Role of electron-transfer processes in reactions of diarylcarbenium ions and related quinone methides with nucleophiles

Second-harmonic alternating current voltammetry has been used to determine one-electron reduction potentials of 15 diarylcarbenium ions and 5 structurally analogous quinone methides, which have been employed as reference electrophiles for the development of nucleophilicity scales. A linear correlation (r(2) = 0.993) between the empirical electrophilicity parameters E and the reduction potentials in acetonitrile (E = 14.091E degrees (red) - 0.279) covering a range of 1.64 V (or 158 kJ mol(-)(1)) has been observed. For a large number of nucleophiles, it has been demonstrated that the observed activation free energies of the electrophile-nucleophile combinations are 61-195 kJ mol(-)(1) smaller than the free energy change of electron transfer from nucleophile to electrophile, which definitely excludes outer-sphere electron transfer occurring during these reactions.     

Transient rheology in a new type of Couette apparatus.

Summary Calculations are performed which concern a new type of air-bearing viscometer. The transient motions of the rotor are studied for Newtonian fluids, and some applications are described for non-Newtonian fluids. It is shown that the rise and relaxation motions may be generally well described by only one exponential function of the time.

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Zusammenfassung Es werden Rechnungen betreffend ein luftgelagertes Viskosimeter neuen Typs durchgeführt. Die nichtstationären Bewegungen des Rotors werden zuerst für newtonsche Flüssigkeiten untersucht, anschließend folgt eine Anwendung auf nicht-newtonsche Flüssigkeiten. Es wird gezeigt, daß sowohl die Anlauf- als auch die Relaxationsbewegung im allgemeinen hinreichend gut durch eine einzige Exponentialfunktion beschrieben werden kann.

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With 10 figures and 1 table     

Über Borel's Verallgemeinerung des Grenzbegriffes

Ohne Zusammenfassung     

Topological and orbital-based mechanisms of the electronic stabilization of bis(diisopropylamino)cyclopropenylidene

Previous analysis of the topology of the electron density of bis(dimethylamino)cyclopropenylidene as a model of the stable bis(diisopropylamino)cyclopropenylidene revealed mechanisms of induction/back-polarization, sigma-aromaticity, and sigma-pi polarization to be responsible for the electronic stabilization of the divalent carbon C2 upon amino substitution on the 3MR. This work presents new data from molecular orbital calculations and a full analysis of the operative natural bond orbitals and their interactions. The discrepancies between these orbital-based stabilization mechanisms and the physical stabilization based upon the quantum theory of atoms in molecules are uncovered through the separation of electron localization and delocalization indices into contributions from orbitals of sigma- and pi-symmetry, as well as calculated nucleus-independent chemical shifts that determine the degree of sigma- and pi-delocalization/aromaticity. Graphical representations of functions of the electron density mapped onto various pi-orbital isosurfaces serve to better visualize the underlying differences between mathematical orbital space and the real space of the electron density. This work also provides new insight into the topological-based mechanism through investigation of the changes in the virial of the electronic forces acting on the interatomic surfaces--forces that govern the bonding and stabilization within a molecule.     

Determination of the electrophilicity parameters of diethyl benzylidenemalonates in dimethyl sulfoxide: reference electrophiles for characterizing strong nucleophiles

The second-order rate constants of the reactions of nine substituted diethyl benzylidenemalonates 1 a-i with the carbanions 2 a-e have been determined spectrophotometrically in dimethyl sulfoxide (DMSO). Product studies show that the nucleophiles attack regioselectively at the electrophilic C==C double bond of the Michael acceptors to form the carbanionic adducts 4. The correlation log k(20 degrees C)=s(N+E) allows the determination of the electrophilicity parameters E for the electrophiles 1 a-i from the rate constants determined in this work and the previously published N and s parameters for the nucleophiles 2 a-e. The electrophilicities E for compounds 1 a-i cover a range of six units (-17.7>E>-23.8) and correlate excellently with Hammett's substituent constants sigma(p). The title compounds are roughly ten orders of magnitude less reactive than analogously substituted benzylidene Meldrum's acids, their cyclic analogues. Due to their low reactivities, compounds 1 a-i are suitable reference electrophiles for determining the reactivities of highly reactive nucleophiles, such as carbanions with 16相似文献