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内容記述 |
IntroductionFree radicals play an important role in various fast reactions undergoing in biological systems. To understand these complicated free radical reactions, kinetic study based on the elementary reaction processes is of importance. So far, there are many literatures about free radical reaction rate constants. However, many of them were not obtained by measuring elementary reaction processes, that is simply because radicals are short-lived intermediates and the time-resolved spectroscopic observation is not straightforward to probe them. Although electronic absorption spectroscopy is known to be sensitive and the time-resolution is adequately high, many radicals have no or weak absorption in UV/VIS wavelength regions.Early this century, a new method to determine radical reaction rate constants have been developed by utilizing pulse ESR combined with pulsed laser [1,2]. This method needs (1) photo-induced spin polarization to enhance sensitivity of ESR and (2) spin coherence of free radicals to perform Stern-Volmer type analysis for the reaction rate measurements. Here, we introduce the method for study of alkyl type radical reactions with various radical scavenger molecules [3].Results and DiscussionTime resolved (TR-) ESR spectra of the intermediate radicals generated in the reactions between radical scavengers and alkane type radicals were measured to elucidate the free radical reaction mechanisms. Photolysis product radical, hydroxycyclohexyl (Hy-CyH) undergoes hydrogen abstraction reaction with tri-tert-butylphenol (TTBP). Hyperfine structure of Hy-CyH radical is clearly seen along with the single peak of benzoyl. Meanwhile, the peak intensity of Hy-CyH is weakened in the presence of the radical scavenger, TTBP. TTBP gives phenoxyl type radical by the reaction and its peaks appear near the benzoyl peak. The expanded view of phenoxyl and benzyol peaks shows the triplet peak with a splitting width of 1.8 G for phenoxyl. This value agrees with the literature value, which is attributed to the protons of benzene ring.After the intermediate radical was assigned, we made rate constant measurements. From the Stern-Volmer plot of the electron spin echo decay rates of Hy-CyH under various TTBP concentrations, the reaction rate constant of Hy-CyH and TTBP was determined to be (6.1+/-0.3) x 10(6) M(-1) s(-1). We carried out similar experiments with various radical scavengers to determine the rate constants. At the conference, we will discuss the radical reaction rate constants with some antioxidant radical quenchers in biological systems.References[1] Weber, M.; Turro, N. J. J. Phys. Chem. A 2003, 107, 3326[2] Takahashi, H.; Marushima, Y.; Tsuji, K.; Shibuya, K.; Kawai, A. J. Phys. Chem. A 2025, 119, 8261.[3] Hirano H.; Marumo, K.; Takahashi, H.; Nakanishi, I.; Kawai, A. J. Chem. Phys. 2025, 162, 124707. |
