this species very reactive toward the proton, as compared to the regular amides.

The enantiodifferentiating photosensitized cyclodimerization of cyclohexa-1,3-diene 1 was performed over a range of temperatures in the presence of chiral arene(poly)carboxylates, giving endo-and exo-[4 ϩ 2] cyclodimers (2a, 2b) and anti-and syn-[2 ϩ 2] cyclodimers (3a, 3b). Among the three chiral cyclodimers (2a, 2b, 3a), only 2b was obtained as an optically active species with an enantiomeric excess (ee) of up to 8.2%. The detailed reaction mechanism and the origin of the enantiodifferentiation have been elucidated, and the crucial role played by the 'microenvironmental polarity' around the chromophore in determining the photoreactivity and the ee of the product is also discussed.

The static and dynamic fluorescence behavior of a series of hexaalkyl benzenehexacarboxylates (R(6)BHC; R = methyl (Me), tert-butyl (tBu), (-)-menthyl (Men), (-)-bornyl (Bor), (-)-1-methylheptyl (MHp), neopentyl (neoPn), and 2-adamantyl (Ad)) was studied by steady-state and time-resolved fluorescence spectroscopy. Dual fluorescence from both the partially relaxed metastable Franck-Condon-like (FC') and the fully relaxed (RX) state was observed for tBu(6)BHC, Men(6)BHC, Bor(6)BHC, MHp(6)BHC, neoPn(6)BHC, and Ad(6)BHC, whereas only single fluorescence from the RX state was observed for Me(6)BHC. Picosecond time-resolved fluorescence spectroscopic measurements clearly demonstrated that the initially formed Franck-Condon (FC) state sequentially converts to the FC' and then to RX state, with the relaxation hindered to such an extent that it shows variation with the steric bulk of the R groups. Thus, the fluorescence lifetimes (tau's) of FC' and RX are critically dependent on the bulkiness of the R groups, varying from 17 to 130 ps and from 0.6 to 1.1 ns, respectively. The relative intensity of FC' and RX fluorescence (I(RX)/I(FC)(')) was found to be dependent on the excitation wavelength, suggesting that the conformational relaxation from the FC' to RX state can compete with the vibrational relaxation of the FC' state. The temperature and pressure dependences were studied by steady-state fluorescence spectroscopy to give the activation energies of 1-3 kcal/mol for the FC'-to-RX relaxation of congested R(6)BHCs, as well as the activation volumes of 2.0, -0.62, and 7.4 mL/mol for tBu(6)BHC, Men(6)BHC, and Bor(6)BHC at room temperature. The fluorescence anisotropy (rho), as a measure of molecular motion, was also determined to be in the ranges of 0.03-0.3 for FC' and 0.003-0.01 for RX. The much larger rho's for the FC' fluorescence by a factor of 2-100 are attributed to the shorter tau's. The I(RX)/I(F' ratio was found to be insensitive to solvent polarity, but critically dependent on solvent viscosity, exhibiting an excellent linear relationship with the reciprocal viscosity. The potential use of these sterically congested R(6)BHCs as microenvironmental viscosity probes is proposed.