Rate constants for reduction of substituted methylperoxyl radicals by ascorbate ions and N,N,N',N'-tetramethyl-p-phenylenediamine

Abstract

Absolute rate constants (k) for reduction of substituted methylperoxyl radicals by ascorbate ions and by TMPD [N,N,-A'.N'-tetramethyl-p-phenylenediamine) in aqueous solutions have been determined by pulse radiolysis. The rate constants vary from 106 to 109 M"1 s'1, increasing as the electron-withdrawing capacity of the substituent on the peroxyl group increases.Linear correlations are observed between log k and the Taft substituent constants * for a wide variety of substituents, but not all substituents fit the same line. In the case of ascorbate as reductant, the points for peroxyl radicals that contain halogens on the -carbon lie on a different line (p* = 0.41) than that for the other substituents (p* = 1.25). In the case of TMPD there are also two families of peroxyl radicals: those comprising the electron-donating groups Me through -Bu (p* = 5.6) and those containing electron-withdrawing substituents (p* = 0.64).

Investigation ofL(+)-Ascorbic acid with Raman spectroscopy in visible and UV light

“…This nonlinear behavior with respect to Raman spectral intensity motivated the present study of ascorbic acid, C 6 H 8 O 6 or AH 2 .…”

“…From comparing three different ascorbic-like compounds, assignments of bands due to AH 2 and AH − were made, and it was concluded that the presence of the ring in the structures gave rise to strong signals in the Raman spectra and that signals from the side chains were not nearly as strong. In 1971 Hvoslef and Klaeboe used a red helium-neon laser to study AH 2 as crystals and in aqueous dilution to supplement infrared absorption experiments.…”

“…Structure clarifications on l-ascorbic acid were later made by infrared (IR) and UV spectroscopy by, for example, Falk and Wojcik (40), Lohmann et al (41,42), and Ferrer and Baran (43). The acid and many salts were studied as solids and in aqueous solutions (also in D 2 O between AgCl windows) by use of IR and 13 C-NMR spectroscopy (17,(34)(35)(36)(37)(41)(42)(43)(44). Panicker et al (45) obtained Fourier transform infrared (FTIR) and FT-Raman spectra including surface-enhanced Raman spectra studies with Ag colloids (probably excited with a 1,064 nm Nd-YAG laser), and many of the vibrational modes of the acid were confirmed or reassigned.…”

“…The influence of high pressures (up to 5.9 GPa) on the hydrogen bonding in AH 2 crystals has been studied (46), and extreme experiments have been performed with simple equipment in cold environments and at high altitudes (47). The temperature dependence (from 15 to 418 K) of the AH 2 Raman spectrum has been investigated, and a crystal phase change in the temperature range 200-270 K has been identified (48). (It is interesting that this phase change, detected by short time scale spectroscopy, did not influence the average structure-based on long timescale diffraction experiments-compare (24)-(26)-but such behaviors have been seen for others crystals, as discussed in, for example, (49)(50)(51).)…”

“…in many redox reactions in natural biological systems and is commonly found in small concentrations in fruits and vegetables (2)(3)(4)(5)(6). The acid is soluble in water and is an important nutrition component for humans, primates, and guinea pigs, which all are incapable of synthesizing it from glucose due to their lack of the enzyme gulonolactone oxidase in the glucoronic acid pathway (7).…”