Primary reactions in crystalline Rochelle salt studied by pulse radiolysis and EPR spectroscopy. The decarboxylation of the acyloxy radical ion

Abstract

The acyloxy radical ion −O2CCHODCHODCȮ2 (I) was identified in irradiated Rochelle salt by combining the techniques of pulse radiolysis and EPR spectroscopy. This primary radical gives hyperfine couplings due to 13C, 1H, and 23Na and has a broad optical absorption band with the peak at 405 nm. It decomposes by decarboxylation in a first order decay. The half life at 22 °C is 55 ns and the activation energy for decarboxylation is 7.5 kcal mol−1. The G value for the CO2 formation is 2.7 and the extinction coefficient of I is 900M−1 cm−1. No effects of the ferroelectricity of the salt on εmaxG were found.

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

“…These observations are similar to those of Samskog et al (Samskog et al, 1978) who performed an EPR single crystal study of Rochelle salt crystals irradiated at 77 K and warmed to 108 K. The central spectral features of the spectra were assigned to couplings from an -and a -proton. Coupling to 13 C in natural abundance was also detected for this resonance and was typical for a carbon-centred -radical.…”

“…On the other hand, decarboxylation of a primary carboxyl oneelectron oxidation product has been shown to be a common process that often is occurring either spontaneously or by very low thermal activation (Lipfert et al, 2004;Øhman et al, 2006). Previous authors have detected formation of CO 2 in irradiated Rochelle salt samples by mass spectroscopy of the gas extracted from the crystals (Samskog et al, 1978). Upon decarboxylation, a radical with an α-coupling and (eventually) one non-exchangeable β-coupling is expected to be formed, with an accompanying reconfiguration from the initial pyramidal sp 3 -hybridized conformation towards the planar sp 2 hybridized conformation.…”

“…Likewise, the dipolar interaction for the R1 -proton coupling with principal values (7.6, -2.7, -4.9) MHz is characteristic for protonated carboxylic anions (Muto, 1991). (Samskog et al, 1978;Samskog et al, 1979). These authors in addition determined the 13 C hyperfine coupling tensor from the central carbon and from that, a 2p-spin density of 0.71 at C 4 could be extracted.…”

“…show that this type of bending will bring the eigenvector of the maximum 13 C coupling (Samskog et al, 1978;Samskog et al, 1979) closer to the lone electron orbital (LEO), the extent of which depends upon the degree of bending. The deviation between the observed and calculated β-coupling may be due to a non-symmetrical bending at C 4 , resulting in an effective reduction of the dihedral angle.…”

“…However, a different decay mechanism involving water elimination from protonated oneelectron reduction products was more recently suggested for the Rochelle salt and for other salts of tartaric acid (Samskog et al, 1978;Samskog et al, 1979). The kinetics of reduction radical decay and the growth of the secondary RĊHCOO -radical were measured both by pulse radiolysis and by EPR.…”