Spectral properties and absolute rate constants for beta-scission of ring-substituted cumyloxyl radicals. A laser flash photolysis study

A laser flash photolysis study of the spectral properties and beta-scission reactions of a series of ring-substituted cumyloxyl radicals has been carried out. All cumyloxyl radicals display a broad absorption band in the visible region of the spectrum, which decays on the microsecond time scale, leading to a strong increase in absorption in the UV region of the spectrum, which is attributed to the corresponding acetophenone formed after beta-scission of the cumyloxyl radicals. The position of the visible absorption band is red-shifted by the presence of electron-donating ring substituents, while a blue-shift is observed in the presence of electron-withdrawing ring substituents, suggesting that + R ring substituents promote charge separation in the excited cumyloxyl radical through stabilization of the partial positive charge on the aromatic ring of an incipient radical zwitterion. Along this line, an excellent Hammett-type correlation between the experimentally measured energies at the visible absorption maxima of the cumyloxyl radicals and sigma(+) substituent constants is obtained. A red-shift is also observed on going from MeCN to MeCN/H(2)O for all cumyloxyl radicals, pointing toward a specific effect of water. The ring substitution does not influence to a significant extent the rate constants for beta-scission of the cumyloxyl radicals, which varies between 7.1 x 10(5) and 1.1 x 10(6) s(-1), a result that suggests that cumyloxyl radical beta-scission is not governed by the stability of the resulting acetophenone. Finally, k(beta) increases on going from MeCN to the more polar MeCN/H(2)O 1:1 for all cumyloxyl radicals, an observation that reflects the increased stabilization of the transition state for beta-scission through increased solvation of the incipient acetophenone product.     

Stringent neutron-star limits on large extra dimensions

Supernovae (SNe) are copious sources for Kaluza-Klein (KK) gravitons which are generic for theories with large extra dimensions. These massive particles are produced with average velocities approximately 0.5c so that many of them are gravitationally retained by the SN core. Every neutron star thus has a halo of KK gravitons which decay into nu(nu), e(+)e(-), and gammagamma on time scales approximately 10(9) years. The EGRET gamma-flux limits (E(gamma) approximately 100 MeV) for nearby neutron stars constrain the compactification scale for n = 2 extra dimensions to M > or = 500 TeV, and M > or = 30 TeV for n = 3. The requirement that neutron stars are not excessively heated by KK decays implies M > or = 1700 TeV for n = 2, and M > or = 60 TeV for n = 3.     

Contacting and forming singularities: Distinguishing examples

A thin film bridge breaks in a way that starts at one equilibrium state and ends at another equilibrium state. The dynamical trajectory that carries it from connected to disconnected provides rare evidence regarding the singularity of passage through topological change. This nonequilibrium trajectory, called a "forming" flow, is discussed in an attempt to frame it within the larger class of singularities for which bounding surfaces do not remain material surfaces. As a contrast, the weaker "contacting" singularity is illustrated by a stagnation flow where material points reach the stagnation point in finite time. A classification scheme based on pathology of the nonunique Lagrangian motions is suggested. New results for the disconnection example include healing of surgery in post-disconnection simulations, different dynamical scalings of the just-disconnected components and a comparison of post-disconnection simulation to experiment. (c) 1999 American Institute of Physics.     

Neutrino masses and the dark energy equation of state: relaxing the cosmological neutrino mass bound

At present, cosmology provides the nominally strongest constraint on the masses of standard model neutrinos. However, this constraint is extremely dependent on the nature of the dark energy component of the Universe. When the dark energy equation of state parameter is taken as a free (but constant) parameter, the neutrino mass bound is sigma m(v) < or = 1.48 eV (95% C.L.), compared with sigma m(v) < or = 0.65 eV (95% C.L.) in the standard model where the dark energy is in the form of a cosmological constant. This has important consequences for future experiments aimed at the direct measurement of neutrino masses. We also discuss prospects for future cosmological measurements of neutrino masses.     

Virtual Algebraic Lie Theory: Tilting Modules and Ringel Duals for Blob Algebras

In this paper we describe a tensor space representation of theblob algebra over a ring allowing base change to every interesting(that is, non-semisimple) specialisation. We show that in quasihereditaryspecialisations this passes to a full tilting module. We considerthe nature of the resultant Ringel dual algebra. 2000 MathematicsSubject Classification 20C08.     

The dual spectral set conjecture

Suppose that where are real numbers such that and The union is not assumed to be disjoint. It is shown that the translates , , tile the real line for some bounded measurable set if and only if the exponentials , , form an orthogonal basis for some bounded measurable set

     

α-Distonic ions as transient species in the reactions of metastable alkyl benzoate radical cations

The key intermediates to the fragmentation of metastable methyl and ethyl benzoate radical cations are α- and β-distonic isomers of the molecular ions. The α-distocic isomers are also formed by fragmentation of longer chain alkyl benzoates, but may not be long-lived, stable species. Rearrangement of the α-distonic ions prior to fragmentation can take place, but (re)formation of the benzoate molecular ions does not occur.     

Effect of particle size on carburization of Fe-Co alloy catalysts

Bimetallic Fe-Co alloy catalysts supported on silica have been studied by use of combined Mössbauer absorption and emisson spectroscopy. Effects of carburization on samples with different particle size are discussed.