Photooxidation and Reduction of Ascorbic Acid Studied by E.S.R.

Abstract

The photooxidation of ascorbic acid in water at pH4.5-11.6 was studied using electron spin resonance spectroscopy. In the presence of oxygen, only one radical is detected and its resonancelines are broad. In the absence of O,, better resolution is obtained and a second radical is observed. When N 2 0 or HzOz is added, the second radical disappears while the concentration of first radical is increased. These results are consistent with a primary photochemical step involving ejection of an electron from the monoanion of ascorbic acid to form the ascorbate. The second radical appears to be formed by addition of a hydrated electron to the monoanion of ascorbic acid.La photooxydation de I'acide ascorbique dans I'eau a des p H 4.5-11.6 a Bte etudike en utilisant la spectroscopie de resonance du spin Blectronique. En presence d'oxygene, on a detect6 un seul radical et ses bandes de resonance sont larges. En absence d'oxygbne, on a obtenu une meilleure r&solution en mCme temps qu'un second radical fut detect&. Lorsque I'on a ajoute du N 2 0 ou du H z 0 2 , le second radical est disparu alors que la concentration du premier radical a augrnente. Ces resultats sont donc compatibles avec la premiere Ctape photochimique qui implique I'expulsion d'un electron a partir du monoanion de I'acide ascorbique pour former I'ascorbate. Le second radical semble donc Ctre form6 par addition d'un electron hydrate au monoanion de I'acide ascorbique.[Traduit par le journal] C:ln.

Micellized Tris(bipyridine)ruthenium Catalysts Affording Preparative Amounts of Hydrated Electrons with a Green Light‐Emitting Diode

“…The p K a of HAsc − is 11.74; hence, the HAsc − concentration is about 7.5 m m in the experiments of this work. Although the elusive product of the reaction between and HAsc − interferes neither spectroscopically nor mechanistically with our desired ‐induced syntheses, it lowers the utilization through (fortunately only inefficient) kinetic competition. The convolution of the fast decay with the 5 ns generating pulse noticeably rounds the cusp ideally expected for the trace.…”

“…This is consistentw ith our earlier investigations, [8,9,14] which pinpointed the captureo fe À aq by residual ascorbate monoanionH Asc À as the lifetime-limiting factor in the absence of deliberately added scavengers.T he pK a of HAsc À is 11.74; [21] hence,t he HAsc À concentration is about 7.5 mm in the experiments of this work. Although the elusive [23] product of the reaction between e À aq and HAsc À interferes neither spectroscopically nor mechanistically with our desired e À aq -induced syntheses, it lowers the e À aq utilization through (fortunately only inefficient) kinetic competition.T he convolution of the fast decay with the 5nsg enerating pulse noticeably rounds the cusp ideally expected for the e À aq trace. To obtain the true initial heights, we therefore extrapolated the electron decays backt ot he center of the laser flash, as seen superimposedo nthe trace.…”

Laboratory-scale photoredox catalysis using hydrated electrons sustainably generated with a single green laser

“…The ascorbate monoanion AscH – quenches 3 MLCT so much slower as to be negligible 27 but scavenges e aq ˙ – . 28 For given ascorbate total, a variation of the relative amounts of Asc 2– and AscH – thus influences the performance of the catalytic system in a two-fold way, through the efficiency of dianion-assisted e aq ˙ – formation as well as through the partitioning of e aq ˙ – between the substrate and the monoanion. However, some systems require AscH – as hydrogen donor in substantial amounts for terminating the intermediate radicals before they can attack the catalyst (Sections 2.3.2 and 2.3.3).…”

First Micelle‐Free Photoredox Catalytic Access to Hydrated Electrons for Syntheses and Remediations with a Visible LED or even Sunlight

“…The secondary chemistryo fu rate redox systems, so far as it is known at all, [20] is similarly complicated as in the case of ascorbate [21,22] but has remained unchartered territory to am uch larger extent. Probingm ore deeply into the mechanismsoft he extrinsicc atalystd ecomposition would thusb et ime-consuming, yeto fu ncertain benefitb ecause with any practical application the substrate concentration will be high enough for capturing as ubstantial fraction of e À aq ,w hich automatically diminishes the catalyst poisoning.…”