B. S. M. Rao

Radiation and Quantum Chemical Studies of Chalcone Derivatives

The reactions of oxidizing radicals ((*)OH, Br(2)(*-), and SO(4)(*-)) with -OH-, -CH(3)-, or -NH(2)-substituted indole chalcones and hydroxy benzenoid chalcones were studied by radiation and quantum chemical methods. The (*)OH radical was found to react by addition at diffusion-controlled rates (k = 1.1-1.7 x 10(10) dm(3) mol(-1) s(-1)), but Br(2)(*-) radical reacted by 2 orders of magnitude lower. Quantum chemical calculations at the B3LYP/6-31+G(d,p) level of theory have shown that the (C2-OH)(*), (C11-OH)(*), and (C10-OH)(*) adducts of the indole chalcones and the (C7-OH)(*) and (C8-OH)(*) adducts of the hydroxy benzenoid chalcones are more stable with DeltaH = -39 to -28 kcal mol(-1) and DeltaG = -32 to -19 kcal mol(-1). This suggests that (*)OH addition to the alpha,beta-unsaturated bond is a major reaction channel in both types of chalcones and is barrierless. The stability and lack of dehydration of the (*)OH adducts arise from two factors: strong frontier orbital interaction due to the low energy gap between interacting orbitals and the negligible Coulombic repulsion due to small absolute values of Mulliken charges. The transient absorption spectrum measured in the (*)OH radical reaction with all the indole chalcone derivatives exhibited a maximum at 390 nm, which is in excellent agreement with the computed value (394 nm). The formation of three phenolic products under steady-state radiolysis is in line with the three stable (*)OH adducts predicted by theory. Independent of the substituent, identical spectra (lambda(max) = 330-360 and approximately 580 nm) were obtained on one-electron oxidation of the three indole chalcones. MO calculations predict the deprotonation from the -NH group is more efficient than from the substituent due to the larger electron density on the N1 atom forming the chalcone indolyl radical. Its reduction potential was determined to be 0.56 V from the ABTS(*-)/ABTS(2-) couple. In benzenoid chalcones, the (*)OH adduct spectrum is characterized by a peak at 270 nm and a broad maximum centered in the range 430-450 nm with an intense bleaching at 340 nm. The spectrum formed by electron transfer in these derivatives with lambda(max) = 280 and 380 nm (epsilon(280) = 5000 dm(3) mol(-1) cm(-1) and epsilon(380) = 700 dm(3) mol(-1) cm(-1)) was assigned to its phenoxyl radical. Our pulse radiolysis experiments in combination with quantum chemical calculations demonstrate that chalcones are efficient scavengers of damaging oxyl radicals.

Redox chemistry of o- and m-hydroxycinnamic acids: A pulse radiolysis study

Radiation chemical reactions of•OH, O•−, N3•and eaqt- witho- and m-hydroxycinnamic acids were studied. The second-orderrateconstantsforthereaction of•OH with ortho and meta isomers in buffer solution at pH7 are 3.9±0.2 × 109 and 4.4 ± 0.3 × 109 dm3 mol-1 s-1 respectively. At pH 3 the rate with the ortho isomer was halved (1.6 ± 0.4 × 109 dm3 mol-1 s-1) but it was unaffected in the case of meta isomer (k = 4.2±0.6 × 109dm3mol-1 s-1). The rate constant in the reaction of N3• with the ortho isomer is lower by an order of magnitude (k = 4.9 ± 0.4 × 108 dm3 mol-1s-1). The rates of the reaction of eaqt- with ortho and meta isomers were found to be diffusion controlled. The transient absorption spectrum measured in the•OH witho-hydroxycinnamic acid exhibited an absorption maximum at 360 nm and in meta isomer the spectrum was blue-shifted (330 nm) with a shoulder at 390 nm. A peak at 420 nm was observed in the reaction of Obb−with theo-isomer whereas the meta isomer has a maximum at 390 and a broad shoulder at 450 nm. In the reaction of the absorption peaks were centred at 370–380 nm in both the isomers. The underlying reaction mechanism is discussed.

Kinetics and spectral properties of electron and .OH adducts of dimethylpyridines: a pulse radiolysis study

The reactions of eaq-, •OH, O•- and SO•-4 with 2,4-, 2,6- and 3,5-dimethylpyridines have been investigated in aqueous solution by pulse radiolysis with optical detection. Both eaq- and •OH radicals have high reactivity toward these compounds with k = (4-8) × 109 dm3 mol-1 s-1. The rates of O•- and SO•4- reactions ((1-3) × 109 dm3 mol-1 s-1) were lower compared to the rate observed with the •OH radical. The transient absorption spectra obtained in the reaction of eaq- with three isomers exhibited a weak broad band around 340-410 nm. The absorption maxima of the intermediates formed in the •OH and SO•4- reactions were centred around 320-330 nm (ε = 2450-3500 dm3 mol-1 cm-1) with an additional broad peak in the range 460-520 nm which are attributed to the corresponding •OH adducts. The spectra in the O•- reaction have absorption maxima between 300 and 320 nm and it reacts both by addition and H-abstraction from the CH3 group. A reaction mechanism consistent with the observed results is proposed.