Free radical products in X-irradiated Rochelle salt: low temperature ENDOR and DFT studies

Abstract

Abstractreactions upon warming. The one-electron gain product was observed and from the hyperfine interaction with a β-proton it was unambiguously centred at the C 4 position of the tartrate moiety. An additional nearly isotropic hyperfine structure of about 21 MHz was tentatively assigned to interaction with a sodium ion exhibiting a close contact to O 3 in the crystal.Evidence was obtained that the one-electron reduced radical had become protonated at one of the C 4 bonded carboxyl oxygens, most probably O 4 . No evidence for the corresponding C 1 -centred reduction product was found. Two resonance lines (R2, A1) were shown by EIE to belong to a species formed by decarboxylation at C 3 , a secondary oxidation product. Two other resonance lines (K1, K2) were assigned to two varieties of another decarboxylation radical, centred at C 2 , distinguished by differences in the potassium ion coordination.Furthermore, one other resonance line (A2) was tentatively ascribed to a third decarboxylation radical, centred at the opposite end of the tartrate moiety. The precursor of these products, that is, the one-electron loss product, was not observed after X-irradiation at 10 K. Thermally induced free radical reactions followed by EPR in the temperature range 12 to 119 K indicate that a water molecule or a hydroxyl ion is eliminated from the one-electron reduction product radical and that a C 3 -centred radical is formed. The reduction and oxidation reaction pathways of hydroxy acid derivatives are discussed.

Rochelle salt – a structural reinvestigation with improved tools. I. The high-temperature paraelectric phase at 308 K

“…Several studies applying various spectroscopic techniques have been carried out to explore the radical chemistry of X-ray irradiated or -irradiated single crystals of RS. A recent study by Sagstuen et al (2012) using both spectroscopy and DFT calculations to characterize the primary-induced radicals at 10 K and secondary species formed at higher T also gives an excellent overview of previous work.…”

Solved? The reductive radiation chemistry of alanine

“…At the same time, the corresponding carboxyl group pyramidalizes -either 'up' or 'down' -to accommodate the unpaired electron, as has been found in other studies. [53][54][55] The a1a up and a1a down models indicated in Figure 1, however, do not constitute minima yet.…”

Radicals in Ammonium Tartrate at 295 K by X-Radiation: Revised Radical Structures by EMR and DFT Analyses

“…Various radical products may result from primary species in the reduction and oxidation chain of events of tartaric acids, as outlined in our previous work on the Rochelle salt (Sagstuen et al, 2012;Samskog et al, 1979). Singly and doubly protonated reduced species (anions) usually are unstable at room temperature (see below) and decay by participation into various sequences of reaction processes.…”

“…the H(-OH) or H(NH 4 + ) protons. Coupling R1(1) exhibits a dipolar coupling tensor similar to that for a carbon-bonded β-proton coupling, whereas R1(3) is characteristic for an oxygen-bonded β-proton coupling (Sagstuen et al, 2012). R1(2), due to its exchangeability and its significantly smaller dipolar coupling, must be ascribed to an oxygen-bonded γ-proton coupling.…”

“…In our previous work on the Rochelle salt, i.e. the (Na + , K + ) salt of tartaric acid a reaction scheme for carboxylic acids, including the dicarboxylic acids, was proposed, based on a review of the literature as well as the new experimental data presented in that work (Sagstuen et al, 2012). Decarboxylation of the one-electron loss radical and protonation of the one-electron gain radical are the two major primary products in the oxidation and reduction chains of processes, respectively.…”