Superacidifiers: assessing the activation and the mode of charge transmission of the extraordinary electron-withdrawing SO2CF3 and S(O)(=NSO2CF3)CF3 substituents in carbanion stabilization

We report on a structural (multinuclear NMR), thermodynamic (pK(a)), and kinetic (Marcus intrinsic reactivity) study of the ionization of benzylic carbon acids activated by an exocyclic (alpha) SO(2)CF(3) group and SO(2)CF(3) or S(O)(=NSO(2)CF(3))CF(3) in the para position of the phenyl ring. The latter exerts an enormous acidifying effect of ca. 8 pK units as compared with 4-H benzyltriflone in Me(2)SO solution, (corresponding to remarkably high Hammett sigma values sigma(p) approximately 1.35, sigma(p)(-) approximately 2.30). In considering the origin of this effect, important information was derived in comparing medium effects on pK(a)'s for NO(2), SO(2)CF(3), and S(O)(=NSO(2)CF(3))CF(3) activated carbon acids. Highly contrasting behavior was thus induced by H(2)O --> Me(2)SO transfer, with a large decrease in acidity of alpha-nitro activated carbon acids but a large increase in acidity of alpha-SO(2)CF(3) analogues, leading to remarkable inversions in C-H acidity. These results support the view that in the case of the triflones the carbanion negative charge resides for the most part at the exocyclic Calpha carbon, implying a major role of a polarizability effect. (1)H, (13)C, and (19)F NMR data fully support this proposal. Most importantly, the intrinsic reactivity (log k(0)) positioning 9 and 10 on the Marcus scale for carbon acids could be kinetically measured in 50%H(2)O-50%Me(2)SO; for 9, log k(0) = 3.80 and for 10, log k(0) = 4.20. Such high log k(0) values correspond to low intrinsic barriers which can only be reconciled on the basis of minimum electronic and structural reorganization in formation of the conjugate carbanions. This further emphasizes polarization as the predominant mechanistic mode of charge stabilization in these species.     

Nuclear spin lattice relaxation of191mIr in Fe

Thermal cycling of the lattice temperature was used to determine the nuclear spin lattice relaxation of^191m IrFe in polarizing fields of 0.05 to 1.3 T. At low temperatures, the relaxation time is not very much shorter than the lifetime of^191m Ir. In the first part of the paper, the master equation formalism is extended to include a finite lifetime. Our result for the reduced relaxation constant, ² C _K =(1.48±0.11)·10¹⁴ K s^–1 T^–2 (high field limit) is in serious disagreement with that of a spin echo measurement of¹⁹³IrFe, but fits much better into the general systematics. A comparison of relaxation rates for 3d-, 4d-, and 5d-impurities in Fe is given. As a by-product, a Kapitza conductivity constant ofl _K =1.5 mW cm^–2 K^–4±30% was found between Fe and dilute³He/⁴He.     

Eine Reaktion zur Unterscheidung von Essigs?ureanhydrid und Eisessig

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Literatur

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