Factors affecting stability and equilibria of free radicals

Reactions of 2,2‐Diphenyl‐1‐picrylhydrazyl (DPPH) with Two Syringylic Phenols or One Aroxide Derivative

“…[56][57][58][59][60] In addition, it has been shown that the steric properties of the picryl group also contribute to the stability of DPPH. [61,62] Convincing experimental evidence for the push-pull (capto-dative [63] or merostabilizing [64] ) effect was provided by the identification of high rotation barriers around the N-bonds of congeneric sulfonyl-hydrazyls. [65,66] The diphenylamino end of the DPPH molecule is depleted in electrons, whereas the picryl group is electron-rich, so induced polarization and orientation effects are likely in solutions containing the ion pairs of the supramolecular complex.…”

The Chemistry of DPPH· Free Radical and Congeners

“…The extreme stability of the DPPH · free radical is easily demonstrated by the comparison with its congeners, 2,2-diphenyl-1-(2,4-dinitrophenyl) hydrazyl 1 and 2,2-diphenyl-1-(2,6-dinitrophenyl)-hydrazyl 2 free radicals ( Figure 3 ). The most important nitro groups are situated in the ortho -position with regard to the nitrogen atom; thus, between the two 2,4-dinitro and 2,6-dinitro isomers 1 and 2 ( Figure 3 ), there is a huge difference in terms of their stability [ 14 , 15 ]. Accordingly, isomer 1 (2,4-dinitro) can exist only in solution (with a half-life of about 90 h) and not in solid state, while isomer 2 (2,6-dinitro) is stable both in solution and in solid state [ 16 ].…”

Synthesis and characterization of a new hydrazyl free radical, a formyl-derivative of DPPH

“…[2][3][4][5] Their stability was a subject of high debate, and it can be mentioned the pioneering work of Prof. Balaban and collaborators that pinpoint the factors affecting the stability and equilibria of such nitrogen centered free radicals. 6,7 * Corresponding author: p_ionita@yahoo.co.uk Some p-phenyl derivatives of DPPH are known, containing methyl-, hydroxyl-, bromo-, or nitrogroups (Figure 1). 8,9 The substitution of a H-atom from the p-phenyl position usually induce a strong change in the physical and chemical behavior.…”